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c 2008 by W. H. Freeman and Company ISBN-13: 978-0-7167-9866-8 ISBN-10: 0-7167-9866-2 All rights reserved Printed in the United States of America

W. H. Freeman and Company, 41 Madison Avenue, New York, NY 10010 Houndmills, Basingstoke RG21 6XS, England www.whfreeman.com

Student’s Solutions Manual to accompany Jon Rogawski’s

Single Variable

CALCULUS BRIAN BRADIE Christopher Newport University With Chapter 12 contributed by

GREGORY P. DRESDEN Washington and Lee University and additional contributions by

Art Belmonte Cindy Chang-Fricke Benjamin G. Jones Kerry Marsack Katherine Socha Jill Zarestky Kenneth Zimmerman

W. H. Freeman and Company New York

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CONTENTS Chapter 1 Precalculus Review 1.1 1.2 1.3 1.4 1.5

2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8

3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9

4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8

5.1 5.2 5.3 5.4 5.5

1

Real Numbers, Functions, and Graphs Linear and Quadratic Functions The Basic Classes of Functions Trigonometric Functions Technology: Calculators and Computers Chapter Review Exercises

1 8 13 16 23 26

Chapter 2 Limits

31

Limits, Rates of Change, and Tangent Lines Limits: A Numerical and Graphical Approach Basic Limit Laws Limits and Continuity Evaluating Limits Algebraically Trigonometric Limits Intermediate Value Theorem The Formal Deﬁnition of a Limit Chapter Review Exercises

31 36 45 48 55 59 65 68 72

Chapter 3 Differentiation

79

Deﬁnition of the Derivative The Derivative as a Function Product and Quotient Rules Rates of Change Higher Derivatives Trigonometric Functions The Chain Rule Implicit Differentiation Related Rates Chapter Review Exercises

79 90 99 105 112 118 123 132 141 148

Chapter 4 Applications of the Derivative

159

Linear Approximation and Applications Extreme Values The Mean Value Theorem and Monotonicity The Shape of a Graph Graph Sketching and Asymptotes Applied Optimization Newton’s Method Antiderivatives Chapter Review Exercises

159 165 173 180 188 202 216 221 228

Chapter 5 The Integral

237

Approximating and Computing Area The Deﬁnite Integral The Fundamental Theorem of Calculus, Part I The Fundamental Theorem of Calculus, Part II Net or Total Change as the Integral of a Rate

237 250 259 266 273

5.6

6.1 6.2 6.3 6.4 6.5

7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9

8.1 8.2 8.3 8.4 8.5 8.6

Substitution Method Chapter Review Exercises

276 283

Chapter 6 Applications of the Integral

292

Area Between Two Curves 292 Setting Up Integrals: Volume, Density, Average Value 300 Volumes of Revolution 308 The Method of Cylindrical Shells 316 Work and Energy 324 Chapter Review Exercises 331

Chapter 7 The Exponential Function

338

Derivative of b x and the Number e Inverse Functions Logarithms and Their Derivatives Exponential Growth and Decay Compound Interest and Present Value Models Involving y = k ( y − b) ˆ L’Hopital’s Rule Inverse Trigonometric Functions Hyperbolic Functions Chapter Review Exercises

338 344 349 357 362 365 369 377 386 392

Chapter 8 Techniques of Integration

404

Numerical Integration Integration by Parts Trigonometric Integrals Trigonometric Substitution The Method of Partial Fractions Improper Integrals Chapter Review Exercises

404 415 426 435 449 469 487

Chapter 9 Further Applications of the Integral and Taylor Polynomials 503 9.1 9.2 9.3 9.4

10.1 10.2 10.3 10.4

Arc Length and Surface Area Fluid Pressure and Force Center of Mass Taylor Polynomials Chapter Review Exercises

503 513 518 526 538

Chapter 10 Introduction to Differential Equations

545

Solving Differential Equations Graphical and Numerical Methods The Logistic Equation First-Order Linear Equations Chapter Review Exercises

545 558 563 568 578

iv

11.1 11.2 11.3 11.4 11.5 11.6 11.7

C A L C U L U S CONTENTS

Chapter 11 Inﬁnite Series

587

Sequences Summing an Inﬁnite Series Convergence of Series with Positive Terms Absolute and Conditional Convergence The Ratio and Root Tests Power Series Taylor Series Chapter Review Exercises

587 598 608 622 628 636 646 660

12.1 12.2 12.3 12.4 12.5

Chapter 12 Parametric Equations, Polar Coordinates, and Conic Sections

674

Parametric Equations Arc Length and Speed Polar Coordinates Area and Arc Length in Polar Coordinates Conic Sections Chapter Review Exercises

674 690 698 711 721 732

P R E CALCULUS 1 REVIEW 1.1 Real Numbers, Functions, and Graphs Preliminary Questions 1. Give an example of numbers a and b such that a < b and |a| > |b|. SOLUTION

Take a = −3 and b = 1. Then a < b but |a| = 3 > 1 = |b|.

2. Which numbers satisfy |a| = a? Which satisfy |a| = −a? What about | − a| = a? SOLUTION

The numbers a ≥ 0 satisfy |a| = a and | − a| = a. The numbers a ≤ 0 satisfy |a| = −a.

3. Give an example of numbers a and b such that |a + b| < |a| + |b|. SOLUTION

Take a = −3 and b = 1. Then |a + b| = | − 3 + 1| = | − 2| = 2,

but

|a| + |b| = | − 3| + |1| = 3 + 1 = 4.

Thus, |a + b| < |a| + |b|. 4. What are the coordinates of the point lying at the intersection of the lines x = 9 and y = −4? SOLUTION

The point (9, −4) lies at the intersection of the lines x = 9 and y = −4.

5. In which quadrant do the following points lie? (a) (1, 4) (c) (4, −3)

(b) (−3, 2) (d) (−4, −1)

SOLUTION

(a) Because both the x- and y-coordinates of the point (1, 4) are positive, the point (1, 4) lies in the ﬁrst quadrant. (b) Because the x-coordinate of the point (−3, 2) is negative but the y-coordinate is positive, the point (−3, 2) lies in the second quadrant. (c) Because the x-coordinate of the point (4, −3) is positive but the y-coordinate is negative, the point (4, −3) lies in the fourth quadrant. (d) Because both the x- and y-coordinates of the point (−4, −1) are negative, the point (−4, −1) lies in the third quadrant. 6. What is the radius of the circle with equation (x − 9)2 + (y − 9)2 = 9? SOLUTION

The circle with equation (x − 9)2 + (y − 9)2 = 9 has radius 3.

7. The equation f (x) = 5 has a solution if (choose one): (a) 5 belongs to the domain of f . (b) 5 belongs to the range of f . SOLUTION

The correct response is (b): the equation f (x) = 5 has a solution if 5 belongs to the range of f .

8. What kind of symmetry does the graph have if f (−x) = − f (x)? SOLUTION

If f (−x) = − f (x), then the graph of f is symmetric with respect to the origin.

Exercises 1. Use a calculator to ﬁnd a rational number r such that |r − π 2 | < 10−4 . r must satisfy π 2 − 10−4 < r < π 2 + 10−4 , or 9.869504 < r < 9.869705. r = 9.8696 = 12337 1250 would be one such number. SOLUTION

In Exercises express interval of an inequality involving absolute value. Let a3–8, = −3 and b the = 2. Whichinofterms the following inequalities are true? (a) a2]< b 3. [−2, (d) 3a < |x| 3b ≤ SOLUTION 5. (0, 4) (−4, 4)

(b) |a| < |b| 2

(e) −4a < −4b

(c) ab > 0 1 1 (f) < a b

2

CHAPTER 1

PRECALCULUS REVIEW SOLUTION The midpoint of the interval is c = (0 + 4)/2 = 2, and the radius is r = (4 − 0)/2 = 2; therefore, (0, 4) can be expressed as |x − 2| < 2.

7. [1, 5] [−4, 0] SOLUTION The midpoint of the interval is c = (1 + 5)/2 = 3, and the radius is r = (5 − 1)/2 = 2; therefore, the interval [1, 5] can be expressed as |x − 3| ≤ 2. In Exercises write the inequality in the form a < x < b for some numbers a, b. (−2, 9–12, 8) 9. |x| < 8 SOLUTION

−8 < x < 8

11. |2x + 1| < 5 |x − 12| < 8 SOLUTION −5 < 2x + 1 < 5 so −6 < 2x < 4 and −3 < x < 2 In Exercises |3x −13–18, 4| < 2express the set of numbers x satisfying the given condition as an interval. 13. |x| < 4 SOLUTION

(−4, 4)

15. |x − 4| < 2 |x| ≤ 9 SOLUTION The expression |x − 4| < 2 is equivalent to −2 < x − 4 < 2. Therefore, 2 < x < 6, which represents the interval (2, 6). 17. |4x − 1| ≤ 8 |x + 7| < 2 7 9 SOLUTION The expression |4x − 1| ≤ 8 is equivalent to −8 ≤ 4x − 1 ≤ 8 or −7 ≤ 4x ≤ 9. Therefore, − 4 ≤ x ≤ 4 , 7 9 which represents the interval [− 4 , 4 ]. In Exercises |3x +19–22, 5| < 1describe the set as a union of ﬁnite or inﬁnite intervals. 19. {x : |x − 4| > 2} SOLUTION

x − 4 > 2 or x − 4 < −2 ⇒ x > 6 or x < 2 ⇒ (−∞, 2) ∪ (6, ∞)

21. {x : |x 2 − 1| > 2}

{x : |2x + 4| > 3} √ √ x 2 − 1 > 2 or x 2 − 1 < −2 ⇒ x 2 > 3 or x 2 < −1 (this will never happen) ⇒ x > 3 or x < − 3 ⇒ √ √ (−∞, − 3) ∪ ( 3, ∞). SOLUTION

23. Match the inequalities (a)–(f) with the corresponding statements (i)–(vi). {x : |x 2 + 2x| > 2} 1 (a) a > 3 (b) |a − 5| < 3 1 (c) a − < 5 (d) |a| > 5 3 (e) |a − 4| < 3 (f) 1 < a < 5 (i) (ii) (iii) (iv) (v) (vi)

a lies to the right of 3. a lies between 1 and 7. The distance from a to 5 is less than 13 . The distance from a to 3 is at most 2. a is less than 5 units from 13 . a lies either to the left of −5 or to the right of 5.

SOLUTION

(a) On the number line, numbers greater than 3 appear to the right; hence, a > 3 is equivalent to the numbers to the right of 3: (i). (b) |a − 5| measures the distance from a to 5; hence, |a − 5| < 13 is satisﬁed by those numbers less than 13 of a unit from 5: (iii). (c) |a − 13 | measures the distance from a to 13 ; hence, |a − 13 | < 5 is satisﬁed by those numbers less than 5 units from 1 : (v). 3 (d) The inequality |a| > 5 is equivalent to a > 5 or a < −5; that is, either a lies to the right of 5 or to the left of −5: (vi). (e) The interval described by the inequality |a − 4| < 3 has a center at 4 and a radius of 3; that is, the interval consists of those numbers between 1 and 7: (ii). (f) The interval described by the inequality 1 < x < 5 has a center at 3 and a radius of 2; that is, the interval consists of those numbers less than 2 units from 3: (iv).

S E C T I O N 1.1

Real Numbers, Functions, and Graphs

3

25. Show that if a > b, then b−1 > a −1, provided that a and b have the same sign. What happens if a > 0 and b < 0? x Describe the set x : < 0 as an interval. b 1 1 SOLUTION Case 1a: If a and b are x+ 1 both positive, then a > b ⇒ 1 > a ⇒ b > a . b Case 1b: If a and b are both negative, then a > b ⇒ 1 < a (since a is negative) ⇒ b1 > a1 (again, since b is negative). Case 2: If a > 0 and b < 0, then a1 > 0 and b1 < 0 so b1 < a1 . (See Exercise 2f for an example of this). 27. Show that if |a − 5||x<−12 3| and − 8||x<−125| , then Which x satisfy < |b 2 and < 1?|(a + b) − 13| < 1. Hint: Use the triangle inequality. SOLUTION

|a + b − 13| = |(a − 5) + (b − 8)| ≤ |a − 5| + |b − 8| < 29. (a) (b)

(by the triangle inequality)

1 1 + = 1. 2 2

Suppose that |x − 4| ≤ 1. Suppose that |a| ≤ 2 and |b| ≤ 3. What is the maximum possible value of |x + 4|? (a) What is the largest possible value of |a + b|? Show that |x 2 − 16| ≤ 9. (b) What is the largest possible value of |a + b| if a and b have opposite signs?

SOLUTION

(a) |x − 4| ≤ 1 guarantees 3 ≤ x ≤ 5. Thus, 7 ≤ x + 4 ≤ 9, so |x + 4| ≤ 9. (b) |x 2 − 16| = |x − 4| · |x + 4| ≤ 1 · 9 = 9. 31. Express r1 = 0.27 as a fraction. Hint: 100r1 − r1 is an integer. Then express r2 = 0.2666 . . . as a fraction. Prove that |x| − |y| ≤ |x − y|. Hint: Apply the triangle inequality to y and x − y. SOLUTION Let r1 = .27. We observe that 100r1 = 27.27. Therefore, 100r1 − r1 = 27.27 − .27 = 27 and r1 =

27 3 = . 99 11

Now, let r2 = .2666. Then 10r2 = 2.666 and 100r2 = 26.666. Therefore, 100r2 − 10r2 = 26.666 − 2.666 = 24 and r2 =

24 4 = . 90 15

33. The text states the following: If the decimal expansions of two real numbers a and b agree to k places, then the Represent 1/7 and 4/27 as repeating decimals. distance |a − b| ≤ 10−k . Show that the converse is not true, that is, for any k we can ﬁnd real numbers a and b whose decimal expansions do not agree at all but |a − b| ≤ 10−k . SOLUTION Let a = 1 and b = .9 (see the discussion before Example 1). The decimal expansions of a and b do not agree, but |1 − .9| < 10−k for all k.

35. Determine the equation of the circle with center (2, 4) and radius 3. Plot each pair of points and compute the distance between them: 2 2 = 32 = 9. SOLUTION (y −(2, 4)1) (a) (1, 4)The andequation (3, 2) of the indicated circle is (x − 2) + (b) and (2, 4) 37. Find all0)points with3)integer coordinates located at a distance from−3) theand origin. (c) (0, and (−2, (d) 5(−3, (−2,Then 3) ﬁnd all points with integer Determine the equation of the circle with center (2, 4) passing through (1, −1). coordinates located at a distance 5 from (2, 3). SOLUTION

• To be located a distance 5 from the origin, the points must lie on the circle x 2 + y 2 = 25. This leads to 12 points

with integer coordinates: (5, 0) (3, 4) (4, 3)

(−5, 0) (−3, 4) (−4, 3)

(0, 5) (3, −4) (4, −3)

(0, −5) (−3, −4) (−4, −3)

• To be located a distance 5 from the point (2, 3), the points must lie on the circle (x − 2)2 + (y − 3)2 = 25, which

implies that we must shift the points listed above two units to the right and three units up. This gives the 12 points: (7, 3) (5, 7) (6, 6)

(−3, 3) (−1, 7) (−2, 6)

(2, 8) (5, −1) (6, 0)

(2, −2) (−1, −1) (−2, 0)

39. Give an example of a function whose domain D has three elements and range R has two elements. Does a function Determine the domain and range of the function exist whose domain D has two elements and range has three elements? f : {r, s, t, u} → { A, B, C, D, E} deﬁned by f (r ) = A, f (s) = B, f (t) = B, f (u) = E.

4

CHAPTER 1

PRECALCULUS REVIEW

Deﬁne f by f : {a, b, c} → {1, 2} where f (a) = 1, f (b) = 1, f (c) = 2. There is no function whose domain has two elements and range has three elements. If that happened, one of the domain elements would get assigned to more than one element of the range, which would contradict the deﬁnition of a function. SOLUTION

In Exercises 40–48, ﬁnd the domain and range of the function. 4 41. g(t)f = (x)t = −x SOLUTION D : all reals; R : {y : y ≥ 0} √ 43. g(t) = 2 −3t f (x) = x SOLUTION D : {t : t ≤ 2}; R : {y : y ≥ 0}

1 45. h(s)f = (x) = |x| s SOLUTION D : {s : s = 0}; R : {y : y = 0} 47. g(t) = 2 +1t 2 √ f (x) = SOLUTION D x: 2all reals; R : {y : y ≥ 2} In Exercises 49–52,1ﬁnd the interval on which the function is increasing. g(t) = cos 49. f (x) = |x + 1|t SOLUTION A graph of the function y = |x + 1| is shown below. From the graph, we see that the function is increasing on the interval (−1, ∞). y 2

1

−3

−2

x

−1

1

51. f (x) = x 4 3 f (x) = x SOLUTION A graph of the function y = x 4 is shown below. From the graph, we see that the function is increasing on the interval (0, ∞). y 12 8 4 −2

x

−1

1

2

In Exercises 53–58, 1 ﬁnd the zeros of the function and sketch its graph by plotting points. Use symmetry and inf (x) = information crease/decrease where appropriate. 2 x +1 53. f (x) = x 2 − 4 Zeros: ±2 Increasing: x > 0 Decreasing: x < 0 Symmetry: f (−x) = f (x) (even function). So, y-axis symmetry.

SOLUTION

y 4 2 −2 −1

x −2

1

2

−4

55. f (x) = x 3 − 4x f (x) = 2x 2 − 4 SOLUTION Zeros: 0, ±2; Symmetry: f (−x) = − f (x) (odd function). So origin symmetry.

Real Numbers, Functions, and Graphs

S E C T I O N 1.1

5

y 10 5 x

−2 −1 −5

1

2

−10

57. f (x) = 2 − x33 f (x) = x √ 3 SOLUTION This is an x-axis reﬂection of x 3 translated up 2 units. There is one zero at x = 2. y 20 10 x

−2 −1−10 −20

1

2

59. Which of the curves in Figure 26 is the graph of a function? 1 f (x) = y y (x − 1)2 + 1 x x

(A)

(B) y

y

x

x

(C)

(D)

FIGURE 26 SOLUTION

(B) is the graph of a function. (A), (C), and (D) all fail the vertical line test.

In Exercises be theisfunction whose graph is shown in Figure 27. State 61–66, whetherletthef (x) function increasing, decreasing, or neither. (a) (b) (c) (d)

Surface area of a sphere as a function of its radius y Temperature at a point on the equator as a function of time 4 Price of an airline ticket as a function of the price of oil 3 Pressure of the gas in a piston as a function of volume 2 1 x

0 1

2

3

4

FIGURE 27

61. What are the domain and range of f (x)? SOLUTION

D : [0, 4]; R : [0, 4]

1 , andf (x) 2 f (x). 63. Sketch the the graphs of fof(2x), f 2) 2 x and Sketch graphs f (x + + 2. SOLUTION The graph of y = f (2x) is obtained by compressing the graph of y = f (x) horizontally by a factor of 2 (see the graph below on the left). The graph of y = f ( 12 x) is obtained by stretching the graph of y = f (x) horizontally by a factor of 2 (see the graph below in the middle). The graph of y = 2 f (x) is obtained by stretching the graph of y = f (x) vertically by a factor of 2 (see the graph below on the right). y

y

y

4

4

8

3

3

6

2

2

4

1

1

2

x 1

2 f(2x)

3

4

x

x 2

4 f(x/2)

6

8

1

2 2 f(x)

3

4

6

CHAPTER 1

PRECALCULUS REVIEW

65. Extend the graph of f (x) to [−4, 4] so that it is an even function. Sketch the graphs of f (−x) and − f (−x). SOLUTION To continue the graph of f (x) to the interval [−4, 4] as an even function, reﬂect the graph of f (x) across the y-axis (see the graph below). y 4 3 2 1 x

−4 −2

2

4

67. Suppose that f (x) has domain [4, 8] and range [2, 6]. What are the domain and range of: Extend the graph of f (x) to [−4, 4] so that it is an odd function. (a) f (x) + 3 (b) f (x + 3) (c) f (3x) (d) 3 f (x) SOLUTION

(a) f (x) + 3 is obtained by shifting f (x) upward three units. Therefore, the domain remains [4, 8], while the range becomes [5, 9]. (b) f (x + 3) is obtained by shifting f (x) left three units. Therefore, the domain becomes [1, 5], while the range remains [2, 6]. (c) f (3x) is obtained by compressing f (x) horizontally by a factor of three. Therefore, the domain becomes [ 43 , 83 ], while the range remains [2, 6]. (d) 3 f (x) is obtained by stretching f (x) vertically by a factor of three. Therefore, the domain remains [4, 8], while the range becomes [6, 18]. 69. Suppose that the graph of f (x) = sin x is compressed horizontally by a factor of 2 and then shifted 5 units to the 2 right. Let f (x) = x . Sketch the graphs of the following functions over [−2, 2]: (a) f is (xthe + 1) (b) f (x) + 1 (a) What equation for the new graph? (c) f is (5x) (d)by52? f (x) (b) What the equation if you ﬁrst shift by 5 and then compress Verify your answers by plotting your equations.

(c) SOLUTION

(a) Let f (x) = sin x. After compressing the graph of f horizontally by a factor of 2, we obtain the function g(x) = f (2x) = sin 2x. Shifting the graph 5 units to the right then yields h(x) = g(x − 5) = sin 2(x − 5) = sin(2x − 10). (b) Let f (x) = sin x. After shifting the graph 5 units to the right, we obtain the function g(x) = f (x − 5) = sin(x − 5). Compressing the graph horizontally by a factor of 2 then yields h(x) = g(2x) = sin(2x − 5). (c) The ﬁgure below at the top left shows the graphs of y = sin x (the dashed curve), the sine graph compressed horizontally by a factor of 2 (the dash, double dot curve) and then shifted right 5 units (the solid curve). Compare this last graph with the graph of y = sin(2x − 10) shown at the bottom left. The ﬁgure below at the top right shows the graphs of y = sin x (the dashed curve), the sine graph shifted to the right 5 units (the dash, double dot curve) and then compressed horizontally by a factor of 2 (the solid curve). Compare this last graph with the graph of y = sin(2x − 5) shown at the bottom right. y

y

1

1

x

−6 −4 −2

2

4

6

x

−6 −4 −2

−1

6

2

4

6

y

y 1

x 2 −1

4

−1

1

−6 −4 −2

2

4

6

x

−6 −4 −2 −1

Figure 28 shows the graph of f (x) = |x| + 1. Match the functions (a)–(e) with their graphs (i)–(v). (a) f (x − 1) (b) − f (x) (c) − f (x) + 2

Real Numbers, Functions, and Graphs

S E C T I O N 1.1

7

71. Sketch the graph of f (2x) and f 12 x , where f (x) = |x| + 1 (Figure 28). SOLUTION The graph of y = f (2x) is obtained by compressing the graph of y = f (x) horizontally by a factor of 2 (see the graph below on the left). The graph of y = f ( 12 x) is obtained by stretching the graph of y = f (x) horizontally by a factor of 2 (see the graph below on the right). y

y 6

6

4

4

2

2 x

−3 −2 −1

1

2

x

−3 −2 −1

3

f(2x)

1

2

3

f (x/2)

73. Deﬁne f (x) to be the larger of x and 2 − x. Sketch the graph of f (x). What are its domain and range? Express f (x) theabsolute functionvalue f (x)function. whose graph is obtained by shifting the parabola y = x 2 three units to the right and four in termsFind of the units down as in Figure 29. SOLUTION y

2 1 x

−1

1

2

3

The graph of y = f (x) is shown above. Clearly, the domain of f is the set of all real numbers while the range is {y | y ≥ 1}. Notice the graph has the standard V-shape associated with the absolute value function, but the base of the V has been translated to the point (1, 1). Thus, f (x) = |x − 1| + 1. 75. Show that the sum of two even functions is even and the sum of two odd functions is odd. For each curve in Figure 30, state whether it is symmetrical with respect to the y-axis, the origin, both, or neither. even SOLUTION Even: ( f + g)(−x) = f (−x) + g(−x) = f (x) + g(x) = ( f + g)(x) odd

Odd: ( f + g)(−x) = f (−x) + g(−x) = − f (x) + −g(x) = −( f + g)(x) 77. Give an example of a curve that is symmetrical with respect to both the y-axis and the origin. Can the graph of a Suppose that f (x) and g(x) are both odd. Which of the following functions are even? Which are odd? function have both symmetries? Hint: Prove algebraically that f (x) = 0 is the only such function. (a) f (x)g(x) (b) f (x)3 SOLUTION A circle of radius 1 with its center at the origin is symmetrical both with respect to the y-axis and the f (x) (c) f (x) − g(x) (d) origin. g(x) The only function having both symmetries is f (x) = 0. For if f is symmetric with respect to the y-axis, then f (−x) = f (x). If f is also symmetric with respect to the origin, then f (−x) = − f (x). Thus f (x) = − f (x) or 2 f (x) = 0. Finally, f (x) = 0.

Further Insights and Challenges 79. Show that if r = a/b is a fraction in lowest terms, then r has a ﬁnite decimal expansion if and only if b = 2n 5m for Prove the triangle inequality by adding the two inequalities some n, m ≥ 0. Hint: Observe that r has a ﬁnite decimal expansion when 10 N r is an integer for some N ≥ 0 (and hence b divides 10 N ). −|a| ≤ a ≤ |a|, −|b| ≤ b ≤ |b| Suppose r has a ﬁnite decimal expansion. Then there exists an integer N ≥ 0 such that 10 N r is an integer, call it k. Thus, r = k/10 N . Because the only prime factors of 10 are 2 and 5, it follows that when r is written in lowest terms, its denominator must be of the form 2n 5m for some integers n, m ≥ 0. a a Conversely, suppose r = a/b in lowest with b = 2n 5m for some integers n, m ≥ 0. Then r = = n m or b 2 5 a2m−n 2n 5m r = a. If m ≥ n, then 2m 5m r = a2m−n or r = and thus r has a ﬁnite decimal expansion (less than or 10m n−m a5 equal to m terms, to be precise). On the other hand, if n > m, then 2n 5n r = a5n−m or r = and once again r has 10n a ﬁnite decimal expansion. SOLUTION

81. is symmetrical with respect vertical line x = a if f (a − x) = f (a + x). be an integer with digits p1 , . . .to, pthe that Let pA=function p1 . . . psf (x) s . Show (a) Draw the graph of a function that is symmetrical with respect to x = 2. p p . . . p = 0. s 1 (b) Show that if f (x) is symmetrical with respect to = 1a, then g(x) = f (x + a) is even. 10xs − SOLUTION

2 Use this ﬁnd the decimal expansion of r may = be . Note that (a) There aretomany possibilities, two of which 11 r=

18 2 = 2 11 10 − 1

8

CHAPTER 1

PRECALCULUS REVIEW y

2 1

−1

x 1

2

3

4

5

y = | x − 2|

(b) Let g(x) = f (x + a). Then g(−x) = f (−x + a) = f (a − x) = f (a + x)

symmetry with respect to x = a

= g(x) Thus, g(x) is even. Formulate a condition for f (x) to be symmetrical with respect to the point (a, 0) on the x-axis.

1.2 Linear and Quadratic Functions Preliminary Questions 1. What is the slope of the line y = −4x − 9? SOLUTION

The slope of the line y = −4x − 9 is −4, given by the coefﬁcient of x.

2. Are the lines y = 2x + 1 and y = −2x − 4 perpendicular? SOLUTION The slopes of perpendicular lines are negative reciprocals of one another. Because the slope of y = 2x + 1 is 2 and the slope of y = −2x − 4 is −2, these two lines are not perpendicular.

3. When is the line ax + by = c parallel to the y-axis? To the x-axis? SOLUTION

The line ax + by = c will be parallel to the y-axis when b = 0 and parallel to the x-axis when a = 0.

4. Suppose y = 3x + 2. What is y if x increases by 3? SOLUTION

Because y = 3x + 2 is a linear function with slope 3, increasing x by 3 will lead to y = 3(3) = 9.

5. What is the minimum of f (x) = (x + 3)2 − 4? SOLUTION

−4.

Because (x + 3)2 ≥ 0, it follows that (x + 3)2 − 4 ≥ −4. Thus, the minimum value of (x + 3)2 − 4 is

6. What is the result of completing the square for f (x) = x 2 + 1? SOLUTION

Because there is no x term in x 2 + 1, completing the square on this expression leads to (x − 0)2 + 1.

Exercises In Exercises 1–4, ﬁnd the slope, the y-intercept, and the x-intercept of the line with the given equation. 1. y = 3x + 12 Because the equation of the line is given in slope-intercept form, the slope is the coefﬁcient of x and the y-intercept is the constant term: that is, m = 3 and the y-intercept is 12. To determine the x-intercept, substitute y = 0 and then solve for x: 0 = 3x + 12 or x = −4. SOLUTION

3. 4x + 9y = 3 y =4−x SOLUTION To determine the slope and y-intercept, we ﬁrst solve the equation for y to obtain the slope-intercept form. This yields y = − 49 x + 13 . From here, we see that the slope is m = − 49 and the y-intercept is 13 . To determine the x-intercept, substitute y = 0 and solve for x: 4x = 3 or x = 34 .

In Exercises 5–8,1ﬁnd the slope of the line. y − 3 = 2 (x − 6) 5. y = 3x + 2 SOLUTION

m=3

7. 3x + 4y = 12 y = 3(x − 9) + 2 3 SOLUTION First solve the equation for y to obtain the slope-intercept form. This yields y = − 4 x + 3. The slope of 3 the line is therefore m = − 4 . 3x + 4y = −8

S E C T I O N 1.2

Linear and Quadratic Functions

9

In Exercises 9–20, ﬁnd the equation of the line with the given description. 9. Slope 3, y-intercept 8 SOLUTION

Using the slope-intercept form for the equation of a line, we have y = 3x + 8.

11. Slope 3, passes through (7, 9) Slope −2, y-intercept 3 SOLUTION Using the point-slope form for the equation of a line, we have y − 9 = 3(x − 7) or y = 3x − 12. 13. Horizontal, passes through (0, −2) Slope −5, passes through (0, 0) SOLUTION A horizontal line has a slope of 0. Using the point-slope form for the equation of a line, we have y − (−2) = 0(x − 0) or y = −2. 15. Parallel to y = 3x − 4, passes through (1, 1) Passes through (−1, 4) and (2, 7) SOLUTION Because the equation y = 3x − 4 is in slope-intercept form, we can readily identify that it has a slope of 3. Parallel lines have the same slope, so the slope of the requested line is also 3. Using the point-slope form for the equation of a line, we have y − 1 = 3(x − 1) or y = 3x − 2. 17. Perpendicular to 3x + 5y = 9, passes through (2, 3) Passes through (1, 4) and (12, −3) SOLUTION We start by solving the equation 3x + 5y = 9 for y to obtain the slope-intercept form for the equation of a line. This yields 3 9 y=− x+ , 5 5 from which we identify the slope as − 35 . Perpendicular lines have slopes that are negative reciprocals of one another, so the slope of the desired line is m ⊥ = 53 . Using the point-slope form for the equation of a line, we have y − 3 = 53 (x − 2) or y = 53 x − 13 . 19. Horizontal, passes through (8, 4) Vertical, passes through (−4, 9) SOLUTION A horizontal line has slope 0. Using the point slope form for the equation of a line, we have y − 4 = 0(x − 8) or y = 4. 21. Find the equation of the perpendicular bisector of joining (1, 2) and (5, 4) (Figure 11). Hint: The the segment Slope 3, x-intercept 6 a+c b+d midpoint Q of the segment joining (a, b) and (c, d) is , . 2 2 y

Perpendicular bisector (5, 4) Q (1, 2) x

FIGURE 11 SOLUTION

The slope of the segment joining (1, 2) and (5, 4) is m=

4−2 1 = 5−1 2

and the midpoint of the segment (Figure 11) is midpoint =

1+5 2+4 , 2 2

= (3, 3)

The perpendicular bisector has slope −1/m = −2 and passes through (3, 3), so its equation is: y − 3 = −2(x − 3) or y = −2x + 9. 23. Find the equation of the line with x-intercept x = 4 and y-intercept y = 3. Intercept-intercept Form Show that if a, b = 0, then the line with x-intercept x = a and y-intercept y = b y x SOLUTION From Exercise has equation (Figure 12) 22, 4 + 3 = 1 or 3x + 4y = 12. y x + cy = 1 25. Determine whether there exists a constant c such thatx the line 1 (3, y) lies on the line. A line of slope m = 2 passes through (1, 4). Find y+such=that a b Passes through (3, 1) (a) Has slope 4 (b) (c) Is horizontal (d) Is vertical SOLUTION

10

CHAPTER 1

PRECALCULUS REVIEW

(a) Rewriting the equation of the line in slope-intercept form gives y = − xc + 1c . To have slope 4 requires − 1c = 4 or c = − 14 . (b) Substituting x = 3 and y = 1 into the equation of the line gives 3 + c = 1 or c = −2. (c) From (a), we know the slope of the line is − 1c . There is no value for c that will make this slope equal to 0. (d) With c = 0, the equation becomes x = 1. This is the equation of a vertical line. 27. Materials expand when heated. Consider a metal rod of length L 0 at temperature T0 . If the temperature is changed Assume that the number N of concert tickets which can be sold at a price of P dollars per ticket is a linear by an amount T , then the rod’s length changes by L = α L 0 T , where α is the thermal expansion coefﬁcient. For function N (P) for 10 ≤ P ≤ 40. Determine N (P) (called the demand function) if N (10) = 500 and N (40) = 0. steel, α = 1.24 × 10−5 ◦ C−1 . What is the decrease N in the number of tickets sold if the price is increased by P = 5 dollars? (a) A steel rod has length L 0 = 40 cm at T0 = 40◦ C. What is its length at T = 90◦ C? (b) Find its length at T = 50◦ C if its length at T0 = 100◦ C is 65 in. (c) Express length L as a function of T if L 0 = 65 in. at T0 = 100◦ C. SOLUTION

(a) With T = 90◦ C and T0 = 40◦ C, T = 50◦ C. Therefore, L = α L 0 T = (1.24 × 10−5 )(40)(50) = .0248

and

L = L 0 + L = 40.0248 cm.

(b) With T = 50◦ C and T0 = 100◦ C, T = −50◦ C. Therefore, L = α L 0 T = (1.24 × 10−5 )(65)(−50) = −.0403

and

L = L 0 + L = 64.9597 in.

(c) L = L 0 + L = L 0 + α L 0 T = L 0 (1 + α T ) = 65(1 + α (T − 100)) 29. Find b such that (2, −1), (3, 2), and (b, 5) lie on a line. Do the points (0.5, 1), (1, 1.2), (2, 2) lie on a line? SOLUTION The slope of the line determined by the points (2, −1) and (3, 2) is 2 − (−1) = 3. 3−2 To lie on the same line, the slope between (3, 2) and (b, 5) must also be 3. Thus, we require 3 5−2 = = 3, b−3 b−3 or b = 4. 31. The period T of a pendulum is measured for pendulums of several different lengths L. Based on the following data, Find an expression for the velocity v as a linear function of t that matches the following data. does T appear to be a linear function of L? t (s) 0 2 4 6 L (ft) 2 3 4 5 v (m/s) 39.2 58.6 78 97.4 T (s) 1.57 1.92 2.22 2.48 SOLUTION

Examine the slope between consecutive data points. The ﬁrst pair of data points yields a slope of 1.92 − 1.57 = .35, 3−2

while the second pair of data points yields a slope of 2.22 − 1.92 = .3, 4−3 and the last pair of data points yields a slope of 2.48 − 2.22 = .26 5−4 Because the three slopes are not equal, T does not appear to be a linear function of L. 33. Find the roots of the quadratic polynomials: Show that f (x) is linear of slope m if and only if (a) 4x 2 − 3x − 1 (b) x 2 − 2x − 1 f (x + h) − f (x) = mh (for all x and h) SOLUTION √ √ 3 ± 9 − 4(4)(−1) 3 ± 25 1 (a) x = = = 1 or − 2(4) 8 4 √ √ √ 2 ± 4 − (4)(1)(−1) 2± 8 (b) x = = =1± 2 2 2

Linear and Quadratic Functions

S E C T I O N 1.2

11

In Exercises 34–41, complete the square and ﬁnd the minimum or maximum value of the quadratic function. 35. y = x 2 −26x + 9 y = x + 2x + 5 SOLUTION y = (x − 3)2 ; therefore, the minimum value of the quadratic polynomial is 0, and this occurs at x = 3. 37. y = x 2 + 6x2+ 2 y = −9x + x SOLUTION y = x 2 + 6x + 9 − 9 + 2 = (x + 3)2 − 7; therefore, the minimum value of the quadratic polynomial is −7, and this occurs at x = −3. 39. y = −4x 2 2+ 3x + 8 y = 2x − 4x − 7 3 9 9 3 137 SOLUTION y = −4x 2 + 3x + 8 = −4(x 2 − 4 x + 64 ) + 8 + 16 = −4(x − 8 )2 + 16 ; therefore, the maximum 3 value of the quadratic polynomial is 137 16 , and this occurs at x = 8 . 2 41. y = 4x − 12x y = 3x 2 + 12x − 5 1 ) + 1 = −12(x − 1 )2 + 1 ; therefore, the maximum value of the SOLUTION y = −12(x 2 − x3 ) = −12(x 2 − x3 + 36 3 6 3 quadratic polynomial is 13 , and this occurs at x = 16 .

43. Sketch the graph of y = x 2 +24x + 6 by plotting the minimum point, the y-intercept, and one other point. Sketch the graph of y = x − 6x + 8 by plotting the roots and the minimum point. SOLUTION y = x 2 + 4x + 4 − 4 + 6 = (x + 2)2 + 2 so the minimum occurs at (−2, 2). If x = 0, then y = 6 and if x = −4, y = 6. This is the graph of x 2 moved left 2 units and up 2 units. 10 8 6 4 2 –4

–3

–2

–1

45. ForIfwhich valuesAofand c does (x) cystic = x2 + cx + 1gene haveoccur a double No real roots? the alleles B off the ﬁbrosis in a root? population with frequencies p and 1 − p (where p is a fraction between 0 and 1), then carriers (carriers SOLUTION A double root occurs whenthe c2frequency − 4(1)(1)of =heterozygous 0 or c2 = 4. Thus, c= ±2. with both alleles) is 2 p(1 − p). Which of proots giveswhen the largest frequency There arevalue no real c2 − 4(1)(1) < 0oforheterozygous c2 < 4. Thus,carriers? −2 < c < 2. 1 2 for all x > 0. Hint: Consider (x 1/2 − x −1/2 )2 . 47. ProveLet that fx(x) + be ≥ x a quadratic function and c a constant. Which is correct? Explain graphically. (a) There is a unique value of c such that y = f (x) − c has a double root. SOLUTION Let x > 0. Then (b) There is a unique value of c such that y = f (x − c) has a double root. 2

1 x 1/2 − x −1/2 = x − 2 + . x Because (x 1/2 − x −1/2 )2 ≥ 0, it follows that x −2+

1 ≥0 x

x+

or

1 ≥ 2. x

49. If objects of weights x and w1 are suspended from the balance in Figure 13(A), the cross-beam is horizontal if √ a+b bx = aw If the a and are geometric known, wemean may useabthis determine an unknown selecting Hint: Use a is equation not largertothan the arithmetic meanweight x. by Let b >lengths 0. Show thatb the 1 . a, 2 First balance x w1 so that the cross-beam is horizontal. If a and b are not known precisely, we might proceed as follows. variation of the hint given in Exercise 47. by w1 on the left as in (A). Then switch places and balance x by w2 on the right as in (B). The average x¯ = 12 (w1 + w2 ) gives an estimate for x. Show that x¯ is greater than or equal to the true weight x. a

a

b

w1

x (A)

b

x

w2 (B)

FIGURE 13

12

CHAPTER 1

PRECALCULUS REVIEW SOLUTION

First note bx = aw1 and ax = bw2 . Thus, x¯ = = = ≥ =

1 (w1 + w2 ) 2 1 bx ax + 2 a b x b a + 2 a b x (2) by Exercise 47 2 x

51. Find a pair of numbers whose sum and product are both equal to 8. Find numbers x and y with sum 10 and product 24. Hint: Find a quadratic polynomial satisﬁed by x. SOLUTION Let x and y be numbers whose sum and product are both equal to 8. Then x + y = 8 and x y = 8. From the second equation, y = 8x . Substituting this expression for y in the ﬁrst equation gives x + 8x = 8 or x 2 − 8x + 8 = 0. By the quadratic formula, √ √ 8 ± 64 − 32 = 4 ± 2 2. x= 2 √ If x = 4 + 2 2, then √ √ 8 8 4−2 2 y= √ = √ · √ = 4 − 2 2. 4+2 2 4+2 2 4−2 2 √ On the other hand, if x = 4 − 2 2, then √ √ 8 8 4+2 2 y= √ = √ · √ = 4 + 2 2. 4−2 2 4−2 2 4+2 2 √ √ Thus, the two numbers are 4 + 2 2 and 4 − 2 2. 2 consists of all points P such that d = d , where d is the distance 1 2 1 from P to (0, 4 ) and d2 is the distance from P to the horizontal line y = − 14 (Figure 14). 53. Show that if f (x) and g(x) are linear, then so is f (x) + g(x). Is the same true of f (x)g(x)?

Show that the graph of the parabola y = x Further Insights and Challenges 1 SOLUTION

If f (x) = mx + b and g(x) = nx + d, then f (x) + g(x) = mx + b + nx + d = (m + n)x + (b + d),

which is linear. f (x)g(x) is not generally linear. Take, for example, f (x) = g(x) = x. Then f (x)g(x) = x 2 . overfthe ] is=not a constant, but=depends 55. Show thatthat the ifratio y/x forare thelinear function f (x) = x 2 that 1 , xf 2(1) Show f (x) and g(x) functions such (0) interval = g(0) [x and g(1), then f (x) g(x). on the interval. Determine the exact dependence of y/x on x 1 and x 2 . SOLUTION

For x 2 ,

x 2 − x 12 y = 2 = x2 + x1 . x x2 − x1

57. Let a, c = 0. Show that the roots of ax 2 + bx + c = 0 and cx 2 +2 bx + a = 0 are reciprocals of each other. Use Eq. (2) to derive the quadratic formula for the roots of ax + bx + c = 0. SOLUTION Let r1 and r2 be the roots of ax 2 + bx + c and r3 and r4 be the roots of cx 2 + bx + a. Without loss of generality, let −b + b2 − 4ac 2a 1 −b − b2 − 4ac r1 = = ⇒ · 2a r1 −b + b2 − 4ac −b − b2 − 4ac −b − b2 − 4ac 2a(−b − b2 − 4ac) = = r4 . = 2c b2 − b2 + 4ac Similarly, you can show

1 = r3 . r2

59. Prove Vi`ete’s Formulas, which state that the quadratic polynomial with given numbers α and β as roots is x 2 + square to show that. the parabolas y = ax 2 + bx + c and y = ax 2 have the same shape (show that bx + c, Complete where b =the −α − β and c = αβ the ﬁrst parabola is congruent to the second by a vertical and horizontal translation). SOLUTION If a quadratic polynomial has roots α and β , then the polynomial is (x − α )(x − β ) = x 2 − α x − β x + αβ = x 2 + (−α − β )x + αβ . Thus, b = −α − β and c = αβ .

S E C T I O N 1.3

The Basic Classes of Functions

13

1.3 The Basic Classes of Functions Preliminary Questions 1. Give an example of a rational function. SOLUTION

One example is

3x 2 − 2 . 7x 3 + x − 1

2. Is |x| a polynomial function? What about |x 2 + 1|? SOLUTION

|x| is not a polynomial; however, because x 2 + 1 > 0 for all x, it follows that |x 2 + 1| = x 2 + 1, which is

a polynomial. 3. What is unusual about the domain of f ◦ g for f (x) = x 1/2 and g(x) = −1 − |x|? SOLUTION Recall that ( f ◦ g)(x) = f (g(x)). Now, for any real number x, g(x) = −1 − |x| ≤ −1 < 0. Because we cannot take the square root of a negative number, it follows that f (g(x)) is not deﬁned for any real number. In other words, the domain of f (g(x)) is the empty set. x 4. Is f (x) = 12 increasing or decreasing? SOLUTION

function.

The function f (x) = ( 12 )x is an exponential function with base b = 12 < 1. Therefore, f is a decreasing

5. Give an example of a transcendental function. SOLUTION

One possibility is f (x) = e x − sin x.

Exercises In Exercises 1–12, determine the domain of the function. 1. f (x) = x 1/4 SOLUTION

x≥0

3. f (x) = x 3 + 3x − 4 g(t) = t 2/3 SOLUTION All reals 1 5. g(t)h(z) = = z 3 + z −3 t +2 SOLUTION

t = −2

1 7. G(u) = 2 1 f (x) u= −2 4 x +4 SOLUTION u = ±2 √(x − 1)−3 9. f (x) = x −4 + x f (x) = 20, 1 SOLUTION x = x −9 √ −1 11. g(y) = 10 y+y s F(s) = sin SOLUTION y > 0 s + 1 In Exercises 13–24, identify each of the following functions as polynomial, rational, algebraic, or transcendental. x + x −1 f (x) =3 2 − 8+ 4) +− 9x3)(x 13. f (x) = 4x (x Polynomial √ 15. f (x) = x f (x) = x −4 SOLUTION Algebraic SOLUTION

x2 17. f (x)f (x) = = 1 − x2 x + sin x SOLUTION

Transcendental

2x 3 x+ 3x 19. f (x)f (x) = =2 2 9 − 7x SOLUTION

Rational

21. f (x) = sin(x 2 ) 3x − 9x −1/2 f (x) = 9 − 7x 2

14

CHAPTER 1

PRECALCULUS REVIEW SOLUTION

Transcendental

23. f (x) = x 2 + 3xx −1 f (x) = √ SOLUTION Rational x +1 2

x a transcendental 25. Is f f(x) function? x) (x)==2sin(3 SOLUTION Yes.

In Exercises 27–34, calculate2 the composite functions f ◦ g and g ◦ f , and determine their domains. Show that f (x) = x + 3x −1 and g(x) = 3x 3 − 9x + x −2 are rational functions (show that each is a quotient √ polynomials). 27. of f (x) = x, g(x) = x + 1 √ √ SOLUTION f (g(x)) = x + 1; D: x ≥ −1, g( f (x)) = x + 1; D: x ≥ 0 29. f (x) = 2x , 1 g(x) = x 2 −4 f (x) = , g(x) = 2x SOLUTION f (g(x)) x = 2x ; D: R,

g( f (x)) = (2x )2 = 22x ; D: R

3 2 31. f (θf)(x) = cos θ , g(x) = |x|, g(θ )==xsin+θ x 3 SOLUTION f (g(x)) = cos(x + x 2 ); D: R,

g( f (θ )) = cos3 θ + cos2 θ ; D: R

1 33. f (t) = √ , g(t) 1 = −t 2 , g(x) = x −2 f (x) =t 2 x +1 1 SOLUTION f (g(t)) = ; D: Not valid for any t, −t 2

2 g( f (t)) = − √1 = − 1t ; D: t > 0 t

kt √ (in millions) of 35. The population 3 a country as a function of time t (years) is P(t) = 30 · 2 , with k = 0.1. Show f (t) = t, g(t) = 1 − t that the population doubles every 10 years. Show more generally that for any nonzero constants a and k, the function g(t) = a2kt doubles after 1/k years. SOLUTION

Let P(t) = 30 · 20.1t . Then P(t + 10) = 30 · 20.1(t+10) = 30 · 20.1t+1 = 2(30 · 20.1t ) = 2P(t).

Hence, the population doubles in size every 10 years. In the more general case, let g(t) = a2kt . Then 1 = a2k(t+1/k) = a2kt+1 = 2a2kt = 2g(t). g t+ k Hence, the function g doubles after 1/k years. x +1 is R. f (x) = 2 x + 2cx + 4 In Exercises 37–43, we deﬁne the ﬁrst difference δ f of a function f (x) by δ f (x) = f (x + 1) − f (x).

Further Insights Challenges Find all valuesand of c such that the domain of

37. Show that if f (x) = x 2 , then δ f (x) = 2x + 1. Calculate δ f for f (x) = x and f (x) = x 3 . f (x) = x 2 : δ f (x) = f (x + 1) − f (x) = (x + 1)2 − x 2 = 2x + 1 f (x) = x: δ f (x) = x + 1 − x = 1 f (x) = x 3 : δ f (x) = (x + 1)3 − x 3 = 3x 2 + 3x + 1

SOLUTION

39. Show that for any two functions f and g, δ ( f + g) = δ f + δ g and δ (c f ) = cδ ( f ), where c is any constant. Show that δ (10x ) = 9 · 10x , and more generally, δ (b x ) = c · b x for some constant c. SOLUTION δ ( f + g) = ( f (x + 1) + g(x + 1)) − ( f (x) − g(x)) = ( f (x + 1) − f (x)) + (g(x + 1) − g(x)) = δ f (x) + δ g(x)

δ (c f ) = c f (x + 1) − c f (x) = c( f (x + 1) − f (x)) = cδ f (x). x(x + 1) 41. First show that P(x)can = be used tosatisﬁes δ P = (x for + 1). 40 toSuppose conclude First differences derive formulas theThen sum apply of theExercise kth powers. wethat can ﬁnd a function 2 P(x) such that δ P = (x + 1)k and P(0) = 0. Prove that P(1) = 1k , P(2) = 1k + 2k , and more generally, for every n(n + 1) whole number n, 1 + 2 + 3 + ··· + n = 2 P(n) = 1k + 2k + · · · + n k SOLUTION Let P(x) = x(x + 1)/2. Then

δ P(x) = P(x + 1) − P(x) =

(x + 1)(x + 2) x(x + 1) (x + 1)(x + 2 − x) − = = x + 1. 2 2 2

Also, note that P(0) = 0. Thus, by Exercise 40, with k = 1, it follows that P(n) =

n(n + 1) = 1 + 2 + 3 + · · · + n. 2

Calculate δ (x 3 ), δ (x 2 ), and δ (x). Then ﬁnd a polynomial P(x) of degree 3 such that δ P = (x + 1)2 and P(0) = 0. Conclude that

S E C T I O N 1.3

The Basic Classes of Functions

15

43. This exercise combined with Exercise 40 shows that for all k, there exists a polynomial P(x) satisfying Eq. (1). The solution requires proof by induction and the Binomial Theorem (see Appendix C). (a) Show that

δ (x k+1 ) = (k + 1) x k + · · · where the dots indicate terms involving smaller powers of x. (b) Show by induction that for all whole numbers k, there exists a polynomial of degree k + 1 with leading coefﬁcient 1/(k + 1): P(x) =

1 x k+1 + · · · k+1

such that δ P = (x + 1)k and P(0) = 0. SOLUTION

(a) By the Binomial Theorem: n+1 n+1 δ (x n+1 ) = (x + 1)n+1 − x n+1 = x n+1 + xn + x n−1 + · · · + 1 − x n+1 1 2 n+1 n+1 xn + x n−1 + · · · + 1 = 1 2 Thus,

δ (x n+1 ) = (n + 1) x n + · · · where the dots indicate terms involving smaller powers of x. (b) For k = 0, note that P(x) = x satisﬁes δ P = (x + 1)0 = 1 and P(0) = 0. Now suppose the polynomial P(x) =

1 k x + pk−1 x k−1 + · · · + p1 x k

which clearly satisﬁes P(0) = 0 also satisﬁes δ P = (x + 1)k−1 . We try to prove the existence of Q(x) =

1 x k+1 + qk x k + · · · + q1 x k+1

such that δ Q = (x + 1)k . Observe that Q(0) = 0. If δ Q = (x + 1)k and δ P = (x + 1)k−1 , then

δ Q = (x + 1)k = (x + 1)δ P = x δ P(x) + δ P By the linearity of δ (Exercise 39), we ﬁnd δ Q − δ P = x δ P or δ (Q − P) = x δ P. By deﬁnition, 1 1 Q−P= x k+1 + qk − x k + · · · + (q1 − p1 )x, k+1 k so, by the linearity of δ ,

δ (Q − P) =

1 1 δ (x k+1 ) + qk − δ (x k ) + · · · + (q1 − p1 ) = x(x + 1)k−1 k+1 k

By part (a),

δ (x k+1 ) = (k + 1)x k + L k−1,k−1 x k−1 + . . . + L k−1,1 x + 1 δ (x k ) = kx k−1 + L k−2,k−2 x k−2 + . . . + L k−2,1 x + 1 .. .

δ (x 2 ) = 2x + 1 where the L i, j are real numbers for each i, j. To construct Q, we have to group like powers of x on both sides of (1). This yields the system of equations 1

(k + 1)x k = x k k+1

(1)

16

CHAPTER 1

PRECALCULUS REVIEW

1 1 L k−1,k−1 x k−1 + qk − kx k−1 = (k − 1)x k−1 k+1 k .. .

1 1 + qk − + (qk−1 − pk−1 ) + · · · + (q1 − p1 ) = 0. k+1 k The ﬁrst equation is identically true, and the second equation can be solved immediately for qk . Substituting the value of qk into the third equation of the system, we can then solve for qk−1 . We continue this process until we substitute the values of qk , qk−1 , . . . q2 into the last equation, and then solve for q1 .

1.4 Trigonometric Functions Preliminary Questions 1. How is it possible for two different rotations to deﬁne the same angle? SOLUTION Working from the same initial radius, two rotations that differ by a whole number of full revolutions will have the same ending radius; consequently, the two rotations will deﬁne the same angle even though the measures of the rotations will be different.

2. Give two different positive rotations that deﬁne the angle π4 . π SOLUTION The angle π /4 is deﬁned by any rotation of the form 4 + 2π k where k is an integer. Thus, two different positive rotations that deﬁne the angle π /4 are

9π π + 2π (1) = 4 4

and

41π π + 2π (5) = . 4 4

3. Give a negative rotation that deﬁnes the angle π3 . SOLUTION The angle π /3 is deﬁned by any rotation of the form π 3 + 2π k where k is an integer. Thus, a negative rotation that deﬁnes the angle π /3 is

5π π + 2π (−1) = − . 3 3 4. The deﬁnition of cos θ using right triangles applies when (choose the correct answer): π (a) 0 < θ < (c) 0 < θ < 2π (b) 0 < θ < π 2 The correct response is (a): 0 < θ < π2 . 5. What is the unit circle deﬁnition of sin θ ?

SOLUTION

SOLUTION Let O denote the center of the unit circle, and let P be a point on the unit circle such that the radius O P makes an angle θ with the positive x-axis. Then, sin θ is the y-coordinate of the point P.

6. How does the periodicity of sin θ and cos θ follow from the unit circle deﬁnition? SOLUTION Let O denote the center of the unit circle, and let P be a point on the unit circle such that the radius O P makes an angle θ with the positive x-axis. Then, cos θ and sin θ are the x- and y-coordinates, respectively, of the point P. The angle θ + 2π is obtained from the angle θ by making one full revolution around the circle. The angle θ + 2π will therefore have the radius O P as its terminal side. Thus

cos(θ + 2π ) = cos θ

and

sin(θ + 2π ) = sin θ .

In other words, sin θ and cos θ are periodic functions.

Exercises 1. Find the angle between 0 and 2π that is equivalent to 13π /4. SOLUTION Because 13π /4 > 2π , we repeatedly subtract 2π until we arrive at a radian measure that is between 0 and 2π . After one subtraction, we have 13π /4 − 2π = 5π /4. Because 0 < 5π /4 < 2π , 5π /4 is the angle measure between 0 and 2π that is equivalent to 13π /4.

3. Convert from radians to degrees: Describe the angle θ = π6 by 5 π an angle of negative radian measure. (c) (a) 1 (b) 3 12

(d) −

3π 4

Trigonometric Functions

S E C T I O N 1.4 SOLUTION

180◦ 180◦ ≈ 57.1◦ = π π 5 180◦ 75◦ (c) ≈ 23.87◦ = 12 π π

180◦ = 60◦ π 3π 180◦ (d) − = −135◦ 4 π

(a) 1

(b)

π 3

5. Find the lengths of the arcs subtended by the angles θ and φ radians in Figure 20. Convert from degrees to radians: (b) 30◦ (c) 25◦ (a) 1◦

(d) 120◦

4 q = 0.9 f =2

FIGURE 20 Circle of radius 4. SOLUTION

s = r θ = 4(.9) = 3.6; s = r φ = 4(2) = 8

7. Fill in the remaining values of (cos θ , sin θ ) for the points in Figure 22. Calculate the values of the six standard trigonometric functions for the angle θ in Figure 21. 3p 4

2p 3

p 2

p 3

5p 6

( 12 , 23 ) p 2 2 , 4 ( 2 2 ) p 3 1 , ( 6 2 2)

p

0 (0, 0)

7p 6

11p 6 5p 4 4p 3

7p 5p 4 3

3p 2

FIGURE 22 SOLUTION

θ

π 2

(cos θ , sin θ )

(0, 1)

2π 3 √

−1 , 3 2 2

θ

5π 4

4π 3

(cos θ , sin θ )

√

√ − 2, − 2 2 2

√

3π 4

√ − 2, 2 2 2 3π 2

√ −1 , − 3 2 2

5π 6

√ − 3, 1 2 2 5π 3 √

1, − 3 2 2

(0, −1)

(−1, 0)

7π 6

√ − 3 , −1 2 2

7π 4

11π 6

π

√

√ 2 − 2 2 , 2

π θsatisfying the given condition. In Exercises use Figure 22 to ﬁnd alltrigonometric angles between 0 and 2at Find 9–14, the values of the six standard functions = 11π /6. 9. cos θ = 12

θ = π3 , 53π 11. tan θ = −1 tan θ = 1 3π 7π SOLUTION θ = 4 , 4 √ 3 13. sin csc x =θ = 2 2 SOLUTION

x = π3 , 23π 15. Fill in the following table of values: sec t = 2 SOLUTION

θ tan θ sec θ SOLUTION

π 6

π 4

π 3

π 2

2π 3

3π 4

5π 6

√

3 −1 2 , 2

17

18

CHAPTER 1

PRECALCULUS REVIEW

π 6

π 4

tan θ

1 √ 3

1

sec θ

2 √ 3

θ

√

2

π 3 √ 3 2

π 2

3π 4

5π 6

und

2π 3 √ − 3

−1

1 −√ 3

und

−2

√ − 2

2 −√ 3

17. Show that if tan = c and 0table ≤ θof<signs π /2, for then θ = 1/ 1 +functions: c2 . Hint: Draw a right triangle whose opposite and Complete theθfollowing thecos trigonometric adjacent sides have lengths c and 1. SOLUTION Because 0 ≤ θ < π /2, deﬁnition the trigonometric in terms of right triangles. θ we can use the sin cos of tan cot sec functions csc tan θ is the ratio of the length of the side opposite the angle θ to the length of the adjacent side. With c = 1c , we label π the length of the opposite side as c 0and length of the θ< + adjacent + side as 1 (see the diagram below). By the Pythagorean < the theorem, the length of the hypotenuse is 12+ c2 . Finally, we use the fact that cos θ is the ratio of the length of the adjacent side to the length of the hypotenuse to obtain π <θ<π 2 1 . cos θ = 3π 1 + c2 π<θ< 2

3π < θ < 2π 2

1 + c2

c

q 1

In Exercises 19–24, assume that 0 ≤ θ < π /2. Suppose that cos θ = 13 . √ √ 5 . and tan = 13 19. Find sin θ that <θ π /2, then sin θ = 2 2/3 and tan θ = 2 2. (a) Show if 0θ≤ifθcos θ and tan if 3π /2below. ≤ θ

12 13

tan θ =

and

13

12 . 5

12

θ 5

21. Find sin θ , sec θ , and cot θ if tan θ 3= 27 . Find cos θ and tan θ if sin θ = 5 . 7 SOLUTION If tan θ = 2 7 , then cot θ = 2 . For the remaining trigonometric functions, consider the triangle below. The lengths of the sides opposite and adjacent to the angle θ have been labeled so that tan θ = 27 . The length of the √ hypotenuse has been calculated using the Pythagorean theorem: 22 + 72 = 53. From the triangle, we see that √ √ 2 53 2 53 and sec θ = . sin θ = √ = 53 7 53 53

2

q 7

1. 23. FindFind cossin 2θ θif, sin 5 sec θ if cot θ = 4. cosθθ ,=and

Using the double angle formula cos 2θ = cos2 θ − sin2 θ and the fundamental identity sin2 θ + cos2 θ = 1, we ﬁnd that cos 2θ = 1 − 2 sin2 θ . Thus, cos 2θ = 1 − 2(1/25) = 23/25. SOLUTION

Find sin 2θ and cos 2θ if tan θ =

√

2.

S E C T I O N 1.4

Trigonometric Functions

19

25. Find cos θ and tan θ if sin θ = 0.4 and π /2 ≤ θ < π . SOLUTION We can determine the “magnitude” of cos θ and tan θ using the triangle shown below. The lengths of the side opposite the angle θ and the hypotenuse have been labeled so that sin θ = 0.4 = 25 . The length of the side adjacent √ to the angle θ was calculated using the Pythagorean theorem: 52 − 22 = 21. From the triangle, we see that √ √ 21 2 21 2 and |tan θ | = √ = . |cos θ | = 5 21 21

Because π /2 ≤ θ < π , both cos θ and tan θ are negative; consequently, √ √ 2 21 21 and tan θ = − . cos θ = − 5 21 5

2

q 21

27. Find the values of sin θ , cos θ , and tan θ at the eight points in Figure 23. Find cos θ and sin θ if tan θ = 4 and π ≤ θ < 3π /2. (0.3965, 0.918)

(A)

(0.3965, 0.918)

(B)

FIGURE 23 SOLUTION

Let’s start with the four points in Figure 23(A).

• The point in the ﬁrst quadrant has coordinates (0.3965, 0.918). Therefore,

0.918 = 2.3153. 0.3965

sin θ = 0.918, cos θ = 0.3965, and tan θ =

• The coordinates of the point in the second quadrant are (−0.918, 0.3965). Therefore,

sin θ = 0.3965, cos θ = −0.918, and tan θ =

0.3965 = −0.4319. −0.918

• Because the point in the third quadrant is symmetric to the point in the ﬁrst quadrant with respect to the origin, its

coordinates are (−0.3965, −0.918). Therefore, sin θ = −0.918, cos θ = −0.3965, and tan θ =

−0.918 = 2.3153. −0.3965

• Because the point in the fourth quadrant is symmetric to the point in the second quadrant with respect to the origin,

its coordinates are (0.918, −0.3965). Therefore, sin θ = −0.3965, cos θ = 0.918, and tan θ =

−0.3965 = −0.4319. 0.918

Now consider the four points in Figure 23(B). • The point in the ﬁrst quadrant has coordinates (0.3965, 0.918). Therefore,

sin θ = 0.918, cos θ = 0.3965, and tan θ =

0.918 = 2.3153. 0.3965

• The point in the second quadrant is a reﬂection through the y-axis of the point in the ﬁrst quadrant. Its coordinates

are therefore (−0.3965, 0.918) and sin θ = 0.918, cos θ = −0.3965, and tan θ =

0.918 = −2.3153. 0.3965

20

CHAPTER 1

PRECALCULUS REVIEW • Because the point in the third quadrant is symmetric to the point in the ﬁrst quadrant with respect to the origin, its

coordinates are (−0.3965, −0.918). Therefore, sin θ = −0.918, cos θ = −0.3965, and tan θ =

−0.918 = 2.3153. −0.3965

• Because the point in the fourth quadrant is symmetric to the point in the second quadrant with respect to the origin,

its coordinates are (0.3965, −0.918). Therefore, sin θ = −0.918, cos θ = 0.3965, and tan θ =

−0.918 = −2.3153. 0.3965

29. Refer to Figure 24(B). Compute cos ψ , sin ψ , cot ψ , and csc ψ . Refer to Figure 24(A). Express the functions sin θ , tan θ , and csc θ in terms of c.

1

1

c

0.3

0.3

(A)

(B)

FIGURE 24

By the Pythagorean theorem, the length of the side opposite the angle ψ in Figure 24 (B) is 1 − 0.32 = 0.91. Consequently, √ 0.3 0.3 1 0.91 √ cos ψ = and csc ψ = √ . = 0.3, sin ψ = = 0.91, cot ψ = √ 1 1 0.91 0.91

SOLUTION

√

graph over

[0,π 2π ]. In Exercises 31–34, sketchπthe and sin θ + in terms of cos θ and sin θ . Hint: Find the relation between the coordiExpress cos θ + 2 2 θ b) and (c, d) in Figure 25. 31. 2nates sin 4(a, SOLUTION

y 2 1 x 1

−1

2

3

4

5

6

−2

π 33. cos 2θ −

π cos 2 2θ − 2 SOLUTION

y 1 0.5 x −0.5

1

2

3

4

5

6

−1

35. How many

points π lie onthe intersection of the horizontal line y = c and the graph of y = sin x for 0 ≤ x < 2π ? +2 sinanswer 2 θ− + πon c. Hint: The depends 2 SOLUTION Recall that for any x, −1 ≤ sin x ≤ 1. Thus, if |c| > 1, the horizontal line y = c and the graph of y = sin x never intersect. If c = +1, then y = c and y = sin x intersect at the peak of the sine curve; that is, they intersect at x = π2 . On the other hand, if c = −1, then y = c and y = sin x intersect at the bottom of the sine curve; that is, they intersect at x = 32π . Finally, if |c| < 1, the graphs of y = c and y = sin x intersect twice. In Exercises solve lie foron 0≤ < 2π (see Example 6). How 37–40, many points theθ intersection of the horizontal line y = c and the graph of y = tan x for 0 ≤ x < 2π ? 37. sin 2θ + sin 3θ = 0

S E C T I O N 1.4

Trigonometric Functions

21

SOLUTION sin α = − sin β when α = −β + 2π k or α = π + β + 2π k. Substituting α = 2θ and β = 3θ , we have either 2θ = −3θ + 2π k or 2θ = π + 3θ + 2π k. Solving each of these equations for θ yields θ = 25 π k or θ = −π − 2π k. The solutions on the interval 0 ≤ θ < 2π are then

θ = 0,

2π 4π 6π 8π , , π, , . 5 5 5 5

39. cos 4θ + cos 2θ = 0 sin θ = sin 2θ SOLUTION cos α = − cos β when α + β = π + 2π k or α = β + π + 2π k. Substituting α = 4θ and β = 2θ , we have either 6θ = π + 2π k or 4θ = 2θ + π + 2π k. Solving each of these equations for θ yields θ = π6 + π3 k or θ = π2 + π k. The solutions on the interval 0 ≤ θ < 2π are then

θ=

π π 5π 7π 3π 11π , , , , , . 6 2 6 6 2 6

In Exercises the identities using the identities listed in this section. = cos 2derive θ sin θ 41–50, 41. cos 2θ = 2 cos2 θ − 1 Starting from the double angle formula for cosine, cos2 θ = 12 (1 + cos 2θ ), we solve for cos 2θ . This gives 2 cos2 θ = 1 + cos 2θ and then cos 2θ = 2 cos2 θ − 1. θ θ 1 −1 cos θ θ + cos 43. sin = 2 cos 2 2 = 2 2 θ 1 SOLUTION Substitute x = θ /2 into the double angle formula for sine, sin2 x = 2 (1 − cos 2x) to obtain sin2 = 2 θ 1 − cos θ 1 − cos θ . Taking the square root of both sides yields sin = . 2 2 2 SOLUTION

45. cos(θ + π ) = − cos θ sin(θ + π ) = − sin θ SOLUTION From the addition formula for the cosine function, we have cos(θ + π ) = cos θ cos π − sin θ sin π = cos θ (−1) = − cos θ 47. tan(π − θ ) = −

tan π θ tan x = cot −x SOLUTION Using Exercises 44 and 45, 2 tan(π − θ ) =

sin(π − θ ) sin(π + (−θ )) − sin(−θ ) sin θ = − tan θ . = = = cos(π − θ ) cos(π + (−θ )) − cos(−θ ) − cos θ

The second to last equality occurs because sin x is an odd function and cos x is an even function. sin 2x 49. tan x = 2 tan x + cos 2x tan 2x1 = 1 − tan2 x SOLUTION Using the addition formula for the sine function, we ﬁnd sin 2x = sin(x + x) = sin x cos x + cos x sin x = 2 sin x cos x. By Exercise 41, we know that cos 2x = 2 cos2 x − 1. Therefore, 2 sin x cos x sin x 2 sin x cos x sin 2x = = = = tan x. 2 2 1 + cos 2x cos x 1 + 2 cos x − 1 2 cos x 51. Use Exercises 44 and 45 to show that tan θ and cot θ are periodic with period π . 1 − cos 4x sin2 x By cos2Exercises x= SOLUTION 44 8 and 45, tan(θ + π ) =

sin(θ + π ) − sin θ = tan θ , = cos(θ + π ) − cos θ

cot(θ + π ) =

cos(θ + π ) − cos θ = cot θ . = sin(θ + π ) − sin θ

and

Thus, both tan θ and cot θ are periodic with period π . Use the addition formula to compute cos

π π π π , noting that = − . 12 12 3 4

22

CHAPTER 1

PRECALCULUS REVIEW

53. Use the Law of Cosines to ﬁnd the distance from P to Q in Figure 26. Q

10

7π/9 8

P

FIGURE 26 SOLUTION

By the Law of Cosines, the distance from P to Q is 102 + 82 − 2(10)(8) cos

7π = 16.928. 9

Further Insights and Challenges 55. Use the addition formulas for sine and cosine to prove Use the addition formula to prove tan a + tan b 3 θ − 3 cos θ tan(a cos+ 3θb)==4 cos 1 − tan a tan b cot(a − b) =

cot a cot b + 1 cot b − cot a

SOLUTION

tan(a + b) =

sin a cos b cos a sin b sin(a + b) tan a + tan b sin a cos b + cos a sin b cos a cos b + cos a cos b = = = cos a cos b sin a sin b cos(a + b) cos a cos b − sin a sin b 1 − tan a tan b − cos a cos b cos a cos b

cot(a − b) =

cos a cos b + sin a sin b cos(a − b) cot a cot b + 1 cos a cos b + sin a sin b sin a sin b sin a sin b = = = sin a cos b cos a sin b sin(a − b) sin a cos b − cos a sin b cot b − cot a sin a sin b − sin a sin b

57. Let L 1 and L 2 be the lines of slope m 1 and m 2 [Figure 27(B)]. Show that the angle θ between L 1 and L 2 satisﬁes Let m 2θm 1be+the 1 angle between the line y = mx + b and the x-axis [Figure 27(A)]. Prove that m = tan θ . . cot θ = m2 − m1 SOLUTION Measured from the positive x-axis, let α and β satisfy tan α = m 1 and tan β = m 2 . Without loss of generality, let β ≥ α . Then the angle between the two lines will be θ = β − α . Then from Exercise 55,

cot θ = cot(β − α ) =

( m11 )( m12 ) + 1 cot β cot α + 1 1 + m1m2 = = 1 − 1 cot α − cot β m2 − m1 m1 m2

59. Apply the double-angle formula to prove: Perpendicular √Lines Use Exercise 57 to prove that two lines with nonzero slopes m 1 and m 2 are perpendicular π 1 (a) cos = if 2m+ = 2−1/m 1 . if and only 2 8 2 √ 1 π 2+ 2+ 2 = (b) cos 16 2 π π and of cos n for all n. Guess the values of cos 32 2 SOLUTION

√

√ 1 + cos π4 1 + 22 1 π π /4 = = = (a) cos = cos 2 + 2. 8 2 2 2 2 √

√ 1 + cos π8 1 + 12 2 + 2 π 1 (b) cos 2 + 2 + 2. = = = 16 2 2 2 π involves three (c) Observe that 8 = 23 and cos π8 involves two nested square roots of 2; further, 16 = 24 and cos 16 5 nested square roots of 2. Since 32 = 2 , it seems plausible that

√ 1 π 2 + 2 + 2 + 2, = cos 32 2

and that cos 2πn involves n − 1 nested square roots of 2. Note that the general case can be proven by induction.

S E C T I O N 1.5

Technology: Calculators and Computers

23

1.5 Technology: Calculators and Computers Preliminary Questions 1. Is there a deﬁnite way of choosing the optimal viewing rectangle, or is it best to experiment until you ﬁnd a viewing rectangle appropriate to the problem at hand? SOLUTION

It is best to experiment with the window size until one is found that is appropriate for the problem at hand.

2. Describe the calculator screen produced when the function y = 3 + x 2 is plotted with viewing rectangle: (a) [−1, 1] × [0, 2] (b) [0, 1] × [0, 4] SOLUTION

(a) Using the viewing rectangle [−1, 1] by [0, 2], the screen will display nothing as the minimum value of y = 3 + x 2 is y = 3. (b) Using the viewing rectangle [0, 1] by [0, 4], the screen will display the portion of the parabola between the points (0, 3) and (1, 4). 3. According to the evidence in Example 4, it appears that f (n) = (1 + 1/n)n never takes on a value greater than 3 for n ≥ 0. Does this evidence prove that f (n) ≤ 3 for n ≥ 0? SOLUTION

No, this evidence does not constitute a proof that f (n) ≤ 3 for n ≥ 0.

4. How can a graphing calculator be used to ﬁnd the minimum value of a function? Experiment with the viewing window to zoom in on the lowest point on the graph of the function. The y-coordinate of the lowest point on the graph is the minimum value of the function.

SOLUTION

Exercises The exercises in this section should be done using a graphing calculator or computer algebra system. 1. Plot f (x) = 2x 4 + 3x 3 − 14x 2 − 9x + 18 in the appropriate viewing rectangles and determine its roots. SOLUTION

Using a viewing rectangle of [−4, 3] by [−20, 20], we obtain the plot below. y 20 10 x

−4 −3 −2 −1 −10

1

2

3

−20

Now, the roots of f (x) are the x-intercepts of the graph of y = f (x). From the plot, we can identify the x-intercepts as −3, −1.5, 1, and 2. The roots of f (x) are therefore x = −3, x = −1.5, x = 1, and x = 2. 3. How many positive solutions does x 3 − 12x + 8 = 0 have? How many solutions does x 3 − 4x + 8 = 0 have? SOLUTION The graph of y = x 3 − 12x + 8 shown below has two x-intercepts to the right of the origin; therefore the equation x 3 − 12x + 8 = 0 has two positive solutions. y 60 40 20 −4

−2

x 2

−20 −40 −60

4

√ 5. Find all the solutions of sin x = x for x > 0. Does cos x + x = 0 have a solution? Does √ it have a positive solution? SOLUTION Solutions to the equation sin x = x correspond to points of intersection between the graphs of y = sin x √ and y = √ x. The two graphs are shown below; the only point of intersection is at x = 0. Therefore, there are no solutions of sin x = x for x > 0. y 2 1 x 1 −1

2

3

4

5

24

CHAPTER 1

PRECALCULUS REVIEW

7. Let f (x) = (x − 100)2 + 1,000. What 2will the display show if you graph f (x) in the viewing rectangle [−10, 10] How many solutions x = x have? by [−10, −10]? What woulddoes be ancos appropriate viewing rectangle? SOLUTION Because (x − 100)2 ≥ 0 for all x, it follows that f (x) = (x − 100)2 + 1000 ≥ 1000 for all x. Thus, using a viewing rectangle of [−10, 10] by [−10, 10] will display nothing. The minimum value of the function occurs when x = 100, so an appropriate viewing rectangle would be [50, 150] by [1000, 2000]. x in + a viewing rectangle that clearly displays the vertical and horizontal asymptotes. 9. Plot the graph of f (x) = 8x 1 Plot the graph of f (x)4 − =x in an appropriate viewing rectangle. What are the vertical and horizontal 8x − x4 asymptotes (seethe Example 3)?y = SOLUTION From graph of shown below, we see that the vertical asymptote is x = 4 and the horizontal 4−x asymptote is y = −1. y 2 −8 −4

x 4

8 12 16

−2

11. Plot f (x) = cos(x 2 ) sin x for 0 ≤ x ≤ 2π2 . Then illustrate local linearity at x = 3.8 by choosing appropriate viewing Illustrate local linearity for f (x) = x by zooming in on the graph at x = 0.5 (see Example 6). rectangles. SOLUTION The following three graphs display f (x) = cos(x 2 ) sin x over the intervals [0, 2π ], [3.5, 4.1] and [3.75, 3.85]. The ﬁnal graph looks like a straight line. y

y

y

1

0.4

1

0.2 x 1

2

3

4

5

x

6

3.5 3.6 3.7 3.8 3.9

−1

4

3.76 3.78 3.8 3.82 3.84 x −0.2

−1

In Exercises 13–18, the behavior of the function or xdeposit grows Plarge by making a table of function values A bank pays investigate r = 5% interest compounded monthly.asIf nyou 0 dollars at time t = 0, then the value of behavior and plotting a graph (see Example 4). Describe the in words. N 0.05 . Find to the nearest integer N the number of months after which your account after N months is P0 1 + 12 13. f (n) = n 1/n the account value doubles. SOLUTION The table and graphs below suggest that as n gets large, n 1/n approaches 1. n

n 1/n

10 102 103 104 105 106

1.258925412 1.047128548 1.006931669 1.000921458 1.000115136 1.000013816

y

y

1

1

x 0

2

4

6

8

10

x 0

200 400 600 800 1000

n 2 4n 1+ 1 15. f (n)f (n) = =1 + 6n n− 5 SOLUTION

The table and graphs below suggest that as n gets large, f (n) tends toward ∞.

Technology: Calculators and Computers

S E C T I O N 1.5

n 10 102 103 104 105 106

2 1 n 1+ n

13780.61234 1.635828711 × 1043 1.195306603 × 10434 5.341783312 × 104342 1.702333054 × 1043429 1.839738749 × 10434294

y

y 1 × 10 43

10,000

x 0

25

2

4

6

8

x 0

10

20

40

60

80 100

1 x 17. f (x) = x tanx + 6 x x f (x) = x −4 SOLUTION The table and graphs below suggest that as x gets large, f (x) approaches 1. 1 x x

x

x tan

10 102 103 104 105 106

1.033975759 1.003338973 1.000333389 1.000033334 1.000003333 1.000000333

y

y

1.5 1.4 1.3 1.2 1.1 1

1.5 1.4 1.3 1.2 1.1 1 x 5

10

15

x

20

20

40

60

80 100

19. The graph of f (θ ) = A2 cos θ + B sin θ is a sinusoidal wave for any constants A and B. Conﬁrm this for ( A, B) = (1, 1), (1, 2), and (3, 4)1by xplotting f (θ ). f (x) = x tan x of f (θ ) = cos θ + sin θ , f (θ ) = cos θ + 2 sin θ and f (θ ) = 3 cos θ + 4 sin θ are shown SOLUTION The graphs below. y

y

y

2

4

1 1 x

−2

2

4

6

8

−2

−1

2 x 2

−1

4

6

8

−2 (A, B) = (1, 1)

−2

x 2

−2

4

6

8

−4 (A, B) = (1, 2)

(A, B) = (3, 4)

21. Find the intervals on which f (x) = x(x + 2)(x − 3) is positive by plotting a graph. Find the maximum value of f (θ ) for the graphs produced in Exercise 19. Can you guess the formula for the SOLUTION The function f (x) = x(x + 2)(x − 3) is positive when the graph of y = x(x + 2)(x − 3) lies above the maximum value in terms of A and B? x-axis. The graph of y = x(x + 2)(x − 3) is shown below. Clearly, the graph lies above the x-axis and the function is positive for x ∈ (−2, 0) ∪ (3, ∞). y 20 −4

−2

x −20

2

4

−40

Find the set of solutions to the inequality (x 2 − 4)(x 2 − 1) < 0 by plotting a graph.

26

CHAPTER 1

PRECALCULUS REVIEW

Further Insights and Challenges 23. Let f 1 (x) = x and deﬁne a sequence of functions by f n+1 (x) = 12 ( f n (x) + x/ f n (x)). For example, √ 1 f 2 (x) = 2 (x + 1). Use a computer algebra system to compute f n (x) for n = 3, 4, 5 and plot f n (x) together with x for x ≥ 0. What do you notice? SOLUTION

With f 1 (x) = x and f 2 (x) = 12 (x + 1), we calculate 1 1 x 2 + 6x + 1 x = (x + 1) + 1 f 3 (x) = 2 2 4(x + 1) 2 (x + 1) ⎛ ⎞ 4 3 2 1 x 2 + 6x + 1 x ⎠ = x + 28x + 70x + 28x + 1 + 2 f 4 (x) = ⎝ x +6x+1 2 4(x + 1) 8(1 + x)(1 + 6x + x 2 ) 4(x+1)

and 1 + 120x + 1820x 2 + 8008x 3 + 12870x 4 + 8008x 5 + 1820x 6 + 120x 7 + x 8 . 16(1 + x)(1 + 6x + x 2 )(1 + 28x + 70x 2 + 28x 3 + x 4 ) √ √ A plot of f 1 (x), f 2 (x), f 3 (x), f 4 (x), f 5 (x) and x is shown below, with the graph of x shown as a dashed curve. It √ seems as if the f n are asymptotic to x. f 5 (x) =

y 12 8 4 x 20 40 60 80 100

Set P0 (x) = 1 and P1 (x) = x. The Chebyshev polynomials (useful in approximation theory) are deﬁned successively by the formula Pn+1 (x) = 2x Pn (x) − Pn−1 (x). CHAPTER REVIEW EXERCISES (a) Show that P2 (x) = 2x 2 − 1. for{x3 :≤|x n− ≤ computer algebra Compute 1. (b) Express (4, 10)Pnas(x) a set a| 6< using c} forasuitable a and c. system or by hand, and plot each Pn (x) over the interval [−1, 1]. 4+10 = 7 and the radius is 10−4 = 3. Therefore, the interval (4, 10) SOLUTION The center of the interval (4, 10) is 2 2 (c) Check that your plots conﬁrm two interesting properties of Chebyshev polynomials: (a) Pn (x) has n real roots is equivalent to the set {x : |x − 7| < 3}. in [−1, 1] and (b) for x ∈ [−1, 1], Pn (x) lies between −1 and 1. 3. Express {x : 2 ≤ |x − 1| ≤ 6} as a union of two intervals. Express as an interval: SOLUTION set<{x4}: 2 ≤ |x − 1| ≤ 6} consists of those numbers at ≤ least (a) {x : |xThe − 5| (b) {x that : |5xare + 3| 2} 2 but at most 6 units from 1. The numbers larger than 1 that satisfy these conditions are 3 ≤ x ≤ 7, while the numbers smaller than 1 that satisfy these conditions are −5 ≤ x ≤ −1. Therefore {x : 2 ≤ |x − 1| ≤ 6} = [−5, −1] ∪ [3, 7]. 5. Describe the pairs of numbers x, y such that |x + y| = x − y. Give an example of numbers x, y such that |x| + |y| = x − y. SOLUTION First consider the case when x + y ≥ 0. Then |x + y| = x + y and we obtain the equation x + y = x − y. The solution of this equation is y = 0. Thus, the pairs (x, 0) with x ≥ 0 satisfy |x + y| = x − y. Next, consider the case when x + y < 0. Then |x + y| = −(x + y) = −x − y and we obtain the equation −x − y = x − y. The solution of this equation is x = 0. Thus, the pairs (0, y) with y < 0 also satisfy |x + y| = x − y. In Exercises 7–10, let f (x) be the function whose graph is shown in Figure 1. Sketch the graph of y = f (x + 2) − 1, where f (x) = x 2 for −2 ≤ x ≤ 2. y 3 2 1 x

0 1

2

3

4

FIGURE 1

7. Sketch the graphs of y = f (x) + 2 and y = f (x + 2). SOLUTION The graph of y = f (x) + 2 is obtained by shifting the graph of y = f (x) up 2 units (see the graph below at the left). The graph of y = f (x + 2) is obtained by shifting the graph of y = f (x) to the left 2 units (see the graph below at the right).

Chapter Review Exercises y

y

5

5

4

4

3

3

2

2

1 −2 −1

27

1 x 1

2

3

x

−2 −1

4

1

f(x) + 2

2

3

4

f(x + 2)

9. Continue the graph of f (x) to the interval [−4, 4] as an even function. Sketch the graphs of y = 12 f (x) and y = f ( 12 x). SOLUTION To continue the graph of f (x) to the interval [−4, 4] as an even function, reﬂect the graph of f (x) across the y-axis (see the graph below). y 3 2 1 x

−4 −3 −2 −1

1

2

3

4

In Exercises 11–14, theofdomain and of the function. Continue theﬁnd graph f (x) to therange interval [−4, 4] as an odd function. √ 11. f (x) = x + 1 √ SOLUTION The domain of the function f (x) = x + 1 is {x : x ≥ −1} and the range is {y : y ≥ 0}. 2 13. f (x) = 4 f (x) 3=− x4 x +1 SOLUTION The domain of the function f (x) =

2 is {x : x = 3} and the range is {y : y = 0}. 3−x

15. Determine whether the function is increasing, decreasing, or neither: 2 −x +5 f (x) = x 1 (b) f (x) = 2 (a) f (x) = 3−x x +1 (c) g(t) = t 2 + t (d) g(t) = t 3 + t SOLUTION

(a) The function f (x) = 3−x can be rewritten as f (x) = ( 13 )x . This is an exponential function with a base less than 1; therefore, this is a decreasing function. (b) From the graph of y = 1/(x 2 + 1) shown below, we see that this function is neither increasing nor decreasing for all x (though it is increasing for x < 0 and decreasing for x > 0). y 1 0.8 0.6 0.4 0.2 x

−3 −2 −1

1

2

3

(c) The graph of y = t 2 + t is an upward opening parabola; therefore, this function is neither increasing nor decreasing for all t. By completing the square we ﬁnd y = (t + 12 )2 − 14 . The vertex of this parabola is then at t = − 12 , so the function is decreasing for t < − 12 and increasing for t > − 12 . (d) From the graph of y = t 3 + t shown below, we see that this is an increasing function. y 20 −3 −2 −1 −20

x 1

In Exercises 17–22,whether ﬁnd thethe equation of is theeven, line.odd, or neither: Determine function (a) f (x) = x 4 − 3x 2 (b) g(x) = sin(x + 1)

2

3

28

CHAPTER 1

PRECALCULUS REVIEW

17. Line passing through (−1, 4) and (2, 6) SOLUTION

The slope of the line passing through (−1, 4) and (2, 6) is m=

2 6−4 = . 2 − (−1) 3

The equation of the line passing through (−1, 4) and (2, 6) is therefore y − 4 = 23 (x + 1) or 2x − 3y = −14. 19. Line of slope 6 through (9, 1) Line passing through (−1, 4) and (−1, 6) SOLUTION Using the point-slope form for the equation of a line, the equation of the line of slope 6 and passing through (9, 1) is y − 1 = 6(x − 9) or 6x − y = 53. 21. Line through (2, 3) parallel to y = 4 − x 3 Line ofThe slope − through (4, −12) SOLUTION equation 2 y = 4 − x is in slope-intercept form; it follows that the slope of this line is −1. Any line parallel to y = 4 − x will have the same slope, so we are looking for the equation of the line of slope −1 and passing through (2, 3). The equation of this line is y − 3 = −(x − 2) or x + y = 5. 23. Does the following table of market data suggest a linear relationship between price and number of homes sold during Horizontal line through (−3, 5) a one-year period? Explain.

SOLUTION

Price (thousands of $)

180

195

220

240

No. of homes sold

127

118

103

91

Examine the slope between consecutive data points. The ﬁrst pair of data points yields a slope of 9 3 118 − 127 =− =− , 195 − 180 15 5

while the second pair of data points yields a slope of 15 3 103 − 118 =− =− 220 − 195 25 5 and the last pair of data points yields a slope of 91 − 103 12 3 =− =− . 240 − 220 20 5 Because all three slopes are equal, the data does suggest a linear relationship between price and the number of homes sold. 2 and sketch its graph. On which intervals is f (x) decreasing? 25. FindDoes the roots of f (x) = x4 − the following table of4x yearly revenue data for a computer manufacturer suggest a linear relation between revenue and SOLUTION Thetime? rootsExplain. of f (x) = x 4 − 4x 2 are obtained by solving the equation x 4 − 4x 2 = x 2 (x − 2)(x + 2) = 0, which yields x = −2, x = 0 and x = 2. The graph of y = f (x) is shown below. From this graph we see that f (x) is Year −1.4 and for x between 1995 0 and 1999 2001 2004 decreasing for x less than approximately approximately 1.4.

Revenue (billions of $)

y

13

18

15

11

20 10 −1

1

−3 −2

x 2

3

27. Let f (x) be the square of the distance from the point (2, 1) to a point (x, 3x + 2) on the line y = 3x + 2. Show that 2 + 12z + 3. Complete the square and ﬁnd the minimum value of h(z). h(z) = 2z f (x) is Let a quadratic function and ﬁnd its minimum value by completing the square. SOLUTION

3x + 2. Then

Let f (x) denote the square of the distance from the point (2, 1) to a point (x, 3x + 2) on the line y = f (x) = (x − 2)2 + (3x + 2 − 1)2 = x 2 − 4x + 4 + 9x 2 + 6x + 1 = 10x 2 + 2x + 5,

which is a quadratic function. Completing the square, we ﬁnd f (x) = 10

1 x2 + x + 5

1 100

1 1 2 49 + . +5− = 10 x + 10 10 10

1 )2 ≥ 0 for all x, it follows that f (x) ≥ 49 for all x. Hence, the minimum value of f (x) is 49 . Because (x + 10 10 10

Prove that x 2 + 3x + 3 ≥ 0 for all x.

Chapter Review Exercises

29

In Exercises 29–34, sketch the graph by hand. 29. y = t 4 y

SOLUTION 1 0.8 0.6 0.4 0.2 −1

x

−0.5

0.5

1

θ 31. y =y sin = t25 y

SOLUTION 1 0.5

x

−5

5

10

−0.5 −1

33. y = x 1/3 −x y = 10 y

SOLUTION 2 1 −4 −3 −2 −1 −1

x 1 2 3 4

−2

35. Show that1 the graph of y = f 13 x − b is obtained by shifting the graph of y = f 13 x to the right 3b units. Use 1 y = 2 to sketch the graph of y = x − 4. this observation 3 x SOLUTION

Let g(x) = f ( 13 x). Then

g(x − 3b) = f

1 1 (x − 3b) = f x −b . 3 3

Thus, the graph of y = f ( 13 x − b) is obtained by shifting the graph of y = f ( 13 x) to the right 3b units. The graph of y = | 13 x − 4| is the graph of y = | 13 x| shifted right 12 units (see the graph below). y 4 3 2 1 x 0

5

10

15

20

37. Find functions f and g such that the function Let h(x) = cos x and g(x) = x −1 . Compute the composite functions h(g(x)) and g(h(x)), and ﬁnd their dof (g(t)) = (12t + 9)4 mains. SOLUTION

One possible choice is f (t) = t 4 and g(t) = 12t + 9. Then f (g(t)) = f (12t + 9) = (12t + 9)4

as desired.

θ sin 2θ + sin to?the following three angles and ﬁnd the values of the six 39. What is thethe period of the function g(θ ) = Sketch points on the unit circle corresponding 2 standard trigonometric functions at each angle: SOLUTION of 4π . Because 4π is a 2π The function sin 2θ has a period 7ofπ π , and the function sin(θ /2) has a 7period π (a) of π , the period of the function g(θ(b) (c) multiple ) = sin 2 θ + sin θ /2 is 4 π . 3 4 6 Assume that sin θ =

4 π , where < θ < π . Find: 5 2

30

CHAPTER 1

PRECALCULUS REVIEW

41. Give an example of values a, b such that (a) cos(a + b) = cos a + cos b

(b) cos

a cos a = 2 2

SOLUTION

(a) Take a = b = π /2. Then cos(a + b) = cos π = −1 but cos a + cos b = cos

π π + cos = 0 + 0 = 0. 2 2

(b) Take a = π . Then cos

a 2

= cos

π 2

=0

but cos a cos π −1 1 = = =− . 2 2 2 2 43. Solve sin 2x + cos x = 0 for 0 ≤ x < 2π . π 1 π . equation as 2 sin x cos x + cos x = Let f (x) = cos x. Sketch the graph of y = 2 f x − forwe0 ≤ x ≤ 6the SOLUTION Using the double angle formula for the sine rewrite 3 function, 4 cos x(2 sin x + 1) = 0. Thus, either cos x = 0 or sin x = −1/2. From here we see that the solutions are x = π /2, x = 7π /6, x = 3π /2 and x = 11π /6. Use a graphing calculator to determine whether the equation cos x = 5x 2 − 8x 4 has any solutions. n2 How does h(n) = n behave for large whole 2 number 4 values of n? Does h(n) tend to inﬁnity? SOLUTION The graphs of 2 y = cos x and y = 5x − 8x are shown below. Because the graphs do not intersect, there are no solutions to the equation cos x = 5x 2 − 8x 4 . 45.

y 1

y = cos x x

−1

1 −1

y = 5x 2 − 8x 4

Using a graphing calculator, ﬁnd the number of real roots and estimate the largest root to two decimal places: (a) f (x) = 1.8x 4 − x 5 − x (b) g(x) = 1.7x 4 − x 5 − x

2 LIMITS 2.1 Limits, Rates of Change, and Tangent Lines Preliminary Questions 1. Average velocity is deﬁned as a ratio of which two quantities? SOLUTION

Average velocity is deﬁned as the ratio of distance traveled to time elapsed.

2. Average velocity is equal to the slope of a secant line through two points on a graph. Which graph? SOLUTION

Average velocity is the slope of a secant line through two points on the graph of position as a function of

time. 3. Can instantaneous velocity be deﬁned as a ratio? If not, how is instantaneous velocity computed? SOLUTION Instantaneous velocity cannot be deﬁned as a ratio. It is deﬁned as the limit of average velocity as time elapsed shrinks to zero.

4. What is the graphical interpretation of instantaneous velocity at a moment t = t0 ? SOLUTION Instantaneous velocity at time t = t0 is the slope of the line tangent to the graph of position as a function of time at t = t0 .

5. What is the graphical interpretation of the following statement: The average ROC approaches the instantaneous ROC as the interval [x 0 , x 1 ] shrinks to x0 ? SOLUTION

The slope of the secant line over the interval [x 0 , x 1 ] approaches the slope of the tangent line at x = x 0 .

6. The ROC of atmospheric temperature with respect to altitude is equal to the slope of the tangent line to a graph. Which graph? What are possible units for this rate? SOLUTION The rate of change of atmospheric temperature with respect to altitude is the slope of the line tangent to the graph of atmospheric temperature as a function of altitude. Possible units for this rate of change are ◦ F/ft or ◦ C/m.

Exercises 1. A ball is dropped from a state of rest at time t = 0. The distance traveled after t seconds is s(t) = 16t 2 ft. (a) How far does the ball travel during the time interval [2, 2.5]? (b) Compute the average velocity over [2, 2.5]. (c) Compute the average velocity over time intervals [2, 2.01], [2, 2.005], [2, 2.001], [2, 2.00001]. Use this to estimate the object’s instantaneous velocity at t = 2. SOLUTION

(a) Galileo’s formula is s(t) = 16t 2 . The ball thus travels s = s(2.5) − s(2) = 16(2.5)2 − 16(2)2 = 36 ft. (b) The average velocity over [2, 2.5] is s(2.5) − s(2) 36 s = = = 72 ft/s. t 2.5 − 2 0.5 (c) time interval

[2, 2.01]

[2, 2.005]

[2, 2.001]

[2, 2.00001]

average velocity

64.16

64.08

64.016

64.00016

The instantaneous velocity at t = 2 is 64 ft/s. √ 3. LetAv wrench = 20 Tisas in Example Estimate theatinstantaneous of vthe with respect to T when T =velocity 300 K. at t = 1, released from 2. a state of rest time t = 0. ROC Estimate wrench’s instantaneous 2 SOLUTION assuming that the distance traveled after t seconds is s(t) = 16t . T interval

[300, 300.01]

[300, 300.005]

average rate of change

0.577345

0.577348

T interval

[300, 300.001]

[300, 300.00001]

average ROC

0.57735

0.57735

32

CHAPTER 2

LIMITS

The instantaneous rate of change is approximately 0.57735 m/(s · K). In Exercises 5–6, y/x a stone is in the [2, air 5], from ground an initial of 15 m/s. time t to is Compute fortossed the interval where y =level 4x −with 9. What is thevelocity instantaneous ROCItsofheight y withatrespect h(t) x=at15t − 2? 4.9t 2 m. x= 5. Compute the stone’s average velocity over the time interval [0.5, 2.5] and indicate the corresponding secant line on a sketch of the graph of h(t). SOLUTION

The average velocity is equal to h(2.5) − h(0.5) = 0.3. 2

The secant line is plotted with h(t) below. h 10 8 6 4 2 t 0.5

1

1.5

2

2.5

3

7. With an initial deposit of $100, the balance in a bank account after t years is f (t) = 100(1.08)t dollars. Compute the stone’s average velocity over the time intervals [1, 1.01], [1, 1.001], [1, 1.0001] and [0.99, 1], (a) What are1],the units of1]. theUse ROC (t)? [0.999, [0.9999, thisoftofestimate the instantaneous velocity at t = 1. (b) Find the average ROC over [0, 0.5] and [0, 1]. (c) Estimate the instantaneous rate of change at t = 0.5 by computing the average ROC over intervals to the left and right of t = 0.5. SOLUTION

(a) The units of the rate of change of f (t) are dollars/year or $/yr. (b) The average rate of change of f (t) = 100(1.08)t over the time interval [t1 , t2 ] is given by f f (t2 ) − f (t1 ) . = t t2 − t 1 time interval

[0, .5]

[0, 1]

average rate of change

7.8461

8

(c) time interval

[.5, .51]

[.5, .501]

[.5, .5001]

average rate of change

8.0011

7.9983

7.9981

time interval

[.49, .5]

[.499, .5]

[.4999, .5]

average ROC

7.9949

7.9977

7.998

The rate of change at t = 0.5 is approximately $8/yr. In Exercises 9–12, estimate the instantaneous rate of change at3 the point indicated. The distance traveled by a particle at time t is s(t) = t + t. Compute the average velocity over the time interval 2 − 3; xthe 4] = and4xestimate velocity at t = 1. = instantaneous 2 9. [1, P(x) SOLUTION

x interval

[2, 2.01]

[2, 2.001]

[2, 2.0001]

[1.99, 2]

[1.999, 2]

[1.9999, 2]

average rate of change

16.04

16.004

16.0004

15.96

15.996

15.9996

The rate of change at x = 2 is approximately 16. 1 11. y(x)f (t) = = 3t − ; x5;=t 2= −9 x +2 SOLUTION

x interval

[2, 2.01]

[2, 2.001]

[2, 2.0001]

[1.99, 2]

[1.999, 2]

[1.9999, 2]

average ROC

−.0623

−.0625

−.0625

−.0627

−.0625

−.0625

S E C T I O N 2.1

Limits, Rates of Change, and Tangent Lines

33

The rate of change at x = 2 is approximately −0.06. 13. The atmospheric temperature T (in ◦ F) above a certain point on earth is T = 59 − 0.00356h, where h is the altitude √ 1; t = 1What are the average and instantaneous rates of change of T with respect to h? Why are y(t) =for h3t≤+37,000). in feet (valid they the same? Sketch the graph of T for h ≤ 37,000. The average and instantaneous rates of change of T with respect to h are both −0.00356◦ F/ft. The rates of change are the same because T is a linear function of h with slope −0.00356. SOLUTION

Temp (°F) 60 40 20

20,000 10,000

−20 −40 −60

15. (a) (b)

30,000

Altitude (ft)

The number P(t) of E. coli cells at time t (hours) in a petri dish is plotted in Figure 9. The height (in feet) at time t (in seconds) of a small weight oscillating at the end of a spring is h(t) = 0.5 cos(8t). Calculate the average ROC of P(t) over the time interval [1, 3] and draw the corresponding secant line. (a) Calculate the weight’s average velocity over the time intervals [0, 1] and [3, 5]. Estimate the slope m of the line in Figure 9. What does m represent? (b) Estimate its instantaneous velocity at t = 3. P(t) 10,000 8,000 6,000 4,000 2,000 1,000 1

2

3 t (hours)

FIGURE 9 Number of E. coli cells at time t. SOLUTION

(a) Looking at the graph, we can estimate P(1) = 2000 and P(3) = 8000. Assuming these values of P(t), the average rate of change is P(3) − P(1) 6000 = = 3000 cells/hour. 3−1 2 The secant line is here: P(t) 10,000 8,000 6,000 4,000 2,000 1,000 1

2

3 t (hours)

(b) The line in Figure 9 goes through two points with approximate coordinates (1, 2000) and (2.5, 4000). This line has approximate slope m=

4000 4000 − 2000 = cells/hour. 2.5 − 1 3

m is close to the slope of the line tangent to the graph of P(t) at t = 1, and so m represents the instantaneous rate of change of P(t) at t = 1 hour. √ T (in seconds) of a pendulum (the√time required for a complete back-and-forth cycle) 17. Assume√that the period √Calculate x + 1 − x for x = 1, 10, 100, 1,000. Does x√change more rapidly when x is large or small? 3 where L is the pendulum’s length (into meters). is T Interpret = 2 L,your answer in terms of tangent lines the graph of y = x. (a) What are the units for the ROC of T with respect to L? Explain what this rate measures. (b) Which quantities are represented by the slopes of lines A and B in Figure 10? (c) Estimate the instantaneous ROC of T with respect to L when L = 3 m.

CHAPTER 2

LIMITS BA Period (s)

34

2

1

3 Length (m)

FIGURE 10 The period T is the time required for a pendulum to swing back and forth. SOLUTION

(a) The units for the rate of change of T with respect to L are seconds per meter. This rate measures the sensitivity of the period of the pendulum to a change in the length of the pendulum. (b) The slope of the line B represents the average rate of change in T from L = 1 m to L = 3 m. The slope of the line A represents the instantaneous rate of change of T at L = 3 m. (c) time interval

[3, 3.01]

[3, 3.001]

[3, 3.0001]

[2.99, 3]

[2.999, 3]

[2.9999, 3]

average velocity

0.4327

0.4330

0.4330

0.4334

0.4330

0.4330

The instantaneous rate of change at L = 1 m is approximately 0.4330 s/m. 19. The graphs in Figure 12 represent the positions s of moving particles as functions of time t. Match each graph with The graphs in Figure 11 represent the positions of moving particles as functions of time. one of the following statements: (a) Do the (a) Speeding upinstantaneous velocities at times t1 , t2 , t3 in (A) form an increasing or decreasing sequence? (b) Is theup particle speeding updown or slowing down in (A)? (b) Speeding and then slowing (c) Is thedown particle speeding up or slowing down in (B)? (c) Slowing (d) Slowing down and then speeding up s

s

t

t (A)

(B)

s

s

t

t

(C)

(D)

FIGURE 12 SOLUTION When a particle is speeding up over a time interval, its graph is bent upward over that interval. When a particle is slowing down, its graph is bent downward over that interval. Accordingly,

• In graph (A), the particle is (c) slowing down. • In graph (B), the particle is (b) speeding up and then slowing down. • In graph (C), the particle is (d) slowing down and then speeding up. • In graph (D), the particle is (a) speeding up.

21. (a) (b) (c)

The fraction of a city’s population infected by a ﬂu virus is plotted as a function of time (in weeks) in Figure 14. An epidemiologist ﬁnds that the percentage N (t) of susceptible children who were infected on day t during the Which quantities are represented by the slopes of lines A and B? Estimate these slopes. ﬁrst three weeks of a measles outbreak is given, to a reasonable approximation, by the formula Is the ﬂu spreading more rapidly at t = 1, 2, or 3? Is the ﬂu spreading more rapidly at t = 4, 5, or 6? 100t 2 N (t) = 3 Fraction infected 2 t + 5t − 100t + 380 A graph of N (t) appears in Figure 13.

0.3 B 0.2 A

(a) Draw the secant line whose slope is the0.1 average rate of increase in infected children over the intervals between days 4 and 6 and between days 12 and 14. Then compute these average rates (in units of percent per day). Weeks (b) Estimate the ROC of N (t) on day 12. 1 2 3 4 5 6 FIGURE 14

Limits, Rates of Change, and Tangent Lines

S E C T I O N 2.1

35

SOLUTION

(a) The slope of line A is the average rate of change over the interval [4, 6], whereas the slope of the line B is the instantaneous rate of change at t = 6. Thus, the slope of the line A ≈ (0.28 − 0.19)/2 = 0.045/week, whereas the slope of the line B ≈ (0.28 − 0.15)/6 = 0.0217/week. (b) Among times t = 1, 2, 3, the ﬂu is spreading most rapidly at t = 3 since the slope is greatest at that instant; hence, the rate of change is greatest at that instant. (c) Among times t = 4, 5, 6, the ﬂu is spreading most rapidly at t = 4 since the slope is greatest at that instant; hence, the rate of change is greatest at that instant. √ 23. Let v =fusarium 20 T as in Example 2. Is athe ROC of v plants with respect T greater at low temperatures high The fungus exosporium infects ﬁeld of ﬂax throughtothe roots and causes the plants or to wilt. temperatures? in terms ofinfected. the graph. Eventually,Explain the entire ﬁeld is The percentage f (t) of infected plants as a function of time t (in days) since planting is shown in Figure 15. SOLUTION (a) What are the units of the rate of change of f (t) with respect to t? What does this rate measure? (b) Use the graph to rank (from smallest (m/s) to largest) the average infection rates over the intervals [0, 12], [20, 32], 350 and [40, 52]. 300 (c) Use the following table to compute 250 the average rates of infection over the intervals [30, 40], [40, 50], [30, 50]: Days Percent

200 150 100 50 infected

0 0

10 18

20 56

30 82

50 100 150 200 250 300

T (K)

40 91

50 96

60 98

(d) Draw the tangent line at t = 40 and estimate its slope. Choose any two points on the tangent line for the computation. As the graph progresses to the right, the graph bends progressively downward, meaning that the slope of the tangent lines becomes smaller. This means that the ROC of v with respect to T is lower at high temperatures. Sketch the graph in of af straight (x) = x(1 x) over 1]. Refer to the graph without anythen computations, 25. If an object moving line−(but with [0, changing velocity) coversand, s feet in tmaking seconds, its average ﬁnd:velocity is v = s/t ft/s. Show that it would cover the same distance if it traveled at constant velocity v over 0 0 (a) The average overof [0,t 1] seconds. This is a justiﬁcation for calling s/t the average velocity. the same timeROC interval (b) The (instantaneous) ROC at x = 12 (c) The values of x at which the ROC is positive SOLUTION y 0.25 0.2 0.15 0.1 0.05 x 0.2

0.4

0.6

0.8

1

(a) f (0) = f (1), so there is no change between x = 0 and x = 1. The average ROC is zero. (b) The tangent line to the graph of f (x) is horizontal at x = 12 ; the instantaneous ROC is zero at this point. (c) The ROC is positive at all points where the graph is rising, because the slope of the tangent line is positive at these points. This is so for all x between x = 0 and x = 0.5. graphand in Figure 16 has the following property: For all x, the average ROC over [0, x] is greater than the Further Which Insights Challenges

instantaneous ROC at x? Explain. 27. The height of a projectile ﬁred in the air vertically with initial velocity 64 ft/s is h(t) = 64t − 16t 2 ft. (a) Compute h(1). Show that h(t) − h(1) can be factored with (t − 1) as a factor. (b) Using part (a), show that the average velocity over the interval [1, t] is −16(t − 3). (c) Use this formula to ﬁnd the average velocity over several intervals [1, t] with t close to 1. Then estimate the instantaneous velocity at time t = 1. SOLUTION

(a) With h(t) = 64t − 16t 2 , we have h(1) = 48 ft, so h(t) − h(1) = −16t 2 + 64t − 48. Taking out the common factor of −16 and factoring the remaining quadratic, we get h(t) − h(1) = −16(t 2 − 4t + 3) = −16(t − 1)(t − 3).

36

CHAPTER 2

LIMITS

(b) The average velocity over the interval [1, t] is h(t) − h(1) −16(t − 1)(t − 3) = = −16(t − 3). t −1 t −1

t

1.1

1.05

1.01

1.001

average velocity over [1, t]

30.4

31.2

31.84

31.984

(c)

The instantaneous velocity is approximately 32 ft/s. Plugging t = 1 second into the formula in (b) yields −16(1 − 3) = 32 ft/s exactly. 29. Show that the average ROC of f (x) = x 3 over [1, x] is equal to x 2 + x + 1. Use this to estimate the instantaneous 2 Q(t) ROC ofLet f (x) at x==t 1.. As in the previous exercise, ﬁnd a formula for the average ROC of Q over the interval [1, t] and use it to estimate the instantaneous ROC at t = 1. Repeat for the interval [2, t] and estimate the ROC at t = 2. SOLUTION The average ROC is f (x) − f (1) x3 − 1 = . x −1 x −1 Factoring the numerator as the difference of cubes means the average rate of change is (x − 1)(x 2 + x + 1) = x2 + x + 1 x −1 (for all x = 1). The closer x gets to 1, the closer the average ROC gets to 12 + 1 + 1 = 3. The instantaneous ROC is 3. √ 31. T = 32 forLthe as in Exercise 17.ofThe numbers the[2, second of estimate Table 4 are and those over x] andcolumn use it to theincreasing instantaneous ROC in at Find Let a formula average ROC f (x) = x 3 in the last are decreasing. Explain why in terms of the graph of T as a function of L. Also, explain graphically why x =column 2. the instantaneous ROC at L = 3 lies between 0.4329 and 0.4331. TABLE 4

Average Rates of Change of T with Respect to L

Interval

Average ROC

Interval

Average ROC

[3, 3.2] [3, 3.1] [3, 3.001] [3, 3.0005]

0.42603 0.42946 0.43298 0.43299

[2.8, 3] [2.9, 3] [2.999, 3] [2.9995, 3]

0.44048 0.43668 0.43305 0.43303

SOLUTION Since the average ROC is increasing on the intervals [3, L] as L get close to 3, we know that the slopes of the secant lines between points on the graph over these intervals are increasing. The more rows we add with smaller intervals, the greater the average ROC. This means that the instantaneous ROC is probably greater than all of the numbers in this column. Likewise, since the average ROC is decreasing on the intervals [L , 3] as L gets closer to 3, we know that the slopes of the secant lines between points over these intervals are decreasing. This means that the instantaneous ROC is probably less than all the numbers in this column. The tangent slope is somewhere between the greatest value in the ﬁrst column and the least value in the second column. Hence, it is between .43299 and .43303. The ﬁrst column underestimates the instantaneous ROC by secant slopes; this estimate improves as L decreases toward L = 3. The second column overestimates the instantaneous ROC by secant slopes; this estimate improves as L increases toward L = 3.

2.2 Limits: A Numerical and Graphical Approach Preliminary Questions 1. What is the limit of f (x) = 1 as x → π ? SOLUTION

limx→π 1 = 1.

2. What is the limit of g(t) = t as t → π ? SOLUTION

limt→π t = π .

3. Can f (x) approach a limit as x → c if f (c) is undeﬁned? If so, give an example.

Limits: A Numerical and Graphical Approach

S E C T I O N 2.2

37

SOLUTION Yes. The limit of a function f as x → c does not depend on what happens at x = c, only on the behavior of f as x → c. As an example, consider the function

f (x) =

x2 − 1 . x −1

The function is clearly not deﬁned at x = 1 but x2 − 1 = lim (x + 1) = 2. x→1 x − 1 x→1

lim f (x) = lim

x→1

4. Is lim 20 equal to 10 or 20? x→10

SOLUTION

limx→10 20 = 20.

5. What does the following table suggest about lim f (x) and lim f (x)? x→1−

x

0.9

0.99

0.999

1.1

1.01

1.001

7

25

4317

3.0126

3.0047

3.00011

f (x) SOLUTION

x→1+

The values in the table suggest that limx→1− f (x) = ∞ and limx→1+ f (x) = 3.

6. Is it possible to tell if lim f (x) exists by only examining values f (x) for x close to but greater than 5? Explain. x→5

SOLUTION No. By examining values of f (x) for x close to but greater than 5, we can determine whether the one-sided limit limx→5+ f (x) exists. To determine whether limx→5 f (x) exists, we must examine value of f (x) on both sides of x = 5.

7. If you know in advance that lim f (x) exists, can you determine its value just knowing the values of f (x) for all x→5

x > 5? SOLUTION

Yes. If limx→5 f (x) exists, then both one-sided limits must exist and be equal.

8. Which of the following pieces of information is sufﬁcient to determine whether lim f (x) exists? Explain. x→5

(a) (b) (c) (d) (e)

The values of The values of The values of The values of f (5)

f (x) for all x f (x) for x in [4.5, 5.5] f (x) for all x in [4.5, 5.5] other than x = 5 f (x) for all x ≥ 5

SOLUTION To determine whether lim x→5 f (x) exists, we must know the values of f (x) for values of x near 5, both smaller than and larger than 5. Thus, the information in (a), (b) or (c) would be sufﬁcient. The information in (d) does not include values of f for x < 5, so this information is not sufﬁcient to determine whether the limit exists. The limit does not depend at all on the value f (5), so the information in (e) is also not sufﬁcient to determine whether the limit exists.

Exercises In Exercises 1–4, ﬁll in the tables and guess the value of the limit. x3 − 1 . 1. lim f (x), where f (x) = 2 x→1 x −1 f (x)

x

f (x)

x

1.002

0.998

1.001

0.999

1.0005

0.9995

1.00001

0.99999

SOLUTION

x

0.998

0.999

0.9995

0.99999

1.00001

1.0005

1.001

1.002

f (x)

1.498501

1.499250

1.499625

1.499993

1.500008

1.500375

1.500750

1.501500

38

CHAPTER 2

LIMITS

The limit as x → 1 is 32 . 2− y−2 ycos t −1 3. limlim f (y), where f (y) = h(t), where h(t) =y 2 + 2y − 6. .Note that h(t) is even, that is, h(t) = h(−t). y→2 t→0 t

t

y±0.002 f (y) y ±0.0001

f (y) ±0.00005

h(t) 2.002

1.998

2.001

1.999

2.0001

1.9999

±0.00001

SOLUTION

y

1.998

1.999

1.9999

2.0001

2.001

2.02

f (y)

0.59984

0.59992

0.599992

0.600008

0.60008

0.601594

The limit as y → 2 is 35 . 5. Determine lim f (x) for the function f (x) shown in Figure 8. sin θ − θ x→0.5 lim f (θ ), where f (θ ) = . θ →0 θ3 y

θ f (θ )

±0.002

±0.0001

±0.00005

±0.00001

1.5

x .5

FIGURE 8 SOLUTION

The graph suggests that f (x) → 1.5 as x → .5.

In Exercises 7–8, of evaluate limit. functions in Figure 9 appear to approach a limit as x → 0? Do either the twothe oscillating 7. lim x x→21

SOLUTION

As x → 21, f (x) = x → 21. You can see this, for example, on the graph of f (x) = x.

√ verify each limit using the limit deﬁnition. For example, in Exercise 9, show that |2x − 6| can be made In Exercises 9–18, lim 3 as smallx→4.2 as desired by taking x close to 3. 9. lim 2x = 6 x→3

SOLUTION

|x − 3| small.

|2x − 6| = 2|x − 3|. |2x − 6| can be made arbitrarily small by making x close enough to 3, thus making

11. lim (4x + 3) = 11 x→2lim 4 = 4 x→3

|(4x + 3) − 11| = |4x − 8| = 4|x − 2|. Therefore, if you make |x − 2| small enough, you can make |(4x + 3) − 11| as small as desired. SOLUTION

13. lim (−2x) = −18 x→9lim (5x − 7) = 8 x→3

We have |−2x − (−18)| = |(−2)(x − 9)| = 2 |x − 9|. If you make |x − 9| small enough, you can make 2 |x − 9| = |−2x − (−18)| as small as desired. SOLUTION

x 2 =(10 − 2x) = 11 15. lim lim x→0 x→−5

As x → 0, we have |x 2 − 0| = |x + 0||x − 0|. To simplify things, suppose that |x| < 1, so that |x + 0||x − 0| = |x||x| < |x|. By making |x| sufﬁciently small, so that |x + 0||x − 0| = x 2 is even smaller, you can make |x 2 − 0| as small as desired. SOLUTION

+ 3) = 3 17. lim (x 2 + 2x x→0lim (2x 2 + 4) = 4 x→0

As x → 0, we have |x 2 + 2x + 3 − 3| = |x 2 + 2x| = |x||x + 2|. If |x| < 1, |x + 2| can be no bigger than 3, so |x||x + 2| < 3|x|. Therefore, by making |x − 0| = |x| sufﬁciently small, you can make |x 2 + 2x + 3 − 3| = |x||x + 2| as small as desired. SOLUTION

lim (x 3 + 9) = 9

x→0

Limits: A Numerical and Graphical Approach

S E C T I O N 2.2

In Exercises 19–32, estimate the limit numerically or state that the limit does not exist. √ x −1 19. lim x→1 x − 1 SOLUTION

x

.9995

.99999

1.00001

1.0005

f (x)

.500063

.500001

.49999

.499938

x

1.999

1.99999

2.00001

2.001

f (x)

1.666889

1.666669

1.666664

1.666445

x

−0.01

−0.005

0.005

0.01

f (x)

1.999867

1.999967

1.999967

1.999867

The limit as x → 1 is 12 . x2 + x − 6 2x 2 − 18 x→2 lim x2 − x − 2 x→−3 x + 3

21. lim

SOLUTION

The limit as x → 2 is 53 . sin 2x 23. lim x 3 − 2x 2 − 9 x→0lim x x→3 x 2 − 2x − 3 SOLUTION

The limit as x → 0 is 2. sin x 25. lim sin 5x x→0limx 2 x→0 x SOLUTION

x

−.01

−.001

−.0001

.0001

.001

.01

f (x)

−99.9983

−999.9998

−10000.0

10000.0

999.9998

99.9983

The limit does not exist. As x → 0−, f (x) → −∞; similarly, as x → 0+, f (x) → ∞. 1 27. lim cos cos θ − 1 h→0lim h θ θ →0 SOLUTION

h

±0.1

±0.01

±0.001

±0.0001

f (h)

−0.839072

0.862319

0.562379

−0.952155

The limit does not exist since cos (1/ h) oscillates inﬁnitely often as h → 0. 2h − 1 29. limlim sin h cos 1 h h→0 h h→0 SOLUTION

h

−.05

−.001

.001

.05

f (h)

.681273

.692907

.693387

.705298

The limit as x → 0 is approximately 0.693. (The exact answer is ln 2.) 5h −x25 x 2 −3 31. lim lim h→2 h − 2 x x→0 SOLUTION

39

40

CHAPTER 2

LIMITS

x

1.95

1.999

2.001

2.05

f (x)

38.6596

40.2036

40.2683

41.8992

The limit as h → 2 is approximately 40.2. (The exact answer is 25 ln 5.) 33. Determine x lim f (x) and lim f (x) for the function shown in Figure 10. lim |x| x→2+ x→2− x→0

y

2

1

x 2

FIGURE 10 SOLUTION

The left-hand limit is lim f (x) = 2, whereas the right-hand limit is lim f (x) = 1. Accordingly, the x→2−

x→2+

two-sided limit does not exist. 35. The greatest integer function is deﬁned by [x] = n, where n is the unique integer such that n ≤ x < n + 1. See Determine the one-sided limits at c = 1, 2, 4, 5 of the function g(t) shown in Figure 11 and state whether the Figure 12. limit exists at these points. (a) For which values of c does lim [x] exist? What about lim [x]? x→c−

x→c+

(b) For which values of c does lim [x] exist? x→c

y 2 1 x

−1

1

2

3

FIGURE 12 Graph of y = [x]. SOLUTION

(a) The one-sided limits exist for all real values of c. (b) For each integer value of c, the one-sided limits differ. In particular, lim [x] = c − 1, whereas lim [x] = c. (For x→c−

x→c+

noninteger values of c, the one-sided limits both equal [c].) The limit lim [x] exists when x→c

lim [x] = lim [x] ,

x→c−

x→c+

namely for noninteger values of c: n < c < n + 1, where n is an integer. In Exercises 37–39, determine the xone-sided limits numerically. −1 and use it to determine the one-sided limits lim f (x) and lim f (x). Draw a graph of f (x) = |x − 1| x→1+ x→1− sin x 37. lim |x| x→0± SOLUTION

x

−.2

−.02

.02

.2

f (x)

−.993347

−.999933

.999933

.993347

The left-hand limit is lim f (x) = −1, whereas the right-hand limit is lim f (x) = 1. x→0−

x→0+

x − sin(|x|) 39. limlim |x|1/x 3 x→0± x→0± x SOLUTION

x

−.1

−.01

.01

.1

f (x)

199.853

19999.8

.166666

.166583

Limits: A Numerical and Graphical Approach

S E C T I O N 2.2

The left-hand limit is lim f (x) = ∞, whereas the right-hand limit is lim f (x) = x→0−

x→0+

1 . 6

41. Determine the one-sided limits of f (x) at c = 2 and c = 4, for the function shown in Figure 14. Determine the one- or two-sided inﬁnite limits in Figure 13. y 15 10 5 x 2

4

−5

FIGURE 14 SOLUTION

• For c = 2, we have lim f (x) = ∞ and lim f (x) = ∞. x→2− x→2+ • For c = 4, we have lim f (x) = −∞ and lim f (x) = 10. x→4− x→4+

In Exercises 43–46,the draw the graph of atwo-sided function with theingiven Determine inﬁnite one- and limits Figurelimits. 15. 43. lim f (x) = 2, lim f (x) = 0, lim f (x) = 4 x→1

x→3−

x→3+

SOLUTION y 6 4 2

x 1

45.

2

3

4

lim f (x) = f (2) = 3, lim f (x) = −1, lim f (x) = 2 = f (4)

lim f (x) = ∞, lim x→2− f (x) = 0, lim fx→4 (x) = −∞ x→2+ x→1 x→3− x→3+

SOLUTION y 3 2 1 x −1

1

2

3

4

5

Inlim Exercises 47–52, use = the−∞ graph to estimate the value of the limit. f (x) = ∞, graph lim fthe (x)function = 3, limandf (x) x→1+

sin 3θ 47. lim θ →0 sin 2θ

x→1−

x→4

SOLUTION y 1.500 y= 1.495 1.490 1.485

The limit as θ → 0 is 32 . 2x − 1 x→0 4x − 1 lim

sin 3 sin 2

41

42

CHAPTER 2

LIMITS

2x − cos x x x→0

49. lim

SOLUTION y 0.6940 0.6935 y=

2x

− cos x x

0.6930

0.6925 0.6920

The limit as x → 0 is approximately 0.693. (The exact answer is ln 2.) cos 3θ − cos 4θ 51. lim sinθ224θ θ →0lim θ →0 cos θ − 1 SOLUTION

y 3.500

y=

cos 3 − cos 4 2

3.475 3.450 3.425

The limit as θ → 0 is 3.5. cos 3θ − cos 5θ FurtherlimInsights and Challenges 2

θ →0 θ 53. Light waves of frequency λ passing through a slit of width a produce a Fraunhofer diffraction pattern of light and dark fringes (Figure 16). The intensity as a function of the angle θ is given by sin(R sin θ ) 2 I (θ ) = I m R sin θ

where R = π a/λ and Im is a constant. Show that the intensity function is not deﬁned at θ = 0. Then check numerically that I (θ ) approaches Im as θ → 0 for any two values of R (e.g., choose two integer values).

a Incident light waves Slit

Viewing screen

Intensity pattern

FIGURE 16 Fraunhofer diffraction pattern. SOLUTION

If you plug in θ = 0, you get a division by zero in the expression

sin R sin θ ; R sin θ

thus, I (0) is undeﬁned. If R = 2, a table of values as θ → 0 follows:

θ

−0.01

−0.005

0.005

0.01

I (θ )

0.998667 Im

0.9999667 Im

0.9999667 Im

0.9998667 Im

The limit as θ → 0 is 1 · Im = Im . If R = 3, the table becomes:

θ

−0.01

−0.005

0.005

0.01

I (θ )

0.999700 Im

0.999925 Im

0.999925 Im

0.999700 Im

S E C T I O N 2.2

Limits: A Numerical and Graphical Approach

43

Again, the limit as θ → 0 is 1Im = Im . bx − 1 sin nlim θ 55. Show numerically that for b = 3, 5 appears to equal ln 3, ln 5, where ln x is the natural logarithm. Then for several values of n and then guess the value in general. Investigate lim x x→0numerically θ θ →0 make a conjecture (guess) for the value in general and test your conjecture for two additional values of b (these results are explained in Chapter 7). SOLUTION

•

−.1

−.01

−.001

.001

.01

.1

1.486601

1.596556

1.608144

1.610734

1.622459

1.746189

−.1

−.01

−.001

.001

.01

.1

1.040415

1.092600

1.098009

1.099216

1.104669

1.161232

x 5x − 1 x We have ln 5 ≈ 1.6094. •

x 3x − 1 x We have ln 3 ≈ 1.0986. • We conjecture that lim x→0

bx − 1 = ln b for any positive number b. Here are two additional test cases. x −.1

−.01

−.001

.001

.01

.1

−.717735

−.695555

−.693387

−.692907

−.690750

−.669670

x 1 x 2

−1

x

We have ln 12 ≈ −0.69315. x

−.1

−.01

−.001

.001

.01

.1

7x − 1 x

1.768287

1.927100

1.944018

1.947805

1.964966

2.148140

We have ln 7 ≈ 1.9459. xn − 1 for (m, n) equal to + (2,nx) 1),1/x (1,. 2), (2, 3), and (3, 2). Then guess the value of the limit in 57. Investigate lim number Let n be x→1 any lim (1 x m − 1 and consider x→0 general and check your guess for at least three additional pairs. (a) Give numerical evidence that for n = 1, the limit is e [where e = exp(1) can be found on any scientiﬁc SOLUTION calculator]. •(b) Estimate the limit for n = 2, 3 and try to express the results in terms of e. Then form a conjecture for the value of the limit in general. Is it valid for noninteger n? x .99 .9999 1.0001 1.01 x −1 x2 − 1

.502513

.500025

.499975

.497512

x

.99

.9999

1.0001

1.01

x2 − 1 x −1

1.99

1.9999

2.0001

2.01

The limit as x → 1 is 12 .

The limit as x → 1 is 2. x

.99

.9999

1.0001

1.01

x2 − 1 x3 − 1

0.670011

0.666700

0.666633

0.663344

44

CHAPTER 2

LIMITS

The limit as x → 1 is 23 . x

.99

.9999

1.0001

1.01

x3 − 1 x2 − 1

1.492513

1.499925

1.500075

1.507512

The limit as x → 1 is 32 .

xn − 1 n • For general m and n, we have lim = . m x→1 x m − 1 • x

.99

.9999

1.0001

1.01

x −1 x3 − 1

.336689

.333367

.333300

.330022

x

.99

.9999

1.0001

1.01

x3 − 1 x −1

2.9701

2.9997

3.0003

3.0301

x

.99

.9999

1.0001

1.01

x3 − 1 x7 − 1

.437200

.428657

.428486

.420058

The limit as x → 1 is 13 .

The limit as x → 1 is 3.

The limit as x → 1 is 37 ≈ 0.428571. 2x − 8 2 Sketch a graph of f (x) = with a graphing calculator. Observe that f (3) is not deﬁned. x − 3 k such that lim sin(sin x) exists. Find by experimentation the positive integers k x→0 x (a) Zoom in on the graph to estimate L = lim f (x).

59.

x→3

(b) Observe that the graph of f (x) is increasing. Explain how this implies that f (2.99999) ≤ L ≤ f (3.00001) Use this to determine L to three decimal places. SOLUTION

(a)

y 5.565 5.555 5.545

y=

2x − 8 x−3

5.535 5.525 x=3

(b) It is clear that the graph of f rises as we move to the right. Mathematically, we may express this observation as: whenever u < v, f (u) < f (v). Because 2.99999 < 3 = lim f (x) < 3.00001, x→3

it follows that f (2.99999) < L = lim f (x) < f (3.00001). x→3

With f (2.99999) ≈ 5.54516 and f (3.00001) ≈ 5.545195, the above inequality becomes 5.54516 < L < 5.545195; hence, to three decimal places, L = 5.545. 21/x − 2−1/x The function f (x) = 1/x is deﬁned for x = 0. −1/x

S E C T I O N 2.3

Basic Limit Laws

45

sin x is equal to the slope of a secant line through the origin and the point (x, sin x) on x the graph of y = sin x (Figure 17). Use this to give a geometric interpretation of lim f (x). Show that f (x) =

61.

x→0

y (x, sin x)

1 sin x

x x

FIGURE 17 Graph of y = sin x.

sin x sin x − 0 = . Since x −0 x f (x) is the slope of the secant line, the limit lim f (x) is equal to the slope of the tangent line to y = sin x at x = 0.

SOLUTION

The slope of a secant line through the points (0, 0) and (x, sin x) is given by x→0

2.3 Basic Limit Laws Preliminary Questions 1. State the Sum Law and Quotient Law. SOLUTION

Suppose limx→c f (x) and limx→c g(x) both exist. The Sum Law states that lim ( f (x) + g(x)) = lim f (x) + lim g(x).

x→c

x→c

x→c

Provided limx→c g(x) = 0, the Quotient Law states that lim

f (x)

x→c g(x)

=

limx→c f (x) . limx→c g(x)

2. Which of the following is a verbal version of the Product Law? (a) The product of two functions has a limit. (b) The limit of the product is the product of the limits. (c) The product of a limit is a product of functions. (d) A limit produces a product of functions. SOLUTION

The verbal version of the Product Law is (b): The limit of the product is the product of the limits.

3. Which of the following statements are incorrect (k and c are constants)? (b) lim k = k (a) lim k = c x→c

x→c

(c) lim x = c2 x→c2

(d) lim x = x x→c

SOLUTION Statements (a) and (d) are incorrect. Because k is constant, statement (a) should read lim x→c k = k. Statement (d) should be limx→c x = c.

4. Which of the following statements are incorrect? (a) The Product Law does not hold if the limit of one of the functions is zero. (b) The Quotient Law does not hold if the limit of the denominator is zero. (c) The Quotient Law does not hold if the limit of the numerator is zero. SOLUTION Statements (a) and (c) are incorrect. The Product Law remains valid when the limit of one or both of the functions is zero, and the Quotient Law remains valid when the limit of the numerator is zero.

Exercises In Exercises 1–22, evaluate the limits using the Limit Laws and the following two facts, where c and k are constants: lim x = c,

x→c

1. lim x x→9

SOLUTION

lim x = 9.

x→9

lim x

x→−3

lim k = k

x→c

46

CHAPTER 2

LIMITS

3. lim 14 x→9

SOLUTION

5.

lim 14 = 14.

x→9

lim (3x + 4)

lim 14 x→−3 x→−3

SOLUTION

We apply the Laws for Sums, Products, and Constants: lim (3x + 4) = lim 3x + lim 4

x→−3

x→−3

x→−3

= 3 lim x + lim 4 = 3(−3) + 4 = −5. x→−3

7.

x→−3

lim (y + 14)

lim 14y y→−3 y→−3

SOLUTION

lim (y + 14) = lim y + lim 14 = −3 + 14 = 11.

y→−3

y→−3

y→−3

9. lim (3t − 14) t→4 lim y(y + 14) y→−3

SOLUTION

lim (3t − 14) = 3 lim t − lim 14 = 3 · 4 − 14 = −2.

t→4

t→4

t→4

11. lim (4x + 1)(2x − 1) x→ 21 lim (x 3 + 2x) x→−5

SOLUTION

lim (4x + 1)(2x − 1) = 4 lim x + lim 1 2 lim x − lim 1

x→1/2

x→1/2

x→1/2

x→1/2

x→1/2

1 1 = 4 +1 2 − 1 = 3 · 0 = 0. 2 2

13. lim x(x + 1)(x + 2) x→2 lim (3x 4 − 2x 3 + 4x) x→−1 SOLUTION We apply the Product Law and Sum Law: lim (x + 1) lim (x + 2) lim x(x + 1)(x + 2) = lim x x→2

x→2

=2

x→2

x→2

lim x + lim 1 lim x + lim 2

x→2

x→2

x→2

= 2(2 + 1)(2 + 2) = 24 t 15. lim lim (x + 1)(3x 2 − 9) t→9 t + 1 x→2

SOLUTION

lim t t 9 9 t→9 = = = . lim t + lim 1 9+1 10 t→9 t + 1 lim

t→9

t→9

1−x 17. lim 3t − 14 1+x x→3lim t→4 t + 1 lim 1 − lim x 1−3 −2 1 1−x x→3 x→3 SOLUTION lim = = = =− . lim 1 + lim x 1+3 4 2 x→3 1 + x x→3

x→3

19. lim t −1 x t→2 lim x→−1 x 3 + 4x SOLUTION

We apply the deﬁnition of t −1 , and then the Quotient Law. lim 1 1 1 t→2 lim t −1 = lim = = . lim t 2 t→2 t→2 t t→2

21. lim (x 2 +−29x −3 ) x→3lim x x→5

x→2

S E C T I O N 2.3 SOLUTION

Basic Limit Laws

47

We apply the Sum, Product, and Quotient Laws. The Product Law is applied to the exponentiations x 2 =

x · x and x 3 = x · x · x. lim (x 2 + 9x −3 ) = lim x 2 + lim 9x −3 =

x→3

x→3

⎛

= 9 + 9⎝

x→3

x→3

⎞

lim 1

x→3

( lim x)3

2 lim x

⎠=9+9

1 27

+9

=

1 x→3 x 3

lim

28 . 3

x→3

23. Use the Quotient Law to prove that if lim f (x) exists and is nonzero, then x→c z −1 + z lim z→1 z + 1 1 1 lim = x→c f (x) lim f (x) x→c

SOLUTION

Since lim f (x) is nonzero, we can apply the Quotient Law: x→c

lim

x→c

1 f (x)

lim 1

1 x→c = =

. lim f (x) lim f (x) x→c x→c

In Exercises 25–28, the=limit assuming Assume that evaluate lim f (x) 4 and compute:that lim f (x) = 3 and lim g(x) = 1. x→−4

x→6

25. (a) lim limf (x)g(x) f (x)2 x→−4

(b) lim

1

(c) lim x f (x)

x→6 f (x)

x→6

SOLUTION

x→−4

x→6

lim f (x)g(x) = lim f (x) lim g(x) = 3 · 1 = 3.

x→−4

x→−4

x→−4

g(x) 27. limlim 2(2 f (x) + 3g(x)) x→−4 x x→−4 SOLUTION

Since lim x 2 = 0, we may apply the Quotient Law, then applying the Product Law (from x 2 = x · x): x→−4

lim g(x) g(x) x→−4 = = x→−4 x 2 lim x 2

1

lim

x→−4

lim x

1 2 = 16 .

x→−4

sin x 29. Can the Quotient ? Explain. f (x) +Law 1 be applied to evaluate lim x→0 x lim x→−4 3g(x) − 9 sin x SOLUTION The limit Quotient Law cannot be applied to evaluate lim since lim x = 0. This violates a condition x→0 x x→0 of the Quotient Law. Accordingly, the rule cannot be employed. 31. Give an example where lim ( f (x) + g(x)) exists but neither lim f (x) nor lim g(x) exists. Show that the Productx→0 Law cannot be used to evaluate the x→0 lim (x − π /2) tan x. x→0 x→π /2

SOLUTION

Let f (x) = 1/x and g(x) = −1/x. Then lim ( f (x) + g(x)) = lim 0 = 0 However, lim f (x) = x→0

lim 1/x and lim g(x) = lim −1/x do not exist.

x→0

x→0

x→0

x→0

x→0

Use the Limit Laws and the result lim lim x n = cn for all whole numbers n. If you are a x −x→c 1 x = c to show that x→c exists and that lim a x = 1 for all a > 0. Prove that L ab = L a + L b Assume that the limit L a = lim familiar with induction, give a formalx→0 proof by x induction. x→0 for a, b > 0. Hint: (ab)x − 1 = a x (b x − 1) + (a x − 1). Verify numerically that L 12 = L 3 + L 4 . SOLUTION Correct answers can vary. An example is given: Let P[n] be the proposition : lim x n = cn , and proceed by induction.

33.

x→c

P[1] is true, as lim x = c. Suppose that P[n] is true, so that lim x n = cn . We must prove P[n + 1], that is, that x→c

x→c

lim x n+1 = cn+1 .

x→c

Applying the Product Law and P[n], we see: lim x n+1 = lim x n · x =

x→c

x→c

lim x n

x→c

lim x = cn c = cn+1 .

x→c

Therefore, P[n] true implies that P[n + 1] true. By induction, the limit is equal to cn for all n. Extend Exercise 33 to negative integers.

48

CHAPTER 2

LIMITS

Further Insights and Challenges 35. Show that if both lim f (x) g(x) and lim g(x) exist and lim g(x) is nonzero, then lim f (x) exists. Hint: Write x→c

x→c

x→c

f (x) = ( f (x) g(x))/g(x) and apply the Quotient Law. SOLUTION

x→c

Given that lim f (x)g(x) = L and lim g(x) = M = 0 both exist, observe that x→c

x→c

lim f (x)g(x) f (x)g(x) L x→c = = x→c g(x) lim g(x) M

lim f (x) = lim

x→c

x→c

also exists. h(t) = 5,=then lim h(t) = 15.exists and equals 4. 37. Prove thatthat if lim 12, then lim g(t) Show if lim tg(t) t→3t→3t t→3 t→3 SOLUTION

h(t) = 5, observe that lim t = 3. Now use the Product Law: t→3 t t→3 h(t) h(t) lim = lim t = 3 · 5 = 15. lim h(t) = lim t t t→3 t→3 t→3 t→3 t

Given that lim

f (x) 39. Prove that if lim f (x) = Lf (x) = 0 and lim g(x) = 0, then lim does not exist. x→cthat lim x→c g(x) Assuming = 1, x→c which of the following statements is necessarily true? Why? x→0 x f (x) (a) f (0) Suppose =0 (b) lim f (x) = 0 SOLUTION that lim exists. Then x→c g(x) x→0 L = lim f (x) = lim g(x) · x→c

x→c

f (x) f (x) f (x) = lim g(x) · lim = 0 · lim = 0. x→c x→c g(x) x→c g(x) g(x)

But, we were given that L = 0, so we have arrived at a contradiction. Thus, lim

f (x)

x→c g(x)

does not exist.

There is a Limit Law for composite functions but it is not stated in the text. Which of the following is the correct statement? Give an intuitive explanation. 2.4 (a)Limits (g(x))Continuity = lim f (x) lim f and x→c

x→c

(b) lim f (g(x)) = lim f (x), where L = lim g(x) Preliminary Questions x→c

x→c

x→L

f (g(x)) where lim f (x) (c) lim 1. Which property of = f (x)lim = g(x), x 3 allows us Lto= conclude that lim x 3 = 8? x→c

x→c

x→L

x→2

Use the correct version to evaluate SOLUTION We can conclude that lim x→2 x 3 = 8 because the function x 3 is continuous at x = 2. π lim sin(g(x)), where lim 1 ? g(x) = f (x) = 2. What can be said about f (3) if f is continuous and lim 6 x→2 x→2 2 x→3

SOLUTION

If f is continuous and limx→3 f (x) = 12 , then f (3) = 12 .

3. Suppose that f (x) < 0 if x is positive and f (x) > 1 if x is negative. Can f be continuous at x = 0? SOLUTION

Since f (x) < 0 when x is positive and f (x) > 1 when x is negative, it follows that lim f (x) ≤ 0

x→0+

and

lim f (x) ≥ 1.

x→0−

Thus, limx→0 f (x) does not exist, so f cannot be continuous at x = 0. 4. Is it possible to determine f (7) if f (x) = 3 for all x < 7 and f is right-continuous at x = 7? SOLUTION No. To determine f (7), we need to combine either knowledge of the values of f (x) for x < 7 with left-continuity or knowledge of the values of f (x) for x > 7 with right-continuity.

5. Are the following true or false? If false, state a correct version. (a) f (x) is continuous at x = a if the left- and right-hand limits of f (x) as x → a exist and are equal. (b) f (x) is continuous at x = a if the left- and right-hand limits of f (x) as x → a exist and equal f (a). (c) If the left- and right-hand limits of f (x) as x → a exist, then f has a removable discontinuity at x = a. (d) If f (x) and g(x) are continuous at x = a, then f (x) + g(x) is continuous at x = a. (e) If f (x) and g(x) are continuous at x = a, then f (x)/g(x) is continuous at x = a. SOLUTION

(a) False. The correct statement is “ f (x) is continuous at x = a if the left- and right-hand limits of f (x) as x → a exist and equal f (a).”

S E C T I O N 2.4

Limits and Continuity

49

(b) True. (c) False. The correct statement is “If the left- and right-hand limits of f (x) as x → a are equal but not equal to f (a), then f has a removable discontinuity at x = a.” (d) True. (e) False. The correct statement is “If f (x) and g(x) are continuous at x = a and g(a) = 0, then f (x)/g(x) is continuous at x = a.”

Exercises 1. Find the points of discontinuity of the function shown in Figure 14 and state whether it is left- or right-continuous (or neither) at these points. y 5 4 3 2 1 x 1

2

3

4

5

6

FIGURE 14 SOLUTION

• The function f is discontinuous at x = 1; it is left-continuous there. • The function f is discontinuous at x = 3; it is neither left-continuous nor right-continuous there. • The function f is discontinuous at x = 5; it is left-continuous there.

In Exercises 2–4, refer to the function f (x) in Figure 15. y 5 4 3 2 1 x 1

2

3

4

5

6

FIGURE 15

3. At which point c does f (x) have a removable discontinuity? What value should be assigned to f (c) to make f Find the points of discontinuity of f (x) and state whether f (x) is left- or right-continuous (or neither) at these continuous at x = c? points. SOLUTION Because limx→3 f (x) exists, the function f has a removable discontinuity at x = 3. Assigning f (3) = 4.5 makes f continuous at x = 3. 5. (a) For the function shown in Figure 16, determine the one-sided limits at the points of discontinuity. Find the point c1 at which f (x) has a jump discontinuity but is left-continuous. What value should be assigned (b) Which discontinuities is removable to f (c1of ) tothese make f right-continuous at x = and c1 ? how should f be redeﬁned to make it continuous at this point? y

6

x

−2

2

4

FIGURE 16 SOLUTION

(a) The function f is discontinuous at x = 0, at which lim f (x) = ∞ and lim f (x) = 2. The function f is also x→0−

discontinuous at x = 2, at which lim f (x) = 6 and lim f (x) = 6. x→2−

x→0+

x→2+

(b) The discontinuity at x = 2 is removable. Assigning f (2) = 6 makes f continuous at x = 2. In Exercises Laws of Continuity Let f7–14, (x) beuse thethe function (Figure 17): and Theorems 2–3 to show that the function is continuous. ⎧ 2 ⎪ ⎨x + 3

for x < 1

50

CHAPTER 2

LIMITS

7. f (x) = x + sin x Since x and sin x are continuous, so is x + sin x by Continuity Law (i).

SOLUTION

9. f (x) = 3x + 4 sin x f (x) = x sin x SOLUTION Since x and sin x are continuous, so are 3x and 4 sin x by Continuity Law (iii). Thus 3x + 4 sin x is continuous by Continuity Law (i). 1 11. f (x)f (x) = =2 3x 3 + 8x 2 − 20x x +1 SOLUTION

• Since x is continuous, so is x 2 by Continuity Law (ii). • Recall that constant functions, such as 1, are continuous. Thus x 2 + 1 is continuous. • Finally,

1 x2 + 1

is continuous by Continuity Law (iv) because x 2 + 1 is never 0.

3x 13. f (x) = x 2x− cos x f (x) 1=+ 4 3 + cos x SOLUTION The functions 3 x , 1 and 4 x are each continuous. Therefore, 1 + 4 x is continuous by Continuity Law (i). 3x Because 1 + 4x is never zero, it follows that is continuous by Continuity Law (iv). 1 + 4x In Exercises 15–32, determine the points at which the function is discontinuous and state the type of discontinuity: x f (x) = 10x cos removable, jump, inﬁnite, or none of these. 15. f (x) = SOLUTION

1 x The function 1/x is discontinuous at x = 0, at which there is an inﬁnite discontinuity.

x −2 17. f (x)f (x) = = |x| |x − 1| SOLUTION

The function

x −2 is discontinuous at x = 1, at which there is an inﬁnite discontinuity. |x − 1|

19. f (x) = [x] x −2 f (x) = SOLUTION This function has a jump discontinuity at x = n for every integer n. It is continuous at all other values of |x − 2| x. For every integer n, lim [x] = n

x→n+

since [x] = n for all x between n and n + 1. This shows that [x] is right-continuous at x = n. On the other hand, lim [x] = n − 1

x→n−

since [x] = n − 1 for all x between n − 1 and n. Thus [x] is not left-continuous. 1 21. g(t) = 2 1 f (x)t =− 1 x 2 1 1 = SOLUTION The function f (t) = is discontinuous at t = −1 and t = 1, at which there are 2 (t − 1)(t + 1) t −1 inﬁnite discontinuities. 23. f (x) = 3x 3/2 − 19x 3 x+ f (x) = SOLUTION The f (x) = 3x 3/2 − 9x 3 is continuous for x > 0. At x = 0 it is right-continuous. (It is not 4xfunction −2 deﬁned for x < 0.) 1 − 2z 25. h(z)g(t) = =2 3t −3/2 − 9t 3 z −z−6 1 − 2z 1 − 2z The function f (z) = 2 is discontinuous at z = −2 and z = 3, at which there = (z + 2)(z − 3) z −z−6 are inﬁnite discontinuities. SOLUTION

x 2 1−−3x2z+ 2 27. f (x) = h(z) = |x2 − 2| z +9

S E C T I O N 2.4

SOLUTION

Limits and Continuity

51

(x − 2)(x − 1) x 2 − 3x + 2 = . For x > 2, the function |x − 2| |x − 2| f (x) =

For x < 2, f (x) =

(x − 2)(x − 1) (x − 2)(x − 1) = = x − 1. |x − 2| (x − 2)

(x − 2)(x − 1) = −(x − 1). This function has a jump discontinuity at x = 2. 2−x

29. f (x) == csctan x 22t g(t)

1 is discontinuous whenever sin(x 2 ) = 0; i.e., whenever x 2 = n π 2) sin(x √ or x = ± n π , where n is a positive integer. At every such value of x there is an inﬁnite discontinuity. SOLUTION

The function f (x) = csc(x 2 ) =

31. f (x) = tan(sin x) 1 f (x) = cosfunction f (x) = tan(sin x) is continuous everywhere. Reason: sin x is continuous everywhere and SOLUTION The x tan u is continuous on − π2 , π2 —and in particular on −1 ≤ u = sin x ≤ 1. Continuity of tan(sin x) follows by the continuity of composite functions. In Exercises the domain of the function and prove that it is continuous on its domain using the Laws of f (x)33–46, = x − determine |x| Continuity and the facts quoted in this section. 33. f (x) = 9 − x 2 √ SOLUTION The domain of 9 − x 2 is all x such that 9 − x 2 ≥ 0, or |x| ≤ 3. Since x and the polynomial 9 − x 2 are both continuous on this domain, so is the composite function 9 − x 2 . √ sin x 35. f (x) = x √ √ f (x) = x 2 + 9 SOLUTION This function is deﬁned as long as x ≥ 0. Since x and sin x are continuous, so is x sin x by Continuity Law (ii). 37. f (x) = x 2/3 2x 2 x 2/3 x 2/3 f (x) = SOLUTION The domain x+ x 1/4 of x 2 is all real numbers as the denominator of the rational exponent is odd. Both x x 2/3 x and 2 are continuous on this domain, so x 2 is continuous by Continuity Law (ii). 39. f (x) = x −4/3 f (x) = x 1/3 + x 3/4 SOLUTION This function is deﬁned for all x = 0. Because the function x 4/3 is continuous and not equal to zero for x = 0, it follows that 1 x −4/3 = 4/3 x is continuous for x = 0 by Continuity Law (iv). 41. f (x) = tan2 x 3 f (x) = cos x SOLUTION The domain of tan2 x is all x = ±(2n − 1)π /2 where n is a positive integer. Because tan x is continuous on this domain, it follows from Continuity Law (ii) that tan2 x is also continuous on this domain. 43. f (x) = (x 4 + 1)3/2 f (x) = cos(2x ) SOLUTION The domain of (x 4 + 1)3/2 is all real numbers as x 4 + 1 > 0 for all x. Because x 3/2 and the polynomial x 4 + 1 are both continuous, so is the composite function (x 4 + 1)3/2 . cos(x 2 ) 45. f (x)f (x) = =2 3−x 2 x −1 The domain for this function is all x = ±1. Because the functions cos x and x 2 are continuous on this domain, so is the composite function cos(x 2 ). Finally, because the polynomial x 2 − 1 is continuous and not equal to zero cos(x 2 ) for x = ±1, the function 2 is continuous by Continuity Law (iv). x −1 SOLUTION

47. Suppose that tan f (x) = 2 for x > 0 and f (x) = −4 for x < 0. What is f (0) if f is left-continuous at x = 0? What is =9 x f (0) if ff (x) is right-continuous at x = 0? SOLUTION

Let f (x) = 2 for positive x and f (x) = −4 for negative x.

• If f is left-continuous at x = 0, then f (0) = limx→0− f (x) = −4. • If f is right-continuous at x = 0, then f (0) = limx→0+ f (x) = 2.

Sawtooth Function Draw the graph of f (x) = x − [x]. At which points is f discontinuous? Is it left- or right-continuous at those points?

52

CHAPTER 2

LIMITS

In Exercises 49–52, draw the graph of a function on [0, 5] with the given properties. 49. f (x) is not continuous at x = 1, but lim f (x) and lim f (x) exist and are equal. x→1+

x→1−

SOLUTION y 4 3 2 1 x 1

2

3

4

5

51. f (x) has a removable discontinuity at x = 1, a jump discontinuity at x = 2, and f (x) is left-continuous but not continuous at x = 2 and right-continuous but not continuous at x = 3. lim f (x) = 2 lim f (x) = −∞, x→3−

x→3+

SOLUTION y 4 3 2 1 x 1

2

3

4

5

53. Each of the following statements is false. For each statement, sketch the graph of a function that provides a counf (x) is right- but not left-continuous at x = 1, left- but not right-continuous at x = 2, and neither left- nor terexample. right-continuous at x = 3. (a) If lim f (x) exists, then f (x) is continuous at x = a. x→a

(b) If f (x) has a jump discontinuity at x = a, then f (a) is equal to either lim f (x) or lim f (x). x→a−

x→a+

(c) If f (x) has a discontinuity at x = a, then lim f (x) and lim f (x) exist but are not equal. x→a−

x→a+

(d) The one-sided limits lim f (x) and lim f (x) always exist, even if lim f (x) does not exist. x→a−

x→a

x→a+

Refer to the four ﬁgures shown below. (a) The ﬁgure at the top left shows a function for which lim f (x) exists, but the function is not continuous at x = a

SOLUTION

x→a

because the function is not deﬁned at x = a. (b) The ﬁgure at the top right shows a function that has a jump discontinuity at x = a but f (a) is not equal to either lim f (x) or lim f (x). x→a−

x→a−

(c) This statement can be false either when the two one-sided limits exist and are equal or when one or both of the one-sided limits does not exist. The ﬁgure at the top left shows a function that has a discontinuity at x = a with both one-sided limits being equal; the ﬁgure at the bottom left shows a function that has a discontinuity at x = a with a one-sided limit that does not exist. (d) The ﬁgure at the bottom left shows a function for which lim f (x) does not exist and one of the one-sided limits x→a

also does not exist; the ﬁgure at the bottom right shows a function for which lim f (x) does not exist and neither of the x→a

one-sided limits exists.

S E C T I O N 2.4

Limits and Continuity

53

y

y

a

x

y

a

x

y

a

x

a

x

In Exercises 55–70,toevaluate using the method. According the Lawstheoflimit Continuity, if substitution f (x) and g(x) are continuous at x = c, then f (x) + g(x) is continuous at x =2 c. Suppose that f (x) and g(x) are discontinuous at x = c. Is it true that f (x) + g(x) is discontinuous at 55. lim x xx→5 = c? If not, give a counterexample. lim x 2 = 52 = 25.

SOLUTION

57.

x→5

lim (2x 33 − 4) lim x x→−1 x→8

lim (2x 3 − 4) = 2(−1)3 − 4 = −6.

SOLUTION

x→−1

x +9 59. limlim (5x − 12x −2 ) x −9 x→0 x→2 0+9 x +9 = = −1. x − 9 0−9 x→0

x 61. lim sin x + − 2π x→πlim 2 x→3 x 2 + 2x x SOLUTION lim sin( − π ) = sin(− π ) = −1. lim

SOLUTION

x→π

2

2

63. lim tan(3x) x→ π4lim sin(x 2 ) x→π

lim tan(3x) = tan(3 · π4 ) = tan( 34π ) = −1

SOLUTION

x→ π4

65. lim x −5/21 x→4lim x→π cos x 1 . SOLUTION lim x −5/2 = 4−5/2 = 32 x→4 67.

− 8x 3 )3/2 lim (1 x 3 + 4x

x→−1 lim x→2

lim (1 − 8x 3 )3/2 = (1 − 8(−1)3 )3/2 = 27.

SOLUTION

x→−1

69. lim 10x −2x 7x + 2 2/3 x→3lim x→2 4 − x 2 2 SOLUTION lim 10x −2x = 103 −2(3) = 1000. 2

x→3

In Exercises 71–74, sketch the graph of the given function. At each point of discontinuity, state whether f is left- or lim 3sin x π right-continuous. x→ 2 for x ≤ 1 x2 71. f (x) = 2 − x for x > 1

54

CHAPTER 2

LIMITS SOLUTION y 1

x

−1

1

2

3

−1

The function f is continuous everywhere. |x⎧− 3| for x ≤ 3 73. f (x) = ⎨x + 1 for x < 1 for x > 3 f (x) =x − 13 ⎩ for x ≥ 1 SOLUTION x y 4

2

x 2

4

6

The function f is continuous everywhere. ⎧ ﬁnd the value of c that makes the function continuous. In Exercises 75–76, ⎪x 3 + 1 for −∞ < x ≤ 0 ⎨ f (x) =x 2 − −xc + 1for x < 5 for 0 < x < 2 ⎪ 75. f (x) = ⎩ 2 10xx − −x2c +for 4x + ≥15 5 for x ≥ 2 As x → 5−, we have x 2 − c → 25 − c = L. As x → 5+, we have 4x + 2c → 20 + 2c = R. Match the limits: L = R or 25 − c = 20 + 2c implies c = 53 . SOLUTION

77. Find all constants a, b such that the following function has no discontinuities: 2x + 9 for x ≤ 3 ⎧ π f (x) = ⎪ ⎪ −4x + c for x > 3 ⎨ax + cos x for x ≤ 4 f (x) = ⎪ π ⎪ ⎩bx + 2 for x > 4 As x → π4 −, we have ax + cos x → a4π + √1 = L. As x → π4 +, we have bx + 2 → b4π + 2 = R. 2 √ Match the limits: L = R or a4π + √1 = b4π + 2. This implies π4 (a − b) = 2 − √1 or a − b = 8−2π 2 . SOLUTION

2

2

In of 1993, the amount T (x) ofwould federalbeincome tax owed on an income of x dollars the 79. Which the following quantities represented by continuous functions of time was and determined which wouldbyhave formula one or more discontinuities? ⎧ (a) Velocity of an airplane during a ﬂight ⎪0.15x for 0 ≤ x < 21,450 ⎨ (b) Temperature in aTroom under ordinary conditions (x) = 3,217.50 + 0.28(x − 21,450) for 21,450 ≤ x < 51,900 ⎪ ⎩ interest paid yearly (c) Value of a bank account with 11,743.50 + 0.31(x − 51,900) for x ≥ 51,900 (d) The salary of a teacher Sketch graph of T (x)ofand if it has any discontinuities. Explain why, if T (x) had a jump discontinuity, it (e)the The population the determine world might be advantageous in some situations to earn less money. SOLUTION T (x), the amount of federal income tax owed on an income of x dollars in 1993, might be a discontinuous function depending upon how the tax tables are constructed (as determined by that year’s regulations). Here is a graph of T (x) for that particular year. y 15,000 10,000 5,000 x 20,000 40,000 60,000

S E C T I O N 2.5

Evaluating Limits Algebraically

55

If T (x) had a jump discontinuity (say at x = c), it might be advantageous to earn slightly less income than c (say c − ) and be taxed at a lower rate than to earn c or more and be taxed at a higher rate. Your net earnings may actually be more in the former case than in the latter one.

Further Insights and Challenges 81. Give an example of functions f (x) and g(x) such that f (g(x)) is continuous but g(x) has at least one discontinuity. If f (x) has a removable discontinuity at x = c, then it is possible to redeﬁne f (c) so that f (x) is continuous SOLUTION Answers vary. The simplest at x = c. Can f (c) may be redeﬁned in more thanexamples one way?are the functions f (g(x)) where f (x) = C is a constant function, and g(x) is deﬁned for all x. In these cases, f (g(x)) = C. For example, if f (x) = 3 and g(x) = [x], g is discontinuous at all integer values x = n, but f (g(x)) = 3 is continuous. 83. Let f (x) = 1 if x is rational and f (x) = −1 if x is irrational. Show that f (x) is discontinuous at all points, whereas Continuous at Only One Point Let f (x) = x for x rational, f (x) = −x for x irrational. Show that f (x)2 isFunction continuous at all points. f is continuous at x = 0 and discontinuous at all points x = 0. SOLUTION lim f (x) does not exist for any c. If c is irrational, then there is always a rational number r arbitrarily x→c

close to c so that | f (c) − f (r )| = 2. If, on the other hand, c is rational, there is always an irrational number z arbitrarily close to c so that | f (c) − f (z)| = 2. f (x)2 , on the other hand, is a constant function that always has value 1, which is obviously continuous.

2.5 Evaluating Limits Algebraically Preliminary Questions 1. Which of the following is indeterminate at x = 1? x2 + 1 , x −1

x2 − 1 , x +2

√

x2 − 1 x +3−2

,

√

x2 + 1 x +3−2

2 At x = 1, √x −1

is of the form 00 ; hence, this function is indeterminate. None of the remaining functions x+3−2 2 +1 2 and √x +1 are undeﬁned because the denominator is zero but the numerator is not, is indeterminate at x = 1: xx−1 x+3−2 2 −1 while xx+2 is equal to 0. SOLUTION

2. Give counterexamples to show that each of the following statements is false: (a) If f (c) is indeterminate, then the right- and left-hand limits as x → c are not equal. (b) If lim f (x) exists, then f (c) is not indeterminate. x→c

(c) If f (x) is undeﬁned at x = c, then f (x) has an indeterminate form at x = c. SOLUTION 2 −1 (a) Let f (x) = xx−1 . At x = 1, f is indeterminate of the form 00 but

x2 − 1 x2 − 1 = lim (x + 1) = 2 = lim (x + 1) = lim . x→1− x − 1 x→1− x→1+ x→1+ x − 1 lim

2 −1 (b) Again, let f (x) = xx−1 . Then

x2 − 1 = lim (x + 1) = 2 x→1 x − 1 x→1

lim f (x) = lim

x→1

but f (1) is indeterminate of the form 00 . (c) Let f (x) = 1x . Then f is undeﬁned at x = 0 but does not have an indeterminate form at x = 0. 3. Although the method for evaluating limits discussed in this section is sometimes called “simplify and plug in,” explain how it actually relies on the property of continuity. SOLUTION If f is continuous at x = c, then, by deﬁnition, lim x→c f (x) = f (c); in other words, the limit of a continuous function at x = c is the value of the function at x = c. The “simplify and plug-in” strategy is based on simplifying a function which is indeterminate to a continuous function. Once the simpliﬁcation has been made, the limit of the remaining continuous function is obtained by evaluation.

56

CHAPTER 2

LIMITS

Exercises In Exercises 1–4, show that the limit leads to an indeterminate form. Then carry out the two-step procedure: Transform the function algebraically and evaluate using continuity. x 2 − 25 x→5 x − 5

1. lim

2 −25 When we substitute x = 5 into xx−5 , we obtain the indeterminate form 00 . Upon factoring the numerator and simplifying, we ﬁnd

SOLUTION

x 2 − 25 (x − 5)(x + 5) = lim = lim (x + 5) = 10. x −5 x→5 x − 5 x→5 x→5 lim

3. lim

2t − 14

x +1

5t − 35 t→7 lim x→−1 x 2 + 2x + 1

0 When we substitute t = 7 into 2t−14 5t−35 , we obtain the indeterminate form 0 . Upon dividing out the common factor of t − 7 from both the numerator and denominator, we ﬁnd SOLUTION

lim

2t − 14

t→7 5t − 35

= lim

2(t − 7)

t→7 5(t − 7)

= lim

2

t→7 5

=

2 . 5

In Exercises 5–32, the limit or state that it does not exist. h +evaluate 3 lim 2 h→−3 x 2 −h64− 9 5. lim x→8 x − 8 SOLUTION

x 2 − 64 (x + 8)(x − 8) = lim = lim (x + 8) = 16. x −8 x→8 x − 8 x→8 x→8 lim

x 2 −23x + 2 7. lim x − 64 x→2lim x − 2 x→8 x − 9 x 2 − 3x + 2 (x − 1)(x − 2) = lim = lim (x − 1) = 1. SOLUTION lim x −2 x −2 x→2 x→2 x→2 x −2 9. lim 3 x 3 − 4x x→2lim x − 4x x→2 x − 2 1 x −2 x −2 1 SOLUTION lim 3 = lim = . = lim 8 x→2 x − 4x x→2 x(x − 2)(x + 2) x→2 x(x + 2) (1 +3h)3 − 1 11. lim x − 2x h h→0lim x→2 x − 2 SOLUTION

(1 + h)3 − 1 1 + 3h + 3h 2 + h 3 − 1 3h + 3h 2 + h 3 = lim = lim h h h h→0 h→0 h→0 lim

= lim 3 + 3h + h 2 = 3 + 3(0) + 02 = 3. h→0

3x 2 − 4x −2 4 13. lim (h + 2) − 9h x→2lim 2x 2 − 8 h−4 h→4 3x 2 − 4x − 4 (3x + 2)(x − 2) 3x + 2 8 = lim SOLUTION lim = lim = = 1. 8 x→2 x→2 2(x − 2)(x + 2) x→2 2(x + 2) 2x 2 − 8 (y − 2)3 15. lim lim3 2t + 4 y→2 y − 5y + 22 t→−2 12 − 3t (y − 2)3 (y − 2)3 (y − 2)2 = lim = lim = 0. SOLUTION lim 3 2 2 y→2 y − 5y + 2 y→2 (y − 2) y + 2y − 1 y→2 y + 2y − 1 1√ 1 x−− 4 3 + 3 lim 17. limx→16 hx − 16 h h→0

S E C T I O N 2.5

1

Evaluating Limits Algebraically

57

1

−3 1 3 − (3 + h) −1 1 lim 3+h = lim = lim =− . h 9 h→0 h→0 h 3(3 + h) h→0 3(3 + h)

SOLUTION

√

2+h−2 1 1 − h 2 4 (h + 2) √ lim hh+2−2 h→0 SOLUTION lim does not exist. h h→0 √ √ √ h + 2 − 2 ( h + 2 + 2) h+2−2 h−2 • As h → 0+, we have = √ = √ → −∞. h h( h + 2 + 2) h( h + 2 + 2) √ √ √ h + 2 − 2 ( h + 2 + 2) h+2−2 h−2 • As h → 0−, we have = √ → ∞. = √ h h( h + 2 + 2) h( h + 2 + 2)

19. lim

h→0

x −2 √ 21. lim √ x√− 6 − 2 x→2limx − 4 − x x − 10 x→10 SOLUTION

√ √ √ √ x −2 (x − 2)( x + 4 − x) (x − 2)( x + 4 − x) lim √ = lim √ = lim √ √ √ √ x − (4 − x) x→2 x − 4 − x x→2 ( x − 4 − x)( x + 4 − x) x→2 √ √ √ √ (x − 2)( x + 4 − x) (x − 2)( x + 4 − x) = lim = lim 2x − 4 2(x − 2) x→2 x→2 √ √ √ √ ( x + 4 − x) 2+ 2 √ = lim = = 2. 2 2 x→2

√ 2−1− √ x√ x +1 1 + x − 1−x 23. lim x −3 x→2lim x x→0 √ √ √ x2 − 1 − x + 1 3− 3 SOLUTION lim = = 0. x −3 −1 x→2 4 √1 25. lim √ 5 − x−− 1 x→4lim x − 2 √ x − 4 x→4 2 − x √ √ 1 x +2−4 x −2 4 1 √ = lim √ √ = . SOLUTION lim √ − = lim √ x −4 4 x −2 x→4 x→4 x→4 x −2 x +2 x −2 x +2 27. lim

cot x

1 x→0 csc x 1 lim √ − 2 x x→0+ cot x x + x cos x

SOLUTION

29. lim

lim

x→0 csc x

= lim

x→0 sin x

· sin x = cos 0 = 1.

sin x − cos x cot θ

x→ π4limπtan x − 1 θ → 2 csc θ

SOLUTION

√ sin x − cos x cos x π 2 (sin x − cos x) cos x · = lim = cos = . cos x sin x − cos x 4 2 x→ π4 tan x − 1 x→ π4 lim

2 x + 3 cos x− 2 2 cos 31. limlimπ sec θ − tan θ 2 cos x − 1 x→θπ3→ 2 SOLUTION

5 2 cos2 x + 3 cos x − 2 π (2 cos x − 1) (cos x + 2) = lim = lim cos x + 2 = cos + 2 = . π π π 2 cos x − 1 2 cos x − 1 3 2 x→ 3 x→ 3 x→ 3 lim

x −2 Use to estimate lim f (x) to two decimal places. Compare with the answer √ cosaθplot − 1of f (x) = √ lim x→2 x − 4−x θ →0 sin θ obtained algebraically in Exercise 21.

33.

√ Let f (x) = √ x−2 . From the plot of f (x) shown below, we estimate lim f (x) ≈ 1.41; to two decimal x− 4−x x→2 √ places, this matches the value of 2 obtained in Exercise 21. SOLUTION

58

CHAPTER 2

LIMITS y 1.414 1.410 1.406 x

1.402 1.6

1.8

2.0

2.2

2.4

In Exercises 35–40, use the identity 1a 3 − b3 =4 (a − b)(a 2 + ab + b2 ). to evaluate lim f (x) numerically. Compare with the answer obtained Use a plot of f (x) = √ − x −2 x −4 x→4 x3 − 1 in Exercise 25. 35. algebraically lim x→1 x − 1

2 +x +1

3 (x − 1) x x −1 SOLUTION lim = lim = lim x 2 + x + 1 = 3. x −1 x→1 x − 1 x→1 x→1 x 2 −33x + 2 37. lim x −8 x→1lim x 3 − 1 x→2 x 2 − 4 x 2 − 3x + 2 (x − 1)(x − 2) x −2 1 = lim SOLUTION lim = lim =− . 3 x→1 x→1 (x − 1) x 2 + x + 1 x→1 x 2 + x + 1 x3 − 1 x4 − 1 3 39. lim 3 x −8 x→1lim x −1 x→2 x 2 − 6x + 8 SOLUTION

4 x4 − 1 (x 2 − 1)(x 2 + 1) (x − 1)(x + 1)(x 2 + 1) (x + 1)(x 2 + 1) = lim = lim = lim = . 3 x→1 x 3 − 1 x→1 (x − 1)(x 2 + x + 1) x→1 (x − 1)(x 2 + x + 1) x→1 (x 2 + x + 1) lim

In Exercises 41–50, evaluate the limits in terms of the constants involved. x − 27 lim 1/3 41. limx→27 (2a +x x) − 3 x→0

lim (2a + x) = 2a.

SOLUTION

x→0

43. lim (4t − 2at + 3a) t→−1lim (4ah + 7a) h→−2

SOLUTION

lim (4t − 2at + 3a) = −4 + 5a.

t→−1

2(x + h)2 −22x 2 2 (3a + h) − 9a h x→0lim h h→0 2(x + h)2 − 2x 2 4hx + 2h 2 = lim = lim (4x + 2h) = 2h. SOLUTION lim h h x→0 x→0 x→0 √ √ x− a 2 47. lim (x + a) − 4x 2 x→alim x − a x→a √ √ x√ −a √ 1 x− a x− a 1 SOLUTION lim = lim √ √ = √ . √ √ √ = lim √ x→a x − a x→a x→a x + a 2 a x− a x+ a 45. lim

√a)3 − a 3 √ (x + 49. lim a + 2h − a x x→0lim h h→0 (x + a)3 − a 3 x 3 + 3x 2 a + 3xa 2 + a 3 − a 3 SOLUTION lim = lim = lim (x 2 + 3xa + 3a 2 ) = 3a 2 . x x x→0 x→0 x→0 1 1 Further Insights and Challenges −

h a lim 51–52, In Exercises ﬁnd all values of c such that the limit exists. h→a h − a x 2 − 5x − 6 x→c x −c

51. lim

x 2 − 5x − 6 will exist provided that x − c is a factor of the numerator. (Otherwise there will be an x→c x −c inﬁnite discontinuity at x = c.) Since x 2 − 5x − 6 = (x + 1)(x − 6), this occurs for c = −1 and c = 6. SOLUTION

lim

lim

x 2 + 3x + c

S E C T I O N 2.6

Trigonometric Limits

59

53. For which sign ± does the following limit exist?

1 1 lim ± x(x − 1) x→0 x

SOLUTION

(x − 1) + 1 1 1 1 + = lim = lim = −1. x x(x − 1) x(x − 1) x − 1 x→0 x→0 1 1 • The limit lim − does not exist. x(x − 1) x→0 x

• The limit lim x→0

1 1 (x − 1) − 1 x −2 − = = → ∞. x x(x − 1) x(x − 1) x(x − 1) 1 1 (x − 1) − 1 x −2 – As x → 0−, we have − = = → −∞. x x(x − 1) x(x − 1) x(x − 1)

– As x → 0+, we have

For which sign ± does the following limit exist?

2.6 Trigonometric Limits

1 1 ± |x| x→0+ x

lim

Preliminary Questions 1. Assume that −x 4 ≤ f (x) ≤ x 2 . What is lim f (x)? Is there enough information to evaluate lim f (x)? Explain. x→ 12

x→0

Since limx→0 −x 4 = limx→0 x 2 = 0, the squeeze theorem guarantees that limx→0 f (x) = 0. Since 1 = 1 = lim 4 limx→ 1 −x = − 16 x 2 , we do not have enough information to determine limx→ 1 f (x). 4 x→ 21 2 2 SOLUTION

2. State the Squeeze Theorem carefully. SOLUTION

Assume that for x = c (in some open interval containing c), l(x) ≤ f (x) ≤ u(x)

and that lim l(x) = lim u(x) = L. Then lim f (x) exists and x→c

x→c

x→c

lim f (x) = L .

x→c

3. Suppose that f (x) is squeezed at x = c on an open interval I by two constant functions u(x) and l(x). Is f (x) necessarily constant on I ? SOLUTION

Yes. Because f (x) is squeezed at x = c on an open interval I by the two functions u(x) and l(x), we know

that lim u(x) = lim l(x).

x→c

x→c

Combining this relation with the fact that u(x) and l(x) are constant functions, it follows that u(x) = l(x) for all x on I . Finally, to have l(x) ≤ f (x) ≤ u(x) = l(x), it must be the case that f (x) = l(x) for all x on I . In other words, f (x) is constant on I . sin 5h , it is a good idea to rewrite the limit in terms of the variable (choose one): h→0 3h 5h (b) θ = 3h (c) θ = 3

4. If you want to evaluate lim (a) θ = 5h SOLUTION

To match the given limit to the pattern of sin θ , θ →0 θ lim

it is best to substitute for the argument of the sine function; thus, rewrite the limit in terms of (a): θ = 5h.

60

CHAPTER 2

LIMITS

Exercises 1. In Figure 6, is f (x) squeezed by u(x) and l(x) at x = 3? At x = 2? y

u(x) f(x) l(x)

1.5

x 1

2

3

4

FIGURE 6 SOLUTION

Because there is an open interval containing x = 3 on which l(x) ≤ f (x) ≤ u(x) and lim l(x) = lim u(x), x→3

x→3

f (x) is squeezed by u(x) and l(x) at x = 3. Because there is an open interval containing x = 2 on which l(x) ≤ f (x) ≤ u(x) but lim l(x) = lim u(x), f (x) is trapped by u(x) and l(x) at x = 2 but not squeezed. x→2

x→2

3. What information about f (x) does the Squeeze Theorem provide if we assume that the graphs of f (x), u(x), and What is lim f (x) if f (x) satisﬁes cos x ≤ f (x) ≤ 1 for x in (−1, 1)? l(x) are related as in Figure 7 and that lim u(x) = lim l(x) = 6? Note that the inequality f (x) ≤ u(x) is not satisﬁed x→0 x→7

x→7

for all x. Does this affect the validity of your conclusion? y f(x)

6 u(x) l(x) x 7

FIGURE 7 SOLUTION The Squeeze Theorem does not require that the inequalities l(x) ≤ f (x) ≤ u(x) hold for all x, only that the inequalities hold on some open interval containing x = c. In Figure 7, it is clear that l(x) ≤ f (x) ≤ u(x) on some open interval containing x = 7. Because lim u(x) = lim l(x) = 6, the Squeeze Theorem guarantees that lim f (x) = 6.

x→7

x→7

x→7

In Exercises use the Theorem tosufﬁcient evaluate the limit. to determine lim State 5–8, whether the Squeeze inequality provides information

x→1 f (x) and, if so, ﬁnd the limit. (a) 4x − 51≤ f (x) ≤ x 2 5. lim x cos 2 (b) x→02x − 1x≤ f (x) ≤ x 2+2 (c) 2x + We 1 ≤prove f (x) the ≤ xlimit SOLUTION to the right ﬁrst, and then the limit to the left. Suppose x > 0. Since −1 ≤ cos 1x ≤ 1, multiplication by x (positive) yields −x ≤ x cos 1x ≤ x. The squeeze theorem implies that lim −x ≤ lim x cos 1x ≤ x→0+

x→0+

lim x, so 0 ≤ lim x cos 1x ≤ 0. This gives us lim x cos 1x = 0. On the other hand, suppose x < 0. −1 ≤ cos 1x ≤ 1

x→0+

x→0+

x→0+

multiplied by x gives us x ≤ x cos 1x ≤ −x, and we can apply the squeeze theorem again to get lim x cos 1x = 0. x→0−

Therefore, lim x cos 1x = 0. x→0

1 1 x 7. lim cos 2 sin x→0lim x xsin x x→0

1 ≤ 1, observe that − sin x ≤ cos 1x sin x ≤ sin x. Since lim sin x = 0 = SOLUTION Since −1 ≤ cos x x→0 lim − sin x, the Squeeze Theorem shows that x→0

lim cos

x→0

1 sin x = 0. x

In Exercises 9–16, evaluate π the limit using Theorem 2 as necessary. lim (x − 1) sin x −1 x→1 sin x cos x 9. lim x x→0 sin x cos x sin x SOLUTION lim = lim · lim cos x = 1 · 1 = 1. x x→0 x→0 x x→0 sin2√t 11. lim t + 2 sin t t→0lim t t t→0

S E C T I O N 2.6

SOLUTION

sin2 t sin t sin t = lim sin t = lim · lim sin t = 1 · 0 = 0. t→0 t t→0 t t→0 t t→0 lim

x2 13. limlim 2tan x x→0 sin xx x→0 SOLUTION

x2

lim

x→0 sin2 x

1 1 1 1 1 = lim sin x sin x = lim sin x · lim sin x = · = 1. 1 1 x→0 x→0 x→0 x x x x

sin t 15. lim 1 − cos t πlimt t→ 4 π t t→ 2 sin t π SOLUTION is continuous at t = . Hence, by substitution t 4 √ √ 2 2 2 sin t 2 . = π = lim π t→ π4 t 4

sin 10x 17. Let L =cos limt − cos2 t . x x→0 lim t t→0 sin θ (a) Show, by letting θ = 10x, that L = lim 10 . θ θ →0 (b) Compute L. SOLUTION

θ . θ → 0 as x → 0, so we get: Since θ = 10x, x = 10

sin θ sin θ sin θ sin 10x = lim · = lim 10 = 10 lim · = 10. x θ x→0 θ →0 (θ /10) θ →0 θ →0 θ lim

In Exercises 19–40, evaluate sin 2h 2 sin 2h 5h sin 2h the limit. . Hint: = . Evaluate lim sin 6h h→0 sin 5h sin 5h 5 2h sin 5h 19. lim h→0 h SOLUTION

lim

h→0

sin 6h sin 6h = lim 6 = 6. h 6h h→0

sin 6h 21. lim 6 sin h h→0lim6h h→0 6h sin 6h sin x SOLUTION Substitute x = 6h. As h → 0, x → 0, so: lim = lim = 1. h→0 6h x→0 x sin 7x 23. lim sin h x→0lim3x h→0 6h sin 7x 7 sin 7x 7 = lim = . SOLUTION lim 3 x→0 3x x→0 3 7x 25. lim

tan 4x

x

x→0lim9x x→ π4 sin 11x

SOLUTION

27. lim

lim

x→0

tan 4t

4 4 tan 4x 1 sin 4x = lim · · = . 9x 4x cos 4x 9 x→0 9

x

t sec t t→0lim x→0 csc 25x

SOLUTION

tan 4t 4 sin 4t 4 cos t sin 4t = lim = lim · = 4. 4t t→0 t sec t t→0 4t cos(4t) sec(t) t→0 cos 4t lim

sin(z/3) sin 2h sin 3h z→0limsin z h→0 h2 sin(z/3) z/3 1 z sin(z/3) 1 SOLUTION lim · = lim · · = . z/3 z→0 3 sin z z/3 3 z→0 sin z

29. lim

31. lim

tan 4x sin(−3θ )

tan 9x x→0lim θ →0 sin(4θ )

SOLUTION

lim

lim

tan 4x

x→0 tan 9x

csc 8t

t→0 csc 4t

= lim

cos 9x

x→0 cos 4x

·

sin 4x 4 9x 4 · · = . 4x 9 sin 9x 9

Trigonometric Limits

61

62

CHAPTER 2

LIMITS

33. lim

sin 5x sin 2x

x→0 sin 3x sin 5x

SOLUTION

lim

sin 5x sin 2x

x→0 sin 3x sin 5x

= lim

x→0

sin 2x 2 3x 2 · · = . 2x 3 sin 3x 3

1 − cos 2h 35. lim sin 3x sin 2x h→0lim h x→0 x sin 5x 1 − cos 2h 1 − cos 2h 1 − cos 2h SOLUTION lim = lim 2 = 2 lim = 2 · 0 = 0. h 2h 2h h→0 h→0 h→0 1 − cos t sin(2h)(1 − cos h) t→0limsin t h h→0 t SOLUTION A single multiplication by t turns this limit into the quotient of two familiar limits.

37. lim

lim

t→0

1 − cos t t (1 − cos t) t (1 − cos t) t (1 − cos t) = lim = lim = lim lim = 1 · 0 = 0. sin t t sin t t t t→0 t→0 sin t t→0 sin t t→0

1 − cos 3h 39. lim cos 2θ − cos θ h h→ π2lim θ θ →0 π SOLUTION The function is continuous at 2 , so we may use substitution: 1 − cos 3 π2 1−0 2 1 − cos 3h = π = . = π π h π h→ 2 2 2 lim

sin x sin x 41. Calculate1 (a) lim and (b) lim . − cos 4θ lim x→0+ |x| x→0− |x| θ →0 sin 3θ SOLUTION

sin x = 1. x sin x sin x (b) lim = −1. = lim x→0− |x| x→0− −x (a)

lim

sin x

x→0+ |x|

= lim

x→0+

1 ≤ |tan x|. Then evaluate lim tan x cos 1 using the Squeeze Theorem. 43. Show thatthe −|tan x| ≤ tan x cosstated x x Prove following result in Theorem 2: x→0

1 1 − cos θ1 SOLUTION Since | cos x | ≤ 1, it follows that | tan lim x cos x =| 0≤ | tan x|, which is equivalent to −| tan x| ≤

θ θ →0 tan x cos 1x ≤ | tan x|. Because limx→0 −| tan x| = limx→0 | tan x| = 0, the Squeeze Theorem gives 1 − cos θ 1 1 − cos2 θ Hint: = · . 1 θ 1 + cos θ θ = 0. lim tan x cos x x→0

45. Plot the graphs of u(x) = 1 + |x − π2 | and l(x) = sin x on the same set of axes. What can you say about 1 using the Squeeze π tan x cos by sinl(x) Evaluate lim f (x) if f (x)lim is squeezed x and u(x) at x = 2 ?Theorem. x→ π2

x→0

SOLUTION y u(x) = 1 + | x −

1

/2 |

l(x) = sin x x /2

lim u(x) = 1 and lim l(x) = 1, so any function f (x) satisfying l(x) ≤ f (x) ≤ u(x) for all x near π /2 will satisfy

x→π /2

lim

x→π /2

f (x) = 1.

x→π /2

Use the Squeeze Theorem to prove that if lim | f (x)| = 0, then lim f (x) = 0. x→c

x→c

S E C T I O N 2.6

Trigonometric Limits

63

Further Insights and Challenges 1 − cos h numerically (and graphically if you have a graphing utility). Then prove that the h2 limit is equal to 12 . Hint: see the hint for Exercise 42.

47.

Investigate lim

h→0

SOLUTION

•

h 1 − cos h h2

−.1

−.01

.01

.1

.499583

.499996

.499996

.499583

The limit is 12 . •

y 0.5 0.4 0.3 0.2 0.1 −2

•

−1

x 1

2

1 1 − cos h 1 − cos2 h 1 sin h 2 = . = lim = lim h 1 + cos h 2 h→0 h→0 h 2 (1 + cos h) h→0 h2 lim

cos 3h − 1 49. Evaluate lim cos 3h −. 1 cos 2h − 1 . Hint: Use the result of Exercise 47. Evaluate h→0lim h→0 h2 SOLUTION

cos 3h − 1 cos 3h − 1 cos 3h + 1 cos 2h + 1 cos2 3h − 1 cos 2h + 1 · = lim · · = lim h→0 cos 2h − 1 h→0 cos 2h − 1 cos 3h + 1 cos 2h + 1 h→0 cos2 2h − 1 cos 3h + 1 lim

= lim

− sin2 3h

h→0 − sin2 2h

=

·

9 2h 2h cos 2h + 1 sin 3h sin 3h cos 2h + 1 = lim · · · cos 3h + 1 4 h→0 3h 3h sin 2h sin 2h cos 3h + 1

2 9 9 ·1·1·1·1· = . 4 2 4

51. Using a diagram of the unit circle and the Pythagorean Theorem, show that Use the result of Exercise 47 to prove that for m = 0, sin2 θ ≤ (1 − cos θ )2 + sin2 θ ≤ θ2 m2 cos mx − 1 lim =− Conclude that sin2 θ ≤ 2(1 − cos θ ) ≤ θ 2 and use this to give 2 proof of Eq. (7) in Exercise 42. Then give an x→0 x 2 an alternate alternate proof of the result in Exercise 47. SOLUTION

• Consider the unit circle shown below. The triangle B D A is a right triangle. It has base 1 − cos θ , altitude sin θ , and

hypotenuse h. Observe that the hypotenuse h is less than the arc length AB = radius · angle = 1 · θ = θ . Apply the Pythagorean Theorem to obtain (1 − cos θ )2 + sin2 θ = h 2 ≤ θ 2 . The inequality sin2 θ ≤ (1 − cos θ )2 + sin2 θ follows from the fact that (1 − cos θ )2 ≥ 0.

B A O

D

• Note that

(1 − cos θ )2 + sin2 θ = 1 − 2 cos θ + cos2 θ + sin2 θ = 2 − 2 cos θ = 2(1 − cos θ ).

64

CHAPTER 2

LIMITS

Therefore, sin2 θ ≤ 2(1 − cos θ ) ≤ θ 2 . • Divide the previous inequality by 2θ to obtain

1 − cos θ sin2 θ θ ≤ ≤ . 2θ θ 2 Because 1 1 sin2 θ sin θ = lim · lim sin θ = (1)(0) = 0, 2 θ →0 θ 2 θ →0 2θ θ →0 lim

and lim

θ

h→0 2

= 0, it follows by the Squeeze Theorem that lim

θ →0

1 − cos θ = 0. θ

• Divide the inequality

sin2 θ ≤ 2(1 − cos θ ) ≤ θ 2 by 2θ 2 to obtain sin2 θ 1 − cos θ 1 ≤ ≤ . 2 2θ 2 θ2 Because sin θ 2 sin2 θ 1 1 1 = = (12 ) = , lim 2 2 θ →0 θ 2 2 θ →0 2θ lim

and lim

1

h→0 2

=

1 , it follows by the Squeeze Theorem that 2 lim

θ →0

1 1 − cos θ = . 2 θ2

Let A(n) be sin the xarea of a regular n-gon inscribed in a unit circle (Figure 8). − sin c numerically for the ﬁve values c = 0, π , π , π , π . (a) Investigate lim1 6 4 3 2 x −2cπ . (a) Prove that A(n) x→c = n sin n for general c? (We’ll see why in Chapter 3.) (b) Can you guess 2the answer (b) Intuitively, might we expect A(n) to converge the values area ofof thec.unit circle as n → ∞? (c) Check why if your answer to part (b) works for two to other (c) Use Theorem 2 to evaluate lim A(n). 53.

n→∞

y

x 1

FIGURE 8 Regular n-gon inscribed in a unit circle. SOLUTION

(a) A(n) = n · a(n), where a(n) is the area of one section of the n-gon as shown. The sections into which the n-gon is divided are identical. By looking at the section lying just atop the x-axis, we can see that each section has a base length of 1 and a height of sin θ , where θ is the interior angle of the triangle.

θ=

2π , n

so a(n) =

2π 1 sin . 2 n

S E C T I O N 2.7

This means

Intermediate Value Theorem

65

2π 1 2π 1 = n sin . A(n) = n sin 2 n 2 n

(b) As n increases, the difference between the n-gon and the unit circle shrinks toward zero; hence, as n increases, A(n) approaches the area of the unit circle. (c) Substitute x = n1 . n = 1x , so A(n) = 12 1x sin(2π x). x → 0 as n → ∞, so lim A(n) = lim

1 1 sin(2π x) = (2π ) = π . x 2

x→0 2

n→∞

2.7 Intermediate Value Theorem Preliminary Questions 1. Explain why f (x) = x 2 takes on the value 0.5 in the interval [0, 1]. Observe that f (x) = x 2 is continuous on [0, 1] with f (0) = 0 and f (1) = 1. Because f (0) < 0.5 < f (1), the Intermediate Value Theorem guarantees there is a c ∈ [0, 1] such that f (c) = 0.5. SOLUTION

2. The temperature in Vancouver was 46◦ at 6 AM and rose to 68◦ at noon. What must we assume about temperature to conclude that the temperature was 60◦ at some moment of time between 6 AM and noon? SOLUTION

We must assume that temperature is a continuous function of time.

3. What is the graphical interpretation of the IVT? SOLUTION If f is continuous on [a, b], then the horizontal line y = k for every k between f (a) and f (b) intersects the graph of y = f (x) at least once.

4. Show that the following statement is false by drawing a graph that provides a counterexample: If f (x) is continuous and has a root in [a, b], then f (a) and f (b) have opposite signs. SOLUTION

f (a) f (b) a

b

Exercises 1. Use the IVT to show that f (x) = x 3 + x takes on the value 9 for some x in [1, 2]. SOLUTION Observe that f (1) = 2 and f (2) = 10. Since f is a polynomial, it is continuous everywhere; in particular on [1, 2]. Therefore, by the IVT there is a c ∈ [1, 2] such that f (c) = 9. 3. Show that g(t) = t 2 tan tt takes on the value 12 for some t in 0, π4 . Show that g(t) = takes on the value 0.499 for some t in [0, 1]. t + 1π π2 π 1 π2 SOLUTION g(0) = 0 and g( 4 ) = 16 . g(t) is continuous for all t between 0 and 4 , and 0 < 2 < 16 ; therefore, by the IVT, there is a c ∈ [0, π4 ] such that g(c) = 12 .

5. Show that cos x = x has a solution in the interval [0, 1]. Hint: Show that f (x) = x − cos x has a zero in [0, 1]. x2 on the value Show that SOLUTION Let f (x) = = x7 − costakes x. Observe that 0.4. f is continuous with f (0) = −1 and f (1) = 1 − cos 1 ≈ .46. x +1 Therefore, by the IVT there is a c ∈ [0, 1] such that f (c) = c − cos c = 0. Thus c = cos c and hence the equation cos x = x has a solution c in [0, 1]. In Exercises 7–14, use the IVT to prove each of the following statements. 1 3 √ Use√the IVT to ﬁnd an interval of length 2 containing a root of f (x) = x + 2x + 1. 7. c + c + 1 = 2 for some number c.

66

CHAPTER 2

LIMITS

√ √ Let f (x) = x + x + 1 − 2. Note that f is continuous on 14 , 1 with f ( 14 ) = 14 + 54 − 2 ≈ −.38 √ √ √ and f (1) = 2 − 1 ≈ .41. Therefore, by the IVT there is a c ∈ 14 , 1 such that f (c) = c + c + 1 − 2 = 0. Thus √ √ √ √ c + c + 1 = 2 and hence the equation x + x + 1 = 2 has a solution c in 14 , 1 . √ 9. 2For exists. Hint: Consider = xx 2for . some x ∈ [0, π ]. all integers n, sin nxf (x) = cos 2 SOLUTION Let f (x) = x . Observe that f is continuous with f (1) = 1 and f (2) = 4. Therefore, by the IVT there is √ a c ∈ [1, 2] such that f (c) = c2 = 2. This proves the existence of 2, a number whose square is 2. SOLUTION

11. ForAallpositive positivenumber integersck,has there x such cos x =integers xk. an exists nth root for that all positive n. (This fact is usually taken for granted, but it k SOLUTION For each positive integer k, let f (x) = x − cos x. Observe that f is continuous on 0, π2 with f (0) = −1 requires proof.) k and f ( π2 ) = π2 > 0. Therefore, by the IVT there is a c ∈ 0, π2 such that f (c) = ck − cos(c) = 0. Thus cos c = ck and hence the equation cos x = x k has a solution c in the interval 0, π2 . 13. 2x =x b has a solution for all b > 0 (treat b ≥ 1 ﬁrst). 2 = bx has a solution if b > 2. SOLUTION Let b ≥ 1. Let f (x) = 2 x . f is continuous for all x. f (0) = 1, and f (x) > b for some x > 0 (since f (x) increases without bound). Hence, by the IVT, f (x) = b for some x > 0. On the other hand, suppose b < 1. Since limx→−∞ 2x = 0, there is some x < 0 such that f (x) = 2x < b. f (0) = 1 > b, so the IVT states that f (x) = b for some x < 0. 15. (a) (b) (c)

Carry of themany Bisection Method for f (x) = 2x − x 3 as follows: tanout x =three x hassteps inﬁnitely solutions. Show that f (x) has a zero in [1, 1.5]. Show that f (x) has a zero in [1.25, 1.5]. Determine whether [1.25, 1.375] or [1.375, 1.5] contains a zero.

SOLUTION

Note that f (x) is continuous for all x.

(a) f (1) = 1, f (1.5) = 21.5 − (1.5)3 < 3 − 3.375 < 0. Hence, f (x) = 0 for some x between 0 and 1.5. (b) f (1.25) ≈ 0.4253 > 0 and f (1.5) < 0. Hence, f (x) = 0 for some x between 1.25 and 1.5. (c) f (1.375) ≈ −0.0059. Hence, f (x) = 0 for some x between 1.25 and 1.375. 17. Find an interval of length 14 in [0,31] containing a root of x 5 − 5x + 1 = 0. Figure 4 shows that f (x) = x − 8x − 1 has a root in the interval [2.75, 3]. Apply the Bisection Method twice SOLUTION f (x)of=length x 5 −15xcontaining + 1. Observe that f is continuous with f (0) = 1 and f (1) = −3. Therefore, this root. to ﬁnd an Let interval 16 by the IVT there is a c ∈ [0, 1] such that f (c) = 0. f (.5) = −1.46875 < 0, so f (c) = 0 for some c ∈ [0, .5]. f (.25) = −.0.249023 < 0, and so f (c) = 0 for some c ∈ [0, .25]. This means that [0, .25] is an interval of length 0.25 containing a root of f (x). In Exercises 19–22, draw the graph of a function f (x) on [0, 4] with the given property. Show that tan3 θ − 8 tan2 θ + 17 tan θ − 8 = 0 has a root in [0.5, 0.6]. Apply the Bisection Method twice to ﬁnd interval of lengthat0.025 root. the conclusion of the IVT. 19. an Jump discontinuity x = 2containing and does this not satisfy SOLUTION The function graphed below has a jump discontinuity at x = 2. Note that while f (0) = 2 and f (4) = 4, there is no point c in the interval [0, 4] such that f (c) = 3. Accordingly, the conclusion of the IVT is not satisﬁed. y 4 3 2 1 x 1

2

3

4

21. Inﬁnite one-sided limits at x = 2 and does not satisfy the conclusion of the IVT. Jump discontinuity at x = 2, yet does satisfy the conclusion of the IVT on [0, 4]. SOLUTION The function graphed below has inﬁnite one-sided limits at x = 2. Note that while f (0) = 2 and f (4) = 4, there is no point c in the interval [0, 4] such that f (c) = 3. Accordingly, the conclusion of the IVT is not satisﬁed. y 6 5 4 3 2 1 x −1

1

2

3

4

S E C T I O N 2.7

Intermediate Value Theorem

67

Corollary 2 islimits not foolproof. Letdoes f (x) = x 2the−conclusion 1 and explain why 23. Inﬁnite one-sided at x = 2, yet satisfy of the IVTtheoncorollary [0, 4]. fails to detect the roots x = ±1 if [a, b] contains [−1, 1]. If [a, b] contains [−1, 1], f (a) > 0 and f (b) > 0. If we were using the corollary, we would guess that f (x) has no roots in [a, b]. However, we can easily see that f (1) = 0 and f (−1) = 0, so that any interval [a, b] containing [−1, 1] contains two roots. SOLUTION

Further Insights and Challenges f (x) is continuous and that ≤ f (x) ≤ 1 on forit0 anywhere. ≤ x ≤ 1 (see Figure Show that f (c) =c 25. Take Assume any mapthat (e.g., of the United States) and0draw a circle Prove that 5). at any moment in time for some in [0,a1]. therecexists pair of diametrically opposite points on that circle corresponding to locations where the temperatures at that moment are equal. Hint: Let θ be an angular coordinate along the circle and let f (θ ) be the difference in y temperatures at the locations corresponding to θ and θ + π . y = x 1

y = f (x)

x c

1

FIGURE 5 A function satisfying 0 ≤ f (x) ≤ 1 for 0 ≤ x ≤ 1. SOLUTION If f (0) = 0, the proof is done with c = 0. We may assume that f (0) > 0. Let g(x) = f (x) − x. g(0) = f (0) − 0 = f (0) > 0. Since f (x) is continuous, the Rule of Differences dictates that g(x) is continuous. We need to prove that g(c) = 0 for some c ∈ [0, 1]. Since f (1) ≤ 1, g(1) = f (1) − 1 ≤ 0. If g(1) = 0, the proof is done with c = 1, so let’s assume that g(1) < 0. We now have a continuous function g(x) on the interval [0, 1] such that g(0) > 0 and g(1) < 0. From the IVT, there must be some c ∈ [0, 1] so that g(c) = 0, so f (c) − c = 0 and so f (c) = c. This is a simple case of a very general, useful, and beautiful theorem called the Brouwer ﬁxed point theorem.

27. Figure 6(B) shows a slice of ham a piece of bread. Prove thatthat it is to θ slice thisθ open-faced Ham Sandwich Theorem Figure 6(A)onshows a slice of ham. Prove forpossible any angle (0 ≤ ≤ π ), it is θ ≤ π a line sandwich so that each part has equal amounts of ham and bread. Hint: By Exercise 26, for all 0 ≤ givenisby the possible to cut the slice of ham in half with a cut of incline θ . Hint: The lines of inclination θ are there L(θ )equations of incliney θ=(which we+assume is unique) into twoline equal pieces. = 0into or πtwo , letpieces B(θ ) (tan θ )x b, where b varies that fromdivides −∞ tothe ∞.ham Each such divides the For hamθslice denote theofamount of bread to theLet leftA(b) of L(beθ )the minus the of amount theleft right. Notice that L( π )amount = L(0) θ = (one which may be empty). amount ham totothe of the line minus the to since the right and0 and let θ =A πbegive the same line, but B( π ) = −B(0) since left and right get interchanged as the angle moves from 0 to the total area of the ham. Show that A(b) = −A if b is sufﬁciently large and A(b) = A if b is sufﬁciently π π . Assume that B( θ ) is continuous and apply the IVT. (By a further extension of this argument, one can prove the full negative. Then use the IVT. This works if θ = 0 or 2 . If θ = 0, deﬁne A(b) as the amount of ham above the “Ham Sandwich Theorem,” which states thatHow if you knifethe to cut at a slant, then itwhen is possible cut a sandwich line y = b minus the amount below. canallow you the modify argument to work θ = πto 2 (in which case consisting of a slice of ham and two slices of bread so that all three layers are divided in half.) tan θ = ∞)? y

( )

L 2

y

L( )

L (0) = L( )

x (A) Cutting a slice of ham at an angle .

x (B) A slice of ham on top of a slice of bread.

FIGURE 6 SOLUTION For each angle θ , 0 ≤ θ < π , let L(θ ) be the line at angle θ to the x-axis that slices the ham exactly in half, as shown in ﬁgure 6. Let L(0) = L(π ) be the horizontal line cutting the ham in half, also as shown. For θ and L(θ ) thus deﬁned, let B(θ ) = the amount of bread to the left of L(θ ) minus that to the right of L(θ ). To understand this argument, one must understand what we mean by “to the left” or “to the right”. Here, we mean to the left or right of the line as viewed in the direction θ . Imagine you are walking along the line in direction θ (directly right if θ = 0, directly left if θ = π , etc). We will further accept the fact that B is continuous as a function of θ , which seems intuitively obvious. We need to prove that B(c) = 0 for some angle c. Since L(0) and L(π ) are drawn in opposite direction, B(0) = −B(π ). If B(0) > 0, we apply the IVT on [0, π ] with B(0) > 0, B(π ) < 0, and B continuous on [0, π ]; by IVT, B(c) = 0 for some c ∈ [0, π ]. On the other hand, if B(0) < 0, then we apply the IVT with B(0) < 0 and B(π ) > 0. If B(0) = 0, we are also done; L(0) is the appropriate line.

68

CHAPTER 2

LIMITS

2.8 The Formal Definition of a Limit Preliminary Questions 1. Given that lim cos x = 1, which of the following statements is true? x→0

(a) (b) (c) (d)

If |cos x − 1| is very small, then x is close to 0. There is an > 0 such that |x| < 10−5 if 0 < |cos x − 1| < . There is a δ > 0 such that |cos x − 1| < 10−5 if 0 < |x| < δ . There is a δ > 0 such that |cos x| < 10−5 if 0 < |x − 1| < δ .

SOLUTION

The true statement is (c): There is a δ > 0 such that |cos x − 1| < 10−5 if 0 < |x| < δ .

2. Suppose that for a given and δ , it is known that | f (x) − 2| < if 0 < |x − 3| < δ . Which of the following statements must also be true? (a) | f (x) − 2| < if 0 < |x − 3| < 2δ (b) | f (x) − 2| < 2 if 0 < |x − 3| < δ δ (c) | f (x) − 2| < if 0 < |x − 3| < 2 2 δ (d) | f (x) − 2| < if 0 < |x − 3| < 2 SOLUTION Statements (b) and (d) are true.

Exercises 1. Consider lim f (x), where f (x) = 8x + 3. x→4

(a) Show that | f (x) − 35| = 8|x − 4|. (b) Show that for any > 0, | f (x) − 35| < if |x − 4| < δ , where δ = 8 . Explain how this proves rigorously that lim f (x) = 35. x→4

SOLUTION

(a) | f (x) − 35| = |8x + 3 − 35| = |8x − 32| = |8(x − 4)| = 8 |x − 4|. (Remember that the last step is justiﬁed because 8 > 0). (b) Let > 0. Let δ = /8 and suppose |x − 4| < δ . By part (a), | f (x) − 35| = 8|x − 4| < 8δ . Substituting δ = /8, we see | f (x) − 35| < 8/8 = . We see that, for any > 0, we found an appropriate δ so that |x − 4| < δ implies | f (x) − 35| < . Hence lim f (x) = 35. x→4

3. Consider lim x 2 = 4 (refer to Example 2). Consider x→2lim f (x), where f (x) = 4x − 1. x→2 (a) Show that |x 2 − 4| < 0.05 if 0 < |x − 2| < 0.01.

(a) Show that | f (x) − 7| < 4δ if |x − 2| < δ . (b) Show thata|xδ 2such − 4|that: < 0.0009 if 0 < |x − 2| < 0.0002. (b) Find (c) Find a value of δ such that |x 2 − 4| is less than 10−4 if 0 < |x − 2| < δ . | f (x) − 7| < 0.01 if |x − 2| < δ SOLUTION |x −rigorously (a) If 2| < δ = that .01, then and x 2 − 4 = |x − 2||x + 2| ≤ |x − 2| (|x| + 2) < 5|x − 2| < .05. (c)0 < Prove lim f|x| (x)<=3 7. x→2

(b) If 0 < |x − 2| < δ = .0002, then |x| < 2.0002 and 2 x − 4 = |x − 2||x + 2| ≤ |x − 2| (|x| + 2) < 4.0002|x − 2| < .00080004 < .0009. (c) Note that x 2 − 4 = |(x + 2)(x − 2)| ≤ |x + 2| |x − 2|. Since |x − 2| can get arbitrarily small, we can require |x − 2| < 1 so that 1 < x < 3. This ensures that |x + 2| is at most 5. Now we know that x 2 − 4 ≤ 5|x − 2|. Let δ = 10−5 . Then, if |x − 2| < δ , we get x 2 − 4 ≤ 5|x − 2| < 5 × 10−5 < 10−4 as desired. 5. Refer to Example 3 to ﬁnd a value of δ > 0 such that With regard to the limit lim x2 = 25, x→5 1 1 −4 if Write 0< −25| 3| < < 10 2− x < 6. Hint: |x |x = δ|x + 5| · |x − 5|. (a) Show that |x 2 − 25| < 11|x −x 5|−if34 < (b) Find a δ such that |x 2 − 25| < 10−3 if |x − 5| < δ . SOLUTION The Example shows that for any > 0 we have2 (c) Give a rigorous proof of the limit by showing that |x − 25| < if 0 < |x − 5| < δ , where δ is the smaller of 1 11 and 1. − 1 ≤ if |x − 3| < δ x 3 where δ is the smaller of the numbers 6 and 1. In our case, we may take δ = 6 × 10−4 .

S E C T I O N 2.8

The Formal Deﬁnition of a Limit

69

Plot f (x) =√tan x together with the horizontal lines y = 0.99 and y = 1.01. Use this plot to ﬁnd a value of 7. π | 0 suchPlot thatf |tan if |x −with δ . horizontal lines y = 2.9 and y = 3.1. Use this plot to ﬁnd a value of √ 4 δ > 0 such that | 2x − 1 − 3| < 0.1 if |x − 5| < δ . π SOLUTION From the plot below, we see that δ = 0.005 will guarantee that |tan x − 1| < 0.01 whenever |x − 4 | ≤ δ . y 1.02 1.01 1 0.99 0.98

x 0.775

0.78

0.785

0.79

0.795

1 9. Consider lim . 4 . Observe that the limit L = lim f (x) has the value L = f ( 12 ) = 16 Let x→2 f (x) x= 2 5 . Let = 0.5 and, x x→ 12 (a) Show that if |x − 2| <+1,1then that |f (x) − 16 | < for |x − 1 | < δ . Repeat for = 0.2 and 0.1. using a plot of f (x), ﬁnd a value of δ > 0 such 5 2 1 − 1 < 1 |x − 2| x 2 2 (b) Let δ be the smaller of 1 and 2. Prove the following statement: 1 − 1 < if 0 < |x − 2| < δ x 2 (c) Find a δ > 0 such that 1x − 12 < 0.01 if |x − 2| < δ . (d) Prove rigorously that lim

1

x→2 x

=

1 . 2

SOLUTION

(a) Since |x − 2| < 1, it follows that 1 < x < 3, in particular that x > 1. Because x > 1, then 1 − 1 = 2 − x = |x − 2| < 1 |x − 2|. x 2 2x 2x 2

1 < 1 and x

(b) Let δ = min{1, 2} and suppose that |x − 2| < δ . Then by part (a) we have 1 − 1 < 1 |x − 2| < 1 δ < 1 · 2 = . x 2 2 2 2 1 1 1 (c) Choose δ = .02. Then − < δ = .01 by part (b). x 2 2 (d) Let > 0 be given. Then whenever 0 < |x − 2| < δ = min {1, 2}, we have 1 − 1 < 1 δ ≤ . x 2 2 Since was arbitrary, we conclude that lim

1

x→2 x

=

1 . 2

11. Based on the information conveyed in Figure 5(A), ﬁnd values of L, , and δ > 0 such that the following statement √ +< 3. δ . lim holds: | Consider f (x) − L|x→1 < ifx|x| √ √ SOLUTION forces < Hint: 4.7. Rewritten, thisinequality means that 0|3< implies that x + 3<−x2|<<.112 |x − 1| 3.3 if |x<− f1|(x) < 4. Multiply the by |x | − x+ + .1 2| and observe (a) ShowWe thatsee | −.1 √ | f (x) − |4| x<+.7. Looking that 3+ 2| > 2.at the limit deﬁnition |x − c| < δ implies | f (x) − L| < , we can replace so that L = 4, √ = .7, c = 0, and δ = .1. (b) Find δ > 0 such that | x + 3 − 2| < 10−4 for |x − 1| < δ . Prove rigorously that the is equal to 2. for5(B), 13. (c) Based A on calculator gives thelimit following values f (x)ﬁnd = sin x: of c, L, , and δ > 0 such that the following the information conveyed in Figure values statement holds: | f (x) −

πL| < if |x − c| < δ . π ≈ 0.707, f − 0.1 ≈ 0.633, f 4 4

f

π 4

+ 0.1 ≈ 0.774

Use these values and the fact that f (x) is increasing on [0, π2 ] to justify the statement

π f (x) − f < 0.08 4

Then draw a ﬁgure like Figure 3 to illustrate this statement.

if

π x − < 0.1 4

70

CHAPTER 2

LIMITS SOLUTION Since f (x) is increasing on the interval, the three f (x) values tell us that .633 ≤ f (x) ≤ .774 for all x between π4 − .1 and π4 + .1. We may subtract f ( π4 ) from the inequality for f (x). This show that, for π4 − .1 < x < π4 + .1, .633 − f ( π4 ) ≤ f (x) − f ( π4 ) ≤ .774 − f ( π4 ). This means that, if |x − π4 | < .1, then .633 − .707 ≤ f (x) − f ( π4 ) ≤ .774 − .707, so −0.074 ≤ f (x) − f ( π4 ) ≤ 0.067. Then −0.08 < f (x) − f ( π4 ) < 0.08 follows from this, so |x − π4 | < 0.1 implies | f (x) − f ( π4 )| < .08. The ﬁgure below illustrates this. y 1 0.8 0.6 0.4 0.2 x 0.25 0.5 0.75

1

1.25 1.5

15. Adapt the the argument in Example 2 to1prove rigorously thatthat lim x 2 = c2 for all c. Adapt argument in Example to prove rigorously x→clim (ax + b) = ac + b, where a, b, c are arbitrary. x→c

SOLUTION

To relate the gap to |x − c|, we take 2 x − c2 = |(x + c)(x − c)| = |x + c| |x − c| .

We choose δ in two steps. First, since we are requiring |x − c| to be small, we require δ < |c|, so that x lies between 0 and 2c. This means that |x + c| < 3|c|, so |x − c||x + c| < 3|c|δ . Next, we require that δ < , so 3|c| |x − c||x + c| <

3|c| = , 3|c|

and we are done. Therefore, given > 0, we let δ = min |c|, . 3|c| Then, for |x − c| < δ , we have |x 2 − c2 | = |x − c| |x + c| < 3|c|δ < 3|c|

= . 3|c|

In Exercises 17–22, use the formal deﬁnition of the limit to prove the statement rigorously. Adapt the argument in Example 3 to prove rigorously that lim x −1 = 1c for all c. x→c √ 17. lim x = 2 x→4

√ √ x +2 Let > 0 be given. We bound | x − 2| by multiplying √ . x +2 √ √ 1 √ x + 2 x − 4 . = √ = |x − 4| √ | x − 2| = x − 2 √ x + 2 x + 2 x + 2 √ √ We can assume δ < 1, so that |x − 4| < 1, and hence x + 2 > 3 + 2 > 3. This gives us 1 √ < |x − 4| 1 . | x − 2| = |x − 4| √ 3 x + 2

SOLUTION

Let δ = min(1, 3). If |x − 4| < δ ,

1 √ < |x − 4| 1 < δ 1 < 3 1 = , | x − 2| = |x − 4| √ 3 3 3 x + 2

thus proving the limit rigorously. 19. lim x 3 = 12 x→1lim (3x + x) = 4 x→1

SOLUTION

Let > 0 be given. We bound x 3 − 1 by factoring the difference of cubes: 3 x − 1 = (x 2 + x + 1)(x − 1) = |x − 1| x 2 + x + 1 .

S E C T I O N 2.8

The Formal Deﬁnition of a Limit

71

Let δ = min(1, 7 ), and assume |x − 1| < δ . Since δ < 1, 0 < x < 2. Since x 2 + x + 1 increases as x increases for x > 0, x 2 + x + 1 < 7 for 0 < x < 2, and so 3 x − 1 = |x − 1| x 2 + x + 1 < 7|x − 1| < 7 = 7 and the limit is rigorously proven. 21. lim |x| = |c| x→c 1 lim x −2 = |x| + |c| 4 x→2 SOLUTION Let > 0 be given. First we bound |x| − |c| by multiplying by . |x| + |c| 2 2 2 2 |x| − |c| < |x| − |c| |x| + |c| = ||x| − |c| | = x − c = (x + c)(x − c) ≤ |x − c| |x + c| . |x| + |c| |x| + |c| |x| + |c| |x| + |c| |x| + |c| Let δ = . We apply the Triangle Inequality—for all a, b: |a + b| ≤ |a| + |b|. This yields: |x| − |c| ≤ |x − c| |x + c| ≤ |x − c| = , |x| + |c| and the limit is rigorously proven. x 23. Let f (x) = 1 . Prove rigorously that lim f (x) does not exist. Hint: Show that no number L qualiﬁes as the limit x→0 lim x sin|x| = 0 x exists some x such that |x| < δ but | f (x) − L| ≥ 1 , no matter how small δ is taken. becausex→0 there always 2 Let L be any real number. Let δ > 0 be any small positive number. Let x = δ2 , which satisﬁes |x| < δ , and f (x) = 1. We consider two cases: SOLUTION

• (| f (x) − L| ≥ 1 ) : we are done. 2 • (| f (x) − L| < 1 ): This means 1 < L < 3 . In this case, let x = − δ . f (x) = −1, and so 3 < L − f (x). 2 2 2 2 2

In either case, there exists an x such that |x| < δ2 , but | f (x) − L| ≥ 12 . 25. Use the identity Prove rigorously that lim sin 1x does not exist. x+y x−y x→0 sin x + sin y = 2 sin cos 2 2 to verify the relation sin(a + h) − sin a = h

sin(h/2) h cos a + h/2 2

6

Conclude that |sin(a + h) − sin a| < |h| for all a and prove rigorously that lim sin x = sin a. You may use the inequality x→a sin x ≤ 1 for x = 0. x SOLUTION

We ﬁrst write sin(a + h) − sin a = sin(a + h) + sin(−a).

Applying the identity with x = a + h, y = −a, yields:

a+h−a 2a + h cos 2 2 h h h h h sin(h/2) h = 2 sin cos a + =2 sin cos a + =h cos a + . 2 2 h 2 2 h/2 2

sin(a + h) − sin a = sin(a + h) + sin(−a) = 2 sin

Therefore,

sin(h/2) cos a + h . |sin(a + h) − sin a| = |h| h/2 2

sin θ < 1 and that |cos θ | ≤ 1, we have Making the substitution h = x − a, we see that this last Using the fact that θ relation is equivalent to |sin x − sin a| < |x − a|.

72

CHAPTER 2

LIMITS

Now, to prove the desired limit, let > 0, and take δ = . If |x − a| < δ , then |sin x − sin a| < |x − a| < δ = , Therefore, a δ was found for arbitrary , and the proof is complete. Use Eq. (6) to prove rigorously that Further Insights and Challenges sin(a + h) − sin 27. Uniqueness of the Limit Show that a function converges to aat most one limiting value. In other words, use the = cos a lim limit deﬁnition to show that if lim f (x) = L 1h→0 and lim f (x) h = L 2 , then L 1 = L 2 . x→c

SOLUTION

x→c

Let > 0 be given. Since lim f (x) = L 1 , there exists δ1 such that if |x − c| < δ1 then | f (x) − L 1 | < . x→c

Similarly, since lim f (x) = L 2 , there exists δ2 such that if |x − c| < δ2 then | f (x) − L 2 | < . Now let |x − c| < x→c min(δ1 , δ2 ) and observe that |L 1 − L 2 | = |L 1 − f (x) + f (x) − L 2 | ≤ |L 1 − f (x)| + | f (x) − L 2 | = | f (x) − L 1 | + | f (x) − L 2 | < 2. So, |L 1 − L 2 | < 2 for any > 0. We have |L 1 − L 2 | = lim |L 1 − L 2 | < lim 2 = 0. Therefore, |L 1 − L 2 | = 0 →0

and, hence, L 1 = L 2 .

→0

In Exercises 28–30, prove the statement using the formal limit deﬁnition. 29. The Squeeze Theorem. (Theorem 1 in Section 2.6, p. 89) The Constant Multiple Law [Theorem 1, part (ii) in Section 2.3, p. 64] SOLUTION Proof of the Squeeze Theorem. Suppose that (i) the inequalities h(x) ≤ f (x) ≤ g(x) hold for all x near (but not equal to) a and (ii) lim h(x) = lim g(x) = L. Let > 0 be given. x→a

x→a

• By (i), there exists a δ1 > 0 such that h(x) ≤ f (x) ≤ g(x) whenever 0 < |x − a| < δ1 . • By (ii), there exist δ2 > 0 and δ3 > 0 such that |h(x) − L| < whenever 0 < |x − a| < δ2 and |g(x) − L| < whenever 0 < |x − a| < δ3 . • Choose δ = min {δ1 , δ2 , δ3 }. Then whenever 0 < |x − a| < δ we have L − < h(x) ≤ f (x) ≤ g(x) < L + ;

i.e., | f (x) − L| < . Since was arbitrary, we conclude that lim f (x) = L. x→a

31. Let f (x) = 1 if x is rational and f (x) = 0 if x is irrational. Prove that lim f (x) does not exist for any c. The Product Law [Theorem 1, part (iii) in Section 2.3, p. 64]. Hint: x→c Use the identity SOLUTION Let c be any number, and let δ > 0 be an arbitrary small number. We will prove that there is an x such f (x)g(x) − L M = ( f (x) − L) g(x) + L(g(x) − M) that |x − c| < δ , but | f (x) − f (c)| > 12 . c must be either irrational or rational. If c is rational, then f (c) = 1. Since the irrational numbers are dense, there is at least one irrational number z such that |z − c| < δ . | f (z) − f (c)| = 1 > 12 , so the function is discontinuous at x = c. On the other hand, if c is irrational, then there is a rational number q such that |q − c| < δ . | f (q) − f (c)| = |1 − 0| = 1 > 12 , so the function is discontinuous at x = c.

Here is a function with strange continuity properties: ⎧ p 1 if x is the rational number in ⎪ ⎨ q q lowest terms f (x) = 1. A particle’s position at time t (s) is s(t) =⎪ ⎩ t 2 + 1 m. Compute its average velocity over [2, 5] and estimate its 0 if x is an irrational number instantaneous velocity at t = 2. (a) ShowLet thats(t) f (x) at c if velocity c is rational. exist irrational numbers arbitrarily close to c. SOLUTION = is discontinuous t 2 + 1. The average over Hint: [2, 5] There is (b) Show that f (x) is continuous at c if c is irrational. Let I be the interval {x : |x − c| < 1}. Show that for √ Hint: p√ q < Q. Conclude that there is a δ such that all any Q > 0, I contains at most ﬁnitely s(5) −many s(2) fractions 26 −q with 5 = ≈ 0.954 m/s. larger than Q. fractions in {x : |x − c| < δ } have a denominator 5−2 3

CHAPTER REVIEW EXERCISES

From the data in the table below, we estimate that the instantaneous velocity at t = 2 is approximately 0.894 m/s. interval average ROC

[1.9, 2]

[1.99, 2]

[1.999, 2]

[2, 2.001]

[2, 2.01]

[2, 2.1]

0.889769

0.893978

0.894382

0.894472

0.894873

0.898727

3. For a whole number n, let P(n) be the number of partitions of n, that is, the number of ways of writing n as a sum The price p ofnumbers. natural gas the United States dollars per 1,000 ft3 ) be on partitioned the ﬁrst dayinofﬁve each monthways: in 2004 of one or more whole Forinexample, P(4) = 5(in since the number 4 can different 4, 3 per month) over the is listed in the table below. Compute the average rate of change of p (in dollars per 1,000 ft 3 + 1, 2 + 2, 2 + 1 + 1, 1 + 1 + 1 + 1. Treating P(n) as a continuous function, use Figure 1 to estimate the rate of quarterly periods April–June, and July–September. change of P(n) at n =January–March, 12. J

F

M

A

M

J

Chapter Review Exercises

73

SOLUTION The tangent line drawn in the ﬁgure appears to pass through the points (15, 140) and (10.5, 40). We therefore estimate that the rate of change of P(n) at n = 12 is

100 200 140 − 40 = = . 15 − 10.5 4.5 9 In Exercises 5–8, estimate the limit numerically to two decimal places or state that the limit does not exist.√ The average velocity v (m/s) of an oxygen molecule in the air at temperature T (◦ C) is v = 25.7 273.15 + T . 3 (x) What1is−the speed at T = 25◦ (room temperature)? Estimate the rate of change of average velocity with cosaverage 5. respect lim to temperature at T = 25◦ . What are the units of this rate? x→0 x2 SOLUTION

3x Let f (x) = 1−cos . The data in the table below suggests that 2 FIGURE 1 Graph of P(n).

x

1 − cos3 x ≈ 1.50. x→0 x2 lim

In constructing the table, we take advantage of the fact that f is an even function. x

±0.001

±0.01

±0.1

f (x)

1.500000

1.499912

1.491275

(The exact value is 32 .) xx − 4 7. limlim2 x 1/(x−1) x→2 x −4 x→1 SOLUTION

x Let f (x) = x 2 −4 . The data in the table below suggests that

x −4

xx − 4 ≈ 1.69. x→2 x 2 − 4 lim

x

1.9

1.99

1.999

2.001

2.01

2.1

f (x)

1.575461

1.680633

1.691888

1.694408

1.705836

1.828386

(The exact value is 1 + ln 2.) In Exercises 9–42, the limit if possible or state that it does not exist. 3x − evaluate 9 lim x 1/2 (3 +5x − 25 ) 9. limx→2 x→4 √ SOLUTION lim (3 + x 1/2 ) = 3 + 4 = 5. x→4

4 11. lim 35 − x 2 x→−2 limx x→1 4x + 7 4 4 1 SOLUTION lim = =− . 2 x→−2 x 3 (−2)3 x3 − x 2 3x + 4x + 1 x −1 x→1 lim x +1 x→−1 x3 − x x(x − 1)(x + 1) SOLUTION lim = lim = lim x(x + 1) = 1(1 + 1) = 2. x −1 x→1 x − 1 x→1 x→1 √ t −33 15. lim x − 2x t −9 t→9lim √ √ x→1 x − 1 1 1 t −3 t −3 1 SOLUTION lim = . = lim √ = √ = lim √ √ 6 t→9 t − 9 t→9 ( t − 3)( t + 3) t→9 t + 3 9+3 √ x +1−2 t −6 17. lim x −3 x→3lim √ t→9 t − 3 13. lim

SOLUTION

√ lim

x→3

x +1−2 = lim x −3 x→3

√

= lim √ x→3

√ x +1−2 x +1+2 (x + 1) − 4 = lim ·√ √ x −3 x + 1 + 2 x→3 (x − 3)( x + 1 + 2) 1 x +1+2

= √

1 3+1+2

=

1 . 4

74

CHAPTER 2

LIMITS

2 − 2a 2 2(a + h) 19. lim 1 − s2 + 1 h h→0lim s→0 s2 SOLUTION

2(a + h)2 − 2a 2 2a 2 + 4ah + 2h 2 − 2a 2 h(4a + 2h) = lim = lim = lim (4a + 2h) = 4a + 2(0) = 4a. h h h h→0 h→0 h→0 h→0 lim

1 21. lim 2 1 t→3 tlim −9 t→−1+ x + 1 SOLUTION Because the one-sided limits 1

lim

= −∞

t→3− t 2 − 9

and

lim

1

t→3+ t 2 − 9

= ∞,

are not equal, the two-sided limit lim

1

does not exist.

t→3 t 2 − 9

a 2 −33ab + 2b2 x − b3 a−b a→blim x→b x − b a 2 − 3ab + 2b2 (a − b)(a − 2b) SOLUTION lim = lim = lim (a − 2b) = b − 2b = −b. a−b a−b a→b a→b a→b 1 1 25. lim x 3−−x(x ax 2++3)ax − 1 x→0lim3x x − 1 x→1 1 (x + 3) − 3 1 1 1 1 SOLUTION lim − = lim = lim = = . x(x + 3) 3(0 + 3) 9 x→0 3x x→0 3x(x + 3) x→0 3(x + 3)

23. lim

27.

lim

[x]

x→1.5 limx x→1+

SOLUTION

1

1

− x− x2 − 1 1 [1.5] 2 [x]1 lim = = = . 1.5 1.5 3 x→1.5 x √

[x] t→0−limx

29. lim

1

x→4.3 x − [x]

SOLUTION

For x sufﬁciently close to zero but negative, [x] = −1. Therefore, lim

[x]

x→0− x

= lim

−1

x→0− x

= ∞.

31. lim sec θ [x] θ → π4 lim t→0+ x SOLUTION

lim sec θ = sec

θ → π4

√ π = 2. 4

cos θ − 2 33. lim lim θ sec θ θ →0 θ → π2 θ SOLUTION

Because the one-sided limits cos θ − 2 =∞ θ θ →0− lim

and

cos θ − 2 = −∞ θ θ →0+ lim

are not equal, the two-sided limit cos θ − 2 θ θ →0 lim

sin 4x 35. lim sin 5θ sin 3x θ →0lim θ →0 θ

does not exist.

Chapter Review Exercises

75

SOLUTION

lim

sin 4θ

θ →0 sin 3θ

=

4 3θ 4 4 sin 4θ sin 4θ 3θ 4 · = lim · lim = (1)(1) = . lim 3 θ →0 4θ sin 3θ 3 θ →0 4θ 3 3 θ →0 sin 3θ

37. lim tan x x→ π2 sin2 t lim 3 t→0 Because t SOLUTION the one-sided limits lim tan x = ∞

lim tan x = −∞

and

x→ π2 −

x→ π2 +

are not equal, the two-sided limit lim tan x

does not exist.

x→ π2

√ 1 t cos1 39. lim lim cos t t→0+ t t→0 SOLUTION For t > 0, −1 ≤ cos so

1 ≤ 1, t

√ √ √ 1 ≤ t. − t ≤ t cos t

Because √ √ t = 0, lim − t = lim

t→0+

t→0+

it follows from the Squeeze Theorem that √ 1 t cos = 0. t t→0+ lim

41. lim

cos x − 1 sin 7x

x→0lim sin x x→0 sin 3x

SOLUTION

cos x − 1 cos x − 1 cos x + 1 − sin2 x sin x 0 = lim · = lim = − lim =− = 0. cos x + 1 x→0 sin x(cos x + 1) 1+1 x→0 sin x x→0 sin x x→0 cos x + 1 lim

43. Find the left- and right-hand limits of the function f (x) in Figure 2 at x = 0, 2, 4. State whether f (x) is left- or tan θ − sin θ right-continuous (or both) at these points. lim θ →0 sin3 θ y

2 1 x 1

2

3

4

5

FIGURE 2 SOLUTION

According to the graph of f (x), lim f (x) = lim f (x) = 1

x→0−

x→0+

lim f (x) = lim f (x) = ∞

x→2−

x→2+

lim f (x) = −∞

x→4−

lim f (x) = ∞.

x→4+

The function is both left- and right-continuous at x = 0 and neither left- nor right-continuous at x = 2 and x = 4.

76

CHAPTER 2

LIMITS

45. Sketch the graph of a function g(x) such that Sketch the graph of a function f (x) such that lim g(x) = −∞, lim g(x) = ∞ lim g(x) = ∞, = 1, lim f (x) = 3 lim f (x) x→−3− x→−3+ x→4 x→2−

x→2+

SOLUTION

and such that lim f (x) but does not equal f (4). x→4

y 10 5 −4

x

−2

2

4

6

−5 −10

47. Find a constant b such that h(x) is continuous at x = 2, where Find the points of discontinuity of g(x) and determine the type of discontinuity, where ⎧x + 1 for π x |x| < 2 h(x) = ⎪ ⎨cos 2 for |x| < 1 b − x 2 for |x| ≥ 2 g(x) = ⎪ ⎩ for |x| ≥ 1 With this choice of b, ﬁnd all points of discontinuity. |x − 1| SOLUTION

To make h(x) continuous at x = 2, we must have the two one-sided limits as x approaches 2 be equal.

With lim h(x) = lim (x + 1) = 2 + 1 = 3

x→2−

x→2−

and lim h(x) = lim (b − x 2 ) = b − 4,

x→2+

x→2+

it follows that we must choose b = 7. Because x + 1 is continuous for −2 < x < 2 and 7 − x 2 is continuous for x ≤ −2 and for x ≥ 2, the only possible point of discontinuity is x = −2. At x = −2, lim

x→−2+

h(x) =

lim (x + 1) = −2 + 1 = −1

x→−2+

and lim

x→−2−

h(x) =

lim (7 − x 2 ) = 7 − (−2)2 = 3,

x→−2−

so h(x) has a jump discontinuity at x = −2. 49. Let f (x) and g(x) be functions such that g(x) = 0 for x = a, and let Calculate the following limits, assuming that B= lim g(x), L= = 4lim A = lim f (x), = x→a 6, lim g(x) lim f (x) x→a x→a x→3

x→3

f (x) g(x)

Prove(a)thatlim if the limits A, B, and L exist and L = 1, then A = (b) B. Hint: cannot use the Quotient Law if B = 0, so 2 f (x) ( f (x) − 2g(x)) lim xYou apply thex→3 Product Law to L and B instead. x→3 f (x) SOLUTION (d)Then lim (2g(x)3 − g(x)2 ) (c) lim Suppose the limits A, B, and L all exist and L = 1. x→3 g(x) + x x→3 f (x) f (x) = lim g(x) = lim f (x) = A. B = B · 1 = B · L = lim g(x) · lim x→a x→a g(x) x→a x→a g(x) 51. Let f (x) be a function deﬁned for all real numbers. Which of the following statements must be true, might be true, In true. the notation of Exercise 49, give an example in which L exists but neither A nor B exists. or are never (a) lim f (x) = f (3). x→3

= 1, then f (0) = 0. (b) If lim f (x) x→0 x 1

1 . 8 (d) If lim f (x) = 4 and lim f (x) = 8, then lim f (x) = 6. (c) If lim f (x) = 8, then lim x→−7

x→5+

x→−7 f (x)

x→5−

(e) If lim f (x) = 1, then lim f (x) = 0 x→0 x x→0

=

x→5

Chapter Review Exercises

77

(f) If lim f (x) = 2, then lim f (x)3 = 8. x→5

x→5

SOLUTION

(a) This statement might be true. If f (x) is continuous at x = 3, then the statement will be true; if f (x) is not continuous at x = 3, then the statement will not be true. (b) This statement might be true. If f (x) is continuous at x = 0, then the statement will be true; if f (x) is not continuous at x = 0, then the statement will not be true. (c) This statement is always true. (d) This statement is never true. If the two one-sided limits are not equal, then the two-sided limit does not exist. (e) This statement is always true. (f) This statement is always true. 1 , where [x] is the greatest integer function. LetSqueeze f (x) = xTheorem 53. x Use the to prove that the function f (x) = x cos 1 for x = 0 and f (0) = 0 is continuous at x

x = 0. the graph of f (x) on the interval [ 1 , 2]. (a) Sketch 4 (b) Show that for x = 0,

1 1 1 −1< ≤ x x x Then use the Squeeze Theorem to prove that 1 =1 x x→0 lim x

SOLUTION

(a) The graph of f (x) = x 1x over [ 14 , 2] is shown below. y 1 0.8 0.6 0.4 0.2 x 0.5

1

1.5

2

(b) Let y be any real number. From the deﬁnition of the greatest integer function, it follows that y − 1 < [y] ≤ y, with equality holding if and only if y is an integer. If x = 0, then 1x is a real number, so 1 1 1 −1< ≤ . x x x Upon multiplying this inequality through by x, we ﬁnd 1−x < x

1 ≤ 1. x

Because lim (1 − x) = lim 1 = 1,

x→0

x→0

it follows from the Squeeze Theorem that 1 = 1. x x→0 lim x

1 . 55. Let f (x) = x+2 Let r1 and r2 be the roots of the quadratic polynomial f (x) = ax 2 − 2x + 20. Observe that f (x) “approaches” |x−2| 1 the linear as < a 1. →Hint: 0. Since r = that 10 is the+unique = 1.0, we might expect (a) Show that function |x 20 − 2| Observe |4(x 2)| > root 12 if of |x L(x) − 2| < f (x) − L(x) 4 < =12−2xif + of f (x) to approach 10 as a → 0 (Figure 3). Prove that the roots can be labeled so that at least one of the roots 1 (b) Find lim rδ1 > = 010such andthat lim rf 2(x) =− ∞.4 < 0.01 for |x − 2| < δ . a→0

a→0

(c) Prove rigorously that lim f (x) = 14 . x→2

SOLUTION

FIGURE 3 Graphs of f (x) = ax 2 − 2x + 20.

78

CHAPTER 2

LIMITS 1 . Then (a) Let f (x) = x+2

f (x) −

1 4 − (x + 2) |x − 2| 1 1 = − = = . 4 x + 2 4 4(x + 2) |4(x + 2)|

If |x − 2| < 1, then 1 < x < 3, so 3 < x + 2 < 5 and 12 < 4(x + 2) < 20. Hence, 1 1 f (x) − 1 < |x − 2| . < and |4(x + 2)| 12 4 12 (b) If |x − 2| < δ , then by part (a),

f (x) −

1 δ < . 4 12

Choosing δ = 0.12 will then guarantee that | f (x) − 14 | < 0.01. (c) Let > 0 and take δ = min{1, 12}. Then, whenever |x − 2| < δ , f (x) − 1 = 1 − 1 = |2 − x| ≤ |x − 2| < δ = . 4 x + 2 4 4|x + 2| 12 12 57. Prove rigorously that lim (4 + 8x) = −4. x→−1 Plot the function f (x) = x 1/3 . Use the zoom feature to ﬁnd a δ > 0 such that |x 1/3 − 2| < 0.05 for δ . |x − 8| < SOLUTION Let > 0 and take δ = /8. Then, whenever |x − (−1)| = |x + 1| < δ , | f (x) − (−4)| = |4 + 8x + 4| = 8|x + 1| < 8δ = . 59. Use the IVT to prove that the curves y = x 2 and y = cos x intersect. Prove rigorously that lim (x 2 − x) = 6. SOLUTION Let f (x) = x 2x→3 − cos x. Note that any root of f (x) corresponds to a point of intersection between the curves y = x 2 and y = cos x. Now, f (x) is continuous over the interval [0, π2 ], f (0) = −1 < 0 and f ( π2 ) = π4 > 0. Therefore, by the Intermediate Value Theorem, there exists a c ∈ (0, π2 ) such that f (c) = 0; consequently, the curves y = x 2 and y = cos x intersect. 2

61. Use the Bisection Method to locate a root of x22 − 7 = 0 to two decimal places. +2 has a root in the interval [0, 2]. Use the IVT to prove that f (x) = x 3 − x x+2 SOLUTION Let f (x) = x 2 − 7. By trial andcos error, we ﬁnd that f (2.6) = −0.24 < 0 and f (2.7) = 0.29 > 0. Because f (x) is continuous on [2.6, 2.7], it follows from the Intermediate Value Theorem that f (x) has a root on (2.6, 2.7). We approximate the root by the midpoint of the interval: x = 2.65. Now, f (2.65) = 0.0225 > 0. Because f (2.6) and f (2.65) are of opposite sign, the root must lie on (2.6, 2.65). The midpoint of this interval is x = 2.625 and f (2.625) < 0; hence, the root must be on the interval (2.625, 2.65). Continuing in this fashion, we construct the following sequence of intervals and midpoints. interval

midpoint

(2.625, 2.65) (2.6375, 2.65) (2.64375, 2.65) (2.64375, 2.646875) (2.6453125, 2.646875)

2.6375 2.64375 2.646875 2.6453125 2.64609375

At this point, we note that, to two decimal places, one root of x 2 − 7 = 0 is 2.65. Give an example of a (discontinuous) function that does not satisfy the conclusion of the IVT on [−1, 1]. Then show that the function ⎧ ⎨sin 1 for x = 0 x f (x) = ⎩ 0 for x = 0 satisﬁes the conclusion of the IVT on every interval [−a, a], even though f is discontinuous at x = 0.

3 DIFFERENTIATION 3.1 Definition of the Derivative Preliminary Questions 1. Which of the lines in Figure 10 are tangent to the curve? D B

C

A

FIGURE 10 SOLUTION

Lines A and D are tangent to the curve.

2. What are the two ways of writing the difference quotient? SOLUTION

The difference quotient may be written either as f (x) − f (a) x −a

or as f (a + h) − f (a) . h 3. For which value of x is f (x) − f (3) f (7) − f (3) = ? x −3 4 SOLUTION

With x = 7, f (7) − f (3) f (x) − f (3) = . x −3 4

4. What do the following quantities represent in terms of the graph of f (x) = sin x? sin 1.3 − sin 0.9 (a) sin 1.3 − sin 0.9 (b) 0.4 (c) f (1.3) Consider the graph of y = sin x. (a) The quantity sin 1.3 − sin .9 represents the difference in height between the points (.9, sin .9) and (1.3, sin 1.3). sin 1.3 − sin .9 (b) The quantity represents the slope of the secant line between the points (.9, sin .9) and (1.3, sin 1.3) .4 on the graph. (c) The quantity f (1.3) represents the slope of the tangent line to the graph at x = 1.3. SOLUTION

5. For which values of a and h is and (5, f (5)) on the graph of f (x)?

f (a + h) − f (a) equal to the slope of the secant line between the points (3, f (3)) h

f (a + h) − f (a) is equal to the slope of the secant line between the points h (3, f (3)) and (5, f (5)) on the graph of f (x). SOLUTION

With a = 3 and h = 2,

6. To which derivative is the quantity tan π4 + 0.00001 − 1 0.00001 a good approximation? SOLUTION

tan( π4 + 0.00001) − 1 0.00001

is a good approximation to the derivative of the function f (x) = tan x at x = π4 .

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D I F F E R E N T I AT I O N

Exercises 1. Let f (x) = 3x 2 . Show that f (2 + h) = 3h 2 + 12h + 12 Then show that f (2 + h) − f (2) = 3h + 12 h and compute f (2) by taking the limit as h → 0. With f (x) = 3x 2 , it follows that

SOLUTION

f (2 + h) = 3(2 + h)2 = 3(4 + 4h + h 2 ) = 12 + 12h + 3h 2 . Using this result, we ﬁnd f (2 + h) − f (2) 12 + 12h + 3h 2 − 3 · 4 12 + 12h + 3h 2 − 12 12h + 3h 2 = = = = 12 + 3h. h h h h As h → 0, 12 + 3h → 12, so f (2) = 12. In Exercises 3–6, compute f (a) in two ways, using Eq. (1) and Eq. (2). Let f (x) = 2x 2 − 3x − 5. Show that the slope of the secant line through (2, f (2)) and (2 + h, f (2 + h)) is + 5. formula to compute the slope of: 9x, this a= 0 3. 2h f (x) = Then x 2 + use (a) The secant line through (2, f (2)) and (3, f (3)) SOLUTION Let f (x) = x 2 + 9x. Then (b) The tangent line at x = 2 (by taking a limit) f (0 + h) − f (0) (0 + h)2 + 9(0 + h) − 0 9h + h 2 = lim = lim = lim (9 + h) = 9. h h h h→0 h→0 h→0 h→0

f (0) = lim Alternately,

f (x) − f (0) x 2 + 9x − 0 = lim = lim (x + 9) = 9. x −0 x x→0 x→0 x→0

f (0) = lim

5. f (x) = 3x 2 + 4x + 2, a = −1 f (x) = x 2 + 9x, a = 2 SOLUTION Let f (x) = 3x 2 + 4x + 2. Then f (−1 + h) − f (−1) 3(−1 + h)2 + 4(−1 + h) + 2 − 1 = lim h h h→0 h→0

f (−1) = lim

3h 2 − 2h = lim (3h − 2) = −2. h h→0 h→0

= lim Alternately,

f (x) − f (−1) 3x 2 + 4x + 2 − 1 = lim x − (−1) x +1 x→−1 x→−1

f (−1) = lim = lim

x→−1

(3x + 1)(x + 1) = lim (3x + 1) = −2. x +1 x→−1

In Exercises 7–10, refer2 to the function whose graph is shown in Figure 11. f (x) = 9 − 3x , a = 0 7. Calculate the slope of the secant line through the points on the graph where x = 0 and x = 2.5. SOLUTION

f (0) ≈ 6 and f (2.5) ≈ 2, so the slope of the secant line connecting (0, f (0)) and (2.5, f (2.5)) is 2−6 −4 f (2.5) − f (0) ≈ = = −1.6. 2.5 − 0 2.5 2.5

f (2 + h) − f (2) Estimate for h = 0.5 and h = −0.5. Are these numbers larger or smaller than f (2)? f (2.5) − f (1) h . What does this quantity represent? Compute 2.5 − 1 Explain. 9.

S E C T I O N 3.1 SOLUTION

Deﬁnition of the Derivative

81

f (2) ≈ 1. If h = .5, f (2 + h) = f (2.5) ≈ 2, so 2−1 f (2 + h) − f (2) ≈ = 2. h .5

This number is probably greater than f (2). Since the curve is bending upward slightly at (2, f (2)), the secant line lies inside and over the curve, showing it has higher slope than the tangent line. If h = −.5, f (2 + h) = f (1.5) ≈ .5, so .5 − 1 f (2 + h) − f (2) ≈ = 1. h −.5 This number is smaller than f (2), since the curve is bending upwards near (2, f (2)). In Exercises 11–14, let f (x) be the function whose graph is shown in Figure 12. Estimate f (2). 5 4 3 2 1

y

x

1 2 3 4 5 6 7 8 9

FIGURE 12 Graph of f (x).

11. Determine f (a) for a = 1, 2, 4, 7. SOLUTION Remember that the value of the derivative of f at x = a can be interpreted as the slope of the line tangent to the graph of y = f (x) at x = a. From Figure 12, we see that the graph of y = f (x) is a horizontal line (that is, a line with zero slope) on the interval 0 ≤ x ≤ 3. Accordingly, f (1) = f (2) = 0. On the interval 3 ≤ x ≤ 5, the graph of y = f (x) is a line of slope 12 ; thus, f (4) = 12 . Finally, the line tangent to the graph of y = f (x) at x = 7 is horizontal, so f (7) = 0.

13. Which is larger: f (5.5) or f (6.5)? Estimate f (6). SOLUTION The line tangent to the graph of y = f (x) at x = 5.5 has a larger slope than the line tangent to the graph of y = f (x) at x = 6.5. Therefore, f (5.5) is larger than f (6.5). In Exercises the limit deﬁnition to ﬁnd the derivative of the linear function. (Note: The derivative does not Show15–18, that f use (3) does not exist. depend on a.) 15. f (x) = 3x − 2 SOLUTION

f (a + h) − f (a) 3(a + h) − 2 − (3a − 2) = lim = lim 3 = 3. h h h→0 h→0 h→0 lim

17. g(t) = 9 − t f (x) = 2 SOLUTION

g(a + h) − g(a) 9 − (a + h) − (9 − a) −h = lim = lim = lim (−1) = −1. h h h→0 h→0 h→0 h h→0 lim

1 1 1 1 Does or + ? Compute the difference quotient for f (x) at a = −2 19. Letk(z) f (x)==16z.+ 9 f (−2 + h) equal x −2 + h −2 h with h = 0.5. SOLUTION

Let f (x) = 1x . Then f (−2 + h) =

1 . −2 + h

With a = −2 and h = .5, the difference quotient is 1 − 1 f (−1.5) − f (−2) 1 f (a + h) − f (a) = = −1.5 −2 = − . h .5 .5 3

Let f (x) = with h = 1.

√

x. Does f (5 + h) equal

√

5 + h or

√

5+

√

h? Compute the difference quotient for f (x) at a = 5

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CHAPTER 3

D I F F E R E N T I AT I O N

21. Let f (x) =

√

x. Show that 1 f (9 + h) − f (9) = √ h 9+h+3

Then use this formula to compute f (9) (by taking the limit). √ SOLUTION We multiply numerator and denominator by 9 + h + 3 to make a difference of squares. Thus, f (9 + h) − f (9) = h =

√ √ √ 9+h−3 ( 9 + h − 3)( 9 + h + 3) = √ h h( 9 + h + 3) 9+h−9 h 1 √ = √ = √ . h( 9 + h + 3) h( 9 + h + 3) 9+h+3

Applying this formula we ﬁnd: f (9) = lim

h→0

1 f (9 + h) − f (9) 1 1 = . = lim √ = √ h 6 h→0 9 + h + 3 9+3

√ In Exercises derivative at x = the limit x at x of = the 4. tangent line. First 23–40, ﬁnd thecompute slope andthethen an equation of atheusing tangent line todeﬁnition the graphand of ﬁnd f (x)an=equation 23. f (x) = 3x 2 + 2x,

a=2

Let f (x) = 3x 2 + 2x. Then

SOLUTION

f (2 + h) − f (2) 3(2 + h)2 + 2(2 + h) − 16 = lim h h h→0 h→0

f (2) = lim

12 + 12h + 3h 2 + 4 + 2h − 16 = lim (14 + 3h) = 14. h h→0 h→0

= lim At a = 2, the tangent line is

y = f (2)(x − 2) + f (2) = 14(x − 2) + 16 = 14x − 12. 25. f (x) = x 3 , a2 = 2 f (x) = 3x + 2x, a = −1 SOLUTION Let f (x) = x 3 . Then f (2 + h) − f (2) (2 + h)3 − 23 8 + 12h + 6h 2 + h 3 − 8 = lim = lim = lim (12 + 6h + h 2 ) = 12 h h h h→0 h→0 h→0 h→0

f (2) = lim

At a = 2, the tangent line is y = f (2)(x − 2) + f (2) = 12(x − 2) + 8 = 12x − 16. 27. f (x) = x 3 +3x, a = 0 f (x) = x , a = 3 SOLUTION Let f (x) = x 3 + x. Then

f (0 + h) − f (0) (0 + h)3 + (0 + h) − 0 h3 + h = lim = lim = lim h 2 + 1 = 1. h h h h→0 h→0 h→0 h→0

f (0) = lim

The tangent line at a = 0 is y = f (0) + f (0)(x − 0) = 0 + 1(x − 0) or y = x. 29. f (x) = x −1 , 3 a = 3 f (t) = 3t + 2t, a = 4 SOLUTION Let f (x) = x −1 . Then

1 − 1 3−3−h 1 f (3 + h) − f (3) −h 3+h 3 3(3+h) = lim = lim = lim =− f (3) = lim h h h 9 h→0 h→0 h→0 h→0 (9 + 3h)h The tangent at a = 3 is 1 1 1 2 y = f (3)(x − 3) + f (3) = − (x − 3) + = − x + . 9 3 9 3 31. f (x) = 9x − 4, a = −7 f (x) = x −2 , a = 1

Deﬁnition of the Derivative

S E C T I O N 3.1 SOLUTION

83

Let f (x) = 9x − 4. Then f (−7) = lim

h→0

= lim

h→0

f (−7 + h) − f (−7) (9(−7 + h) − 4) − (−67) = lim h h h→0 (−63 + 9h − 4) + 67 9h = lim = lim 9 = 9. h h→0 h h→0

The tangent line at a = −7 is y = f (−7)(x + 7) + f (−7) = 9(x + 7) − 67 = 9x − 4. In general, we can take a shorter path. The tangent line to a line is the line itself. 1√ a =a−2 33. f (x)f (t) = = t ,+ 1, =0 x +3 SOLUTION

1 . Then Let f (x) = x+3 1 1 −1 −1 f (−2 + h) − f (−2) −h −1 = lim −2+h+3 = lim 1+h = lim = lim = −1. h h h h→0 h→0 h→0 h→0 h(1 + h) h→0 1 + h

f (−2) = lim

The tangent line at a = −2 is y = f (−2)(x + 2) + f (−2) = −1(x + 2) + 1 = −x − 1. 2 35. f (t) = x ,+ 1a = −1 , a=3 f (x)1=− t x −1 2 SOLUTION Let f (t) = 1−t . Then 2 f (−1 + h) − f (−1) 2 − (2 − h) 1 1 1−(−1+h) − 1 = lim = lim = lim = . h h 2 h→0 h→0 h→0 h(2 − h) h→0 2 − h

f (−1) = lim

At a = −1, the tangent line is 1 1 3 y = f (−1)(x + 1) + f (−1) = (x + 1) + 1 = x + . 2 2 2 37. f (t) = t −3 , 1a = 1 f (t) = , a=2 t +f (t) 9 = 1 . Then SOLUTION Let t3 f (1 + h) − f (h) = lim h h→0 h→0

f (1) = lim

1 −1 (1+h)3

h

= lim

h→0

−h 3+3h+h 2 3 (1+h)

h

−(3 + 3h + h 2 ) = −3. h→0 (1 + h)3

= lim

The tangent line at a = 1 is y = f (1)(t − 1) + f (1) = −3(t − 1) + 1 = −3t + 4. 1 39. f (x)f (t) == √ t 4, , aa==92 x SOLUTION

1 Let f (x) = √ . Then x f (9 + h) − f (9) = lim h h→0 h→0

f (9) = lim

√1 − 13 9+h

h

= lim

h→0

√ √ 3−√ 9+h 3+√9+h · 3 9+h 3+ 9+h

h

= lim

h→0

−1

1 = lim √ =− . 54 h→0 9 9 + h + 3(9 + h) At a = 9 the tangent line is 1 1 1 1 y = f (9)(x − 9) + f (9) = − (x − 9) + = − x + . 54 3 54 2 41. What is an equation of the tangent line at x = 3, assuming that f (3) = 5 and f (3) = 2? f (x) = (x 2 + 1)−1 , a = 0

√ 9−9−h 9 9+h+3(9+h)

h

84

CHAPTER 3

D I F F E R E N T I AT I O N SOLUTION By deﬁnition, the equation of the tangent line to the graph of f (x) at x = 3 is y = f (3) + f (3)(x − 3) = 5 + 2(x − 3) = 2x − 1.

43. Consider the “curve” y = 2x + 8. What is the tangent line at the point (1, 10)? Describe the tangent line at an Suppose that y = 5x + 2 is an equation of the tangent line to the graph of y = f (x) at a = 3. What is f (3)? arbitrary point. What is f (3)? SOLUTION Since y = 2x + 8 represents a straight line, the tangent line at any point is the line itself, y = 2x + 8. 1 1 Verify thatf (x) P = and L(x) = 12 + m(x − 1) for every on the f (x) = 2 + 5h. Suppose that is a 1, function thatgraphs f (2 + of h) both − f (2) = 3h 2 liessuch 1 + x2 slope(a) m. What Plot fis(x)f and (2)? L(x) on the same axes for several values of m. Experiment until you ﬁnd a value of m for which y = (b) L(x)What appears tangent to the What(2, is your f (1)? is the slope of the graph secantof linef (x). through f (2))estimate and (6, for f (6))?

45.

SOLUTION

Let f (x) =

1 and L(x) = 12 + m(x − 1). Because 1 + x2 f (1) =

1 1 = 2 1 + 12

L(1) =

and

1 1 + m(1 − 1) = , 2 2

it follows that P = (1, 12 ) lies on the graphs of both functions. A plot of f (x) and L(x) on the same axes for several values of m is shown below. The graph of L(x) with m = − 12 appears to be tangent to the graph of f (x) at x = 1. We therefore estimate f (1) = − 12 . y 1 m = −1

0.8 0.6

m = −1/4

0.4 0.2 0.5

1

m = −1/2 x 2

1.5

Plot f (x) = xx x and the line y = x + c on the same set of axes for several values of c. Experiment until you 47. Plot f (x) that = x theforline 0 ≤is xtangent ≤ 1.5.to the graph. Then: ﬁnd a value c0 such are there thatthat f (xf0 )(x=)0? (a) How many points x (a) Use your plot to estimate0 the value such x 0 such 0 = 1. plotting horizontal lines y = c together with the plot of f (x) until you ﬁnd a value of c (b) Estimatef (x x 00 by + h) − f (xseveral 0) (b) Verify thatthe line is tangent to the is close for which graph.to 1 for h = 0.01, 0.001, 0.0001. h SOLUTION The ﬁgure below shows the graphs of the function f (x) = x x together with the lines y = x − 0.5, y = x, and y = x + 0.5. y 2 1.5 1 0.5 x 0.2

0.4

0.6

0.8

1

1.2

1.4

−0.5

(a) The graph of y = x appears to be tangent to the graph of f (x) at x = 1. We therefore estimate that f (1) = 1. (b) With x 0 = 1, we generate the table h

0.01

0.001

0.0001

( f (x 0 + h) − f (x 0 ))/ h

1.010050

1.001001

1.000100

(1)net Thefollowing vapor pressure water isa deﬁned as the quotients atmospheric at whichf no takes 49. Use the table toof calculate few difference of fpressure (x). ThenP estimate by evaporation taking an average place. The following table and Figure 13 give P (in atmospheres) as a function of temperature T in kelvins. of difference quotients at h and −h. For example, take the average for h = 0.03 and −0.03, etc. Recall that the a +or b P (350)? Answer by referring to the graph. (a) Which is larger: P (300) average of a and b is . 2 303, 313, 323, 333, 343 using the table and the average of the difference quotients for (b) Estimate P (T ) for T = h = 10 and −10: x 1.03 1.02 1.01 1 P(T + 10) − P(T − 10) )≈ 4 f (x)P (T0.5148 0.5234 0.5319 0.5403 20 x 0.97 0.98 0.99

f (x)

0.5653

0.557

0.5486

Deﬁnition of the Derivative

S E C T I O N 3.1

T (K)

293

303

313

323

333

343

85

353

P (atm) 0.0278 0.0482 0.0808 0.1311 0.2067 0.3173 0.4754 P (atm) 1 0.75 0.5 0.25 293 313 333 353 373 T (K)

FIGURE 13 Vapor pressure of water as a function temperature T in kelvins. SOLUTION

(a) Consider the graph of vapor pressure as a function of temperature. If we draw the tangent line at T = 300 and another at T = 350, it is clear that the latter has a steeper slope. Therefore, P (350) is larger than P (300). (b) Using equation (4), P (303) ≈ P (313) ≈ P (323) ≈ P (333) ≈ P (343) ≈

P(313) − P(293) 20 P(323) − P(303) 20 P(333) − P(313) 20 P(343) − P(323) 20 P(353) − P(333) 20

= = = = =

.0808 − .0278 20 .1311 − .0482 20 .2067 − .0808 20 .3173 − .1311 20 .4754 − .2067 20

= .00265 atm/K; = .004145 atm/K; = .006295 atm/K; = .00931 atm/K; = .013435 atm/K

In Exercises 50–51, trafﬁc speed S along a certain road (in mph) varies as a function of trafﬁc density q (number of cars per mile on the road). Use the following data to answer the questions: q (density)

100

110

120

130

140

S (speed)

45

42

39.5

37

35

(q) when The Squantity V q==q120 S is cars called volume. why V is equal to the atnumber of cars 51. Estimate pertrafﬁc mile using the Explain average of difference quotients h and −h as inpassing Exercisea (q) when q = 120. particular point per hour. Use the data to compute values of V as a function of q and estimate V 48. SOLUTION The trafﬁc speed S has units of miles/hour, and the trafﬁc density has units of cars/mile. Therefore, the trafﬁc volume V = Sq has units of cars/hour. A table giving the values of V follows.

q

100

110

120

130

140

V

4500

4620

4740

4810

4900

h

−20

−10

10

20

V (120 + h) − V (120) h

12

12

7

8

To estimate d V /dq, we take the difference quotient.

The mean of the difference quotients is 9.75. Hence d V /dq ≈ 9.75 mph when q = 120. and a.the derivative is positive. In Exercises 53–58, of the 14, limits represents derivative f (a). For the grapheach in Figure determine theaintervals along the Find x-axisf (x) on which (5 + h)3 − 125 h h→0

53. lim

SOLUTION

The difference quotient

sin( π63 + h) − .5 x − 125 h h→0lim x→5 x − 5

55. lim

(5 + h)3 − 125 f (a + h) − f (a) has the form where f (x) = x 3 and a = 5. h h

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CHAPTER 3

D I F F E R E N T I AT I O N

SOLUTION

The difference quotient

sin( π6 + h) − .5 h

has the form

f (a + h) − f (a) where f (x) = sin x and a = π6 . h

52+h −1 − 25 x −4 h→0lim h 1 x→ 41 x − 4

57. lim

5(2+h) − 25 f (a + h) − f (a) has the form where f (x) = 5x and a = 2. h h 59. Sketch the graph of f (x) = sin x on [0, π ] and guess the value of f π2 . Then calculate the slope of the secant h π π 5 −1 line between lim x = 2 and x = 2 + h for at least three small positive and negative values of h. Are these calculations h h→0 consistent with your guess? SOLUTION

The difference quotient

SOLUTION

Here is the graph of y = sin x on [0, π ]. y 1 0.8 0.6 0.4 0.2 x 0.5 1 1.5 2 2.5 3

At x = π2 , we’re at the peak of the sine graph. The tangent line appears to be horizontal, so the slope is 0; hence, f ( π2 ) appears to be 0. h

−.01

−.001

−.0001

.0001

.001

.01

sin( π2 + h) − 1

.005

.0005

.00005

−.00005

−.0005

−.005

h

These numerical calculations are consistent with our guess. 4 √ Let15(A) f (x) = .graph of f (x) = x. The close-up in (B) shows that the graph is nearly a straight line Figure shows the 1 + 2x near x = 16. Estimate the slope of this line and take it as an estimate for f (16). Then compute f (16) and compare (a) Plot f (x) over [−2, 2]. Then zoom in near x = 0 until the graph appears straight and estimate the slope f (0). with your estimate. (b) Use your estimate to ﬁnd an approximate equation to the tangent line at x = 0. Plot this line and the graph on the same set of axes.

61.

SOLUTION

4 over [−2, 2]. The ﬁgure below at the right is a close-up (a) The ﬁgure below at the left shows the graph of f (x) = 1+2 x near x = 0. From the close-up, we see that the graph is nearly straight and passes through the points (−0.22, 2.15) and (0.22, 1.85). We therefore estimate

f (0) ≈

1.85 − 2.15 −0.3 = = −0.68 0.22 − (−0.22) 0.44

y

y

3 2.5

2.4 2.2

2 1.5 1 0.5 −2

−1

2.0 1.8 x 1

−0.2

2

−0.1

x 0.1

0.2

(b) Using the estimate for f (0) obtained in part (a), the approximate equation of the tangent line is y = f (0)(x − 0) + f (0) = −0.68x + 2. The ﬁgure below shows the graph of f (x) and the approximate tangent line. y 3 2.5 2 1.5 1 0.5 −2

−1

x 1

2

Deﬁnition of the Derivative

S E C T I O N 3.1

87

Let f (x) = cot x. Estimate f ( π2 ) graphically by zooming in on a plot of f (x) near x = π2 . Repeat Exercise 61 with f (x) = (2 + x)x . π SOLUTION The ﬁgure below shows a close-up of the graph of f (x) = cot x near x = 2 ≈ 1.5708. From the closeup, we see that the graph is nearly straight and passes through the points (1.53, 0.04) and (1.61, −0.04). We therefore estimate

π −0.04 − 0.04 −0.08 f ≈ = = −1 2 1.61 − 1.53 0.08 63.

y 1 0.05 1.58 1.6

x

1.54 1.56

−0.05 −1

65. Apply the method of Example 6 to f (x) = sin x to determine f π4 accurately to four decimal places. For each graph in Figure 16, determine whether f (1) is larger or smaller than the slope of the secant line SOLUTION between xWe = know 1 and that x = 1 + h for h > 0. Explain. √ f (π /4 + h) − f (π /4) sin(π /4 + h) − 2/2 f (π /4) = lim = lim . h h h→0 h→0 Creating a table of values of h close to zero: h

√

sin( π4 + h) − ( 2/2) h

−.001

−.0001

−.00001

.00001

.0001

.001

.7074602

.7071421

.7071103

.7071033

.7070714

.7067531

Accurate up to four decimal places, f ( π4 ) ≈ .7071. 67. Sketch the graph of f (x) = x 5/2 on [0, 6]. Apply the method of Example 6 to f (x) = cos x to determine f ( π5 ) accurately to four decimal places. Use a (a) Use theofsketch to explain justify the h > in 0: this case. graph f (x) to howinequalities the methodfor works f (4 + h) − f (4) f (4) − f (4 − h) ≤ f (4) ≤ h h (b) Use part (a) to compute f (4) to four decimal places. (c) Use a graphing utility to plot f (x) and the tangent line at x = 4 using your estimate for f (4). SOLUTION

(a) The slope of the secant line between points (4, f (4)) and (4 + h, f (4 + h)) is f (4 + h) − f (4) . h x 5/2 is a smooth curve increasing at a faster rate as x → ∞. Therefore, if h > 0, then the slope of the secant line is greater than the slope of the tangent line at f (4), which happens to be f (4). Likewise, if h < 0, the slope of the secant line is less than the slope of the tangent line at f (4), which happens to be f (4). (b) We know that f (4 + h) − f (4) (4 + h)5/2 − 32 = lim . h h h→0 h→0

f (4) = lim

Creating a table with values of h close to zero: h

−.0001

−.00001

.00001

.0001

(4 + h)5/2 − 32 h

19.999625

19.99999

20.0000

20.0000375

Thus, f (4) ≈ 20.0000. (c) Using the estimate for f (4) obtained in part (b), the equation of the line tangent to f (x) = x 5/2 at x = 4 is y = f (4)(x − 4) + f (4) = 20(x − 4) + 32 = 20x − 48.

88

CHAPTER 3

D I F F E R E N T I AT I O N y 80 60 40 20 x 1

− 20 − 40 − 60

2

3

4

5

6

69. The graph in Figure 18 (basedx on data collected by the biologist Julian Huxley, 1887–1975) gives the average antler of fas(x) = 2 is shown 17.the slope of the tangent line to the graph at t = 4. For which weight W The of agraph red deer a function of ageint. Figure Estimate (a)ofReferring to the explain thetoinequality values t is the slope ofgraph, the tangent linewhy equal zero? For which values is it negative? f (h) − f (0) Antler 8 f (0) ≤ Weight 7 h (kg) 6 5 holds for h > 0 and the opposite inequality holds for h < 0. 4 (b) Use part (a) to show that 0.69314 ≤ f3 (0) ≤ 0.69315. 2 (c) Use the same procedure to determine 1 f (x) to four decimal places for x = 1, 2, 3, 4. 0 0 x 2= 1,4 2, 3, 6 4. Can 8 10 14 an (approximate) you 12 guess (d) Now compute the ratios f (x)/ f (0) for Age (years) general x?

formula for f (x) for

FIGURE 18 SOLUTION Let W (t) denote the antler weight as a function of age. The “tangent line” sketched in the ﬁgure below passes through the points (1, 1) and (6, 5.5). Therefore

W (4) ≈

5.5 − 1 = 0.9 kg/year. 6−1

If the slope of the tangent is zero, the tangent line is horizontal. This appears to happen at roughly t = 10 and at t = 11.6. The slope of the tangent line is negative when the height of the graph decreases as we move to the right. For the graph in Figure 18, this occurs for 10 < t < 11.6. y 8 7 6 5 4 3 2 1 0

0

2

4

6

8

10

12

x 14

In Exercises 70–72, i(t) is the current (in amperes) at time t (seconds) ﬂowing in the circuit shown in Figure 19. According to Kirchhoff’s law, i(t) = Cv (t) + R −1 v(t), where v(t) is the voltage (in volts) at time t, C the capacitance (in farads), and R the resistance (in ohms, ). i + v

R

−

C

FIGURE 19

71. Use the following table to estimate v (10). For a better estimate, take the average of the difference quotients for h Calculate the current at t = 3 if v(t) = 0.5t + 4 V, C = 0.01 F, and R = 100 . and −h as described in Exercise 48. Then estimate i(10), assuming C = 0.03 and R = 1,000. t v(t) SOLUTION

9.8

9.9

10

10.1

10.2

256.52

257.32

258.11

258.9

259.69

We generate a table of difference quotients at 10 using the following table: h

−0.2

−0.1

0.1

0.2

v(10 + h) − v(10) h

7.95

7.9

7.9

7.9

Deﬁnition of the Derivative

S E C T I O N 3.1

89

The mean of the difference quotients is 7.9125, so v (10) ≈ 7.9125 volts/s. Thus, i(10) = 0.03(7.9125) +

1 (258.11) = 0.495485 amperes. 1000

Assume that Rand = 200 but C is unknown. Use the following data to estimate v (4) as in Exercise 71 and deduce Further Insights Challenges an approximate value for the capacitance C. In Exercises 73–76, we deﬁne the symmetric difference quotient (SDQ) at x = a for h = 0 by 3.8f (a + 3.9 h) − f (a4− h) 4.1 2h 420 436.2 388.8 404.2

t v(t)

73. Explain how SDQ can be interpreted slope of a secant34.1 line. 34.98 i(t)as the 32.34 33.22 SOLUTION The symmetric difference quotient

4.2

5

452.8 35.86

f (a + h) − f (a − h) 2h is the slope of the secant line connecting the points (a − h, f (a − h)) and (a + h, f (a + h)) on the graph of f ; the difference in the function values is divided by the difference in the x-values. 75. Showusually that if gives f (x) is a quadratic polynomial, then the SDQthan at xdoes = athe (for any h = 0) is equal to f (a). The SDQ a better approximation to the derivative ordinary difference quotient. Let x Explain of this the result. = 0. Compute SDQ with h = 0.001 and the ordinary difference quotients with h = ±0.001. f (x)the=graphical 2 and a meaning Compare with the actual value of f (0) that, to eight decimal places, is 0.69314718. SOLUTION Let f (x) = px 2 + qx + r be a quadratic polynomial. We compute the SDQ at x = a. f (a + h) − f (a − h) p(a + h)2 + q(a + h) + r − ( p(a − h)2 + q(a − h) + r ) = 2h 2h pa 2 + 2 pah + ph 2 + qa + qh + r − pa 2 + 2 pah − ph 2 − qa + qh − r 2h 4 pah + 2qh 2h(2 pa + q) = = = 2 pa + q 2h 2h =

Since this doesn’t depend on h, the limit, which is equal to f (a), is also 2 pa + q. Graphically, this result tells us that the secant line to a parabola passing through points chosen symmetrically about x = a is always parallel to the tangent line at x = a. 77. Which of the two functions in Figure 20 satisﬁes the inequality Let f (x) = x −2 . Compute f (1) by taking the limit of the SDQs (with a = 1) as h → 0. f (a + h) − f (a) f (a + h) − f (a − h) ≤ 2h h for h > 0? Explain in terms of secant lines. y

y

x

x a

a (A)

(B)

FIGURE 20 SOLUTION

Figure (A) satisﬁes the inequality f (a + h) − f (a − h) f (a + h) − f (a) ≤ 2h h

since in this graph the symmetric difference quotient has a larger negative slope than the ordinary right difference quotient. [In ﬁgure (B), the symmetric difference quotient has a larger positive slope than the ordinary right difference quotient and therefore does not satisfy the stated inequality.]

90

CHAPTER 3

D I F F E R E N T I AT I O N

3.2 The Derivative as a Function Preliminary Questions 1. What is the slope of the tangent line through the point (2, f (2)) if f is a function such that f (x) = x 3 ? SOLUTION

The slope of the tangent line through the point (2, f (2)) is given by f (2). Since f (x) = x 3 , it follows

that f (2) = 23 = 8.

2. Evaluate ( f − g) (1) and (3 f + 2g) (1) assuming that f (1) = 3 and g (1) = 5. Can we evaluate ( f g) (1) using the information given and the rules presented in this section? SOLUTION ( f − g) (1) = f (1) − g (1) = 3 − 5 = −2 and (3 f + 2g) (1) = 3 f (1) + 2g (1) = 3(3) + 2(5) = 19. Using the rules of this section, we cannot evaluate ( f g) (1).

3. Which of the following functions can be differentiated using the rules covered in this section? Explain. (b) f (x) = 2x (a) f (x) = x 2 1 (c) f (x) = √ (d) f (x) = x −4/5 x (e) f (x) = sin x (f) f (x) = (x + 5)3 SOLUTION

(a) Yes. x 2 is a power function, so the Power Rule can be applied. (b) No. 2x is an exponential function (the base is constant while the exponent is a variable), so the Power Rule does not apply. 1 (c) Yes. √ can be rewritten as x −1/2 . This is a power function, so the Power Rule can be applied. x −4/5 (d) Yes. x is a power function, so the Power Rule can be applied. (e) No. The Power Rule does not apply to the trigonometric function sin x. (f) Yes. (x + 5)3 can be expanded to x 3 + 15x 2 + 75x + 125. The Power Rule, Rule for Sums, and Rule for Constant Multiples can be applied to this function. 4. Which algebraic identity is used to prove the Power Rule for positive integer exponents? Explain how it is used. SOLUTION

The algebraic identity x n − a n = (x − a)(x n−1 + x n−2 a + x n−3 a 2 + · · · + xa n−2 + a n−1 )

is used to prove the Power Rule for positive integer exponents. With this identity, the difference quotient x n − an x −a can be simpliﬁed to x n−1 + x n−2 a + x n−3 a 2 + · · · + xa n−2 + a n−1 . This expression is continuous at x = a, so the limit as x → a can be evaluated by substitution. √ 5. Does the Power Rule apply to f (x) = 5 x? Explain. √ SOLUTION Yes. 5 x = x 1/5 , which is a power function. 6. In which of the following two cases does the derivative not exist? (a) Horizontal tangent (b) Vertical tangent SOLUTION

The derivative does not exist when there is a vertical tangent. At a horizontal tangent, the derivative is zero.

Exercises In Exercises 1–8, compute f (x) using the limit deﬁnition. 1. f (x) = 4x − 3 SOLUTION

Let f (x) = 4x − 3. Then, f (x) = lim

h→0

2 3. f (x) = 1 − 2x f (x) = x 2 + x

f (x + h) − f (x) 4(x + h) − 3 − (4x − 3) 4h = lim = lim = 4. h h h→0 h→0 h

The Derivative as a Function

S E C T I O N 3.2 SOLUTION

91

Let f (x) = 1 − 2x 2 . Then, f (x + h) − f (x) 1 − 2(x + h)2 − (1 − 2x 2 ) = lim h h h→0 h→0

f (x) = lim

−4xh − 2h 2 = lim (−4x − 2h) = −4x. h h→0 h→0

= lim 5. f (x) = x −1 3 f (x) = x SOLUTION Let f (x) = x −1 . Then,

x−x−h 1 − 1 f (x + h) − f (x) −1 1 x(x+h) x = lim x+h = lim = lim = − 2. h h h h→0 h→0 h→0 h→0 x(x + h) x

f (x) = lim

√ 7. f (x) = x x √ f (x) = SOLUTION Letx − f (x) 1 = x. Then, f (x + h) − f (x) = lim h h→0 h→0

f (x) = lim = lim

√

x +h− h

√

x

√ = lim

h→0

x +h− h

√

x

√ ·

√

√ x √ x +h+ x x +h+

1 (x + h) − x 1 √ √ = lim √ √ = √ . 2 x x + h + x) h→0 x + h + x

h→0 h(

In Exercises 9–16,−1/2 use the Power Rule to compute the derivative. f (x) = x d 4 9. x d x x=−2 d 4 d 4 3 x = 4x so x SOLUTION = 4(−2)3 = −32. dx d x x=−2 d 2/3 11. t d −4 dt x d x t=8x=3 d 2/3 2 1 d 2/3 2 −1/3 t SOLUTION so = (8)−1/3 = . t = t dt 3 dt 3 3 t=8 d 0.35 xd −2/3 dx t

dt d t=1 x 0.35 = 0.35(x 0.35−1 ) = 0.35x −0.65 . SOLUTION dx d √17 15. t d 14/3 dt x dx d √17 √ √17−1 SOLUTION = 17t t dt 13.

In Exercises compute f (a) and ﬁnd an equation of the tangent line to the graph at x = a. 2 d −17–20, t π dt= x 5 , a = 1 17. f (x) Let f (x) = x 5 . Then, by the Power Rule, f (x) = 5x 4 . The equation to the tangent line to the graph of f (x) at x = 1 is

SOLUTION

y = f (1)(x − 1) + f (1) = 5(x − 1) + 1 = 5x − 4. √ 19. f (x) = 3 x + 8x, a = 9 f (x) = x −2 , a = 3 3 17 SOLUTION Let f (x) = 3x 1/2 + 8x. Then f (x) = 2 x −1/2 + 8. In particular, f (9) = 2 . The tangent line at x = 9 is 17 17 9 y = f (9)(x − 9) + f (9) = (x − 9) + 81 = x+ . 2 2 2 √ In Exercises a = 8 the derivative of the function. f (x)21–32, = 3 x,calculate 21. f (x) = x 3 + x 2 − 12

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CHAPTER 3

D I F F E R E N T I AT I O N

SOLUTION

d 3 x + x 2 − 12 = 3x 2 + 2x. dx

23. f (x) = 2x 3 −310x −12 f (x) = 2x − 3x + 2x d 2x 3 − 10x −1 = 6x 2 + 10x −2 . SOLUTION dx 25. g(z) = 7z −3 5+ z 2 +2 5 f (x) = x − 7x + 10x + 9 d SOLUTION 7z −3 + z 2 + 5 = −21z −4 + 2z. dz √ √ 3 27. f (s) = 4 s + √ s 1 √ +√ h(t) =f6(s)t = 4√ SOLUTION st + 3 s = s 1/4 + s 1/3 . In this form, we can apply the Sum and Power Rules. 1 1 1 1 d 1/4 + s 1/3 = (s (1/4)−1 ) + (s (1/3)−1 ) = s −3/4 + s −2/3 . s ds 4 3 4 3 29. f (x) = (x + 1)43 (Hint: Expand) + 7y 2/3 W (y) = 6y d 3 d

SOLUTION (x + 1)3 = x + 3x 2 + 3x + 1 = 3x 2 + 6x + 3. dx dx 31. P(z) = (3z − 1)(2z + 1) R(s) = (5s + 1)2 d d 2 SOLUTION 6z + z − 1 = 12z + 1. ((3z − 1)(2z + 1)) = dz dz √ calculate the derivative indicated. In Exercises 33–38, q(t) = t(t + 1) 3 33. f (2), f (x) = 4 x SOLUTION

With f (x) = 3x −4 , we have f (x) = −12x −5 , so 3 f (2) = −12(2)−5 = − . 8

d T √ , T = 3Cx2/3 +1 dCy C=8 (16), y = x dT SOLUTION With T (C) = 3C 2/3 , we have = 2C −1/3 . Therefore, dC d T = 2(8)−1/3 = 1. dC

35.

C=8

ds d P , s = 4z − 16z72 dz z=2 , P= d V V =−2 V ds SOLUTION With s = 4z − 16z 2 , we have = 4 − 32z. Therefore, dz ds = 4 − 32(2) = −60. dz z=2

37.

39. Match the functions in graphs (A)–(D) with their derivatives (I)–(III) in Figure 11. Note that two of the functions R derivative. Explain have thedsame why. , R = Wπ d W W =1 y

y

y

y x

x

x x (A)

(B)

(C)

y

y

(D)

y x

x (I)

x (II)

(III)

FIGURE 11 SOLUTION

S E C T I O N 3.2

The Derivative as a Function

93

• Consider the graph in (A). On the left side of the graph, the slope of the tangent line is positive but on the right

side the slope of the tangent line is negative. Thus the derivative should transition from positive to negative with increasing x. This matches the graph in (III). • Consider the graph in (B). This is a linear function, so its slope is constant. Thus the derivative is constant, which matches the graph in (I). • Consider the graph in (C). Moving from left to right, the slope of the tangent line transitions from positive to negative then back to positive. The derivative should therefore be negative in the middle and positive to either side. This matches the graph in (II). • Consider the graph in (D). On the left side of the graph, the slope of the tangent line is positive but on the right side the slope of the tangent line is negative. Thus the derivative should transition from positive to negative with increasing x. This matches the graph in (III). Note that the functions whose graphs are shown in (A) and (D) have the same derivative. This happens because the graph in (D) is just a vertical translation of the graph in (A), which means the two functions differ by a constant. The derivative of a constant is zero, so the two functions end up with the same derivative. 41. Sketch the graph of f (x) for f (x) as in Figure 13, omitting points where f (x) is not differentiable. Assign the labels f (x), g(x), and h(x) to the graphs in Figure 12 in such a way that f (x) = g(x) and g (x) = h(x). y 5 4 3 2 1

x

1 2 3 4 5 6 7 8 9

FIGURE 13 Graph of f (x).

On the interval 0 ≤ x ≤ 3, the function is constant, so f (x) = 0. On the interval 3 ≤ x ≤ 5, the function is linear with slope 12 , so f (x) = 12 . From x = 5 to x = 7, the derivative should be positive but decreasing to zero at x = 7. Finally, for 7 < x ≤ 9, the derivative is negative and decreasing. A sketch of f (x) is given below. SOLUTION

y

2 1 x

−1 −2

1 2 3 4 5 6 7 8 9

43. Let R be a variable and r a constant. Compute the derivatives f (x). Does f (x) appear to increased or decrease as a function of x? d The table below lists values of a function d (a) Is (A) R or (B) in Figure 14 the graph of (b)f (x)?rExplain. (c) r 2 R3 dR dR dR SOLUTION

x 0 0.5 1 1.5 2 d (a) R = 1, since R is a linear of R with f (x) function 10 55 98 slope 139 1. 177 dR d r = 0, since r is a constant. (b) dR (c) We apply the Linearity and Power Rules:

2.5

3

3.5

4

210

237

257

268

d 3 d 2 3 r R = r2 R = r 2 3(R 2 ) = 3r 2 R 2 . dR dR 45. Find the points on the curve y = x 2 + 23x − 7 at which the slope of the tangent line is equal to 4. Sketch the graph of f (x) = x − 3x and ﬁnd the values of x for which the tangent line is horizontal. 1 SOLUTION Let y = x 2 + 3x − 7. Solving d y/d x = 2x + 3 = 4 yields x = 2 . 47. Find all values of x where the tangent lines to y = x 3 and y = x 4 are parallel. Sketch the graphs of f (x) = x 2 − 5x + 4 and g(x) = −2x + 3. Find the value of x at which the graphs have SOLUTION Let f (x) = x 3 and let g(x) = x 4 . The two graphs have parallel tangent lines at all x where f (x) = g (x). parallel tangent lines. f (x) = g (x) 3x 2 = 4x 3 3x 2 − 4x 3 = 0 x 2 (3 − 4x) = 0 hence, x = 0 or x = 34 . Show that there is a unique point on the graph of the function f (x) = ax 2 + bx + c where the tangent line is horizontal (assume a = 0). Explain graphically.

94

CHAPTER 3

D I F F E R E N T I AT I O N

49. Determine coefﬁcients a and b such that p(x) = x 2 + ax + b satisﬁes p(1) = 0 and p (1) = 4. SOLUTION Let p(x) = x 2 + ax + b satisfy p(1) = 0 and p (1) = 4. Now, p (x) = 2x + a. Therefore 0 = p(1) = 1 + a + b and 4 = p (1) = 2 + a; i.e., a = 2 and b = −3.

m > −3, there are precisely two points 51. Let f (x) = x 3 − 3x + 1. Show that f (x) ≥ −3 for all x, and that for every + 11x + 2 isfor steeper than of themtangent line values of x the such that theoftangent line toand thethe graph of y = 4x 2tangent (x) all where fFind = m. Indicate position these points corresponding lines one value in a sketch 3 to y = x . of the graph of f (x). SOLUTION

Let P = (a, b) be a point on the graph of f (x) = x 3 − 3x + 1.

• The derivative satisﬁes f (x) = 3x 2 − 3 ≥ −3 since 3x 2 is nonnegative. • Suppose the slope m of the tangent line is greater than −3. Then f (a) = 3a 2 − 3 = m, whence

m+3 a2 = >0 3

and thus

a=±

m+3 . 3

• The two parallel tangent lines with slope 2 are shown with the graph of f (x) here. y

4 2 −2

1 −1

x

2

−2

Hint: Show that 53. Compute the derivative of f (x) = x −2 using the limit1 deﬁnition. Show that if the tangent lines to the graph of y = 3 x 3 − x 2 at x = a and at x = b are parallel, then either a = b or a + b = 2. (x + h)2 − x 2 1 f (x + h) − f (x) =− 2 h h x (x + h)2 SOLUTION

1

1

x 2 −(x+h)2

− 2 f (x + h) − f (x) 1 (x+h)2 x x 2 (x+h)2 = = = h h h h

x 2 − (x + h)2 1

1 x 2 (x + h)2

1 (x + h)2 − x 2 =− 2 . h x (x + h)2 We now compute the limits as h → 0 of each factor. By continuity, we see that, if x = 0, lim

−1

h→0 x 2 (x + h)2

1 = − 4. x

The second factor can be recognized easily as the limit deﬁnition of the derivative of x 2 , which is 2x. Applying the Product Rule, we get that: 1 2 d −2 = − 4 (2x) = − 3 . x dx x x √ 55. The average speed (in meters per second) of a gas molecule is vavg = 8RT /(π M), where T is the temperature (in 3/2 limit deﬁnition. the derivative of f (x)and = xR =using kelvin),Compute M is the molar mass (kg/mol) 8.31. the Calculate dvavg /d THint: at T Show = 300that K for oxygen, which has a molar mass of 0.032 kg/mol. f (x + h) − f (x) (x + √ h)3 − x 3 1 1/2 = 1/2 , where M √ and R are constant, we use the SOLUTION Using the form vav = (8RT /(π M))= h h8R/(π M)T (x + h)3 + x 3 Power Rule to compute the derivative dvav /d T . d d 1/2 1 = 8R/(π M) (T (1/2)−1 ). 8R/(π M)T 1/2 = 8R/(π M) T dT dT 2 In particular, if T = 300◦ K, 1 d vav = 8(8.31)/(π (0.032)) (300)−1/2 = 0.74234 m/(s · K). dT 2 57. Some studies suggest that kidney mass K in mammals (in kilograms) is related to body mass m (in kilograms) by Biologists have observed that the pulse rate P (in beats per minute) in animals is related to body mass (in the approximate formula K = 0.007m 0.85 . Calculate d K /dm at m = 68. Then calculate the derivative with respect to kilograms) by the approximate formula P = 200m −1/4 . This is one of many allometric scaling laws prevalent in m of the relative kidney-to-mass ratio K /m at m = 68. biology. Is the absolute value |d P/dm| increasing or decreasing as m increases? Find an equation of the tangent line at the points on the graph in Figure 15 that represent goat (m = 33) and man (m = 68).

S E C T I O N 3.2

The Derivative as a Function

95

SOLUTION

dK = 0.007(0.85)m −0.15 = 0.00595m −0.15 ; dm hence, d K = 0.00595(68)−0.15 = 0.00315966. dm m=68 Because K m 0.85 = 0.007 = 0.007m −0.15 , m m we ﬁnd d dm

K m

and d dm

= 0.007

d −0.15 m = −0.00105m −1.15 , dm

K = −8.19981 × 10−6 kg−1 . m m=68

59. Let L be a tangent line to the hyperbola x y = 1 at x = a, where a > 0. Show that the area of the triangle bounded The relation between the vapor pressure P (in atmospheres) of water and the temperature T (in kelvin) is given by L and the coordinate axes does not depend on a. by the Clausius–Clapeyron law: 1 1 SOLUTION Let f (x) = x −1 . The tangent line to f at x = a is y = f (a)(x − a) + f (a) = − 2 (x − a) + a . The a P dP 2 = k 2y = 0) is 2a. Hence the area of the triangle bounded y-intercept of this line (where x = 0) is a . Its x-intercept (where

dT T 1 1 by the tangent line and the coordinate axes is A = 2 bh = 2 (2a) a2 = 2, which is independent of a. where k is a constant. Use the table below and the approximation y dP P(T + 10) − P(T − 10) ≈ dT 20 1 y=

x

to estimate d P/d T for T = 303, 313, 323, 333, 343. Do your estimates seem to conﬁrm the Clausius–Clapeyron 2 = (0, a )are the units of k? law? What is the approximate value of k?P What

( 1)

R = a, a

T

293

303

313

323

333

343

353

x

P 0.0278 0.0482 0.0808 0.1311 Q = (2a, 0)0.2067 0.3173 0.4754 61. Match the functions (A)–(C) with their derivatives (I)–(III) in Figure 16. In the notation of Exercise 59, show that the point of tangency is the midpoint of the segment of L lying in the ﬁrst quadrant. y y

x

x

(A)

(I) y

y

x x

(B)

(II)

y

y

x x (C)

(III)

FIGURE 16

96

CHAPTER 3

D I F F E R E N T I AT I O N SOLUTION Note that the graph in (A) has three locations with a horizontal tangent line. The derivative must therefore cross the x-axis in three locations, which matches (III). The graph in (B) has only one location with a horizontal tangent line, so its derivative should cross the x-axis only once. Thus, (I) is the graph corresponding to the derivative of (B). Finally, the graph in (B) has two locations with a horizontal tangent line, so its derivative should cross the x-axis twice. Thus, (II) is the graph corresponding to the derivative of (C).

63. Make a rough sketch of the graph of the derivative of the function shown in Figure 17(A). Plot the derivative f (x) of f (x) = 2x 3 − 10x −1 for x > 0 (set the bounds of the viewing box appropriately) SOLUTION Thethat graph has>a 0. tangent negative slope onthe thegraph interval 3.6), andPlot has af (x) tangent and observe f (x) What line doeswith the positivity of f approximately (x) tell us about of (1, f (x) itself? and line with a positive slope elsewhere. This implies that the derivative must be negative on the interval (1, 3.6) and positive conﬁrm. elsewhere. The graph may therefore look like this: y

x 1

65.

2

3

4

thederivative two functions and g inshown Figurein18, which is theomitting derivative of thewhere other? your is answer. GraphOfthe of the ffunction Figure 17(B), points theJustify derivative not deﬁned. y f(x)

2

g(x) x

−1

1

FIGURE 18 SOLUTION g(x) is the derivative of f (x). For f (x) the slope is negative for negative values of x until x = 0, where there is a horizontal tangent, and then the slope is positive for positive values of x. Notice that g(x) is negative for negative values of x, goes through the origin at x = 0, and then is positive for positive values of x.

In Exercises 67–72, ﬁnd is thethe points c (if in any) such19 that f (c) does not At which points function Figure discontinuous? Atexist. which points is it nondifferentiable? 67. f (x) = |x − 1| SOLUTION y 2 1.5 1 0.5 x

−1

1

2

3

Here is the graph of f (x) = |x − 1|. Its derivative does not exist at x = 1. At that value of x there is a sharp corner. 2/3 69. f (x)f (x) = x= [x]

SOLUTION Here is the graph of f (x) = x 2/3 . Its derivative does not exist at x = 0. At that value of x, there is a sharp corner or “cusp”. y 1.5 1

−2

−1

x 1

2

71. f (x) = |x 2 −3/2 1| f (x) = x SOLUTION Here is the graph of f (x) = x 2 − 1. Its derivative does not exist at x = −1 or at x = 1. At these values of x, the graph has sharp corners.

S E C T I O N 3.2

The Derivative as a Function

97

y 3 2 1

−2

x

−1

1

2

f (x) = |x −73–78, 1|2 zoom in on a plot of f (x) at the point (a, f (a)) and state whether or not f (x) appears to be In Exercises differentiable at x = a. If nondifferentiable, state whether the tangent line appears to be vertical or does not exist. 73. f (x) = (x − 1)|x|,

a=0

SOLUTION The graph of f (x) = (x − 1)|x| for x near 0 is shown below. Because the graph has a sharp corner at x = 0, it appears that f is not differentiable at x = 0. Moreover, the tangent line does not exist at this point. y x

− 0.2 −0.1

0.1

0.2

−0.1 −0.2 −0.3

75. f (x) = (x − 3)1/3 ,5/3a = 3 f (x) = (x − 3) , a = 3 SOLUTION The graph of f (x) = (x − 3)1/3 for x near 3 is shown below. From this graph, it appears that f is not differentiable at x = 3. Moreover, the tangent line appears to be vertical.

2.9

2.95

3

3.05

3.1

77. f (x) = | sin x|, a = 0 f (x) = sin(x 1/3 ), a = 0 SOLUTION The graph of f (x) = | sin x| for x near 0 is shown below. Because the graph has a sharp corner at x = 0, it appears that f is not differentiable at x = 0. Moreover, the tangent line does not exist at this point. y 0.1 0.08 0.04

−0.1 −0.05

79.

x 0.05

0.1

thatx|, a nondifferentiable function is not continuous? If not, give a counterexample. f (x)Is=it|xtrue − sin a=0

SOLUTION

This statement is false. Indeed, the function |x| is not differentiable at x = 0 but it is continuous.

Sketch the graph of y = x |x| and show that it is differentiable for all x Further Insights and Challenges x < 0, x = 0, and x > 0).

(check differentiability separately for

81. Prove the following theorem of Apollonius of Perga (the Greek mathematician born in 262 BCE who gave the parabola, ellipse, and hyperbola their names): The tangent to the parabola y = x 2 at x = a intersects the x-axis at the midpoint between the origin and (a, 0). Draw a diagram. SOLUTION

Let f (x) = x 2 . The tangent line to f at x = a is y = f (a)(x − a) + f (a) = 2a(x − a) + a 2 = 2ax − a 2 .

The x-intercept of this line (where y = 0) is a2 , which is halfway between the origin and the point (a, 0).

98

CHAPTER 3

D I F F E R E N T I AT I O N y

(a, a 2) y=

x2

(–,a2 0)

x

83. Plot the graph of f (x) = (4 − x 2/3 )3/2 (the “astroid”). Let L be a tangent line to a point on the graph in 3 Apollonius’s in Exercise canﬁrst be generalized. that the tangent the ﬁrst quadrant. ShowTheorem that the portion of L 81 in the quadrant hasShow a constant length 8. to y = x at x = a intersects the x-axis at the point x = 23 a. Then formulate the general statement for the graph of y = x n and prove it. SOLUTION

• Here is a graph of the astroid. y 10

x

−10

10

−10

• Let f (x) = (4 − x 2/3 )3/2 . Since we have not yet encountered the Chain Rule, we use Maple throughout this

exercise. The tangent line to f at x = a is

4 − a 2/3 2/3 3/2 . y=− (x − a) + 4 − a a 1/3

The y-intercept of this line is the point P = 0, 4 4 − a 2/3 , its x-intercept is the point Q = 4a 1/3 , 0 , and the distance between P and Q is 8.

85. A vase is formed by rotating y = x 2 around the y-axis. If we drop in a marble, it will either the bottom point 2 fortouch −1marble ≤ x ≤be1 to and the Twoorsmall arches have thethe shape of parabolas. Thetheﬁrst is given by21). f (x) = small 1 − xmust of the vase be suspended above bottom by touching sides (Figure How the touch 2 for 2 ≤ x ≤ 6. A board is placed on top of these arches so it rests on both (Figure second by g(x) = 4 − (x − 4) the bottom? 20). What is the slope of the board? Hint: Find the tangent line to y = f (x) that intersects y = g(x) in exactly one point. FIGURE 20

FIGURE 21

Suppose a circle is tangent to the parabola y = x 2 at the point (t, t 2 ). The slope of the parabola at this 1 (since it is perpendicular to the tangent line of the point is 2t, so the slope of the radius of the circle at this point is − 2t 1 (x − t) + t 2 crosses the y-axis. We can circle). Thus the center of the circle must be where the line given by y = − 2t 1 2 ﬁnd the y-coordinate by setting x = 0: we get y = 2 + t . Thus, the radius extends from (0, 12 + t 2 ) to (t, t 2 ) and SOLUTION

r=

2 1 1 2 2 2 +t = +t −t + t 2. 2 4

This radius is greater than 12 whenever t > 0; so, if a marble has radius > 1/2 it sits on the edge of the vase, but if it has radius ≤ 1/2 it rolls all the way to the bottom. 87. Negative Exponents Let n be a whole number. Use the Power Rule for x n to calculate the derivative of f (x) = Let f (x) thatbe a differentiable function and set g(x) = f (x + c), where c is a constant. Use the limit deﬁnition to x −n by showing show that g (x) = f (x + c). Explain this result graphically, recalling that the graph of g(x) is obtained by shifting (x + − 0)f (x) the graph of f (x) c units to the fleft (ifh) c> or right (if −1 c < 0). (x + h)n − x n = n h x (x + h)n h

Product and Quotient Rules

S E C T I O N 3.3 SOLUTION

99

Let f (x) = x −n where n is a positive integer.

• The difference quotient for f is 1

1

x n −(x+h)n

(x + h)−n − x −n f (x + h) − f (x) (x+h)n − x n x n (x+h)n = = = h h h h (x + h)n − x n −1 . = n x (x + h)n h • Therefore,

f (x + h) − f (x) −1 (x + h)n − x n = lim n n h h h→0 h→0 x (x + h)

f (x) = lim

= lim

−1

lim

h→0 x n (x + h)n h→0

(x + h)n − x n d n = −x −2n x . h dx

d n x = −x −2n · nx n−1 = −nx −n−1 . Since n is a positive integer, dx d −n d k d xk = x x = kx k−1 = −nx −n−1 = kx k−1 ; i.e. k = −n is a negative integer and we have dx dx dx for negative integers k.

• From above, we continue: f (x) = −x −2n

89. Inﬁnitely Rapid Oscillations Deﬁne Verify the Power Rule for the exponent 1/n, where n is a positive integer, using the following trick: Rewrite the difference quotient for y = x 1/n at x = b in terms⎧ of u =1 (b + h)1/n and a = b1/n . ⎪ ⎨x sin if x = 0 x f (x) = ⎪ ⎩0 if x = 0 Show that f (x) is continuous at x = 0 but f (0) does not exist (see Figure 10).

if x = 0 x sin 1x SOLUTION Let f (x) = . As x → 0, 0 if x = 0 1 1 | f (x) − f (0)| = x sin − 0 = |x| sin →0 x x since the values of the sine lie between −1 and 1. Hence, by the Squeeze Theorem, lim f (x) = f (0) and thus f is continuous at x = 0. As x → 0, the difference quotient at x = 0,

x→0

x sin 1x − 0 1 f (x) − f (0) = = sin x −0 x −0 x does not converge to a limit since it oscillates inﬁnitely through every value between −1 and 1. Accordingly, f (0) does not exist.

3.3 Product and Quotient Rules Preliminary Questions 1. (a) (b) (c)

Are the following statements true or false? If false, state the correct version. The notation f g denotes the function whose value at x is f (g(x)). The notation f /g denotes the function whose value at x is f (x)/g(x). The derivative of the product is the product of the derivatives.

SOLUTION

(a) False. The notation f g denotes the function whose value at x is f (x)g(x). (b) True. (c) False. The derivative of the product f g is f (x)g(x) + f (x)g (x). 2. Are the following equations true or false? If false, state the correct version. d (a) = f (4)g (4) − g(4) f (4) ( f g) dx x=4

100

CHAPTER 3

D I F F E R E N T I AT I O N

d f f (4)g (4) + g(4) f (4) = d x g x=4 (g(4))2 d (c) = f (0)g (0) + g(0) f (0) ( f g) dx x=0

(b)

SOLUTION

d = f (4)g (4) + g(4) f (4). ( f g) dx x=4 d = [g(4) f (4) − f (4)g (4)]/g(4)2 . (b) False. ( f /g) dx x=4 (c) True. (a) False.

3. What is the derivative of f /g at x = 1 if f (1) = f (1) = g(1) = 2, and g (1) = 4? d SOLUTION ( f /g)x=1 = [g(1) f (1) − f (1)g (1)]/g(1)2 = [2(2) − 2(4)]/22 = −1. dx 4. Suppose that f (1) = 0 and f (1) = 2. Find g(1), assuming that ( f g) (1) = 10. ( f g) (1) = f (1)g (1) + f (1)g(1), so 10 = 0 · g (1) + 2g(1) and g(1) = 5.

SOLUTION

Exercises In Exercises 1–4, use the Product Rule to calculate the derivative. 1. f (x) = x(x 2 + 1) Let f (x) = x(x 2 + 1). Then

SOLUTION

f (x) = x

3.

d 2 d (x + 1) + (x 2 + 1) x = x(2x) + (x 2 + 1) = 3x 2 + 1. dx dx

d y √ (t 2 + 1)(t (x), = y =x(1 − x 4 ) + 9) dt ft=3 Let y = (t 2 + 1)(t + 9). Then

SOLUTION

d d dy = (t 2 + 1) (t + 9) + (t + 9) (t 2 + 1) = (t 2 + 1) + (t + 9)(2t) = 3t 2 + 18t + 1. dt dt dt Therefore, d y = 3(3)2 + 18(3) + 1 = 82. dt t=3 In Exercises Rule to calculate the derivative. dh 5–8, use the Quotient −1/2 + 2x)(7 − x −1 ) , h(x) = (x dx x 5. f (x) =x=4 x −2 x . Then Let f (x) = x−2

SOLUTION

f (x) =

(x − 2) ddx x − x ddx (x − 2) (x − 2) − x −2 = = . (x − 2)2 (x − 2)2 (x − 2)2

dg t2 + 1 x+ , g(t) =4 2 (x) = dt ft=−2 x 2 + x +t 1− 1 t2 + 1 SOLUTION Let g(t) = . Then t2 − 1 7.

d (t 2 + 1) − (t 2 + 1) d (t 2 − 1) (t 2 − 1) dt (t 2 − 1)(2t) − (t 2 + 1)(2t) 4t dg dt = =− 2 . = dt (t 2 − 1)2 (t 2 − 1)2 (t − 1)2

Therefore, 4(−2) 8 dg =− = . dt t=−2 9 ((−2)2 − 1)2 dw

, w= √

z2

S E C T I O N 3.3

Product and Quotient Rules

101

In Exercises 9–12, calculate the derivative in two ways: First use the Product or Quotient Rule, then rewrite the function algebraically and apply the Power Rule directly. 9. f (t) = (2t + 1)(t 2 − 2) SOLUTION

Let f (t) = (2t + 1)(t 2 − 2). Then, using the Product Rule, f (t) = (2t + 1)(2t) + (t 2 − 2)(2) = 6t 2 + 2t − 4.

Multiplying out ﬁrst, we ﬁnd f (t) = 2t 3 + t 2 − 4t − 2. Therefore, f (t) = 6t 2 + 2t − 4. x 3 + 2x 2 + 3x −1 11. g(x)f (x) = = x 2 (3 + x −1 ) x SOLUTION

3 2 −1 Let g(x) = x +2x x+3x . Using the quotient rule and the sum and power rules, and simplifying

g (x) =

x(3x 2 + 4x − 3x −2 ) − (x 3 + 2x 2 + 3x −1 )1 1 3 2 − 6x −1 = 2x + 2 − 6x −3 . 2x = + 2x x2 x2

Simplifying ﬁrst yields g(x) = x 2 + 2x + 3x −2 , from which we calculate g (x) = 2x + 2 − 6x −3 . In Exercises 13–32, calculate the derivative using the appropriate rule or combination of rules. t2 − 1 h(t) = 4 1 2 + x + 1) 13. f (x) = (x t−−4)(x

SOLUTION Let f (x) = x 4 − 4 x 2 + x + 1 . Then f (x) = (x 4 − 4)(2x + 1) + (x 2 + x + 1)(4x 3 ) = 6x 5 + 5x 4 + 4x 3 − 8x − 4. 15.

d y 1 2 + 9)(x + x −1 ) (x), = y(x= d x fx=2 x +4

SOLUTION

1 . Using the quotient rule: Let y = x+4

1 (x + 4)(0) − 1(1) dy =− . = dx (x + 4)2 (x + 4)2 Therefore, d y 1 1 =− =− . d x x=2 36 (2 + 4)2 √ √ 17. f (x) = ( x + 1)( x − 1) x dz z == 2(√x + 1)(√x − 1). Multiplying through ﬁrst yields f (x) = x − 1 for x ≥ 0. Therefore, SOLUTION Let , f (x) d x x=−1 x +1 f (x) = 1 for x ≥ 0. If we carry out the product rule on f (x) = (x 1/2 + 1)(x 1/2 − 1), we get 1 −1/2 1 −1/2 1 1 1 1 f (x) = (x 1/2 + 1) ) + (x 1/2 − 1) (x x = + x −1/2 + − x −1/2 = 1. 2 2 2 2 2 2 √ x4 − 4 d y 2 , y3 =x − (x) = d x fx=2 x2 − 5 x x4 − 4 SOLUTION Let y = . Then x2 − 5

x 2 − 5 4x 3 − x 4 − 4 (2x) 2x 5 − 20x 3 + 8x dy = . = 2 2 dx x2 − 5 x2 − 5

19.

Therefore, 2(2)5 − 20(2)3 + 8(2) d y = = −80. d x x=2 (22 − 5)2 21.

dz 1 4 + 2x + 1 , zx = d x fx=1 x3 + 1 (x) = x +1

102

CHAPTER 3

D I F F E R E N T I AT I O N SOLUTION

Let z =

1 . Using the quotient rule: x 3 +1

3x 2 (x 3 + 1)(0) − 1(3x 2 ) dz = − . = dx (x 3 + 1)2 (x 3 + 1)2 Therefore, 3(1)2 3 dz = − =− . 3 2 d x x=1 4 (1 + 1) t 23. h(t) = 4 3x 32 − 7x 2 + 2 (t + t )(t f (x) = √ + 1) x t t = SOLUTION Let h(t) = . Then t4 + t2 t7 + 1 t 11 + t 9 + t 4 + t 2

t 11 + t 9 + t 4 + t 2 (1) − t 11t 10 + 9t 8 + 4t 3 + 2t 10t 11 + 8t 9 + 3t 4 + t 2 =− h (t) = 2 2 . t 11 + t 9 + t 4 + t 2 t 11 + t 9 + t 4 + t 2 4 25. f (t) = 31/2 ·3/2 51/2 − 3x 2x √ f (x) = x √ + x −1/2 SOLUTION Let f (t) = 3 5. Then f (t) = 0, since f (t) is a constant function! 27. f (x) = (x + 3)(x − 1)(x − 5) h(x) = π 2 (x − 1) SOLUTION Let f (x) = (x + 3)(x − 1)(x − 5). Using the Product Rule inside the Product Rule with a ﬁrst factor of (x + 3) and a second factor of (x − 1)(x − 5), we ﬁnd f (x) = (x + 3) ((x − 1)(1) + (x − 5)(1)) + (x − 1)(x − 5)(1) = 3x 2 − 6x − 13. Alternatively,

f (x) = (x + 3) x 2 − 6x + 5 = x 3 − 3x 2 − 13x + 15. Therefore, f (x) = 3x 2 − 6x − 13. z2 − 1 z2 − 4 (x) = x(x 2 + 1)(x + 4) 29. g(z)f = z−1 z+2 SOLUTION

Hint: Simplify ﬁrst.

Let g(z) =

z2 − 4 z−1

z2 − 1 z+2

=

(z + 2)(z − 2) (z + 1)(z − 1) z−1 z+2

= (z − 2)(z + 1)

for z = −2 and z = 1. Then, g (z) = (z + 1)(1) + (z − 2)(1) = 2z − 1. d xt − 4 d (x constant) 2 (ax (a, b constants) dt t 2 − x + b)(abx + 1) dx xt−4 SOLUTION Let f (t) = 2 . Using the quotient rule:

31.

t −x

f (t) =

(t 2 − x)(x) − (xt − 4)(2t) xt 2 − x 2 − 2xt 2 + 8t −xt 2 + 8t − x 2 = = . (t 2 − x)2 (t 2 − x)2 (t 2 − x)2

x the derivative over [−4, 4]. Use the graph to determine the intervals on which f (x) > of f (x) = 2 d Plot ax + b x +1 (a, b, c, d constants) x

f (x) =

(x 2 + 1)(1) − x(2x) 1 − x2 = . (x 2 + 1)2 (x 2 + 1)2

The derivative is shown in the ﬁgure below at the left. From this plot we see that f (x) > 0 for −1 < x < 1 and f (x) < 0 for |x| > 1. The original function is plotted in the ﬁgure below at the right. Observe that the graph of f (x) is increasing whenever f (x) > 0 and that f (x) is decreasing whenever f (x) < 0.

Product and Quotient Rules

S E C T I O N 3.3 y

y 1 0.8 0.6 0.4 0.2

0.4 0.2

1

2

3

x

−4 −3 −2 −1 − 0.2

x −4 −3 −2 −1

103

1

2

3

4

−0.4

4

V2R 5. Calculate dP/dr, assuming that rthe is plot variable and R is whether constant. f (x) is positive 35. Let P Plot = f (x) =2 as xin Example (in a suitably bounded viewing box). Use to determine (R + r ) x 2 − 1 or negative on its {x :constant. x = ±1}.Let Then compute f (x) and conﬁrm your conclusion algebraically. SOLUTION Note thatdomain V is also f (r ) =

V2R V2R = 2 . 2 (R + r ) R + 2Rr + r 2

Using the quotient rule: f (r ) =

(R 2 + 2Rr + r 2 )(0) − (V 2 R)(2R + 2r ) 2V 2 R(R + r ) 2V 2 R = − = − . (R + r )4 (R + r )4 (R + r )3

x at volts), x = 2. and resistance R (in ohms) in a circuit are 37. FindAccording an equation of the tangent line to Ithe y= to Ohm’s Law, current (ingraph amps), voltage V (in x + x −1 related by I = V /R. x . The equation of the tangent line to the graph of f (x) at x = 2 is y = f (2)(x − 2) + SOLUTION Let f (x) = d I x+x −1 , assuming that V has the constant value V = 24. Calculate f (2).(a) Using the quotient rule, we compute: d R R=6

dV 2 (x+ x −1 )(1) − (x)(1 − x −2 ) 1 −1 − x + x −1 = (b) Calculate , assuming that I has the=constant value Ix = 4. + x . f (x) = d R R=6 (x + x −1 )2 (x + x −1 )2 x(x + x −1 )2 2 = 4 . This makes the equation of the tangent line Therefore, f (2) = 25 25 2

y=

2 12 4 4 (x − 2) + 5 = x+ . 25 25 5 2

39. Let f (x) = g(x) = x. Show that ( f /g) = f /g . 1 = 0.ofOn The curve Agnesi (Figure 3) (after Maria Agnesi SOLUTION ( f /g)y = (x/x) =is 1,called so ( fthe /g)witch the other hand, f /g the ) =Italian (x /x mathematician ) = (1/1) = 1. We see that x2 + 1 0 = 1. (1718–1799) 3who wrote one of the ﬁrst books on calculus. This strange name is the result of a mistranslation of the = 3 f 2 f . 41. Show that ( fla)versiera, Italian meaning “that( which Useword the Product Rule to show that f 2 ) =turns.” 2 f f . Find equations of the tangent lines at x = ±1. SOLUTION Let g = f 3 = f f f . Then

g = f 3 = [ f ( f f )] = f f f + f f + f f ( f ) = 3 f 2 f . In Exercises 42–45, use the following function values: f (4)

f (4)

g(4)

g (4)

2

−3

5

−1

43. Calculate F (4), where F(x) = x f (x). Find the derivative of fg and f /g at x = 4. SOLUTION Let F(x) = x f (x). Then F (x) = x f (x) + f (x), and F (4) = 4 f (4) + f (4) = 4(−3) + 2 = −10. x 45. Calculate H (4), where H (x) = = xg(x) f .(x). where G(x) Calculate G (4), g(x) f (x) x SOLUTION Let H (x) = . Then g(x) f (x)

g(x) f (x) · 1 − x g(x) f (x) + f (x)g (x) . H (x) = (g(x) f (x))2 Therefore, H (4) =

(5)(2) − 4 · ((5)(−3) + (2)(−1)) ((5)(2))2

=

78 39 = . 100 50

104

CHAPTER 3

D I F F E R E N T I AT I O N

47. Proceed as in Exercise 46 to calculate F (0), where Calculate F (0), where 3x 5 + 5x 4 + 5x + 1 9+ x 5/3 F(x) = 1 + x + x 4/3x+ x 8 + 4x 5 9− 7x 4 8x − 7x + 1 F(x) = 4 x − 3x 2 + 2x + 1 SOLUTION Write F(x) = f (x)(g(x)/ h(x)), where Hint: Do not calculate F (x). Instead, write F(x) = f (x)/g(x) and express F (0) directly in terms of f (0), f (0), g(0), g (0). f (x) = (1 + x + x 4/3 + x 5/3 ) g(x) = 3x 5 + 5x 4 + 5x + 1 and h(x) = 8x 9 − 7x 4 + 1. 1 2 Now, f (x) = 1 + 43 x 3 + 53 x 3 , g (x) = 15x 4 + 20x 3 + 5, and h (x) = 72x 8 − 28x 3 . Moreover, f (0) = 1, f (0) = 1, g(0) = 1, g (0) = 5, h(0) = 1, and h (0) = 0. From the product and quotient rules,

F (0) = f (0)

h(0)g (0) − g(0)h (0) 1(5) − 1(0) + f (0)(g(0)/ h(0)) = 1 + 1(1/1) = 6. 2 1 h(0)

Further Insights and Challenges 49. Prove the Quotient Rule using the limit deﬁnition of the derivative. Let f , g, h be differentiable functions. Show that ( f gh) (x) is equal to f . Suppose that f and g are differentiable at x = a and that g(a) = 0. Then SOLUTION Let p = g f (x)g(x)h (x) + f (x)g (x)h(x) + f (x)g(x)h(x) f (a + h) f (a + h)g(a) − f (a)g(a + h) f (a) Hint: Write f gh as f (gh). − p(a + h) − p(a) g(a + h) g(a) g(a + h)g(a) p (a) = lim = lim = lim h h h h→0 h→0 h→0 f (a + h)g(a) − f (a)g(a) + f (a)g(a) − f (a)g(a + h) hg(a + h)g(a) 1 f (a + h) − f (a) g(a + h) − g(a) = lim g(a) − f (a) h h h→0 g(a + h)g(a) 1 f (a + h) − f (a) g(a + h) − g(a) = lim g(a) lim − f (a) lim h h h→0 g(a + h)g(a) h→0 h→0

= lim

h→0

=

1 (g(a))2

In other words, p =

g(a) f (a) − f (a)g (a) g(a) f (a) − f (a)g (a) = (g(a))2

f g f − f g = . g g2

51. Derive the Quotient Rule using Eq. (6) and the Product Rule. Derivative of the Reciprocal Use the limit deﬁnition to show that if f (x) is differentiable and f (x) = 0, then f (x) 1 SOLUTION h(x) = g(x) 1/ f (x) is Let differentiable and. We can write h(x) = f (x) g(x) . Applying Eq. (6), 1 f (x) 1 1 d g−(x) − f (x)g (x) + f (x)g(x) f (x) = h (x) = f (x) . = − f (x) = + f (x) + 2 f 2(x) g(x) g(x) g(x) d x f (x) (g(x)) (g(x))2 Hint: Show that the difference quotient for 1/ f (x) is equal to 53. Carry out the details of Agnesi’s proof of the Quotient Rule from her book on calculus, published in 1748: Assume Show that Eq. (6) is a special case of the Quotient Rule. − f (x of + h) that f , g, and h = f /g are differentiable. Compute thef (x) derivative hg = f using the Product Rule and solve for h . h f (x) f (x + h) SOLUTION Suppose that f , g, and h are differentiable functions with h = f /g. • Then hg = f and via the product rule hg + gh = f .

f − hg • Solving for h yields h = = g

f − g

f g g

=

g f − f g . g2

In Exercises 55–56, let f (x) be a polynomial. A basic fact of algebra states that c is a root of f (x) if and only if The Power Rule Revisited If you are familiar with proof by induction, use induction to2prove the Power Rule h(x), where h(x) is a f (x) = (x − c)g(x) for some polynomial g(x). We say that c is a multiple root if f (x)n = (x − c) for all whole numbers n. Show that the Power Rule holds for n = 1, then write x as x · x n−1 and use the Product polynomial. Rule. 55. Show that c is a multiple root of f (x) if and only if c is a root of both f (x) and f (x).

S E C T I O N 3.4

Rates of Change

105

SOLUTION Assume ﬁrst that f (c) = f (c) = 0 and let us show that c is a multiple root of f (x). We have f (x) = (x − c)g(x) for some polynomial g(x) and so f (x) = (x − c)g (x) + g(x). However, f (c) = 0 + g(c) = 0, so c is also a root of g(x) and hence g(x) = (x − c)h(x) for some polynomial h(x). We conclude that f (x) = (x − c)2 h(x), which shows that c is a multiple root of f (x). Conversely, assume that c is a multiple root. Then f (c) = 0 and f (x) = (x − c)2 g(x) for some polynomial g(x). Then f (x) = (x − c)2 g (x) + 2g(x)(x − c). Therefore, f (c) = (c − c)2 g (c) + 2g(c)(c − c) = 0.

57. Figure 4 55 is the graph of awhether polynomial at A, B, and of these is a multiple root? Explain Use Exercise to determine c = with −1 isroots a multiple root ofC. theWhich polynomials 5 4 3 2 your(a) reasoning using the result of Exercise 55. x + 2x − 4x − 8x − x + 2 (b) x 4 + x 3 − 5x 2 − 3x + 2 y

x A

B

C

FIGURE 4 SOLUTION A on the ﬁgure is a multiple root. It is a multiple root because f (x) = 0 at A and because the tangent line to the graph at A is horizontal, so that f (x) = 0 at A. For the same reasons, f also has a multiple root at C.

3.4 Rates of Change Preliminary Questions 1. What units might be used to measure the ROC of: (a) Pressure (in atmospheres) in a water tank with respect to depth? (b) The reaction rate of a chemical reaction (the ROC of concentration with respect to time), where concentration is measured in moles per liter? SOLUTION

(a) The rate of change of pressure with respect to depth might be measured in atmospheres/meter. (b) The reaction rate of a chemical reaction might be measured in moles/(liter·hour). 2. Suppose that f (2) = 4 and the average ROC of f between 2 and 5 is 3. What is f (5)? SOLUTION

The average rate of change of f between 2 and 5 is given by f (5) − 4 f (5) − f (2) = . 5−2 3

Setting this equal to 3 and solving for f (5) yields f (5) = 13. 3. Two trains travel from New Orleans to Memphis in 4 hours. The ﬁrst train travels at a constant velocity of 90 mph, but the velocity of the second train varies. What was the second train’s average velocity during the trip? SOLUTION

Since both trains travel the same distance in the same amount of time, they have the same average velocity:

90 mph. 4. Estimate f (26), assuming that f (25) = 43 and f (25) = 0.75. SOLUTION

f (x) ≈ f (25) + f (25)(x − 25), so f (26) ≈ 43 + 0.75(26 − 25) = 43.75.

5. The population P(t) of Freedonia in 1933 was P(1933) = 5 million. (a) What is the meaning of the derivative P (1933)? (b) Estimate P(1934) if P (1933) = 0.2. What if P (1933) = 0? SOLUTION

(a) Because P(t) measures the population of Freedonia as a function of time, the derivative P (1933) measures the rate of change of the population of Freedonia in the year 1933. (b) P(1934) ≈ P(1933) + P (1933). Thus, if P (1933) = 0.2, then P(1934) ≈ 5.2 million. On the other hand, if P (1933) = 0, then P(1934) ≈ 5 million.

CHAPTER 3

D I F F E R E N T I AT I O N

Exercises 1. Find the ROC of the area of a square with respect to the length of its side s when s = 3 and s = 5. Let the area be A = f (s) = s 2 . Then the rate of change of A with respect to s is d/ds(s 2 ) = 2s. When s = 3, the area changes at a rate of 6 square units per unit increase. When s = 5, the area changes at a rate of 10 square units per unit increase. (Draw a 5 × 5 square on graph paper and trace the area added by increasing each side length by 1, excluding the corner, to see what this means.) SOLUTION

3. FindFind the the ROC of yof=the x −1 with respect to with x forrespect x = 1, to 10.the length of its side s when s = 3 and s = 5. ROC volume of a cube 1 . −2 SOLUTION The ROC of change is d y/d x = −x . If x = 1, the ROC is −1. If x = 10, the ROC is − 100 √ In Exercises 5–8,rate calculate the ROC. At what is the cube root 3 x changing with respect to x when x = 1, 8, 27? dV 5. , where V is the volume of a cylinder whose height is equal to its radius (the volume of a cylinder of height h dr and radius r is π r 2 h) SOLUTION

The volume of the cylinder is V = π r 2 h = π r 3 . Thus d V /dr = 3π r 2 .

4 3 7. ROC of the volume V of (the volume ROC of the volume V aofsphere a cubewith withrespect respecttotoitsitsradius surface area A of a sphere is V = 3 π r ) SOLUTION

The volume of a sphere of radius r is V = 43 π r 3 . Thus d V /dr = 4π r 2 .

In Exercises d A 9–10, refer to Figure 10, which shows the graph of distance (in kilometers) versus time (in hours) for2 a car , where A is the surface area of a sphere of diameter D (the surface area of a sphere of radius r is 4π r ) trip. dD Distance 150 (km) 100 50 0.5

1

1.5

2

2.5

3

Time (hours)

FIGURE 10 Graph of distance versus time for a car trip.

9. (a) Estimate the average velocity over [0.5, 1]. (b) Is average velocity greater over [1, 2] or [2, 3]? (c) At what time is velocity at a maximum? SOLUTION

(a) The average velocity over the interval [.5, 1] is 50 − 25 = 50 km/h. 1 − .5 (b) The average velocity over the interval [1, 2], 75 − 50 = 25 km/h, 2−1 which is less than that over the interval [2, 3], 150 − 75 = 75 km/h. 3−2 (c) The car’s velocity is maximum when the slope of the distance versus time curve is most positive. This appears to happen when t = 0.5 h, t = 1.25 h, or t = 2.5 h. 11. Figure 11 displays the voltage across a capacitor as a function of time while the capacitor is being charged. Estimate Match the description with the interval (a)–(d). the ROC of voltage at t = 20 s. Indicate the values in your calculation and include proper units. Does voltage change (i) Velocity increasing more quickly or more slowly as time goes on? Explain in terms of tangent lines. (ii) Velocity decreasing 4 (iii) Velocity negative 3 (iv) Average velocity of 50 kph 2 (a) [0, 0.5] 1 (b) [0, 1] (c) [1.5, 2] 10 20 30 40 (d) [2.5, 3] Time (s) Voltage (V)

106

FIGURE 11

S E C T I O N 3.4

Rates of Change

107

SOLUTION The tangent line sketched in the ﬁgure below appears to pass through the points (10, 3) and (30, 4). Thus, the ROC of voltage at t = 20 seconds is approximately

4−3 = 0.05 V/s. 30 − 10 As we move to the right of the graph, the tangent lines to it grow shallower, indicating that the voltage changes more slowly as time goes on. y 4 3 2 1

x 10

13. (a) (b) (c)

20

30

40

A stone is tossed vertically upward with an initial velocity of 25 ft/s from the top of a 30-ft building. Use Figure 12 to estimate d T /dh at h = 30 and 70, where T is atmospheric temperature (in degrees Celsius) What height the stone after 0.25iss?d T /dh equal to zero? and h is is the altitude (inofkilometers). Where Find the velocity of the stone after 1 s. When does the stone hit the ground?

1 SOLUTION We employ Galileo’s formula, s(t) = s0 + v0 t − 2 gt 2 = 30 + 25t − 16t 2 , where the time t is in seconds (s) and the height s is in feet (ft). (a) The height of the stone after .25 seconds is s(.25) = 35.25 ft. (b) The velocity at time t is s (t) = 25 − 32t. When t = 1, this is −7 ft/s. √ −25 ± 2545 2 or (c) When the stone hits the ground, its height is zero. Solve 30 + 25t − 16t = 0 to obtain t = −32 t ≈ 2.36 s. (The other solution, t ≈ −0.79, we discard since it represents a time before the stone was thrown.)

30tft.+Find 340 15. TheThe temperature an object (in degrees Fahrenheit) as a after function of time minutes) T (t) = 34 t 2−−15t height (inof feet) of a skydiver at time t (in seconds) opening his (in parachute is is h(t) = 2,000 for 0the ≤ tskydiver’s ≤ 20. At velocity what rateafter doesthe theparachute object cool after 10 min (give correct units)? opens. SOLUTION

Let T (t) = 34 t 2 − 30t + 340, 0 ≤ t ≤ 20. Then T (t) = 32 t − 30, so T (10) = −15◦ F/min.

2.99 × 1016 velocity centimetersforce per second) ﬂowing through a capillary radius 0.008 cm is 17. TheThe earth exerts a(in gravitational of F(r ) of = a blood2 molecule (in Newtons) on an object with a of mass of 75 kg, where r r is the distance from the molecule to the center of the given by the formula v = 6.4 × 10−8 − 0.001r 2 , where r is the distance (inthe meters) the center the earth. Find thewhen ROC rof=force with capillary. Find ROCfrom of velocity as a of function of distance 0.004 cm.respect to distance at the surface of the earth, assuming the radius of the earth is 6.77 × 106 m. SOLUTION

The rate of change of force is F (r ) = −5.98 × 1016 /r 3 . Therefore, F (6.77 × 106 ) = −5.98 × 1016 /(6.77 × 106 )3 = −1.93 × 10−4 N/m.

2.25R 7 −1/2 19. TheThe power delivered byata abattery to ranmeters apparatus R (in ohms) is P= = W. Find rate of (2.82 m/s.the Calculate escape velocity distance fromof theresistance center of the earth is vesc 2 (R × + 10 0.5))r the of rate at which with respect surface of the earth. esc changes change power withvrespect to resistance for Rto=distance 3 and Rat=the 5 . SOLUTION

P (R) =

(R + .5)2 2.25 − 2.25R(2R + 1) . (R + .5)4

Therefore, P (3) = −0.1312 W/ and P (5) = −0.0609 W/. 21. By Faraday’s Law, if a conducting wire of length meters moves at velocity v m/s perpendicular to a magnetic ﬁeld 2 − t + 10 cm. TheBposition of a aparticle line duringina 5-s s(t) = t that of strength (in teslas), voltagemoving of size in V a=straight −Bv is induced the trip wire.isAssume B = 2 and = 0.5. (a) What is the averagedvelocity (a) Find the rate of change V /dv. for the entire trip? (b) Isthe there at which instantaneous velocity to this average velocity? If so, ﬁnd it. (b) Find rateaoftime change of V the with respect to time t if v is = equal 4t + 9. SOLUTION

(a) Assuming that B = 2 and l = 0.5, V = −2(.5)v = −v. Therefore, dV = −1. dv (b) If v = 4t + 9, then V = −2(.5)(4t + 9) = −(4t + 9). Therefore, ddtV = −4. The height (in feet) of a helicopter at time t (in minutes) is s(t) = −3t 3 + 400t for 0 ≤ t ≤ 10.

D I F F E R E N T I AT I O N

23. The population P(t) of a city (in millions) is given by the formula P(t) = 0.00005t 2 + 0.01t + 1, where t denotes the number of years since 1990. (a) How large is the population in 1996 and how fast is it growing? (b) When does the population grow at a rate of 12,000 people per year? SOLUTION

Let P(t) = (0.00005)t 2 + (0.01)t + 1 be the population of a city in millions. Here t is the number of years

past 1990. (a) In 1996 (t = 6 years after 1990), the population is P(6) = 1.0618 million. The rate of growth of population is P (t) = 0.0001t + 0.01. In 1996, this corresponds to a growth rate of P (6) = 0.0106 million per year or 10,600 people per year. (b) When the growth rate is 12,000 people per year (or 0.012 million per year), we have P (t) = 0.0001t + 0.01 = 0.012. Solving for t gives t = 20. This corresponds to the year 2010; i.e., 20 years past 1990. Ethan to ﬁnds that with hours of tutoring, he isI able answer correctly S(h) percent of thebyproblems on a 25. According Ohm’s Law,hthe voltage V , current , andtoresistance R in a circuit are related the equation (h)? Which would you expect to be larger: S (3) or S (30)? Explain. mathVexam. What is the the derivative = I R, where themeaning units areofvolts, amperes, Sand ohms. Assume that voltage is constant with V = 12 V. Calculate (specifying thederivative units): S (h) measures the rate at which the percent of problems Ethan answers correctly changes SOLUTION The The to average ROC of hours I withof respect to R the interval from R = 8 to R = 8.1 with (a) respect the number tutoring hefor receives. One of S(h) is shown in the Rﬁgure (b) possible The ROCgraph of I with respect to R when = 8 below on the left. This graph indicates that in the early hours of working withROC the tutor, makes to rapid progress (c) The of R Ethan with respect I when I = in 1.5learning the material but eventually approaches either the limit of his ability to learn the material or the maximum possible score on the exam. In this scenario, S (3) would be larger than S (30). An alternative graph of S(h) is shown below on the right. Here, in the early hours of working with the tutor little progress is made (perhaps the tutor is assessing how much Ethan already knows, his learning style, his personality, etc.). This is followed by a period of rapid improvement and ﬁnally a leveling off as Ethan reaches his maximum score. In this scenario, S (3) and S (30) might be roughly equal. Percentage correct

CHAPTER 3

Percentage correct

108

Hours of tutoring

Hours of tutoring

Table 2 gives the total month 1999 as determined Department of 27. Suppose θ (t) measures the U.S. anglepopulation between a during clock’seach minute andof hour hands. What is θ by (t)the at 3U.S. o’clock? Commerce. (a) Estimate P (t) for each of the months January–November. (b) Plot these data points for P (t) and connect the points by a smooth curve. (c) Write a newspaper headline describing the information contained in this plot. Total U.S. Population in 1999

TABLE 2

t

P(t) in Thousands

January February March April May June July August September October November December

271,841 271,987 272,142 272,317 272,508 272,718 272,945 273,197 273,439 273,672 273,891 274,076

The table in the text gives the growing population P(t) of the United States. (a) Here are estimates of P (t) in thousands/month for January–November. The estimates are computed using the estimate f (t + 1) − f (t). SOLUTION

t P (t)

Jan

Feb Mar Apr May Jun

146 155

175

191

210

Jul

227 252

Aug Sep Oct Nov 242

233 219

185

S E C T I O N 3.4

Rates of Change

109

(b) Here is a plot of these estimates (1 = Jan, 2 = Feb, etc.) P 250 200 150 100 50 t 2

4

6

8

10

(c) “U.S. Growth Rate Declines After Midsummer Peak” 29. According to a formula widely used by doctors√to determine drug dosages, a person’s body surface area (BSA) (in steeper as xh increases. At what rate do the and slopes tangent The tangent lines to of f BSA (x) ==x 2 grow hw/60, where is the height in centimeters w of thethe weight in meters squared) is given bythe thegraph formula lines increase? kilograms. Calculate the ROC of BSA with respect to weight for a person of constant height h = 180. What is this ROC for w = 70 and w = 80? Express your result in the correct units. Does BSA increase more rapidly with respect to weight at lower or higher body weights? √ √ 5 SOLUTION Assuming constant height h = 180 cm, let f (w) = hw/60 = 10 w be the formula for body surface area in terms of weight. The ROC of BSA with respect to weight is √ √ 5 1 −1/2 5 w = √ . f (w) = 10 2 20 w If w = 70 kg, this is f (70) =

√

√ 5 14 m2 ≈ 0.0133631 . √ = 280 kg 20 70

If w = 80 kg, √

5 1 1 m2 . √ = √ = 80 kg 20 80 20 16 √ Because the rate of change of BSA depends on 1/ w, it is clear that BSA increases more rapidly at lower body weights. f (80) =

31. What is the velocity of an object dropped from a height of 300 m when it hits the ground? A slingshot is used to shoot a pebble in the air vertically from ground level with an initial velocity 200 m/s. Find SOLUTION Wemaximum employ Galileo’s s(t) = s0 + v0 t − 12 gt 2 = 300 − 4.9t 2 , where the time t is in seconds (s) the pebble’s velocityformula, and height. and the height s is in meters (m). When the ball hits the ground its height is 0. Solve s(t) = 300 − 4.9t 2 = 0 to obtain t ≈ 7.8246 s. (We discard the negative time, which took place before the ball was dropped.) The velocity at impact is v(7.8246) = −9.8(7.8246) ≈ −76.68 m/s. This signiﬁes that the ball is falling at 76.68 m/s. 33. A ball is tossed up vertically from ground level and returns to earth 4 s later. What was the initial velocity of the It takes a stone 3 s to hit the ground when dropped from the top of a building. How high is the building and what stone and how high did it go? is the stone’s velocity upon impact? 1 SOLUTION Galileo’s formula gives s(t) = s0 + v0 t − 2 gt 2 = v0 t − 4.9t 2 , where the time t is in seconds (s) and the height s is in meters (m). When the ball hits the ground after 4 seconds its height is 0. Solve 0 = s(4) = 4v0 − 4.9(4)2 to obtain v0 = 19.6 m/s. The stone reaches its maximum height when s (t) = 0, that is, when 19.6 − 9.8t = 0, or t = 2 s. At this time, t = 2 s, 1 s(2) = 0 + 19.6(2) − (9.8)(4) = 19.6 m. 2 35. A man on the tenth ﬂoor of a building sees a bucket (dropped by a window washer) pass his window and notes that An object is tossed up vertically from ground level and hits the ground T s later. Show that its maximum height it hits the ground 1.5 s later. Assuming a ﬂoor is 16 ft high (and neglecting air friction), from which ﬂoor was the bucket was reached after T /2 s. dropped? A falling object moves 16t 2 feet in t seconds. Suppose H is the unknown height from which the bucket fell starting at time zero. The man saw it at some time t (also unknown to us) and it hit the ground, 160 feet down at time t + 1.5. Thus SOLUTION

(H − 16t 2 ) − (H − 16(t + 1.5)2 ) = 160. The H ’s cancel as do the t 2 terms and solving for t gives t = 31/12 s. Thus the bucket fell 16(31/12)2 ≈ 16 · 6.67 feet before the man saw it. Since there are 16 feet in a ﬂoor the bucket was dropped from 6.67 ﬂoors above the 10th: either the 16th or 17th ﬂoor. 37. Show that for an object rising and falling according to Galileo’s formula in Eq. (3), the average velocity over any Which the following statements is true for an object falling under the inﬂuence of gravity near the surface time interval [t1 , tof 2 ] is equal to the average of the instantaneous velocities at t1 and t2 . of the earth? Explain. (a) The object covers equal distance in equal time intervals.

110

CHAPTER 3

D I F F E R E N T I AT I O N SOLUTION

The simplest way to proceed is to compute both values and show that they are equal. The average velocity

over [t1 , t2 ] is (s0 + v0 t2 − 12 gt22 ) − (s0 + v0 t1 − 12 gt12 ) v0 (t2 − t1 ) + g2 (t2 2 − t1 2 ) s(t2 ) − s(t1 ) = = t2 − t 1 t2 − t1 t2 − t 1 v (t − t1 ) g g = 0 2 − (t2 + t1 ) = v0 − (t2 + t1 ) t2 − t 1 2 2 Whereas the average of the instantaneous velocities at the beginning and end of [t1 , t2 ] is 1 g g s (t1 ) + s (t2 ) 1

= (v0 − gt1 ) + (v0 − gt2 ) = (2v0 ) − (t2 + t1 ) = v0 − (t2 + t1 ). 2 2 2 2 2 The two quantities are the same. In Exercises 39–46, use Eq.up (2)and to estimate A weight oscillates down at the the unit end change. of a spring. Figure 13 shows the height y of the weight through one √ oscillation. √ √ √ sketch of the graph of the velocity as a function of time. cycle of the Make a rough 39. Estimate 2 − 1 and 101 − 100. Compare your estimates with the actual values. √ SOLUTION Let f (x) = x = x 1/2 . Then f (x) = 12 (x −1/2 ). We are using the derivative to estimate the average ROC. That is, √ √ x +h− x ≈ f (x), h so that √

√

x ≈ h f (x). √ √ √ Thus, 2 − 1 ≈ 1 f (1) = 12 (1) = 12 . The actual value, to six decimal places, is 0.414214. Also, 101 − 100 ≈

1 = .05. The actual value, to six decimal places, is 0.0498756. 1 f (100) = 12 10 √

x +h−

as in Example 3. Calculate F(65) and estimate the increase in 41. Let F(s) = 1.1s + 0.03s 2 be the stopping distance −x Suppose that f (x) is a function with f (x) = 2 . Estimate f (7) − f (6). Then estimate f (5), assuming that stopping distance if speed is increased from 65 to 66 mph. Compare your estimate with the actual increase. f (4) = 7. SOLUTION Let F(s) = 1.1s + .03s 2 be as in Example 3. F (s) = 1.1 + 0.06s. • Then F(65) = 198.25 ft and F (65) = 5.00 ft/mph. • F (65) ≈ F(66) − F(65) is approximately equal to the change in stopping distance per 1 mph increase in speed

when traveling at 65 mph. Increasing speed from 65 to 66 therefore increases stopping distance by approximately 5 ft. • The actual increase in stopping distance when speed increases from 65 mph to 66 mph is F(66) − F(65) = 203.28 − 198.25 = 5.03 feet, which differs by less than one percent from the estimate found using the derivative. 3 Determine the cost of producing 43. TheAccording dollar costtoofKleiber’s producing x bagels is C(x)rate = 300 0.25x − 0.5(x/1,000) Law, the metabolic P (in+kilocalories per day) and.body mass m (in kilograms) of an 2,000animal bagelsare and estimate cost of the 2001st your estimate the withincrease the actual cost of therate 2001st bagel. . Estimate in metabolic when body related by athe three-quarter power bagel. law P Compare = 73.3m 3/4 SOLUTION Expanding thetopower mass increases from 60 61 kg.of

3 yields C(x) = 300 + .25x − 5 × 10−10 x 3 .

This allows us to get the derivative C (x) = .25 − 1.5 × 10−9 x 2 . The cost of producing 2000 bagels is C(2000) = 300 + 0.25(2000) − 0.5(2000/1000)3 = 796 dollars. The cost of the 2001st bagel is, by deﬁnition, C(2001) − C(2000). By the derivative estimate, C(2001) − C(2000) ≈ C (2000)(1), so the cost of the 2001st bagel is approximately C (2000) = .25 − 1.5 × 10−9 (20002 ) = $.244. C(2001) = 796.244, so the exact cost of the 2001st bagel is indistinguishable from the estimated cost. The function is very nearly linear at this point. 45. The demand for a commodity generally decreases as the price is raised. Suppose that the demand for oil (per capita 2 + 10−8 x 3 . of producing x video cameras is C(x) = 500xFind − 0.003x per year)Suppose is D( p)the = dollar 900/ pcost barrels, where p is the price per barrel in dollars. the demand when p = $40. Estimate (a) Estimate the marginal at production level if x p=is5,000 and to compare the decrease in demand if p risescost to $41 and the increase decreased $39. it with the actual cost C(5,001) − C(5,000). SOLUTION D( p) = 900 p −1 , so D ( p) = −900 p −2 . When the price is $40 a barrel, the per capita demand is (b)=Compare the marginal cost at = 5,000inwith average cost camera, D(40) 22.5 barrels per year. With anxincrease pricethe from $40 to $41per a barrel, thedeﬁned changeasinC(x)/x. demand D(41) − D(40) −2 is approximately D (40) = −900(40 ) = −.5625 barrels a year. With a decrease in price from $40 to $39 a barrel, the change in demand D(39) − D(40) is approximately −D (40) = +.5625. An increase in oil prices of a dollar leads to a decrease in demand of .5625 barrels a year, and a decrease of a dollar leads to an increase in demand of .5625 barrels a year.

S E C T I O N 3.4

Rates of Change

111

of A(s + 1) − melanogaster, A(s) provided grown by Eq.in(2) has error exactly equal to 1. Explain 47. LetThe A =reproduction s 2 . Show that ratethe of estimate the fruit ﬂy Drosophila bottles in a laboratory, decreases as the this result using Figure 14. bottle becomes more crowded. A researcher has found that when a bottle contains p ﬂies, the number of offspring per female per day is f ( p) = (34 − 0.612 p) p −0.658 1 2

(a) Calculate f (15) and f (15). (b) Estimate the decrease in daily offspring per female when p is increased from 15 to 16. Is this estimate larger or s smaller than the actual value f (16) − f (15)? (c) Plot f ( p) for 5 ≤ p ≤ 25 and verify that f ( p) is a decreasing function of p. Do you expect f ( p) to be 14 positive or negative? Plot f ( p) and conﬁrm yourFIGURE expectation. SOLUTION

Let A = s 2 . Then A(s + 1) − A(s) = (s + 1)2 − s 2 = s 2 + 2s + 1 − s 2 = 2s + 1,

while A (s) = 2s. Therefore, regardless of the value of s, A(s + 1) − A(s) − A (s) = (2s + 1) − 2s = 1. To understand this result, consider Figure 14, which illustrates a square of side length s expanded to a square of side length s + 1 by increasing the side length by 1/2 unit in each direction. The difference A(s + 1) − A(s) is the total area between the two squares. On the other hand, A (s) = 2s is the total area of the four rectangles of dimension s by 1/2 bordering the sides of the original square. The error in approximating A(s + 1) − A(s) by A (s) is given by the four 1/2 by 1/2 squares in the corners of the ﬁgure. The total area of these four small squares is always exactly 1. 49. Let M(t) be the mass (in kilograms) of a plant as a function of time (in years). Recent studies by Niklas and Enquist According to aSteven’s Law wide in psychology, the perceived magnitude of to a stimulus (howthe strong a person feels have suggested that for remarkably range of plants (from algae and grass palm trees), growth rate during the span stimulus be) is proportional to a power ofpower the actual I of the Although not an C. exact law, some constant the life of thetoorganism satisﬁes a three-quarter law, intensity that is, d M/dt = stimulus. C2/3 M 3/4 for experiments show that the perceived brightness B of a light satisﬁes B = k I , where I is the light intensity, (a) If a tree has a growth rate of 6 kg/year when M = 100 kg, what is its growth rate when M = 125 kg? whereas the perceived heaviness H of a weight W satisﬁes H = kW 3/2 (k is a constant that is different in the (b) If M = 0.5 kg, how much more mass must the plant acquire to double its growth rate? two cases). Compute d B/d I and d H /d W and state whether they are increasing or decreasing functions. Use this to SOLUTION justify the statements: √ (a) Suppose a tree increase has a growth rate d M/dtisoffelt6 more kg/yr strongly when Mwhen = 100, = C(100 = 10C 10, so that (a)√ A one-unit in light intensity I is then small6than when 3/4 I is)large. 3 10 another pound to a load W is felt more strongly when W is large than when W is small. C =(b) . When M = 125, 50Adding √ dM 3 10 = C(1253/4 ) = 25(51/4 ) = 7.09306. dt 50 (b) The growth rate when M = .5 kg is d M/dt = C(.53/4 ). To double the rate, we must ﬁnd M so that d M/dt = C M 3/4 = 2C(.53/4 ). We solve for M. C M 3/4 = 2C(.53/4 ) M 3/4 = 2(.53/4 ) M = (2(.53/4 ))4/3 = 1.25992. The plant must acquire the difference 1.25992 − .5 = .75992 kg in order to double its growth rate. Note that a doubling of growth rate requires more than a doubling of mass.

Further Insights and Challenges dP 51. TheAs size a certainspreads animalthrough population P(t) at time (in months)p satisﬁes = 0.2(300at−time P). t (in days) satisﬁes anof epidemic a population, the tpercentage of infected individuals dt thePequation a differential (a) Is growing(called or shrinking when Pequation) = 250? when P = 350? (b) Sketch the graph of d P/dt as a functiondof p P for 0 ≤ P ≤2300. = 4 p − 0.06 p 0 ≤ p ≤ 100 (c) Which of the graphs in Figure 15 is the graph dt of P(t) if P(0) = 200? (a) (b) (c) (d)

How fast is the epidemic spreading when p = 10% and Pwhen p = 70%? P For which p is the epidemic 300 neither spreading nor diminishing? 300 Plot d p/dt as a function of p. 200 200 What is the maximum possible rate of increase and for which p does this occur? 100

100 t

t 4

8

4

(A)

8 (B)

FIGURE 15

112

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D I F F E R E N T I AT I O N SOLUTION

Let P (t) = d P/dt = 0.2(300 − P).

(a) Since P (250) = 10, the population is growing when P = 250. Since P (350) = −10, the population is shrinking when P = 350. (b) Here is a graph of d P/dt for 0 ≤ t ≤ 300. dP/dt 60 50 40 30 20 10 50 100 150 200 250 300

P

(c) If P(0) = 200, as in graph (A), then P (0) = 20 > 0 and the population is growing as depicted. Accordingly, graph (A) has the correct shape for P(t). If P(0) = 200, as in graph (B), then P (0) = 20 > 0 and the population is growing, contradicting what is depicted. Thus graph (B) cannot be the correct shape for P(t). In Exercises 53–54, the average cost per unit at production level x is deﬁned as Cavg (x) = C(x)/x, where C(x) is the Studies of internet usage show that website popularity is described quite well by Zipf’s Law, according to cost function. Average cost is a measure of the efﬁciency of the production process. which the nth most popular website receives roughly the fraction 1/n of all visits. Suppose that on a particular day, the nththat most popular hadtoapproximately (n) = 106 /nthe visitors ≤ 15,000). (x) is site equal the slope of theV line through origin(for andn the point (x, C(x)) on the graph of C(x). 53. Show C avg (a)this Verify that the top 50 websites received nearly of the visits. LetatTpoints (N ) denote sum V (n)16. for Using interpretation, determine whether average cost45% or marginal cost isHint: greater A, B, the C, D in of Figure 1 ≤ n ≤ N . Use a computer algebra system to compute T (45) and T (15,000). Cost (b) Verify, by numerical experimentation, that when Eq. (2) is used to estimate V (n + 1) − V (n), the error in the D estimate decreases as n grows larger. Find (again, by experimentation) an N such that the error is at most 10 for n ≥ N. C received at most 100 more visitors than the (n + 1)st web B (c) Using Eq. (2), show that for n ≥ 100, the nth A website site. Production level

FIGURE 16 Graph of C(x). SOLUTION

By deﬁnition, the slope of the line through the origin and (x, C(x)), that is, between (0, 0) and (x, C(x))

is C(x) − 0 C(x) = = Cav . x −0 x At point A, average cost is greater than marginal cost, as the line from the origin to A is steeper than the curve at this point (we see this because the line, tracing from the origin, crosses the curve from below). At point B, the average cost is still greater than the marginal cost. At the point C, the average cost and the marginal cost are nearly the same, since the tangent line and the line from the origin are nearly the same. The line from the origin to D crosses the cost curve from above, and so is less steep than the tangent line to the curve at D; the average cost at this point is less than the marginal cost. The cost in dollars of producing alarm clocks is

3.5 Higher Derivatives

C(x) = 50x 3 − 750x 2 + 3,740x + 3,750

where x is in units of 1,000. (a) Calculate the average cost at x = 4, 6, 8, and 10. 1. An economist who announces that “America’s economic growth is slowing” is making a statement about the gross (b) Use the graphical interpretation of average cost to ﬁnd the production level x0 at which average cost is lowest. national product (GNP) as a function of time. Is the second derivative of the GNP positive? What about the ﬁrst derivative? What is the relation between average cost and marginal cost at x 0 (see Figure 17)? SOLUTION If America’s economic growth is slowing, then the GNP is increasing, but the rate of increase is decreasing. Thus, the second derivative of GNP is negative, while the ﬁrst derivative is positive. FIGURE 17 Cost function C(x) = 50x 3 − 750x 2 + 3,740x + 3,750. 2. On September 4, 2003, the Wall Street Journal printed the headline “Stocks Go Higher, Though the Pace of Their Gains Slows.” Rephrase as a statement about the ﬁrst and second time derivatives of stock prices and sketch a possible graph.

Preliminary Questions

SOLUTION Because stocks are going higher, stock prices are increasing and the ﬁrst derivative of stock prices must therefore be positive. On the other hand, because the pace of gains is slowing, the second derivative of stock prices must be negative.

S E C T I O N 3.5

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113

Stock price

Time

3. Is the following statement true or false? The third derivative of position with respect to time is zero for an object falling to earth under the inﬂuence of gravity. Explain. SOLUTION This statement is true. The acceleration of an object falling to earth under the inﬂuence of gravity is constant; hence, the second derivative of position with respect to time is constant. Because the third derivative is just the derivative of the second derivative and the derivative of a constant is zero, it follows that the third derivative is zero.

4. Which type of polynomial satisﬁes f (x) = 0 for all x? SOLUTION The second derivative of all linear polynomials (polynomials of the form ax + b for some constants a and b) is equal to 0 for all x.

Exercises In Exercises 1–12, calculate the second and third derivatives. 1. y = 14x 2 SOLUTION

Let y = 14x 2 . Then y = 28x, y = 28, and y = 0.

2 25x 3. y =y x=4 − 7− 2x + 2x SOLUTION Let y = x 4 − 25x 2 + 2x. Then y = 4x 3 − 50x + 2, y = 12x 2 − 50, and y = 24x.

4 3 5. y =y =π4t r 3 − 9t 2 + 7 3 SOLUTION

Let y = 43 π r 3 . Then y = 4π r 2 , y = 8π r , and y = 8π .

4/5 − 6t 2/3 7. y = 20t √ y= x 16 4 SOLUTION Let y = 20t 4/5 − 6t 2/3 . Then y = 16t −1/5 − 4t −1/3 , y = − 5 t −6/5 + 3 t −4/3 , and y = 96 t −11/15 − 16 t −7/3 . 25 9

1 9. y =y z=−x −9/5 z SOLUTION

Let y = z − z −1 . Then y = 1 + z −2 , y = −2z −3 , and y = 6z −4 .

11. y = (x 2 + x)(x 3 + 1) y = t 2 (t 2 + t) SOLUTION Since we don’t want to apply the product rule to an ever growing list of products, we multiply through ﬁrst. Let y = (x 2 + x)(x 3 + 1) = x 5 + x 4 + x 2 + x. Then y = 5x 4 + 4x 3 + 2x + 1, y = 20x 3 + 12x 2 + 2, and y = 60x 2 + 24x. In Exercises 13–24, calculate the derivative indicated. 1 y= x = x4 13. f (4) (1), 1 +f (x) Let f (x) = x 4 . Then f (x) = 4x 3 , f (x) = 12x 2 , f (x) = 24x, and f (4) (x) = 24. Thus f (4) (1) = 24. d 2 y 15. = 4t=−3 + 3t 2 4t −3 g (1),, yg(t) dt 2 t=1 SOLUTION

SOLUTION

2 Let y = 4t −3 + 3t 2 . Then ddty = −12t −4 + 6t and d 2y = 48t −5 + 6. Hence

d 2 y dt 2

dt

= 48(1)−5 + 6 = 54. t=1

√ 17. h (9), h(x) = x 4 d f √ f (t)== 6tx9 − SOLUTION4 Let, h(x) = 2tx51/2 . Then h (x) = 12 x −1/2 , h (x) = − 14 x −3/2 , and h (x) = 38 x −5/2 . Thus dt t=1 1 h (9) = 648 . g (9),

g(x) = x −1/2

114

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D I F F E R E N T I AT I O N

d 4 x 19. , dt 4 t=16 SOLUTION

Thus

x = t −3/4

2 −11/4 , d 3 x = − 231 t −15/4 , and d 4 x = 3465 t −19/4 . Let x(t) = t −3/4 . Then ddtx = − 34 t −7/4 , d 2x = 21 16 t 64 256 dt dt 3 dt 4

d 4 x dt 4

=

t=16

3465 −19/4 3465 16 . = 256 134217728

x 21. g (1), g(x) = 2 f (4), f (t)x=+2t 1 −t x SOLUTION Let g(x) = . Then x +1 g (x) =

(x + 1)(1) − (x)(1) 1 1 = = 2 (x + 1)2 (x + 1)2 x + 2x + 1

and g (x) =

(x 2 + 2x + 1)(0) − 1(2x + 2) 2x + 2 2 =− =− . (x 2 + 2x + 1)2 (x + 1)3 (x + 1)4

1 Thus, g (1) = − . 4 1 23. h (1), h(x) = √ 1 1 f (1), f (t) =x + t3 + 1 1 SOLUTION Let h(x) = √ . Then x +1 − 1 x −1/2 − 12 x −1/2 h (x) = √2 , √ 2 = x +2 x +1 x +1 and √

3 x −1/2 + x −1 + 1 x −3/2 x + 1) 14 x −3/2 + 12 x −1/2 (1 + x −1/2 ) 4 4 = √ √ (x + 2 x + 1)2 ( x + 1)4

√ 3 x −1 + 1 x −3/2 ( x + 1) 34 x −1 + 14 x −3/2 4 = = . √ √ 4 ( x + 1)4 ( x + 1)3

(x + 2 h (x) =

= 18 . Accordingly, h (1) = (3/4)+(1/4) 23 25. Calculate y (k) (0) for 0 ≤2 k ≤ 5, where y = x 4 + ax 3 + bx 2 + cx + d (with a, b, c, d the constants). x F(x) =the power, constant multiple, and sum rules at each stage, we get (note y (0) is y by convention): F (2),Applying SOLUTION x −3 k

y (k)

0

x 4 + ax 3 + bx 2 + cx + d

1

4x 3 + 3ax 2 + 2bx + c

2

12x 2 + 6ax + 2b

3

24x + 6a

4

24

5

0

from which we get y (0) (0) = d, y (1) (0) = c, y (2) (0) = 2b, y (3) (0) = 6a, y (4) (0) = 24, and y (5) (0) = 0. d 6 −1 (k) 27. UseWhich the result in Example to ﬁnd satisfy x .f (x) = 0 for all k ≥ 6? of the following2functions dx6 (b) f (x) = x 3 − 2 (a) f (x) = 7x 4 + 4 + x −1 SOLUTION The√equation in Example 2 indicates that (d) f (x) = 1 − x 6 (c) f (x) = x (e) f (x) = x 9/5

d 6 −1 (f) f (x) = 2x 2 + 3x 5 x = (−1)6 6!x −6−1 . 6 dx

S E C T I O N 3.5

Higher Derivatives

115

(−1)6 = 1 and 6! = 6 × 5 × 4 × 3 × 2 × 1 = 720, so d 6 −1 x = 720x −7 . dx6 √ In Exercises 29–32, general formulaoffor f (n) Calculate theﬁnd ﬁrstaﬁve derivatives f (x) =(x).x. (n) −n+1/2 . 29. (a) f (x)Show = (x that + 1)f−1 (x) is a multiple of x (n) 1 as (−1)n−1 for n ≥ 1. −2 −3 (b) ShowLet thatf (x) f (x) SOLUTION = (xalternates + 1)−1 in = sign x+1 . By Exercise 12, f (x) = −1(x + 1) , f (x) = 2(x + 1) , f (x) = 2n − 3 −4 (4) −5 −6(x(c) + Find 1) ,a fformula (x) = . . 2. From we conclude that the nth can (x)1)for ,n. ≥ Hint:this Verify that the coefﬁcient ofderivative x −n+1/2 is ±1be· 3written · 5 · · · as fn(n) (x) . = for24(x f (n)+ 2 n −(n+1) . (−1) n!(x + 1) 31. f (x) = x −1/2 f (x) = x −2 −1 −3/2 SOLUTION f (x) = 2 x . We will avoid simplifying numerators and denominators to ﬁnd the pattern: f (x) =

−3 −1 −5/2 3×1 = (−1)2 2 x −5/2 x 2 2 2

f (x) = −

5 3 × 1 −7/2 5 × 3 × 1 −7/2 x = (−1)3 x 2 22 23

.. . f (n) (x) = (−1)n

(2n − 1) × (2n − 3) × . . . × 1 −(2n+1)/2 x . 2n

33. (a) Find the −3/2 acceleration at time t = 5 min of a helicopter whose height (in feet) is h(t) = −3t 3 + 400t. f (x) = x (b) Plot the acceleration h (t) for 0 ≤ t ≤ 6. How does this graph show that the helicopter is slowing down during this time interval? SOLUTION

(a) Let h(t) = −3t 3 + 400t, with t in minutes and h in feet. The velocity is v(t) = h (t) = −9t 2 + 400 and acceleration is a(t) = h (t) = −18t. Thus a(5) = −90 ft/min2 . (b) The acceleration of the helicopter for 0 ≤ t ≤ 6 is shown in the ﬁgure below. As the acceleration of the helicopter is negative, the velocity of the helicopter must be decreasing. Because the velocity is positive for 0 ≤ t ≤ 6, the helicopter is slowing down. y 1

2

3

4

5

6

x

−20 −40 −60 −80 −100

35. Figure 5 shows f , f , and f . Determine which is which. Find an equation of the tangent to the graph of y = f (x) at x = 3, where f (x) = x 4 . y

y

y

1

2

3

x

1

(A)

2

3

x

1

2

3

x

(C)

(B)

FIGURE 5

(a) f (b) f (c) f . The tangent line to (c) is horizontal at x = 1 and x = 3, where (b) has roots. The tangent line to (b) is horizontal at x = 2 and x = 0, where (a) has roots. 37. Figure 7 shows the graph of the position of an object as a function of time. Determine the intervals on which the The second derivative f is shown in Figure 6. Determine which graph, (A) or (B), is f and which is f . acceleration is positive. SOLUTION

Position

10

20

30

40 Time

FIGURE 7

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CHAPTER 3

D I F F E R E N T I AT I O N SOLUTION Roughly from time 10 to time 20 and from time 30 to time 40. The acceleration is positive over the same intervals over which the graph is bending upward.

+ frespect (x) = xto2 .the length of a side. 39. FindFind a polynomial (x) satisfying equation f (x)with the second fderivative of thethe volume of axcube SOLUTION Since x f (x) + f (x) = x 2 , and x 2 is a polynomial, it seems reasonable to assume that f (x) is a polynomial of some degree, call it n. The degree of f (x) is n − 2, so the degree of x f (x) is n − 1, and the degree of x f (x) + f (x) is n. Hence, n = 2, since the degree of x 2 is 2. Therefore, let f (x) = ax 2 + bx + c. Then f (x) = 2ax + b and f (x) = 2a. Substituting into the equation x f (x) + f (x) = x 2 yields ax 2 + (2a + b)x + c = x 2 , an identity in x. Equating coefﬁcients, we have a = 1, 2a + b = 0, c = 0. Therefore, b = −2 and f (x) = x 2 − 2x. 41. A servomotor controls the vertical movement of a drill bit that will drill a pattern of holes in sheet metal. The the following descriptions could and not apply Figurethe 8? Explain. maximumWhich verticalofspeed of the drill bit is 4 in./s, while to drilling hole, it must move no more than 2.6 in./s (a) Graph of acceleration when velocity to avoid warping the metal. During a cycle, is theconstant bit begins and ends at rest, quickly approaches the sheet metal, and quickly to its initialwhen position after the hole is drilled. Sketch possible graphs of the drill bit’s vertical velocity and (b) returns Graph of velocity acceleration is constant acceleration. Label the point where the bit enters the sheet metal. (c) Graph of position when acceleration is zero SOLUTION There will be multiple cycles, each of which will be more or less identical. Let v(t) be the downward vertical velocity of the drill bit, and let a(t) be the vertical acceleration. From the narrative, we see that v(t) can be no greater than 4 and no greater than 2.6 while drilling is taking place. During each cycle, v(t) = 0 initially, v(t) goes to 4 quickly. When the bit hits the sheet metal, v(t) goes down to 2.6 quickly, at which it stays until the sheet metal is drilled through. As the drill pulls out, it reaches maximum non-drilling upward speed (v(t) = −4) quickly, and maintains this speed until it returns to rest. A possible plot follows: y

4 Metal 2 x

0.5

1

1.5

2

−2 −4

A graph of the acceleration is extracted from this graph: y

40 20 0.5

1

x

1.5

2

−20 − 40 Metal

In Exercises 42–43, refer to the following. In their 1997 study, Boardman and Lave related the trafﬁc speed S on a twolane road to trafﬁc density Q (number of cars per mile of road) by the formula S = 2,882Q −1 − 0.052Q + 31.73 for 60 ≤ Q ≤ 400 (Figure 9). Speed S 70 (mph) 60 50 40 30 20 10 100

200

300

400

Density Q

FIGURE 9 Speed as a function of trafﬁc density.

43. (a) CalculateExplain d S/d Q intuitively and d 2 S/dwhy Q 2 . we should expect that d S/d Q < 0. (b) Show that d 2 S/d Q 2 > 0. Then use the fact that d S/d Q < 0 and d 2 S/d Q 2 > 0 to justify the following statement: A one-unit increase in trafﬁc density slows down trafﬁc more when Q is small than when Q is large. Plot d S/d Q. Which property of this graph shows that d 2 S/d Q 2 > 0? (c) SOLUTION

S E C T I O N 3.5

Higher Derivatives

117

(a) Trafﬁc speed must be reduced when the road gets more crowded so we expect d S/d Q to be negative. This is indeed the case since d S/d Q = −.052 − 2882/Q 2 < 0. (b) The decrease in speed due to a one-unit increase in density is approximately d S/d Q (a negative number). Since d 2 S/d Q 2 = 5764Q −3 > 0 is positive, this tells us that d S/d Q gets larger as Q increases—and a negative number which gets larger is getting closer to zero. So the decrease in speed is smaller when Q is larger, that is, a one-unit increase in trafﬁc density has a smaller effect when Q is large. (c) d S/d Q is plotted below. The fact that this graph is increasing shows that d 2 S/d Q 2 > 0. y 100

200

300

400

x

−0.2 −0.4 −0.6 −0.8 −1 −1.2

Use a computer algebra system to compute f (k) (x) for k = 1, 2, 3 for the functions: According to one model that attempts to account for air resistance, the distance s(t) (in feet) traveled by a 5/3 (a) falling f (x) =raindrop (1 + x 3 )satisﬁes 1 − x4 (b) f (x) = 0.0005 ds 2 d 2s 1 − 5x − 6x 2 = g − D dt dt 2 45.

SOLUTION

where D= is the and g = algebra 32 ft/s2 system, . Terminal velocity v 1/2 is deﬁned as the velocity at which the (a) Let f (x) (1 +raindrop x 3 )5/3 . diameter Using a computer drop has zero acceleration (one can show that velocity approaches v 1/2 as time proceeds). 2 3 2/3 √ = 5x 2000g D. (1 + x ) ; (a) Show that v 1/2 = f (x) (b) Find v 1/2 for drops (x)diameter = 10x(10.003 + x 3and )2/30.0003 + 10x 4ft.(1 + x 3 )−1/3 ; and f of (c) In this model, do fraindrops accelerate or lower (x) = 10(1 2/3 +rapidly 6 (1 + x 3 )−4/3 . + x 3 )more 60x 3 (1at+higher x 3 )−1/3 − 10xvelocities? (b) Let f (x) =

1 − x4 . Using a computer algebra system, 1 − 5x − 6x 2 f (x) =

12x 3 − 9x 2 + 2x + 5 ; (6x − 1)2

f (x) =

2(36x 3 − 18x 2 + 3x − 31) ; and (6x − 1)3

f (x) =

1110 . (6x − 1)4

x +2 Further Let Insights . Use a CAS to compute the f (x) = and Challenges x −1 47. Find the 100th derivative of f (k) (x)?

f (k) (x) for 1 ≤ k ≤ 4. Can you ﬁnd a general formula for

p(x) = (x + x 5 + x 7 )10 (1 + x 2 )11 (x 3 + x 5 + x 7 ) SOLUTION

This is a polynomial of degree 70 + 22 + 7 = 99, so its 100th derivative is zero.

49. Use the Product Rule twice to ﬁnd a formula for ( f g) in terms of the ﬁrst and second derivative of f and g. What is the p(99) (x) for p(x) as in Exercise 47? SOLUTION Let h = f g. Then h = f g + g f = f g + f g and h = f g + g f + f g + g f = f g + 2 f g + f g . 51. Compute Use the Product Rule to ﬁnd a formula for ( f g) and compare your result with the expansion of (a + b)3 . Then f (x + h) + f (x − h) − 2 f (x) try to guess the general formula for ( f g)(n) . f (x) = lim h→0 h2 for the following functions: (a) f (x) = x (b) f (x) = x 2 Based on these examples, can you formulate a conjecture about what f is?

(c) f (x) = x 3

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D I F F E R E N T I AT I O N SOLUTION

For f (x) = x, we have f (x + h) + f (x − h) − 2 f (x) = (x + h) + (x − h) − 2x = 0.

Hence, (x) = 0. For f (x) = x 2 , f (x + h) + f (x − h) − 2 f (x) = (x + h)2 + (x − h)2 − 2x 2 = x 2 + 2xh + h 2 + x 2 − 2xh + h 2 − 2x 2 = 2h 2 , so (x 2 ) = 2. Working in a similar fashion, we ﬁnd (x 3 ) = 6x. One can prove that for twice differentiable functions, f = f . It is an interesting fact of more advanced mathematics that there are functions f for which f exists at all points, but the function is not differentiable.

3.6 Trigonometric Functions Preliminary Questions 1. Determine the sign ± that yields the correct formula for the following: d (a) (sin x + cos x) = ± sin x ± cos x dx d (b) tan x = ± sec2 x dx d (c) sec x = ± sec x tan x dx d (d) cot x = ± csc2 x dx The correct formulas are d (sin x + cos x) = − sin x + cos x dx d tan x = sec2 x dx d sec x = sec x tan x dx d cot x = − csc2 x dx

SOLUTION

(a) (b) (c) (d)

2. Which of the following functions can be differentiated using the rules we have covered so far? (c) y = x 2 cos x (a) y = 3 cos x cot x (b) y = cos(x 2 ) SOLUTION

(a) 3 cos x cot x is a product of functions whose derivatives are known. This function can therefore be differentiated using the Product Rule. (b) cos(x 2 ) is a composition of the functions cos x and x 2 . We have not yet discussed how to differentiate composite functions. (c) x 2 cos x is a product of functions whose derivatives are known. This function can therefore be differentiated using the Product Rule. 3. Compute SOLUTION

d (sin2 x + cos2 x) without using the derivative formulas for sin x and cos x. dx Recall that sin2 x + cos2 x = 1 for all x. Thus, d d (sin2 x + cos2 x) = 1 = 0. dx dx

4. How is the addition formula used in deriving the formula (sin x) = cos x? SOLUTION The difference quotient for the function sin x involves the expression sin(x + h). The addition formula for the sine function is used to expand this expression as sin(x + h) = sin x cos h + sin h cos x.

S E C T I O N 3.6

Trigonometric Functions

Exercises In Exercises 1–4, ﬁnd an equation of the tangent line at the point indicated. 1. y = sin x, SOLUTION

x=

π 4

Let f (x) = sin x. Then f (x) = cos x and the equation of the tangent line is √ √ √

π √2

π

π π 2 2 2 π x− + f = x− + = x+ 1− . y = f 4 4 4 2 4 2 2 2 4

π 3. y = tan x, x = π y = cos x, x 4= 3 SOLUTION Let f (x) = tan x. Then f (x) = sec2 x and the equation of the tangent line is y = f

π

4

x−

π

π π π + f =2 x− + 1 = 2x + 1 − . 4 4 4 2

In Exercises 5–26, use theπProduct and Quotient Rules as necessary to ﬁnd the derivative of each function. y = sec x, x = 6 5. f (x) = sin x cos x SOLUTION

Let f (x) = sin x cos x. Then f (x) = sin x(− sin x) + cos x(cos x) = − sin2 x + cos2 x.

7. f (x) = sin2 x2 f (x) = x cos x SOLUTION Let f (x) = sin2 x = sin x sin x. Then f (x) = sin x(cos x) + sin x(cos x) = 2 sin x cos x. x x + 12 cot x 9. f (x)f (x) = x=3 sin 9 sec SOLUTION Let f (x) = x 3 sin x. Then f (x) = x 3 cos x + 3x 2 sin x. 11. f (θ ) = tan θ sec θ sin x f (x) = SOLUTION Let xf (θ ) = tan θ sec θ . Then

f (θ ) = tan θ sec θ tan θ + sec θ sec2 θ = sec θ tan2 θ + sec3 θ = tan2 θ + sec2 θ sec θ . 13. h(θ ) = cos2 θθ g(θ ) = SOLUTION Let h(θθ ) = cos2 θ = cos θ cos θ . Then cos h (θ ) = cos θ (− sin θ ) + cos θ (− sin θ ) = −2 cos θ sin θ . 15. f (x) = (x −2x 2 )2cot x k(θ ) = θ sin θ SOLUTION Let f (x) = (x − x 2 ) cot x. Then f (x) = (x − x 2 )(− csc2 x) + cot x(1 − 2x). sec x 17. f (x)f (z) = = 2z tan z x SOLUTION

Let f (x) =

2 sec x (x) = (x ) sec x tan x − 2x sec x = x sec x tan x − 2 sec x . . Then f x2 x4 x3

2 19. g(t) = sin t −x f (x) = cos t tan x SOLUTION

Let g(t) = sin t −

x 21. f (x) = cos y−1 sin x R(y) = + 2 sin y

2 = sin t − 2 sec t. Then g (t) = cos t − 2 sec t tan t. cos t

119

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D I F F E R E N T I AT I O N SOLUTION

Let f (x) =

x . Then 2 + sin x f (x) =

(2 + sin x) (1) − x cos x (2 + sin x)2

=

2 + sin x − x cos x (2 + sin x)2

.

1 + sin x 23. f (x)f (x) = = (x 2 − x − 4) csc x 1 − sin x 1 + sin x SOLUTION Let f (x) = . Then 1 − sin x f (x) =

(1 − sin x) (cos x) − (1 + sin x) (− cos x) (1 − sin x)2

=

2 cos x . (1 − sin x)2

sec x 25. g(x) = 1 + tan x f (x) =x 1 − tan x sec x SOLUTION Let g(x) = . Then x g (x) =

x sec x tan x − (sec x) (1) (x tan x − 1) sec x = . x2 x2

In Exercises 27–30, the second derivative. sincalculate x h(x) = 4 x++ cos x x 27. f (x) = 3 sin 4 cos SOLUTION

Let f (x) = 3 sin x + 4 cos x. Then f (x) = 3 cos x − 4 sin x and f (x) = −3 sin x − 4 cos x.

29. g(θ ) = θ sin θ f (x) = tan x SOLUTION Let g(θ ) = θ sin θ . Then, by the product rule, g (θ ) = θ cos θ + sin θ and g (θ ) = −θ sin θ + cos θ + cos θ = 2 cos θ − θ sin θ . In Exercises h(θ ) 31–36, = csc θﬁnd an equation of the tangent line at the point speciﬁed. 31. y = x 2 + sin x, SOLUTION

x =0

Let f (x) = x 2 + sin x. Then f (x) = 2x + cos x and f (0) = 1. The tangent line at x = 0 is y = f (0) (x − 0) + f (0) = 1 (x − 0) + 0 = x.

π 33. y = 2 sin x + 3 cos x, πx = 3 y = θ tan θ , θ = √ 4 π 3 3 SOLUTION Let f (x) = 2 sin x + 3 cos x. Then f (x) = 2 cos x − 3 sin x and f 3 = 1 − 2 . The tangent line at x = π3 is √

π

π 3 3 3

π π √ x− + f = 1− x− + 3+ y= f 3 3 3 2 3 2 √ √ √ 3 3 3 3 π π− . = 1− x + 3+ + 2 2 2 3 π 35. y =y csc sinx,θ , xθ== 40 = θx2−+cot SOLUTION Let f (x) = csc x − cot x. Then f (x) = csc2 x − csc x cot x and f

π 4

=2−

√

2·1=2−

√ 2.

Hence the tangent line is

π

π

√

π π √ x− + f = 2− 2 x − + y = f 2−1 4 4 4 4

√ √ π √ 2−2 . = 2− 2 x + 2−1+ 4

S E C T I O N 3.6

Trigonometric Functions

121

In Exercises 37–39, using (sin x) = cos x and (cos x) = − sin x. cos θ verify the formula π y= , θ= 1 + sin θ 3 d 37. cot x = − csc2 x dx cos x . Using the quotient rule and the derivative formulas, we compute: SOLUTION cot x = sin x d d cos x sin x(− sin x) − cos x(cos x) −(sin2 x + cos2 x) −1 cot x = = = = = − csc2 x. dx d x sin x sin2 x sin2 x sin2 x d csc d x = − csc x cot x dx sec x = sec x tan x dx 1 , we can apply the quotient rule and the two known derivatives to get: SOLUTION Since csc x = sin x

39.

d − cos x cos x 1 d 1 sin x(0) − 1(cos x) = =− csc x = = = − cot x csc x. dx d x sin x sin x sin x sin2 x sin2 x 41. Calculate thevalues ﬁrst ﬁve cos x. determine f (8)graph and of f (37) . sin x cos x is horizontal. where theThen tangent line to the y= Find the of xderivatives between 0of andf (x) 2π = SOLUTION Let f (x) = cos x. • Then f (x) = − sin x, f (x) = − cos x, f (x) = sin x, f (4) (x) = cos x, and f (5) (x) = − sin x. • Accordingly, the successive derivatives of f cycle among

{− sin x, − cos x, sin x, cos x} in that order. Since 8 is a multiple of 4, we have f (8) (x) = cos x. • Since 36 is a multiple of 4, we have f (36) (x) = cos x. Therefore, f (37) (x) = − sin x. 43. Calculate f (x) and f (x), where f (x) = tan x. Find y (157) , where y = sin x. SOLUTION Let f (x) = tan x. Then f (x) = sec2 x = sec x sec x f (x) = 2 sec x sec x tan x = 2 sec2 x tan x, and

f (x) = 2 sec2 x sec2 x + (tan x)(2 sec2 x tan x) = 2 sec4 x + 4 sec2 x tan2 x

sin x LetLet g(t)f (x) = t= − sinx t. for x = 0 and f (0) = 1. (a) Plot (a) Plot f (x)the ongraph [−3π ,of3πg].with a graphing utility for 0 ≤ t ≤ 4π . (b) Show that that f (c) 0 ifofc the = tangent tan c. Use numerical root and ﬁnder on this a computer (b) Show the=slope linethe is always positive verify on your algebra graph. system to ﬁnd a good approximation to the smallest valuerange c0 such that f (c0 )line = 0. (c) For which values of t positive in the given is the tangent horizontal? (c) Verify that the horizontal line y = f (c0 ) is tangent to the graph of y = f (x) at x = c0 by plotting them on the same set of axes. 45.

SOLUTION

(a) Here is the graph of f (x) over [−3π , 3π ]. y 1 0.8 0.4 −5

5

−10

(b) Let f (x) =

x 10

sin x . Then x f (x) =

x cos x − sin x . x2

To have f (c) = 0, it follows that c cos c − sin c = 0, or tan c = c. Using a computer algebra system, we ﬁnd that the smallest positive value c0 such that f (c0 ) = 0 is c0 = 4.493409.

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D I F F E R E N T I AT I O N

(c) The horizontal line y = f (c0 ) = −0.217234 and the function y = f (x) are both plotted below. The horizontal line is clearly tangent to the graph of f (x). y 1 0.8 0.4 −5

5

x

−10

10

47. The height at time t (s) of a weight, oscillating up and down at the end of a spring, is s(t) = 300 + 40 sin t cm. Find π the Show no tangent graph of f (x) = tan x has zero slope. What is the least slope of a tangent line? the velocity and that acceleration at tline = to 3 s. Justify your response by sketching the graph of (tan x) . SOLUTION Let s(t) = 300 + 40 sin t be the height. Then the velocity is v(t) = s (t) = 40 cos t and the acceleration is a(t) = v (t) = −40 sin t. √ At t = π3 , the velocity is v 3 = 20 cm/sec and the acceleration is a π3 = −20 3 cm/sec2 . π

If you stand 1 m Rfrom wall and mark off points on the wall increments δ of angular 49. θ and initial velocityelevation v0 m/s The horizontal range of aa projectile launched from ground levelatatequal an angle (Figure these sin points grow increasingly farθapart. howwill thisthe illustrates the fact or thatdecrease the derivative of tan is θ is R4),=then (v02 /9.8) θ cos θ . Calculate d R/d . If θ Explain = 7π /24, range increase if the angle is increasing. increased slightly? Base your answer on the sign of the derivative. h4

h3 h2 h1 1

FIGURE 4 SOLUTION Let D(θ ) = tan θ represent the distance from the horizontal base to the point on the wall at angular elevation θ . Based on the derivative estimate of the rate of change, for a small change δ in the angle of elevation, the change in the distance can be approximated by:

D(θ + δ ) − D(θ ) d ≈ D (θ ) = tan θ . δ dθ The change in distance resulting from a δ increase in angle is then D(θ + δ ) − D(θ ) ≈ δ

d tan θ . dθ

Knowing that D(θ + δ ) − D(θ ) increases as θ increases, it follows that D (θ ) = ddθ tan θ must increase as θ increases.

Further Insights and Challenges 51. Show that a nonzero polynomial function y = f (x) cannot satisfy the equation y = −y. Use this to prove that Usex the of the derivative and the addition law for the cosine to prove that (cos x) = − sin x. neither sin norlimit cos xdeﬁnition is a polynomial. SOLUTION

• Let p be a nonzero polynomial of degree n and assume that p satisﬁes the differential equation y + y = 0. Then p + p = 0 for all x. There are exactly three cases.

(a) If n = 0, then p is a constant polynomial and thus p = 0. Hence 0 = p + p = p or p ≡ 0 (i.e., p is equal to 0 for all x or p is identically 0). This is a contradiction, since p is a nonzero polynomial. (b) If n = 1, then p is a linear polynomial and thus p = 0. Once again, we have 0 = p + p = p or p ≡ 0, a contradiction since p is a nonzero polynomial. (c) If n ≥ 2, then p is at least a quadratic polynomial and thus p is a polynomial of degree n − 2 ≥ 0. Thus q = p + p is a polynomial of degree n ≥ 2. By assumption, however, p + p = 0. Thus q ≡ 0, a polynomial of degree 0. This is a contradiction, since the degree of q is n ≥ 2.

S E C T I O N 3.7

The Chain Rule

123

CONCLUSION: In all cases, we have reached a contradiction. Therefore the assumption that p satisﬁes the differential equation y + y = 0 is false. Accordingly, a nonzero polynomial cannot satisfy the stated differential equation. • Let y = sin x. Then y = cos x and y = − sin x. Therefore, y = −y. Now, let y = cos x. Then y = − sin x and y = − cos x. Therefore, y = −y. Because sin x and cos x are nonzero functions that satisfy y = −y, it follows that neither sin x nor cos x is a polynomial. 53. Let f (x) = x sin x and g(x) = x cos x. Verify the following identity and use it to give another proof of the formula sin x = cos x: (a) Show that f (x) = g(x) + sin x and g (x) = − f (x) + cos x.

x and g (x) =x−g(x) − 2xsin (b) Verify that f (x) = − f (x) + 2 cos sin(x + h) − sin = 2 cos +x.12 h sin 12 h (c) By further experimentation, try to ﬁnd formulas for all higher derivatives of f and g. Hint: The kth derivative depends on whether k =the 4n,addition 4n + 1,formula 4n + 2,toorprove 4n + that 3. sin(a + b) − sin(a − b) = 2 cos a sin b. Hint: Use Let f (x) = x sin x and g(x) = x cos x. (a) We examine ﬁrst derivatives: f (x) = x cos x + (sin x) · 1 = g(x) + sin x and g (x) = (x)(− sin x) + (cos x) · 1 = − f (x) + cos x; i.e., f (x) = g(x) + sin x and g (x) = − f (x) + cos x. (b) Now look at second derivatives: f (x) = g (x) + cos x = − f (x) + 2 cos x and g (x) = − f (x) − sin x = −g(x) − 2 sin x; i.e., f (x) = − f (x) + 2 cos x and g (x) = −g(x) − 2 sin x. • The third derivatives are f (x) = − f (x) − 2 sin x = −g(x) − 3 sin x and g (x) = −g (x) − 2 cos x = (c) f (x) − 3 cos x; i.e., f (x) = −g(x) − 3 sin x and g (x) = f (x) − 3 cos x. • The fourth derivatives are f (4) (x) = −g (x) − 3 cos x = f (x) − 4 cos x and g (4) (x) = f (x) + 3 sin x = g(x) + 4 sin x; i.e., f (4) = f (x) − 4 cos x and g (4) (x) = g(x) + 4 sin x. • We can now see the pattern for the derivatives, which are summarized in the following table. Here n = 0, 1, 2, . . . SOLUTION

k

4n

4n + 1

4n + 2

4n + 3

f (k) (x)

f (x) − k cos x

g(x) + k sin x

− f (x) + k cos x

−g(x) − k sin x

g (k) (x)

g(x) + k sin x

− f (x) + k cos x

−g(x) − k sin x

f (x) − k cos x

Show that if π /2 < θ < π , then the distance along the x-axis between θ and the point where the tangent line intersects the x-axis is equal to |tan θ | (Figure 5).

3.7 The Chain Rule

Preliminary Questions 1. Identify the outside and inside functions for each of these composite functions. 4x + 9x 2 (b) y = tan(x 2 + 1) (c) y = sec5 x

(a) y =

SOLUTION

√ (a) The outer function is x, and the inner function is 4x + 9x 2 . (b) The outer function is tan x, and the inner function is x 2 + 1. (c) The outer function is x 5 , and the inner function is sec x. 2. Which of the following can be differentiated easily without using the Chain Rule? y = tan(7x 2 + 2), y=

√

x · sec x,

x , x +1 √ y = x cos x y=

x can be differentiated using the Quotient Rule, and the function √ x · sec x can be differThe function x+1 √ entiated using the Product Rule. The functions tan(7x 2 + 2) and x cos x require the Chain Rule. SOLUTION

3. Which is the derivative of f (5x)? (a) 5 f (x) SOLUTION

(b) 5 f (5x)

(c) f (5x)

The correct answer is (b): 5 f (5x).

4. How many times must the Chain Rule be used to differentiate each function? (b) y = cos((x 2 + 1)4 ) (a) y = cos(x 2 + 1) (c) y = cos((x 2 + 1)4 ) SOLUTION

(a) To differentiate cos(x 2 + 1), the Chain Rule must be used once. (b) To differentiate cos((x 2 + 1)4 ), the Chain Rule must be used twice.

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(c) To differentiate

cos((x 2 + 1)4 ), the Chain Rule must be used three times.

5. Suppose that f (4) = g(4) = g (4) = 1. Do we have enough information to compute F (4), where F(x) = f (g(x))? If not, what is missing? SOLUTION If F(x) = f (g(x)), then F (x) = f (g(x))g (x) and F (4) = f (g(4))g (4). Thus, we do not have enough information to compute F (4). We are missing the value of f (1).

Exercises In Exercises 1–4, ﬁll in a table of the following type: f (g(x))

1. f (u) = u 3/2 ,

f (u)

f (g(x))

g (x)

( f ◦ g)

g(x) = x 4 + 1

SOLUTION

f (g(x))

f (u)

f (g(x))

g (x)

( f ◦ g)

(x 4 + 1)3/2

3 u 1/2 2

3 (x 4 + 1)1/2 2

4x 3

6x 3 (x 4 + 1)1/2

3. f (u) = tan u,3 g(x) = x 4 f (u) = u , g(x) = 3x + 5 SOLUTION

f (g(x))

f (u)

f (g(x))

g (x)

( f ◦ g)

tan(x 4 )

sec2 u

sec2 (x 4 )

4x 3

4x 3 sec2 (x 4 )

In Exercises 5–6, 4write the function as a composite f (g(x)) and compute the derivative using the Chain Rule. f (u) = u + u, g(x) = cos x 5. y = (x + sin x)4 SOLUTION

Let f (x) = x 4 , g(x) = x + sin x, and y = f (g(x)) = (x + sin x)4 . Then dy = f (g(x))g (x) = 4(x + sin x)3 (1 + cos x). dx

d 3cos u for the following choices of u(x): 7. Calculate y = cos(x dx ) 2 (b) u = x −1 (a) u = 9 − x

(c) u = tan x

SOLUTION

(a) cos(u(x)) = cos(9 − x 2 ). d cos(u(x)) = − sin(u(x))u (x) = − sin(9 − x 2 )(−2x) = 2x sin(9 − x 2 ). dx (b) cos(u(x)) = cos(x −1 ). d 1 sin(x −1 ) . cos(u(x)) = − sin(u(x))u (x) = − sin(x −1 ) − 2 = dx x x2 (c) cos(u(x)) = cos(tan x). d cos(u(x)) = − sin(u(x))u (x) = − sin(tan x)(sec2 x) = − sec2 x sin(tan x). dx In Exercises 9–14, duse the General Power Rule or the Shifting and Scaling Rule to ﬁnd the derivative. Calculate f (x 2 + 1) for the following choices of f (u): 9. y = (x 2 + 9)4d x (a) f (u) = sin u (b) f (u) = 3u 3/2 SOLUTION Let g(x) = x 2 + 9. We apply the general power rule. 2 (c) f (u) = u − u d 2 d d g(x)4 = (x + 9)4 = 4(x 2 + 9)3 (x 2 + 9) = 4(x 2 + 9)3 (2x) = 8x(x 2 + 9)3 . dx dx dx y = sin5 x

S E C T I O N 3.7

11. y =

√

The Chain Rule

125

11x + 4

SOLUTION

Let g(x) = 11x + 4. We apply the general power rule.

1 d d √ 1 11 d g(x)1/2 = 11x + 4 = (11x + 4)−1/2 (11x + 4) = (11x + 4)−1/2 (11) = √ . dx dx 2 dx 2 2 11x + 4 √ Alternately, let f (x) = x and apply the shifting and scaling rule. Then 1 d √ d 11 11x + 4 = (11) f (11x + 4) = (11x + 4)−1/2 = √ . dx dx 2 2 11x + 4 13. y = sin(1 − 4x) y = (7x − 9)5 SOLUTION Let f (x) = sin x. We apply the shifting and scaling rule. d d f (1 − 4x) = sin(1 − 4x) = −4 cos(1 − 4x). dx dx In Exercises 15–18, ﬁnd the derivative of f ◦ g. y = (x 4 − x + 2)−3/2 15. f (u) = sin u, g(x) = 2x + 1 SOLUTION

Let h(x) = f (g(x)) = sin(2x + 1). Then, applying the shifting and scaling rule, h (x) = 2 cos(2x + 1).

Alternately, d f (g(x)) = f (g(x))g (x) = cos(2x + 1) · 2 = 2 cos(2x + 1). dx 9 , g(x) = x + x −1 17. f (u)f (u) = u= 2u + 1, g(x) = sin x SOLUTION

Let h(x) = f (g(x)) = (x + x −1 )9 . Then, applying the general power rule, h (x) = 9(x + x −1 )8 (1 − 12 ). x

Alternately, d f (g(x)) = f (g(x))g (x) = 9(x + x −1 )8 (1 − x −2 ). dx In Exercises 19–20,u ﬁnd the derivatives of f (g(x)) and g( f (x)). f (u) = , g(x) = csc x u−1 19. f (u) = cos u, g(x) = x 2 + 1 SOLUTION

d f (g(x)) = f (g(x))g (x) = − sin(x 2 + 1)(2x) = −2x sin(x 2 + 1). dx d g( f (x)) = g ( f (x)) f (x) = 2(cos x)(− sin x) = −2 sin x cos x. dx In Exercises 21–32, use the Chain 1 Rule to ﬁnd the derivative. f (u) = u 3 , g(x) = x +1 21. y = sin(x 2 )

SOLUTION Let y = sin x 2 . Then y = cos x 2 · 2x = 2x cos x 2 . 23. y = cot(4t 22 + 9) y = sin x

SOLUTION Let y = cot 4t 2 + 9 . Then

y = − csc2 4t 2 + 9 · 8t = −8t csc2 4t 2 + 9 . 25. y = (t 2 + 3t + 1)−5/2 y = 1 − t2

−5/2 SOLUTION Let y = t 2 + 3t + 1 . Then y = −

y = (x 4 − x 3 − 1)2/3

−7/2 5 2 5 (2t + 3) t + 3t + 1 (2t + 3) = − 7/2 . 2 2 t 2 + 3t + 1

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27. y =

x +1 4 x −1

SOLUTION

Let y =

x +1 4 . Then x −1

y = 4

x + 1 3 (x − 1) · 1 − (x + 1) · 1 8 (x + 1)3 8(1 + x)3 · =− = . 2 5 x −1 (1 − x)5 (x − 1) (x − 1)

29. y = cos3 (θ 21)

3 y = sec SOLUTION Letx y = cos3 θ 2 = cos θ 2 . Here, we note that calculating the derivative of the inside function, cos(θ 2 ), requires the chain rule. After two applications of the chain rule, we have

2

y = 3 cos θ 2 − sin θ 2 · 2θ = −6θ sin θ 2 cos2 θ 2 . 1 31. y =y = tan(θ + cos θ ) cos(x 2 ) + 1

−1/2 SOLUTION Let y = 1 + cos x 2 . Here, we note that calculating the derivative of the inside function, 1 + cos(x 2 ), requires the chain rule. After two applications of the chain rule, we have

−3/2

x sin x 2 1 y = − 1 + cos x 2 · − sin x 2 · (2x) = 3/2 . 2 1 + cos x 2

√ In Exercises 33–62, ﬁnd the derivative using the appropriate rule or combination of rules. y = ( x + 1 − 1)3/2 33. y = tan 5x SOLUTION

Let y = tan 5x. By the scaling and shifting rule, dy = 5 sec2 (5x). dx

35. y = x cos(1 − 3x) y = sin(x 2 + 4x) SOLUTION Let y = x cos (1 − 3x). Applying the product rule and then the scaling and shifting rule, y = x (− sin (1 − 3x)) · (−3) + cos (1 − 3x) · 1 = 3x sin (1 − 3x) + cos (1 − 3x) . 37. y = (4t + 9)1/2 y = sin(x 2 ) cos(x 2 ) SOLUTION Let y = (4t + 9)1/2 . By the shifting and scaling rule, dy 1 =4 (4t + 9)−1/2 = 2(4t + 9)−1/2 . dt 2 3 + cos x)−4 39. y =y (x = sin(cos θ) SOLUTION Let y = (x 3 + cos x)−4 . By the general power rule,

y = −4(x 3 + cos x)−5 (3x 2 − sin x) = 4(sin x − 3x 2 )(x 3 + cos x)−5 . √ 41. y =y =sin x cos x x)) sin(cos(sin SOLUTION We start by using a trig identity to rewrite √ 1 1 y = sin x cos x = sin 2x = √ (sin 2x)1/2 . 2 2 Then, after two applications of the chain rule, 1 1 cos 2x . y = √ · (sin 2x)−1/2 · cos 2x · 2 = √ 2 sin 2x 2 2 y = x 2 tan 2x

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127

43. y = (z + 1)4 (2z − 1)3 SOLUTION

Let y = (z + 1)4 (2z − 1)3 . Applying the product rule and the general power rule,

dy = (z + 1)4 (3(2z − 1)2 )(2) + (2z − 1)3 (4(z + 1)3 )(1) = (z + 1)3 (2z − 1)2 (6(z + 1) + 4(2z − 1)) dz = (z + 1)3 (2z − 1)2 (14z + 2). √ 45. y = (x + x −1 )√ x + 1 y = 3 + 2s s √ SOLUTION Let y = (x + x −1 ) x + 1. Applying the product rule and the shifting and scaling rule, and then factoring, we get: √ 1 1 (x + x −1 + 2(x + 1)(1 − x −2 )) y = (x + x −1 ) (x + 1)−1/2 + x + 1(1 − x −2 ) = √ 2 2 x +1 =

2x 2

1 1 √ √ (x 3 + x + 2x 3 + 2x 2 − 2x − 2) = (3x 3 + 2x 2 − x − 2). 2 x +1 2x x + 1

47. y = (cos 6x2 + sin x 2 )1/2 y = cos (8x) SOLUTION Let y = (cos 6x + sin(x 2 ))1/2 . Applying the general power rule followed by both the scaling and shifting rule and the chain rule, y =

−1/2 x cos(x 2 ) − 3 sin 6x 1 − sin 6x · 6 + cos(x 2 ) · 2x = . cos 6x + sin(x 2 ) 2 cos 6x + sin(x 2 )

3) 49. y = tan3 x + tan(x (x + 1)1/2 y = Let y = tan3 x + tan(x 3 ) = (tan x)3 + tan(x 3 ). Applying the general power rule to the ﬁrst term and the SOLUTION x +2 chain rule to the second term, y = 3(tan x)2 sec2 x + sec2 (x 3 ) · 3x 2 = 3 x 2 sec2 (x 3 ) + sec2 x tan2 x .

z√ +1 51. y =y = 4 − 3 cos x z−1 z + 1 1/2 SOLUTION Let y = . Applying the general power rule followed by the quotient rule, z−1 dy 1 = dz 2

−1 z + 1 −1/2 (z − 1) · 1 − (z + 1) · 1 · = √ . 2 z−1 z + 1 (z − 1)3/2 (z − 1)

cos(1 + x) 53. y =y = (cos3 x + 3 cos x + 7)9 1 + cos x SOLUTION

Let y=

cos(1 + x) . 1 + cos x

Then, applying the quotient rule and the shifting and scaling rule, dy −(1 + cos x) sin(1 + x) + cos(1 + x) sin x cos(1 + x) sin x − cos x sin(1 + x) − sin(1 + x) = = dx (1 + cos x)2 (1 + cos x)2 =

sin(−1) − sin(1 + x) . (1 + cos x)2

The last line follows from the identity sin( A − B) = sin A cos B − cos A sin B with A = x and B = 1 + x. ) 55. y = cot7 (x 5 y = sec( t 2 − 9)

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Let y = cot7 x 5 . Applying the general power rule followed by the chain rule,

dy = 7 cot6 x 5 · − csc2 x 5 · 5x 4 = −35x 4 cot6 x 5 csc2 x 5 . dx

57. y = (1 + (x 2 2+ 2)5 )3 cos(x ) y = Let y2 = (1 + (x 2 + 2)5 )3 . Then, applying the general power rule twice, we obtain: SOLUTION 1+x dy = 3(1 + (x 2 + 2)5 )2 (5(x 2 + 2)4 (2x)) = 30x(1 + (x 2 + 2)5 )2 (x 2 + 2)4 . dx

√

59. y =y =1 +1 +1cot +5 (xx4 + 1) 9 1/2 1/2

SOLUTION Let y = 1 + 1 + x 1/2 . Applying the general power rule twice,

1/2 −1/2 1

−1/2 1 dy 1 1 + x 1/2 · · x −1/2 = = 1 + 1 + x 1/2 dx 2 2 2

1

. √ √ √ 8 x 1+ x 1+ 1+ x

√ 61. y = kx + √ b; k and b any constants x +1+1 y= SOLUTION Let y = (kx + b)1/2 , where b and k are constants. By the scaling and shifting rule, y =

1 k . (kx + b)−1/2 · k = √ 2 2 kx + b

df df du 63. Compute 1if = 2 and = 6. du; k, b constants, dx not both zero y = d x kt 4 + b SOLUTION Assuming f is a function of u, which is in turn a function of x, df d f du = · = 2(6) = 12. dx du d x 65. With notation as in Example 5, calculate √ molecular velocity v of a gas in a certain container is given by v = 29 T m/s, where T is the d The average d (a) sin θ (b) θ + tan θ dv dθ d θ (in atmospheres) . temperature in ◦kelvins. The temperature is related to the pressure ◦ T = 200P. Find θ =60 θ =45by d P P=1.5 SOLUTION

(a)

π

π

π d d π 1 π sin θ = sin θ = cos (60) = = . θ =60◦ d θ θ =60◦ dθ 180 180 180 180 2 360

(b)

π

π d

π π d θ + tan θ = θ + tan θ =1+ sec2 =1+ . ◦ ◦ θ =45 θ =45 dθ dθ 180 180 4 90

67. Compute the derivative of h(sin x) at x = π6 , assuming that h (0.5) = 10. Assume that f (0) = 2 and f (0) = 3. Find the derivatives of ( f (x))3 and f (7x) at x = 0. SOLUTION Let u = sin x and suppose that h (0.5) = 10. Then d dh du dh = cos x. (h(u)) = dx du d x du √ When x = π6 , we have u = .5. Accordingly, the derivative of h(sin x) at x = π6 is 10 cos π6 = 5 3. In Exercises 69–72, the table of values to calculate function at the given and f (g(2)) from the Let F(x) = use f (g(x)), where the graphs of f andthe g derivative are shown of in the Figure 1. Estimate g (2) point. graph and compute F (2).

69. f (g(x)),

x =6

x

1

4

6

f (x) f (x) g(x) g (x)

4 5 4 5

0 7 1

6 4 6 3

1 2

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The Chain Rule

129

d = f (g(6))g (6) = f (6)g (6) = 4 × 3 = 12. f (g(x)) dx x=6

√ 71. g( x), x = 16 sin( f (x)), x =4 √ 1 1 d √ 1 1 1 g( x) . SOLUTION = g (4) (1/ 16) = = dx 2 2 2 4 16 x=16 In Exercises compute f (2x73–76, + g(x)), x = the 1 indicated higher derivatives. d2 sin(x 2 ) dx2

73.

SOLUTION

Let f (x) = sin x 2 . Then, by the chain rule, f (x) = 2x cos x 2 and, by the product rule and the chain

rule,

f (x) = 2x − sin x 2 · 2x + 2 cos x 2 = 2 cos x 2 − 4x 2 sin x 2 . d3 2 d(3x + 9)11 d x 3 2 (x 2 + 9)5 dx SOLUTION Let f (x) = (3x + 9)11 . Then, by repeated use of the scaling and shifting rule,

75.

f (x) = 11(3x + 9)10 · 3 = 33(3x + 9)10 f (x) = 330(3x + 9)9 · 3 = 990(3x + 9)9 , f (x) = 8910(3x + 9)8 · 3 = 26730 (3x + 9)8 . Use a computer algebra system to compute f (k) (x) for k = 1, 2, 3 for the following functions: d3 2 sin(2x) (a) f (x) (b) f (x) = x 3 + 1 d x=3 cot(x )

77.

SOLUTION

(a) Let f (x) = cot(x 2 ). Using a computer algebra system, f (x) = −2x csc2 (x 2 ); f (x) = 2 csc2 (x 2 )(4x 2 cot(x 2 ) − 1); and

f (x) = −8x csc2 (x 2 ) 6x 2 cot2 (x 2 ) − 3 cot(x 2 ) + 2x 2 . (b) Let f (x) =

x 3 + 1. Using a computer algebra system, 3x 2 ; f (x) = 2 x3 + 1 f (x) =

3x(x 3 + 4) ; and 4(x 3 + 1)3/2

f (x) = −

3(x 6 + 20x 3 − 8) . 8(x 3 + 1)5/2

79. Compute the second derivative of sin(g(x)) at x = 2, assuming that g(2) = π4 , g (2) = 5, and g (2) = 3. Use the Chain Rule to express the second derivative of f ◦ g in terms of the ﬁrst and second derivatives of f and g. SOLUTION Let f (x) = sin(g(x)). Then f (x) = cos(g(x))g (x) and f (x) = cos(g(x))g (x) + g (x)(− sin(g(x)))g (x) = cos(g(x))g (x) − (g (x))2 sin(g(x)). Therefore, √ √ 2 f (2) = g (2) cos (g(2)) − g (2) sin (g(2)) = 3 cos π4 − (5)2 sin π4 = −22 · 22 = −11 2

where is at resistance i the current. d P/dt at t volume. = 2 if RFind = 1,000 81. TheAn power P in a sphere circuit is = Ri 2r, = expanding hasPradius 0.4t Rcm time t (inand seconds). Let V Find be the sphere’s d V /dt and iwhen varies according to i = sin(4 π t) (time in seconds). (a) r = 3 and (b) t = 3. di d 2 Ri SOLUTION = 2Ri = 2(1000)4π sin(4π t) cos(4π t)|t=2 = 0. dt dt t=2 t=2 The price (in dollars) of a computer component is P = 2C − 18C −1 , where C is the manufacturer’s cost to produce it. Assume that cost at time t (in years) is C = 9 + 3t −1 and determine the ROC of price with respect to

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83. The force F (in Newtons) between two charged objects is F = 100/r 2 , where r is the distance (in meters) between them. Find d F/dt at t = 10 if the distance at time t (in seconds) is r = 1 + 0.4t 2 . Let F(r ) = 100/r 2 , and let r (t) = 1 + .4t 2 . We compute F (r ) = −200/r 3 , and r (t) = .8t. This gives us r (10) = 41 and r (10) = 8. SOLUTION

dF d F dr = , dt dr dt so

d F = −200(41)−3 (8) ≈ −0.0232 N/s. dt t=10

85. Conservation of Energy The position at time t (in seconds) of a weight of mass m oscillating at the end of a According to the U.S. standard atmospheric model, developed by the National Oceanic and Atmospheric Adspring is x(t) = L sin(2πω t). Here, L is the maximum length of the spring, and ω the frequency (number of oscillations ministration for use in aircraft and rocket design, atmospheric temperature T (in degrees Celsius), pressure P per second). Let v and a be the velocity and acceleration of the weight. (kPa = 1,000 Pascals), and altitude h (meters) are related by the formulas (valid in the troposphere h ≤ 11,000): (a) By Hooke’s Law, the spring exerts a force of magnitude where k is the spring constant. √ F = −kx onthe weight, Use Newton’s Second Law, F = ma, to show that 2πω = k/m. T + 273.1 5.256 T = 15.04 0.000649h, = 101.29 1 mv 2 and potential Penergy U =+12 kx 2 .288.08 Prove that the total energy E = K + U (b) The weight has kinetic energy K =− 2 is conserved, that is, d E/dt = 0. Calculate d P/dh. Then estimate the change in P (in Pascals, Pa) per additional meter of altitude when h = 3,000. SOLUTION

(a) Let x(t) = L sin(2πω t). Then v(t) = x (t) = 2πω L cos(2πω t) and a(t) = v (t) = −(2πω )2 L sin(2πω t). Equating Hooke’s Law, F = −kx, to Newton’s second law, F = ma, yields −k L sin(2πω t) = −m(2πω )2 L sin(2πω t) or k/m = (2πω )2 . The desired result follows upon taking square roots. (b) We have dx dU = kx = kxv dt dt and dK dv = mv = mva dt dt But F = ma = −kx by Hooke’s Law, so a = (−k/m)x and we get dK = mva = mv(−k/m)x = −kvx. dt This shows that the derivative of U + K is zero.

Further Insights and Challenges 87. if f (−x) = fthen (x) and odd if f (−x) = − f (x). ShowRecall that ifthat f , g,f (x) andish even are differentiable, (a) Sketch a graph of any even function and explain graphically why its derivative is an odd function. [ f (g(h(x)))] = f (g(h(x)))g (h(x))h (x) (b) Show that differentiation reverses parity: If f is even, then f is odd, and if f is odd, then f is even. Hint: Differentiate f (−x). (c) Suppose that f is even. Is f necessarily odd? Hint: Check if this is true for linear functions. A function is even if f (−x) = f (x) and odd if f (−x) = − f (x). (a) The graph of an even function is symmetric with respect to the y-axis. Accordingly, its image in the left half-plane is a mirror reﬂection of that in the right half-plane through the y-axis. If at x = a ≥ 0, the slope of f exists and is equal to m, then by reﬂection its slope at x = −a ≤ 0 is −m. That is, f (−a) = − f (a). Note: This means that if f (0) exists, then it equals 0. SOLUTION

y 4 3 2 1 −2

−1

x 1

2

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131

(b) By the chain rule, ddx f (−x) = − f (−x). Now suppose that f is even. Then f (−x) = f (x) and d d f (−x) = f (x) = f (x). dx dx Hence, when f is even, − f (−x) = f (x) or f (−x) = − f (x) and f is odd. On the other hand, suppose f is odd. Then f (−x) = − f (x) and d d f (−x) = − f (x) = − f (x). dx dx Hence, when f is odd, − f (−x) = − f (x) or f (−x) = f (x) and f is even. (c) Suppose that f is even. Then f is not necessarily odd. Let f (x) = 4x + 7. Then f (x) = 4, an even function. But f is not odd. For example, f (2) = 15, f (−2) = −1, but f (−2) = − f (2). In Exercises 89–91, use the following fact (proved in Chapter 4):q If a differentiable function f satisﬁes f (x) = 0 for all Power Rule for Fractional Exponents Let f (u) = u and g(x) = x p/q . Show that f (g(x)) = x p (recall the x, then f is a constant function. laws of exponents). Apply the Chain Rule and the Power Rule for integer exponents to show that f (g(x)) g (x) = px p−1 . ThenEquation derive theofPower RuleCosine for x p/qSuppose . 89. Differential Sine and that f (x) satisﬁes the following equation (called a differential equation): f (x) = − f (x)

3

(a) Show that f (x)2 + f (x)2 = f (0)2 + f (0)2 . Hint: Show that the function on the left has zero derivative. (b) Verify that sin x and cos x satisfy Eq. (3) and deduce that sin2 x + cos2 x = 1. SOLUTION

(a) Let g(x) = f (x)2 + f (x)2 . Then g (x) = 2 f (x) f (x) + 2 f (x) f (x) = 2 f (x) f (x) + 2 f (x)(− f (x)) = 0, where we have used the fact that f (x) = − f (x). Because g (0) = 0 for all x, g(x) = f (x)2 + f (x)2 must be a constant function. In other words, f (x)2 + f (x)2 = C for some constant C. To determine the value of C, we can substitute any number for x. In particular, for this problem, we want to substitute x = 0 and ﬁnd C = f (0)2 + f (0)2 . Hence, f (x)2 + f (x)2 = f (0)2 + f (0)2 . (b) Let f (x) = sin x. Then f (x) = cos x and f (x) = − sin x, so f (x) = − f (x). Next, let f (x) = cos x. Then f (x) = − sin x, f (x) = − cos x, and we again have f (x) = − f (x). Finally, if we take f (x) = sin x, the result from part (a) guarantees that sin2 x + cos2 x = sin2 0 + cos2 0 = 0 + 1 = 1. 91. Use the result of Exercise 90 to show the following: f (x) = sin x is the unique solution of Eq. (3) such that f (0) = 0 = g (0). Suppose thatg(x) both=f cos andxgissatisfy Eq. (3) and have thethat same initial that=is,0.f This (0) =result g(0)provides and f (0) = 1; and the unique solution such g(0) = 1values, and g (0) a means of and f (0) Show that f (x) = g(x) for all x. Hint: Show that f − g satisﬁes Eq. (3) and apply Exercise 89(a). deﬁning and developing all the properties of the trigonometric functions without reference to triangles or the unit circle. SOLUTION In part (b) of Exercise 89, it was shown that f (x) = sin x satisﬁes Eq. (3), and we can directly calculate that f (0) = sin 0 = 0 and f (0) = cos 0 = 1. Suppose there is another function, call it F(x), that satisﬁes Eq. (3) with the same initial conditions: F(0) = 0 and F (0) = 1. By Exercise 90, it follows that F(x) = sin x for all x. Hence, f (x) = sin x is the unique solution of Eq. (3) satisfying f (0) = 0 and f (0) = 1. The proof that g(x) = cos x is the unique solution of Eq. (3) satisfying g(0) = 1 and g (0) = 0 is carried out in a similar manner.

93. Chain Rule This exercise proves the Chain Rule without the special assumption made in the text. For any number lim g (x), where b, deﬁneAaDiscontinuous new function Derivative Use the limit deﬁnition to show that g (0) exists but g (0) = x→0 f (u) ⎧ − f (b) 1 ⎪ F(u) = for all u = b ⎨ 2 u −xb sin x if x = 0 g(x) = ⎪ Observe that F(u) is equal to the slope of the secant line⎩through (b,iff x(b)) 0 = 0and (u, f (u)). (a) Show that if we deﬁne F(b) = f (b), then F(u) is continuous at u = b. (b) Take b = g(a). Show that if x = a, then for all u, u − g(a) f (u) − f (g(a)) = F(u) x −a x −a Note that both sides are zero if u = g(a).

4

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(c) Substitute u = g(x) in Eq. (4) to obtain g(x) − g(a) f (g(x)) − f (g(a)) = F(g(x)) x −a x −a Derive the Chain Rule by computing the limit of both sides as x → a. SOLUTION

For any differentiable function f and any number b, deﬁne F(u) =

f (u) − f (b) u−b

for all u = b. (a) Deﬁne F(b) = f (b). Then lim F(u) = lim

u→b

u→b

f (u) − f (b) = f (b) = F(b), u−b

i.e., lim F(u) = F(b). Therefore, F is continuous at u = b. u→b

(b) Let g be a differentiable function and take b = g(a). Let x be a number distinct from a. If we substitute u = g(a) into Eq. (4), both sides evaluate to 0, so equality is satisﬁed. On the other hand, if u = g(a), then f (u) − f (g(a)) f (u) − f (g(a)) u − g(a) f (u) − f (b) u − g(a) u − g(a) = = = F(u) . x −a u − g(a) x −a u−b x −a x −a (c) Hence for all u, we have u − g(a) f (u) − f (g(a)) = F(u) . x −a x −a (d) Substituting u = g(x) in Eq. (4), we have g(x) − g(a) f (g(x)) − f (g(a)) = F(g(x)) . x −a x −a Letting x → a gives f (g(x)) − f (g(a)) g(x) − g(a) = lim F(g(x)) = F(g(a))g (a) = F(b)g (a) = f (b)g (a) x→a x→a x −a x −a lim

= f (g(a))g (a) Therefore ( f ◦ g) (a) = f (g(a))g (a), which is the Chain Rule.

3.8 Implicit Differentiation Preliminary Questions 1. Which differentiation rule is used to show SOLUTION

d dy sin y = cos y ? dx dx

The chain rule is used to show that ddx sin y = cos y dd xy .

2. One of (a)–(c) is incorrect. Find the mistake and correct it. (a)

d sin(y 2 ) = 2y cos(y 2 ) dy

d sin(x 2 ) = 2x cos(x 2 ) dx d sin(y 2 ) = 2y cos(y 2 ) (c) dx

(b)

SOLUTION

(a) This is correct. Note that the differentiation is with respect to the variable y. (b) This is correct. Note that the differentiation is with respect to the variable x. (c) This is incorrect. Because the differentiation is with respect to the variable x, the chain rule is needed to obtain d dy sin(y 2 ) = 2y cos(y 2 ) . dx dx

S E C T I O N 3.8

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133

3. On an exam, Jason was asked to differentiate the equation x 2 + 2x y + y 3 = 7 What are the errors in Jason’s answer? 2x + 2x y + 3y 2 = 0 There are two mistakes in Jason’s answer. First, Jason should have applied the product rule to the second

SOLUTION

term to obtain d dy (2x y) = 2x + 2y. dx dx Second, he should have applied the general power rule to the third term to obtain dy d 3 y = 3y 2 . dx dx 4. Which of (a) or (b) is equal to (a) (x cos t) SOLUTION

d (x sin t)? dx

dt dt (b) (x cos t) + sin t dx dx Using the product rule and the chain rule we see that dt d (x sin t) = x cos t + sin t, dx dx

so the correct answer is (b).

Exercises 1. Show that if you differentiate both sides of x 2 + 2y 3 = 6, the result is 2x + 6y 2 y = 0. Then solve for y and calculate d y/d x at the point (2, 1). SOLUTION

d d 2 (x + 2y 3 ) = 6 dx dx 2x + 6y 2 y = 0 2x + 6y 2 y = 0 6y 2 y = −2x y =

−2x . 6y 2

2 At (2, 1), dd xy = −4 6 = −3.

In Exercises the expression withofrespect to x. x y + 3x + y = 1, the result is (x + 1)y + y + 3 = 0. Show3–8, thatdifferentiate if you differentiate both sides the equation Then solve for y and calculate d y/d x at the point (1, −1). 3. x 2 y 3 SOLUTION

Assuming that y depends on x, then d 2 3 x y = x 2 · 3y 2 y + y 3 · 2x = 3x 2 y 2 y + 2x y 3 . dx

5.

y3 3 x x y

SOLUTION

Assuming that y depends on x, then d −y 3 3y 2 y y3 x(3y 2 y ) − y 3 = 2 + = . 2 dx x x x x

7. z +tan(xt) z2 SOLUTION

Assuming that z depends on x, then

(x 2 + y 2 )3/2

d (z + z 2 ) = z + 2z(z ) dx

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In Exercises 9–24, calculate the derivative of y (or other variable) with respect to x. 9. 3y 3 + x 2 = 5 SOLUTION

2x Let 3y 3 + x 2 = 5. Then 9y 2 y + 2x = 0, and y = − 2 . 9y

11. x 2 y + 2x y 2 = x + y y4 − y = x 3 + x SOLUTION Let x 2 y + 2x y 2 = x + y. Then x 2 y + 2x y + 2x · 2yy + 2y 2 = 1 + y x 2 y + 4x yy − y = 1 − 2x y − 2y 2 y =

1 − 2x y − 2y 2 . x 2 + 4x y − 1

13. x 2 ysin(xt) + y 4 −=3x t =8 SOLUTION We apply the product rule and the chain rule together: d d 2 (x y + y 4 − 3x) = 8 dx dx x 2 y + 2x y + 4y 3 y − 3 = 0 x 2 y + 4y 3 y = 3 − 2x y y (x 2 + 4y 3 ) = 3 − 2x y 3 − 2x y . y = 2 x + 4y 3 15. x 4 +3z 45= 1 x R =1 SOLUTION Let x 4 + z 4 = 1. Then 4x 3 + 4z 3 z = 0, and z = −x 3 /z 3 . 3/2 = 1 17. y −3/2 y +xx + = 2y x yLet y −3/2 + x 3/2 = 1. Then − 32 y −5/2 y + 32 x 1/2 = 0, and y = x 1/2 y 5/2 = x y 5 . SOLUTION

19.

√

1 1 1/2 x x+ s= +=x+y + yx2/3 s

SOLUTION

Let (x + s)1/2 = x −1 + s −1 . Then 1 (x + s)−1/2 1 + s = −x −2 − s −2 s . 2

√ Multiplying by 2x 2 s 2 x + s and then solving for s gives √ √ x 2 s 2 1 + s = −2s 2 x + s − 2x 2 s x + s √ √ x 2 s 2 s + 2x 2 s x + s = −2s 2 x + s − x 2 s 2

√ √ x 2 s 2 + 2 x + s s = −s 2 x 2 + 2 x + s

√ s2 x 2 + 2 x + s . s = − 2 2 √ x s +2 x +s x 1 √ 21. y +√x = y + y + y = 2x SOLUTION

Let y + x y −1 = 1. Then y + x −y −2 y + y −1 · 1 = 0, and y =

23. sin(x + y) = x + cos y y2 x + =0 y x +1

y −1 y y2 = . · x y −2 − 1 y 2 x − y2

S E C T I O N 3.8 SOLUTION

Implicit Differentiation

135

Let sin(x + y) = x + cos y. Then (1 + y ) cos(x + y) = 1 − y sin y cos(x + y) + y cos(x + y) = 1 − y sin y (cos(x + y) + sin y) y = 1 − cos(x + y) y =

1 − cos(x + y) . cos(x + y) + sin y

In Exercises 25–26, ﬁnd d y/d x at the given point. tan(x 2 y) = x + y 25. (x + 2)2 − 6(2y + 3)2 = 3, SOLUTION

(1, −1)

By the scaling and shifting rule, 2(x + 2) − 24(2y + 3)y = 0.

If x = 1 and y = −1, then 2(3) − 24(1)y = 0. so that 24y = 6, or y = 14 . In Exercises 27–30, ﬁnd anequation 2 − π πof the tangent line at the given point. , sin2 (3y) = x + y; 4 4 27. x y − 2y = 1, (3, 1) SOLUTION

Taking the derivative of both sides of x y − 2y = 1 yields x y + y − 2y = 0. Substituting x = 3, y = 1

yields 3y + 1 − 2y = 0. Solving, we get:

3y + 1 − 2y = 0 y + 1 = 0 y = −1. Hence, the equation of the tangent line at (3, 1) is y − 1 = −(x − 3), or y = 4 − x. 29. x 2/3 2+ 3y 2/3 = 2, (1, 1) x y + 2y = 3x, (2, 1) SOLUTION Taking the derivative of both sides of x 2/3 + y 2/3 = 2 yields 2 −1/3 2 −1/3 + y y = 0. x 3 3 Substituting x = 1, y = 1 yields 23 + 23 y = 0, so that 1 + y = 0, or y = −1. Hence, the equation of the tangent line at (1, 1) is y − 1 = −(x − 1), or y = 2 − x. 31. Find 1/2 the points on the graph of y 2 = x 3 − 3x + 1 (Figure 5) where the tangent line is horizontal. + y −1/2 = 2x y, 2 (1, 1) x (a) First show that 2yy = 3x − 3, where y = d y/d x. (b) Do not solve for y . Rather, set y = 0 and solve for x. This gives two possible values of x where the slope may be zero. (c) Show that the positive value of x does not correspond to a point on the graph. (d) The negative value corresponds to the two points on the graph where the tangent line is horizontal. Find the coordinates of these two points. y 2

x

−2 −1

1

2

−2

FIGURE 5 Graph of y 2 = x 3 − 3x + 1. SOLUTION

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D I F F E R E N T I AT I O N

(a) Applying implicit differentiation to y 2 = x 3 − 3x + 1, we have 2y

dy = 3x 2 − 3. dx

(b) Setting y = 0 we have 0 = 3x 2 − 3, so x = 1 or x = −1. (c) If we return to the equation y 2 = x 3 − 3x + 1 and substitute x = 1, we obtain the equation y 2 = −1, which has no real solutions. (d) Substituting x = −1 into y 2 = x 3 − 3x + 1 yields y 2 = (−1)3 − 3(−1) + 1 = −1 + 3 + 1 = 3, √ √ √ √ so y = 3 or − 3. The tangent is horizontal at the points (−1, 3) and (−1, − 3). 33. Show that no point on the graph of x 22 − 3x y2 + y 2 = 1 has a horizontal tangent line. Find all points on the graph of 3x + 4y + 3x y = 24 where the tangent line is horizontal (Figure 6). SOLUTION Let the implicit curve x 2 − 3x y + y 2 = 1 be given. Then (a) By differentiating the equation of the curve implicitly and setting y = 0, show that if the tangent line is horizontal at (x, y), then y = −2x. 2x − 3x y − 3y + 2yy = 0, (b) Solve for x by substituting y = −2x in the equation of the curve. so y =

2x − 3y . 3x − 2y

Setting y = 0 leads to y = 23 x. Substituting y = 23 x into the equation of the implicit curve gives 2 2 2 = 1, x + x x 2 − 3x 3 3 or − 59 x 2 = 1, which has no real solutions. Accordingly, there are no points on the implicit curve where the tangent line has slope zero. 35. If the derivative d x/d y exists at a point and d x/d y = 0, then the tangent line is vertical. Calculate d x/d y for the 4 + x y = x 3 − x + 2. Find d y/d x at the two points on the graph with x-coordinate thex 2graph of ythe equationFigure y 4 + 11 shows = y2 + and ﬁnd points on the graph where the tangent line is vertical. 0 and ﬁnd an equation of 2the tangent line at (1, 1). 4 2 SOLUTION Let y + 1 = y + x . Differentiating this equation with respect to y yields 4y 3 = 2y + 2x

dx , dy

so 4y 3 − 2y y(2y 2 − 1) dx = = . dy 2x x √ 2 dx Thus, = 0 when y = 0 and when y = ± . Substituting y = 0 into the equation y 4 + 1 = y 2 + x 2 gives dy 2 √ √ 2 3 1 = x 2 , so x = ±1. Substituting y = ± , gives x 2 = 3/4, so x = ± . Thus, there are six points on the graph of 2 2 4 2 2 y + 1 = y + x where the tangent line is vertical: √ √ √ √ √ √ √ √ 3 2 3 2 3 2 3 2 (1, 0), (−1, 0), , , − , , ,− , − ,− . 2 2 2 2 2 2 2 2 37. Differentiate the equation x 3 + 3x y 2 = 1 with respect to the variable t and express d y/dtd yin termsy of d xd x/dt, as in =− . the equation x y = 1 with respect to the variable t and derive the relation ExerciseDifferentiate 36. dt x dt 3 2 SOLUTION Let x + 3x y = 1. Then 3x 2

dy dx dx + 6x y + 3y 2 = 0, dt dt dt

and x 2 + y2 d x dy =− . dt 2x y dt In Exercises 38–39, differentiate the equation with respect to t to calculate d y/dt in terms of d x/dt. x 2 − y2 = 1

S E C T I O N 3.8

Implicit Differentiation

137

39. y 4 + 2x y + x 2 = 0 SOLUTION

Let y 4 + 2x y + x 2 = 0. Then 4y 3

dy dx dx dy + 2x + 2y + 2x = 0, dt dt dt dt

and x + y dx dy =− . dt x + 2y 3 dt discussed in 1638 by 41. The folium of Descartes is the curve with equation x 3 + y 3 = 3x y (Figure 7). It was ﬁrst The volume V and pressure P of gas in a piston (which vary in time t) satisfy P V 3/2 = C, where C is a the French philosopher-mathematician Ren´e Descartes. The name “folium” means leaf. Descartes’s scientiﬁc colleague constant. Prove that Gilles de Roberval called it the “jasmine ﬂower.” Both men believed incorrectly that the leaf shape in the ﬁrst quadrant was repeated in each giving the appearancedof petals of3a ﬂower. Find an equation of the tangent line to this P P/dt quadrant, =− curve at the point 23 , 43 . d V /dt 2 V The ratio of the derivatives is negative. Could you havey predicted this from the relation P V 3/2 = C? 2

x

−2

2

−2

FIGURE 7 Folium of Descartes: x 3 + y 3 = 3x y. SOLUTION

x2 − y 2 , 4 , we have Let x 3 + y 3 = 3x y. Then 3x 2 + 3y 2 y = 3x y + 3y, and y = . At the point 3 3 x − y2 4 − 4 − 89 4 3 y = 29 16 = 10 = . 5 −9 3 − 9

The tangent line at P is thus y − 43 = 45 x − 23 or y = 45 x + 45 .

43. Plot the equation x 3 + y 3 = 3x y + b for several values of b. Find all points on the folium x 3 + y 3 = 3x y at which the tangent line is horizontal. (a) Describe how the graph changes as b → 0. (b) Compute d y/d x (in terms of b) at the point on the graph where y = 0. How does this value change as b → ∞? Do your plots conﬁrm this conclusion? SOLUTION

(a) Consider the ﬁrst row of ﬁgures below. When b < 0, the graph of x 3 + y 3 = 3x y + b consists of two pieces. As b → 0−, the two pieces move closer to intersecting at the origin. From the second row of ﬁgures, we see that the graph of x 3 + y 3 = 3x y + b when b > 0 consists of a single piece that has a “loop” in the ﬁrst quadrant. As b → 0+, the loop comes closer to “pinching off” at the origin. b = −0.1

b = − 0.001

b = − 0.01

y

y

y

1.5

1.5

1.5

1

1

1

0.5

0.5

0.5

x

− 0.5

0.5 − 0.5

1

1.5

x

− 0.5

0.5 − 0.5

1

1.5

x

− 0.5

0.5 − 0.5

1

1.5

138

CHAPTER 3

D I F F E R E N T I AT I O N b = 0.1

b = 0.001

b = 0.01

y

y

y

1.5

1.5

1.5

1

1

1

0.5

0.5

0.5

x

− 0.5

0.5

1

1.5

x

−0.5

− 0.5

0.5

1

x

−0.5

1.5

0.5

1

1.5

−0.5

−0.5

(b) Differentiating the equation x 3 + y 3 = 3x y + b with respect to x yields 3x 2 + 3y 2 y = 3x y + 3y, so y − x2 y = 2 . y −x Substituting y = 0 into x 3 + y 3 = 3x y + b leads to x 3 = b, or x =

√ 3

b. Thus, at the point on the graph where y = 0,

√ 0 − x2 3 y = 2 = x = b. 0 −x Consequently, as b → ∞, y → ∞ at the point on the graph where y = 0. This conclusion is supported by the ﬁgures shown below, which correspond to b = 1, b = 10, and b = 100.

−4

b = 100 y

b = 01 y

b = 10 y

4

4

4

2

2

2

x

−2

2

4

−4

x

−2

2

−4

4

x

−2

2

−2

−2

−2

−4

−4

−4

4

√ a trident curve (Figure 9), named by Isaac Newton in his treatise on 45. The equation x y = x 3 − 5x 2 + 2x − 1 deﬁnes Show thatin the1710. tangent lines x = 1where ± 2the to the so-called 8) with equation (x − 1)2 (x 2 + y 2 ) = curves 2published Find theatpoints tangent to theconchoid trident is(Figure horizontal as follows. 2x are vertical. (a) Show that x y + y = 3x 2 − 10x + 2. (b) Set y = 0 in (a), replace y by x −1 (x 3 − 5x 2 + 2x − 1), and solve the resulting equation for x. y 20 4 −2

2

x 6

8

−20

FIGURE 9 Trident curve: x y = x 3 − 5x 2 + 2x − 1. SOLUTION

Consider the equation of a trident curve: x y = x 3 − 5x 2 + 2x − 1

or y = x −1 (x 3 − 5x 2 + 2x − 1). (a) Taking the derivative of x y = x 3 − 5x 2 + 2x − 1 yields x y + y = 3x 2 − 10x + 2.

S E C T I O N 3.8

Implicit Differentiation

139

(b) Setting y = 0 in (a) gives y = 3x 2 − 10x + 2. Thus, we have x −1 (x 3 − 5x 2 + 2x − 1) = 3x 2 − 10x + 2. Collecting like terms and setting to zero, we have 0 = 2x 3 − 5x 2 + 1 = (2x − 1)(x 2 − 2x − 1). Hence, x = 12 , 1 ±

√

2.

47. Find equations of the tangent lines at the points where x = 1 on the so-called folium (Figure 11): Find an equation of the tangent line at the four points on the curve (x 2 + y 2 − 4x)2 = 2(x 2 + y 2 ), where x = 1. 25 2 after the father of the French philosopher Blaise This curve (Figure 10) is an example of a limac xy )2 = named (x¸2on+ofy 2Pascal, 4 Pascal, who ﬁrst described it in 1650. y 2

x 1

−2

FIGURE 11 SOLUTION

2 First, ﬁnd the points (1, y) on the curve. Setting x = 1 in the equation (x 2 + y 2 )2 = 25 4 x y yields

25 2 y 4 25 2 y 4 + 2y 2 + 1 = y 4 (1 + y 2 )2 =

4y 4 + 8y 2 + 4 = 25y 2 4y 4 − 17y 2 + 4 = 0 (4y 2 − 1)(y 2 − 4) = 0 y2 =

1 or y 2 = 4 4

Hence y = ± 12 or y = ±2. Taking ddx of both sides of the original equation yields 25 2 25 y + x yy 4 2 25 2 25 y + x yy 4(x 2 + y 2 )x + 4(x 2 + y 2 )yy = 4 2 25 25 2 (4(x 2 + y 2 ) − x)yy = y − 4(x 2 + y 2 )x 2 4 2(x 2 + y 2 )(2x + 2yy ) =

25 y 2 − 4(x 2 + y 2 )x y = 4 y(4(x 2 + y 2 ) − 25 2 x) • At (1, 2), x 2 + y 2 = 5, and 25

22 − 4(5)(1) 1 y = 4 = . 25 3 2(4(5) − 2 (1)) Hence, at (1, 2), the equation of the tangent line is y − 2 = 13 (x − 1) or y = 13 x + 53 . • At (1, −2), x 2 + y 2 = 5 as well, and 25

(−2)2 − 4(5)(1) 1 y = 4 =− . 25 3 −2(4(5) − 2 (1)) Hence, at (1, −2), the equation of the tangent line is y + 2 = − 13 (x − 1) or y = − 13 x − 53 .

140

CHAPTER 3

D I F F E R E N T I AT I O N • At (1, 1 ), x 2 + y 2 = 5 , and 2 4

− 4 54 (1) 11

= y = . 1 4 5 − 25 (1) 12 2 4 2 25 4

2 1 2

11 5 Hence, at (1, 12 ), the equation of the tangent line is y − 12 = 11 12 (x − 1) or y = 12 x − 12 . • At (1, − 1 ), x 2 + y 2 = 5 , and 2 4

− 4 54 (1) 11

=− . y = 1 5 25 12 − 2 4 4 − 2 (1) 25 − 1 4 2

2

11 5 Hence, at (1, − 12 ), the equation of the tangent line is y + 12 = − 11 12 (x − 1) or y = − 12 x + 12 . The folium and its tangent lines are plotted below: y 2 1 x 1

0.5

1.5

2

−1 −2

Use a computer algebra system to plot y 2 = x 3 − 4x for −4 ≤ x, y ≤ 4. Show that if d x/d y = 0, then 49. Use a computer algebra system to plot (x 2 + y 2 )2 = 12(x 2 − y 2 ) + 2 for −4 ≤ x, y ≤ 4. How many y = 0. Conclude that the tangent line is vertical at the points where the curve intersects the x-axis. Does your plot conﬁrm horizontal tangent lines does the curve appear to have? Find the points where these occur. this conclusion? SOLUTION

A plot of the curve y 2 = x 3 − 4x is shown below. y 2 1

−2

x

−1

1

2

3

−1 −2

Differentiating the equation y 2 = x 3 − 4x with respect to y yields 2y = 3x 2

dx dx −4 , dy dy

or dx 2y . = 2 dy 3x − 4 From here, it follows that dd xy = 0 when y = 0, so the tangent line to this curve is vertical at the points where the curve intersects the x-axis. This conclusion is conﬁrmed by the plot of the curve shown above. In Exercises 50–53, use implicit differentiation to calculate higher derivatives. 51. Use the method of the previous exercise to show that y = −y −3 if x 2 + y 2 = 1. Consider the equation y 3 − 32 x 2 = 1. x SOLUTION Let x 2 + y 2 =21. Then 2x + 2yy = 0, and y = − . Thus (a) Show that y = x/y and differentiate again to show that y

2 y −yx =− xyy − 2x yy y · 1 − x y y2 + x 2 1 4 y =− =− = = − 3 = −y −3 . y− 2 2 3 y y y y (b) Express y in terms of x and y using part (a). Calculate y at the point (1, 1) on the curve x y 2 + y − 2 = 0 by the following steps:

S E C T I O N 3.9

Related Rates

141

53. Use the method of the previous exercise to compute y at the point 23 , 43 on the folium of Descartes with the equation x 3 + y 3 = 3x y.

x2 − y 2 , 4 , we ﬁnd SOLUTION Let x 3 + y 3 = 3x y. Then 3x 2 + 3y 2 y = 3x y + 3y, and y = . At y) = (x, 3 3 x − y2 4 − 4 4 − 12 −8 4 3 y = 29 16 = = = . 6 − 16 −10 5 3 − 9

Similarly,

y =

x − y2

2x − y − x 2 − y 1 − 2yy 162 =− 2 125 2 x−y

when (x, y) = 23 , 43 and y = 45 .

Further Insights and Challenges 55. The lemniscate curve (x 2 + y 2 )2 = 4(x 2 − y 2 ) was discovered by Jacob Bernoulli in 1694, who noted that it is if P8, lies the or intersection two curves x 2coordinates − y 2 = c of andthe x yfour = dpoints (c, d atconstants), the “shapedShow like athat ﬁgure or aon knot, the bow ofofa the ribbon.” Find the which thethen tangent tangents to the curves at P are perpendicular. line is horizontal (Figure 12). y 1 x

−1

1 −1

FIGURE 12 Lemniscate curve: (x 2 + y 2 )2 = 4(x 2 − y 2 ).

2

Consider the equation of a lemniscate curve: x 2 + y 2 = 4 x 2 − y 2 . Taking the derivative of both sides of this equation, we have

2 x 2 + y 2 2x + 2yy = 4 2x − 2yy . SOLUTION

Therefore,

x 2 + y2 − 2 x 8x − 4x x 2 + y 2 . =− y = 8y + 4y x 2 + y 2 x 2 + y2 + 2 y If y = 0, then either x = 0 or x 2 + y 2 = 2.

• If x = 0 in the lemniscate curve, then y 4 = −4y 2 or y 2 y 2 + 4 = 0. If y is real, then y = 0. The formula for y

in (a) is not deﬁned at the origin (0/0). An alternative parametric analysis shows that the slopes of the tangent lines to the curve at the origin are ±1.

• If x 2 + y 2 = 2 or y 2 = 2 − x 2 , then plugging this into the lemniscate equation gives 4 = 4 2x 2 − 2 which

√ √ yields x = ± 32 = ± 26 . Thus y = ± 12 = ± 22 . Accordingly, the four points at which the tangent lines to the √ √ √ √ √

√ √

√ lemniscate curve are horizontal are − 26 , − 22 , − 26 , 22 , 26 , − 22 , and 26 , 22 . Divide the curve in Figure 13 into ﬁve branches, each of which is the graph of a function. Sketch the branches.

3.9 Related Rates Preliminary Questions 1. Assign variables and restate the following problem in terms of known and unknown derivatives (but do not solve it): How fast is the volume of a cube increasing if its side increases at a rate of 0.5 cm/s? SOLUTION

Let s and V denote the length of the side and the corresponding volume of a cube, respectively. Determine

d V if ds = 0.5 cm/s. dt dt

142

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D I F F E R E N T I AT I O N

2. What is the relation between d V /dt and dr/dt if

4 V = πr 3 3

SOLUTION

Applying the general power rule, we ﬁnd ddtV = 4π r 2 dr dt .

In Questions 3–4, suppose that water pours into a cylindrical glass of radius 4 cm. The variables V and h denote the volume and water level at time t, respectively. 3. Restate in terms of the derivatives d V /dt and dh/dt: How fast is the water level rising if water pours in at a rate of 2 cm3 /min? dV 3 Determine dh dt if dt = 2 cm /min. 4. Repeat the same for this problem: At what rate is water pouring in if the water level rises at a rate of 1 cm/min?

SOLUTION

SOLUTION

Determine ddtV if dh dt = 1 cm/min.

Exercises In Exercises 1–2, consider a rectangular bathtub whose base is 18 ft2 . 1. How fast is the water level rising if water is ﬁlling the tub at a rate of 0.7 ft3 /min? Let h be the height of the water in the tub and V be the volume of the water. Then V = 18h and dh dV = 18 . Thus dt dt

SOLUTION

1 dV 1 dh = = (.7) ≈ .039 ft/min. dt 18 dt 18 3. The radius of a circular oil slick expands at a rate of 2 m/min. At what rate is water pouring into the tub if the water level rises at a rate of 0.8 ft/min? (a) How fast is the area of the oil slick increasing when the radius is 25 m? (b) If the radius is 0 at time t = 0, how fast is the area increasing after 3 min? Let r be the radius of the oil slick and A its area. dA dr = 2π r . Substituting r = 25 and dr (a) Then A = π r 2 and dt = 2, we ﬁnd dt dt

SOLUTION

dA = 2π (25) (2) = 100π ≈ 314.16 m2 /min. dt (b) Since dr dt = 2 and r (0) = 0, it follows that r (t) = 2t. Thus, r (3) = 6 and dA = 2π (6) (2) = 24π ≈ 75.40 m2 /min. dt In Exercises 5–8,rate assume the radius r ofincreasing a sphere isif expanding at increasing a rate of 14 volume of a sphere is At what is the that diagonal of a cube its edges are at in./min. a rate of The 2 cm/s? V = 43 π r 3 and its surface area is 4π r 2 . 5. Determine the rate at which the volume is changing with respect to time when r = 8 in. d As the radius is expanding at 14 inches per minute, we know that dr dt = 14 in./min. Taking dt of the equation V = 43 π r 3 yields dr dr dV 4 = π 3r 2 = 4π r 2 . dt 3 dt dt SOLUTION

Substituting r = 8 and dr dt = 14 yields dV = 4π (8)2 (14) = 3584π in.3 /min. dt 7. Determine the rate at which the surface area is changing when the radius is r = 8 in. Determine the rate at which the volume is changing with respect to time at t = 2 min, assuming that r = 0 at dA dr dr 2 SOLUTION t = 0. Taking the derivative of both sides of A = 4π r with respect to t yields dt = 8π r dt . dt = 14, so dA = 8π (8)(14) = 896π in.2 /min. dt Determine the rate at which the surface area is changing with respect to time at t = 2 min, assuming that r = 3 at t = 0.

S E C T I O N 3.9

Related Rates

143

9. A road perpendicular to a highway leads to a farmhouse located 1 mile away (Figure 9). An automobile travels past the farmhouse at a speed of 60 mph. How fast is the distance between the automobile and the farmhouse increasing when the automobile is 3 miles past the intersection of the highway and the road?

l 60 mph

Automobile

FIGURE 9 SOLUTION Let l denote the distance between the automobile and the farmhouse, and let s denote the distance past the intersection of the highway and the road. Then l 2 = 1 + s 2 . Taking the derivative of both sides of this equation yields dl = 2s ds , so 2l dt dt

s ds dl = . dt l dt When the auto is 3 miles past the intersection, we have √ 3 · 60 dl 180 = √ = 18 10 ≈ 56.92 mph. = 2 2 dt 10 1 +3 11. Follow the same set-up as Exercise 10, but assume that the water level is rising at a rate of 0.3 m/min when it is 2 m. 3 conical tankﬂowing has height At whatArate is water in? 3 m and radius 2 m at the top. Water ﬂows in at a rate of 2 m /min. How fast is the water level rising when it is 2 m? SOLUTION Consider the cone of water in the tank at a certain instant. Let r be the radius of its (inverted) base, h its 4 π h 3 . Accordingly, height, and V its volume. By similar triangles, hr = 23 or r = 23 h and thus V = 13 π r 2 h = 27 dV 4 dh = π h2 . dt 9 dt Substituting h = 2 and dh dt = 0.3 yields dV 4 = π (2)2 (.3) ≈ 1.68 m3 /min. dt 9 13. Answer (a) and (b) in Exercise 12 assuming that Sonya begins moving 1 minute after Isaac takes off. Sonya and Isaac are in motorboats located at the center of a lake. At time t = 0, Sonya begins traveling south at SOLUTION IsaacAtx the miles easttime, of the center of the and Sonya miles south its center, let h be the distance a speed ofWith 32 mph. same Isaac takes off, lake heading east at ya speed of 27 of mph. between them. (a) How far have Sonya and Isaac each traveled after 12 min? 12 = 1 hour, Isaac has traveled 1 × 27 = 27 miles. After 11 minutes or 11 hour, Sonya has (a) After minutes (b) At12what rate isorthe between them increasing 60 distance 5 5 at t = 125 min? 60 11 88 traveled 60 × 32 = 15 miles. dh dx dy = 2x + 2y . Thus, (b) We have h 2 = x 2 + y 2 and 2h dt dt dt x d x + y ddty x d x + y ddty dh . = dt = dt dt h x 2 + y2 dy dx 88 Substituting x = 27 5 , dt = 27, y = 15 , and dt = 32 yields

dh = dt

27 (27) + 5

27 5

2

88 15

+ 88 15

(32)

2

5003 = √ ≈ 41.83 mph. 14305

15. At a given moment, a plane passes directly above a radar station at an altitude of 6 miles. A 6-ft man walks away from a 15-ft lamppost at a speed of 3 ft/s (Figure 10). Find the rate at which his shadow (a) If the plane’sinspeed is 500 mph, how fast is the distance between the plane and the station changing half an hour is increasing length. later? (b) How fast is the distance between the plane and the station changing when the plane passes directly above the station? SOLUTION

Let x be the distance of the plane from the station along the ground and h the distance through the air.

144

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D I F F E R E N T I AT I O N

(a) By the Pythagorean Theorem, we have h 2 = x 2 + 62 = x 2 + 36. dh dx dh x dx = 2x , and = . After an half hour, x = 12 × 500 = 250 miles. With x = 250, h = 2502 + 36, dt dt dt h dt and ddtx = 500, Thus 2h

250 dh = × 500 ≈ 499.86 mph. dt 2502 + 36 (b) When the plane is directly above the station, x = 0, so the distance between the plane and the station is not changing, for at this instant we have 0 dh = × 500 = 0 mph. dt 6 17. A hot air balloon rising vertically is tracked by an observer located 2 miles from the lift-off point. At a certain In the setting of Exercise 15, suppose that the line through the radar station and the plane makes an angle θ with moment, the angle between the observer’s line-of-sight and the horizontal is π5 , and it is changing at a rate of 0.2 rad/min. the horizontal. How fast is θ changing 10 min after the plane passes over the radar station? How fast is the balloon rising at this moment? SOLUTION

Let y be the height of the balloon (in miles) and θ the angle between the line-of-sight and the horizontal. y . Therefore, 2

Via trigonometry, we have tan θ =

sec2 θ ·

1 dy dθ = , dt 2 dt

and dy dθ =2 sec2 θ . dt dt Using ddtθ = 0.2 and θ = π5 yields dy 1 ≈ .61 mi/min. = 2 (.2) 2 dt cos (π /5) In Exercises referaway to a 16-ft down wall, Figuresmoves 1 and twice 2. Theasvariable h isdoes. the height the As a 19–23, man walks from ladder a 12-ftsliding lamppost, thea tip of as hisinshadow fast as he What of is the ladder’s topheight? at time t, and x is the distance from the wall to the ladder’s bottom. man’s 19. Assume the bottom slides away from the wall at a rate of 3 ft/s. Find the velocity of the top of the ladder at t = 2 if the bottom is 5 ft from the wall at t = 0. SOLUTION Let x denote the distance from the base of the ladder to the wall, and h denote the height of the top of the ladder from the ﬂoor. The ladder is 16 ft long, so h 2 + x 2 = 162 . At any time t, x = 5 + 3t. Therefore, at time t = 2, the base is x = 5 + 3(2) = 11 ft from the wall. Furthermore, we have

2h Substituting x = 11, h =

dh dx + 2x = 0 so dt dt

162 − 112 and ddtx = 3, we obtain

dh x dx =− . dt h dt

11 dh 11 (3) = − √ ft/s ≈ −2.84 ft/s. = − dt 15 162 − 112 21. Suppose that h(0) = 12 and the top slides down the wall at a rate of 4 ft/s. Calculate x and d x/dt at t = 2 s. Suppose that the top is sliding down the wall at a rate of 4 ft/s. Calculate d x/dt when h = 12. SOLUTION Let h and x be the height of the ladder’s top and the distance from the wall of the ladder’s bottom, respectively. After 2 seconds, h = 12 + 2 (−4) = 4 ft. Since h 2 + x 2 = 162 , √ x = 162 − 42 = 4 15 ft. Furthermore, we have 2h ﬁnd

√ dh dx dx h dh + 2x = 0, so that =− . Substituting h = 4, x = 4 15, and dh dt = −4, we dt dt dt x dt dx 4 4 = − √ (−4) = √ ≈ 1.03 ft/s. dt 4 15 15

What is the relation between h and x at the moment when the top and bottom of the ladder move at the same speed?

S E C T I O N 3.9

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145

23. Show that the velocity dh/dt approaches inﬁnity as the ladder slides down to the ground (assuming d x/dt is constant). This suggests that our mathematical description is unrealistic, at least for small values of h. What would, in fact, happen as the top of the ladder approaches the ground? SOLUTION Let L be the (constant) length of the ladder, let x be the distance from the base of the ladder to the point of contact of the ladder with the ground, and let h be the height of the point of contact of the ladder with the wall. By Pythagoras’ theorem,

x 2 + h2 = L 2. Taking derivatives with respect to t yields: 2x

dh dx + 2h = 0. dt dt

Therefore, dh x dx =− . dt h dt As ddtx is constant and positive and x approaches the length of the ladder as it falls, − hx ddtx gets arbitrarily large as h → 0. In a real situation, the top of the ladder would slide down the wall only part of the way. At some point it would lose contact with the wall and fall down freely with acceleration g. 25. Suppose that both the radius r and height h of a circular cone change at a rate of 2 cm/s. How fast is the volume of The radius r of a right circular cone of ﬁxed height h = 20 cm is increasing at a rate of 2 cm/s. How fast is the the cone increasing when r = 10 and h = 20? volume increasing when r = 10? 1 SOLUTION Let r be the radius, h be the height, and V be the volume of a right circular cone. Then V = 3 π r 2 h, and dV dh 1 dr = π r2 + 2hr . dt 3 dt dt dh When r = 10, h = 20, and dr dt = dt = 2, we ﬁnd 1000π dV π 2 10 · 2 + 2 · 20 · 10 · 2 = = ≈ 1047.20 cm3 /s. dt 3 3

27. A searchlight rotates at a rate of 3 revolutions per minute. The beam hits a wall located 10 miles away and produces 2 + 16y 2 = 25 (Figure 11). particle the How ellipse 9xis a dot ofAlight that moves moves counterclockwise horizontally alongaround the wall. fast this dot moving when the angle θ between the beam π and the line through the searchlight perpendicular to the wall is 6 ? Note that d θ /dt = 3(2π ) = 6π . SOLUTION Let y be the distance between the dot of light and the point of intersection of the wall and the line through the searchlight perpendicular to the wall.is Let be the angle between the Explain beam ofyour lightanswer. and the line. Using trigonometry, (a) In which of the four quadrants the θderivative d x/dt positive? y we have tan θ = 10 . Therefore, (b) Find a relation between d x/dt and d y/dt. (c) At what rate is the x-coordinate changing when the the point (1, 1) if its y-coordinate is increasing d θparticle 1 passes dy = , sec2 θ · at a rate of 6 ft/s? dt 10 dt and (d) What is d y/dt when the particle is at the top and bottom of the ellipse?

dy dθ = 10 sec2 θ . dt dt With θ = π6 and ddtθ = 6π , we ﬁnd 1 dy = 80π ≈ 251.33 mi/min ≈ 15,079.64 mph. = 10 (6π ) dt cos2 (π /6) 29. A plane traveling at an altitude of 20,000 ft passes directly overhead at time t = 0. One minute later you observe A rocket travels vertically at a speed of 800 mph. The rocket is tracked through a telescope by an observer located that the angle between the vertical and your line of sight to the plane is 1.14 rad and that this angle is changing at a rate 10 miles from the launching pad. Find the rate at which the angle between the telescope and the ground is increasing of 0.38 rad/min. Calculate the velocity of the airplane. 3 min after lift-off. SOLUTION Let x be the distance of the plane from you along the ground and θ the angle between the vertical and your x line of sight to the plane. Then tan θ = 20000 and dθ dx = 20000 sec2 θ · . dt dt Substituting θ = 1.14 and ddtθ = 0.38, we ﬁnd dx 20000 = (.38) ≈ 43581.69 ft/min dt cos2 (1.14) or roughly 495.25 mph.

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31. A jogger runs around a circular track of radius 60 ft. Let (x, y) be her coordinates, where the origin is at the center 2 /s) at which area is swept out by the second hand of a circular clock as a function of Calculate (in cmcoordinates of the track. Whenthe therate jogger’s are (36, 48), her x-coordinate is changing at a rate of 14 ft/s. Find d y/dt. the clock’s radius. dx dy dy x dx SOLUTION We have x 2 + y 2 = 602 . Thus 2x + 2y = 0, and =− . With x = 36, y = 48, and dt dt dt y dt d x = 14, dt dy 36 21 = − (14) = − = −10.5 ft/s. dt 48 2 3 ) of an expanding gas are related In Exercises the pressure P (in volume cmthe A car33–34, travelsassume down a that highway at 55 mph. Ankilopascals) observer is and standing 500Vft(in from highway. b by P(a) V How = C, fast where b and C arebetween constants holds and in adiabatic expansion,atwithout heat gain or passes loss). in front of the is the distance the(this observer the car increasing the moment the car

observer? Can answer without relying 2 , and on 33. Find d P/dt if byou = justify 1.2, P your = 8 kPa, V = 100 cm d Vany /dt calculations? = 20 cm3 /min. (b) How fast is the distance between the observer and the car increasing 1 min later? b SOLUTION Let P V = C. Then PbV b−1

dV dP + Vb = 0, dt dt

and Pb d V dP =− . dt V dt Substituting b = 1.2, P = 8, V = 100, and ddtV = 20, we ﬁnd dP (8) (1.2) =− (20) = −1.92 kPa/min. dt 100 35. A point moves along the parabola y = x 2 + 1. Let (t) be the2 distance between the point and the origin. Calculate , and /dt = 20 cm3 /min. Find b ifthat P =the25x-coordinate kPa, d P/dt of = the 12 kPa/min, V = 100atcm (t), assuming point is increasing a rate ofd9Vft/s. A point moves along the parabola y = x 2 + 1. Let be the distance between the point and the origin. By the distance formula, we have = (x 2 + (x 2 + 1)2 )1/2 . Hence SOLUTION

−1/2 d x

(2x 2 + 3)x ddtx dx 1 2 d = x + (x 2 + 1)2 + 2 x 2 + 1 · 2x = 2x . dt 2 dt dt x 4 + 3x 2 + 1 With ddtx = 9, we have 9x(2x 2 + 3) (t) = ft/s. x 4 + 3x 2 + 1 37. A water tank in the shape of a right circular cone of radius 300 cm and height 500 cm leaks water from the vertex at The base x of the right triangle in Figure 12 increases at a rate of 5 cm/s, while the height remains constant at a rate of 10 cm3 /min. Find the rate at which the water level is decreasing when it is 200 cm. h = 20. How fast is the angle θ changing when x = 20? SOLUTION Consider the cone of water in the tank at a certain instant. Let r be the radius of its (inverted) base, h its 3 1 2 3 3 height, and V its volume. By similar triangles, hr = 300 500 or r = 5 h and thus V = 3 π r h = 25 π h . Therefore, dV 9 dh π h2 , = dt 25 dt and 25 d V dh = . dt 9π h 2 dt We are given ddtV = −10. When h = 200, it follows that dh 25 1 = ≈ −2.21 × 10−4 cm/min. (−10) = − 2 dt 1440 π 9π (200) Thus, the water level is decreasing at the rate of 2.21 × 10−4 cm/min. Two parallel paths 50 ft apart run through the woods. Shirley jogs east on one path at 6 mph, while Jamail walks west on the other at 4 mph. If they pass each other at time t = 0, how far apart are they 3 s later, and how fast is the distance between them changing at that moment?

S E C T I O N 3.9

Related Rates

147

Further Insights and Challenges 39. Henry is pulling on a rope that passes through a pulley on a 10-ft pole and is attached to a wagon (Figure 13). Assume that the rope is attached to a loop on the wagon 2 ft off the ground. Let x be the distance between the loop and the pole. (a) Find a formula for the speed of the wagon in terms of x and the rate at which Henry pulls the rope. (b) Find the speed of the wagon when it is 12 ft from the pole, assuming that Henry pulls the rope at a rate of 1.5 ft/sec.

2 x

FIGURE 13 SOLUTION Let h be the distance from the pulley to the loop on the wagon. (Note that the rate at which Henry pulls the rope is the rate at which h is decreasing.) Using the Pythagorean Theorem, we have h 2 = x 2 + (10 − 2)2 = x 2 + 82 . dh dx dx h dh (a) Thus 2h = 2x , and = . dt dt dt x dt (b) As Henry pulls the rope at the rate of 1.5 = 32 ft/s, the distance h is decreasing at that rate; i.e., dh/dt = − 32 . When the wagon is 12 feet from the pole, we thus have √ dx 122 + 82 3 = − = − 13/2 ft/s. dt 12 2 √ 13 Thus, the speed of the wagon is ft/s. 2

41. Using a telescope, you track a rocket that was launched 2 miles away, recording the angle θ between the telescope A roller coaster has the shape of the graph in Figure 14. Show that when the roller coaster passes the point and the ground at half-second intervals. Estimate the velocity of the rocket if θ (10) = 0.205 and θ (10.5) = 0.225. (x, f (x)), the vertical velocity of the roller coaster is equal to f (x) times its horizontal velocity. h SOLUTION Let h be the height of the vertically ascending rocket. Using trigonometry, tan θ = , so 2 dθ dh = 2 sec2 θ · . dt dt We are given θ (10) = 0.205, and we can estimate θ (10.5) − θ (10) d θ = 0.04. ≈ dt t=10 0.5 Thus, dh = 2 sec2 (0.205) · (0.04) ≈ 0.083 mi/s, dt or roughly 300 mph. 43. A baseball player runs from home plate toward ﬁrst base at 20 ft/s. How fast is the player’s distance from second Two trains leave a station at t = 0 and travel with constant velocity v along straight tracks that make an angle θ . base changing when the player is halfway to ﬁrst base? See Figure 15. √ (a) Show that the trains are separating from each other at a rate of v 2 − 2 cos θ . (b) What does this formula give for θ = π ?

20 ft/s

FIGURE 15 Baseball diamond. SOLUTION In baseball, the distance between bases is 90 feet. Let x be the distance of the player from home plate and h the player’s distance from second base. Using the Pythagorean theorem, we have h 2 = 902 + (90 − x)2 . Therefore, dh dx 2h = 2 (90 − x) − , dt dt

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and dh 90 − x d x =− . dt h dt We are given ddtx = 20. When the player is halfway to ﬁrst base, x = 45 and h =

902 + 452 , so

√ 45 dh = − (20) = −4 5 ≈ −8.94 ft/s. 2 2 dt 90 + 45 45. A spectator seated 300 m away from the center of a circular track of radius 100 m watches an athlete run laps at As the wheel of radius r cm in Figure 16 rotates, the rod of length L attached at the point P drives a piston back a speed of 5 m/s. How fast is the distance between the spectator and athlete changing when the runner is approaching and forth in a straight line. Let x be the distance from the origin to the point Q at the end of the rod as in the ﬁgure. the spectator and the distance between them is 250 m? Hint: The diagram for this problem is similar to Figure 16, with (a) Use Theorem to show that r = 100 and xthe=Pythagorean 300. SOLUTION

2 θ those of Q are (x, 0). From the diagram, the coordinates cosθθ)2, r+sin ) and L 2 of =P (xare − r(rcos r 2 θsin

• The distance formula gives (b) Differentiate Eq. (8) with respect to t to prove that

d x − r cosdθθ)2 + (−r2 sin θ )2 . d θ L 2(x − r cos θ ) = (x + r sin θ + 2r sin θ cos θ =0 dt dt dt

Thus, (c) Calculate the speed of the piston when θ = π2 , assuming that r = 10 cm, L = 30 cm, and the wheel rotates at 4 L 2 = (x − r cos θ )2 + r 2 sin2 θ . revolutions per minute. Note that x (the distance of the spectator from the center of the track) and r (the radius of the track) are constants. • Differentiating with respect to t gives

2L

dθ dθ dL = 2 (x − r cos θ ) r sin θ + 2r 2 sin θ cos θ . dt dt dt

Thus, dθ rx dL = sin θ . dt L dt

θ , namely s = r θ . Thus

• Recall the relation between arc length s and angle ds = −5, we have dt

dθ 1 ds = . Given r = 100 and dt r dt

dθ 1 1 = rad/s. (−5) = − dt 100 20 (Note: In this scenario, the runner traverses the track in a clockwise fashion and approaches the spectator from Quadrant 1.) • Next, the Law of Cosines gives L 2 = r 2 + x 2 − 2r x cos θ , so cos θ =

r 2 + x2 − L2 1002 + 3002 − 2502 5 = = . 2r x 2 (100) (300) 8

Accordingly, sin θ =

√ 2 5 39 1− = . 8 8

• Finally

dL (300) (100) = dt 250

√

39 8

−

1 20

=−

√ 3 39 ≈ −4.68 m/s. 4

CHAPTER REVIEW EXERCISES In Exercises 1–4, refer to the function f (x) whose graph is shown in Figure 1.

Chapter Review Exercises

7 6 5 4 3 2 1

149

y

0.5

1.0

1.5

x 2.0

FIGURE 1

1. Compute the average ROC of f (x) over [0, 2]. What is the graphical interpretation of this average ROC? SOLUTION

The average rate of change of f (x) over [0, 2] is f (2) − f (0) 7−1 = = 3. 2−0 2−0

Graphically, this average rate of change represents the slope of the secant line through the points (2, 7) and (0, 1) on the graph of f (x). f (0.7 + h) − f (0.7) for h+= this number larger or smaller than f (0.7)? 3. Estimate f (0.7 h)0.3. − fIs(0.7) equal to the slope of the secant line between the points where For which valuehof h is h SOLUTION Forxh==1.1. 0.3, x = 0.7 and f (0.7 + h) − f (0.7) f (1) − f (0.7) 2.8 − 2 8 = ≈ = . h 0.3 0.3 3 Because the curve is concave up, the slope of the secant line is larger than the slope of the tangent line, so the value of the difference quotient should be larger than the value of the derivative. using the limit deﬁnition and ﬁnd an equation of the tangent line to the graph of f (x) In Exercises 5–8, fcompute f (a) (0.7) and Estimate f (1.1). at x = a. 5. f (x) = x 2 − x, SOLUTION

a=1

Let f (x) = x 2 − x and a = 1. Then f (a + h) − f (a) (1 + h)2 − (1 + h) − (12 − 1) = lim h h h→0 h→0

f (a) = lim

1 + 2h + h 2 − 1 − h = lim (1 + h) = 1 h h→0 h→0

= lim

and the equation of the tangent line to the graph of f (x) at x = a is y = f (a)(x − a) + f (a) = 1(x − 1) + 0 = x − 1. −1 , a = 4 7. f (x)f (x) = x= 5 − 3x, a = 2 SOLUTION Let f (x) = x −1 and a = 4. Then 1 − 1 f (a + h) − f (a) 4 − (4 + h) 4 = lim 4+h = lim h h h→0 h→0 h→0 4h(4 + h)

f (a) = lim

−1 1 1 =− =− 4(4 + 0) 16 h→0 4(4 + h)

= lim

and the equation of the tangent line to the graph of f (x) at x = a is 1 1 1 1 y = f (a)(x − a) + f (a) = − (x − 4) + = − x + . 16 4 16 2 In Exercises 9–12,3 compute d y/d x using the limit deﬁnition. f (x) = x , a = −2 9. y = 4 − x 2 SOLUTION

Let y = 4 − x 2 . Then

4 − (x + h)2 − (4 − x 2 ) 4 − x 2 − 2xh − h 2 − 4 + x 2 dy = lim = lim = lim (−2x − h) = −2x − 0 = −2x. dx h h h→0 h→0 h→0 y=

√ 2x + 1

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11. y =

1 2−x

SOLUTION

Let y =

1 . Then 2−x 1

1

dy 1 (2 − x) − (2 − x − h) 1 2−(x+h) − 2−x . = lim = lim = lim = dx h h→0 h→0 h(2 − x − h)(2 − x) h→0 (2 − x − h)(2 − x) (2 − x)2 In Exercises 13–16, 1 express the limit as a derivative. y√ = 1)21 1(x +− h− 13. lim h h→0 √ SOLUTION Let f (x) = x. Then √ lim

h→0

15. lim

1+h−1 f (1 + h) − f (1) = lim = f (1). h h h→0

sin t cos t

3 t→π limt −xπ + 1 x→−1 x + 1

SOLUTION

Let f (t) = sin t cos t and note that f (π ) = sin π cos π = 0. Then lim

t→π

sin t cos t f (t) − f (π ) = lim = f (π ). t→π t −π t −π

17. Find f (4) and f (4) if the tangent line to the graph of f (x) at x = 4 has equation y = 3x − 14. cos θ − sin θ + 1 lim SOLUTION θ − π of the tangent line to the graph of f (x) at x = 4 is y = f (4)(x − 4) + f (4) = f (4)x + θ →π The equation ( f (4) − 4 f (4)). Matching this to y = 3x − 14, we see that f (4) = 3 and f (4) − 4(3) = −14, so f (4) = −2. 19. Which of (A), (B), or (C) is the graph of the derivative of the function f (x) shown in Figure 3? Each graph in Figure 2 shows the graph of a function f (x) and its derivative f (x). Determine which is the function and which is the derivative. y y = f(x) x

−2 −1 y

2

y x

−2 −1

1

1

2

x

−2 −1

(A)

y

1

2

x

−2 −1

(B)

1

2

(C)

FIGURE 3

The graph of f (x) has four horizontal tangent lines on [−2, 2], so the graph of its derivative must have four x-intercepts on [−2, 2]. This eliminates (B). Moreover, f (x) is increasing at both ends of the interval, so its derivative must be positive at both ends. This eliminates (A) and identiﬁes (C) as the graph of f (x). SOLUTION

21. A girl’s height h(t) (in centimeters) is measured at time t (years) for 0 ≤ t ≤ 14: Let N (t) be the percentage of a state population infected with a ﬂu virus on week t of an epidemic. What = 1.2? percentage is likely to be infected week 4 if 96.7, N (3) 104.5, = 8 and N (3) 52,in75.1, 87.5, 111.8, 118.7, 125.2, 131.5, 137.5, 143.3, 149.2, 155.3, 160.8, 164.7 (a) What is the girl’s average growth rate over the 14-year period? (b) Is the average growth rate larger over the ﬁrst half or the second half of this period? (c) Estimate h (t) (in centimeters per year) for t = 3, 8. SOLUTION

(a) The average growth rate over the 14-year period is 164.7 − 52 = 8.05 cm/year. 14

Chapter Review Exercises

151

(b) Over the ﬁrst half of the 14-year period, the average growth rate is 125.2 − 52 ≈ 10.46 cm/year, 7 which is larger than the average growth rate over the second half of the 14-year period: 164.7 − 125.2 ≈ 5.64 cm/year. 7 (c) For t = 3, h (3) ≈

h(4) − h(3) 104.5 − 96.7 = = 7.8 cm/year; 4−3 1

h (8) ≈

h(9) − h(8) 137.5 − 131.5 = = 6.0 cm/year. 9−8 1

for t = 8,

In Exercises 23–24, use the following table of values for the number A(t) of automobiles (in millions) manufactured3/2in A planet’s period P (time in years to complete one revolution around the sun) is approximately 0.0011 A , the United States in year t. where A is the average distance (in millions of miles) from the planet to the sun. (a) Calculate P and d P/d Mars using 142. 1974 1975 1976 t A for1970 1971the value 1972 A = 1973 (b) Estimate the increase in P if A is increased to 143. A(t) 6.55 8.58 8.83 9.67 7.32 6.72 8.50 23. What is the interpretation of A (t)? Estimate A (1971). Does A (1974) appear to be positive or negative? Because A(t) measures the number of automobiles manufactured in the United States in year t, A (t) measures the rate of change in automobile production in the United States. For t = 1971, SOLUTION

A (1971) ≈

A(1972) − A(1971) 8.83 − 8.58 = = 0.25 million automobiles/year. 1972 − 1971 1

Because A(t) decreases from 1973 to 1974 and from 1974 to 1975, it appears that A (1974) would be negative. In Exercises compute derivative. Given25–50, the data, whichthe of (A)–(C) in Figure 4 could be the graph of the derivative A (t)? Explain. 25. y = 3x 5 − 7x 2 + 4 SOLUTION

Let y = 3x 5 − 7x 2 + 4. Then dy = 15x 4 − 14x. dx

27. y = t −7.3 −3/2 y = 4x SOLUTION Let y = t −7.3 . Then dy = −7.3t −8.3 . dt x +1 29. y =y =2 4x 2 − x −2 x +1 SOLUTION

x +1 . Then Let y = 2 x +1 1 − 2x − x 2 (x 2 + 1)(1) − (x + 1)(2x) dy = . = 2 2 dx (x + 1) (x 2 + 1)2

6 31. y = (x 4 3t −− 9x) 2 y= SOLUTION 4tLet − 9y = (x 4 − 9x)6 . Then

d dy = 6(x 4 − 9x)5 (x 4 − 9x) = 6(4x 3 − 9)(x 4 − 9x)5 . dx dx 33. y = (2 + 9x2 2 )3/2 −3 6 y = (3t + 20t )

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Let y = (2 + 9x 2 )3/2 . Then 3 d dy = (2 + 9x 2 )1/2 (2 + 9x 2 ) = 27x(2 + 9x 2 )1/2 . dx 2 dx

z 35. y = √ 3 4 y =1(x−+ z 1) (x + 4) SOLUTION

Let y = √

z 1−z

. Then √

dy = dz

1 − z − (− 2z ) √ 1

1−z

1−z

=

1 − z + 2z (1 − z)3/2

=

2−z . 2(1 − z)3/2

√ x4 + x 3 37. y = 1 y = x 12 + x SOLUTION Let y=

√ x4 + x = x 2 + x −3/2 . x2

Then dy 3 = 2x − x −5/2 . dx 2 39. y = tan(t −3 ) 1 y= √ −3 SOLUTION (1Let y = 2tan(t − x) − x ). Then d dy = sec2 (t −3 ) t −3 = −3t −4 sec2 (t −3 ). dt dt 2x 41. y =y sin(2x) cos− = 4 cos(2 3x) SOLUTION Let y = sin(2x) cos2 x = 2 sin x cos3 x. Then

dy = −6 sin2 x cos2 x + 2 cos4 x. dx 43. y = tan3 θ y = sin( x 2 + 1) SOLUTION Let y = tan3 θ . Then d dy = 3 tan2 θ tan θ = 3 tan2 θ sec2 θ . dθ dθ t 45. y = sec t4 y 1=+sin θ SOLUTION Let y =

t . Then 1 + sec t 1 + sec t − t sec t tan t dy . = dt (1 + sec t)2

8 47. y =y = z csc(9z + 1) 1 + cot θ SOLUTION

Let y =

8 = 8(1 + cot θ )−1 . Then 1 + cot θ dy 8 csc2 θ d = −8(1 + cot θ )−2 (1 + cot θ ) = . dθ dθ (1 + cot θ )2

y x)√x + x+ 49. y = =xtan(cos

Chapter Review Exercises

SOLUTION

Let y =

x+

x+

√

153

x. Then

√ −1/2 d √ 1 dy = x+ x+ x x+ x+ x dx 2 dx −1/2

√ √ −1/2 d √ 1 1 = x+ x+ x x+ x x+ x 1+ 2 2 dx −1/2

√ √ −1/2 1 1 1 = x+ x+ x x+ x 1+ 1 + x −1/2 . 2 2 2 In Exercises 51–56, use the table of values to calculate the derivative of the given function at x = 2. y = cos(cos(cos( θ ))) x

f (x)

g(x)

f (x)

g (x)

2

5

4

−3

9

4

3

2

−2

3

51. S(x) = 3 f (x) − 2g(x) SOLUTION

Let S(x) = 3 f (x) − 2g(x). Then S (x) = 3 f (x) − 2g (x) and S (2) = 3 f (2) − 2g (2) = 3(−3) − 2(9) = −27.

f (x) 53. R(x) = = f (x)g(x) H (x) g(x) SOLUTION

Let R(x) = f (x)/g(x). Then R (x) =

g(x) f (x) − f (x)g (x) g(x)2

and R (2) =

g(2) f (2) − f (2)g (2) 4(−3) − 5(9) 57 = =− . 2 2 16 g(2) 4

55. F(x) = f (g(2x)) G(x) = f (g(x)) SOLUTION Let F(x) = f (g(2x)). Then F (x) = 2 f (g(2x))g (2x) and F (2) = 2 f (g(4))g (4) = 2 f (2)g (4) = 2(−3)(3) = −18. In Exercise 57–60, 2 ) f (x) = x 3 − 3x 2 + x + 4. K (x) = f (xlet 57. Find the points on the graph of f (x) where the tangent line has slope 10. Let f (x) = x 3 − 3x 2 + x + 4. Then f (x) = 3x 2 − 6x + 1. The tangent line to the graph of f (x) will have slope 10 when f (x) = 10. Solving the quadratic equation 3x 2 − 6x + 1 = 10 yields x = −1 and x = 3. Thus, the points on the graph of f (x) where the tangent line has slope 10 are (−1, −1) and (3, 7). SOLUTION

59. Find all values of b such that y = 25x + b is tangent to the graph of f (x). For which values of x are the tangent lines to the graph of f (x) horizontal? SOLUTION Let f (x) = x 3 − 3x 2 + x + 4. The equation y = 25x + b represents a line with slope 25; in order for this line to be tangent to the graph of f (x), we must therefore have f (x) = 3x 2 − 6x + 1 = 25. Solving for x yields x = 4 and x = −2. The equation of the line tangent to the graph of f (x) at x = 4 is y = f (4)(x − 4) + f (4) = 25(x − 4) + 24 = 25x − 76, while the equation of the tangent line at x = −2 is y = f (−2)(x + 2) + f (−2) = 25(x + 2) − 18 = 25x + 32. Thus, when b = −76 and when b = 32, the line y = 25x + b is tangent to the graph of f (x). 61. (a) Find Show there a unique of aonly suchone that f (x) line = x 3of−slope 2x 2 k. + x + 1 has the same slope at both a and allthat values of is k such that value f (x) has tangent a + 1. (b) Plot f (x) together with the tangent lines at x = a and x = a + 1 and conﬁrm your answer to part (a).

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(a) Let f (x) = x 3 − 2x 2 + x + 1. Then f (x) = 3x 2 − 4x + 1 and the slope of the tangent line at x = a is f (a) = 3a 2 − 4a + 1, while the slope of the tangent line at x = a + 1 is f (a + 1) = 3(a + 1)2 − 4(a + 1) + 1 = 3(a 2 + 2a + 1) − 4a − 4 + 1 = 3a 2 + 2a. In order for the tangent lines at x = a and x = a + 1 to have the same slope, we must have f (a) = f (a + 1), or 3a 2 − 4a + 1 = 3a 2 + 2a. The only solution to this equation is a = 16 . (b) The equation of the tangent line at x = 16 is y = f

1 1 5 1 241 5 113 1 x− + f = x− + = x+ , 6 6 6 12 6 216 12 108

and the equation of the tangent line at x = 76 is 7 7 5 7 223 5 59 7 x− + f = x− + = x+ . y = f 6 6 6 12 6 216 12 108 The graphs of f (x) and the two tangent lines appear below. y 3 2 −1

x −1 −2 −3

0.5

1

1.5

2

In Exercises 63–68, calculate y . The following table gives the percentage of voters supporting each of two candidates in the days before an − 5x 2 +the 3x average ROC of A’s percentage over the intervals from day 20 to 15, day 15 to 10, and day 10 63. election. y = 12x 3Compute to 5. If this trend continues over the last 5 days before the election, will A win? SOLUTION Let y = 12x 3 − 5x 2 + 3x. Then Days before2Election 20 15 10 5 y = 36x − 10x + 3 and y = 72x − 10. Candidate A 44.8% 46.8% 48.3% 49.3%

√ Candidate B 65. y = 2x −2/5 +3 y=x √ SOLUTION Let y = 2x + 3 = (2x + 3)1/2 . Then y =

1 d (2x + 3)−1/2 (2x + 3) = (2x + 3)−1/2 2 dx

55.2% 53.2% 51.7% 50.7%

and

1 d y = − (2x + 3)−3/2 (2x + 3) = −(2x + 3)−3/2 . 2 dx

2) 67. y = tan(x4x y= SOLUTION x Let + 1y = tan(x 2 ). Then

y = 2x sec2 (x 2 ) and d sec(x 2 ) + 2 sec2 (x 2 ) = 8x 2 sec2 (x 2 ) tan(x 2 ) + 2 sec2 (x 2 ). y = 2x 2 sec(x 2 ) dx 69. In Figure 5, label the graphs f , f , and f . y = sin2 (x + 9) y

y

x

FIGURE 5

x

Chapter Review Exercises

155

SOLUTION First consider the plot on the left. Observe that the green curve is nonnegative whereas the red curve is increasing, suggesting that the green curve is the derivative of the red curve. Moreover, the green curve is linear with negative slope for x < 0 and linear with positive slope for x > 0 while the blue curve is a negative constant for x < 0 and a positive constant for x > 0, suggesting the blue curve is the derivative of the green curve. Thus, the red, green and blue curves, respectively, are the graphs of f , f and f . Now consider the plot on the right. Because the red curve is decreasing when the blue curve is negative and increasing when the blue curve is positive and the green curve is decreasing when the red curve is negative and increasing when the red curve is positive, it follows that the green, red and blue curves, respectively, are the graphs of f , f and f .

q of frozen chocolate cakes a commercial can sell p, per week depends on the price 71. Let qThe be anumber differentiable function of p, say, q =that f ( p). If p changesbakery by an amount then the percentage change p. Ininthis the percentage ROC of q withpercentage respect to change p is called price elasticity of demand Assume p issetting, (100p)/ p, and the corresponding in q the is (100q)/q, where q = f E( ( p p). + p) − f (that p). q = The 50 p(10 − p) forrate 5

2(8) − 10 = −3. 8 − 10

Thus, with the price set at $8, a 1% increase in price results in a 3% decrease in demand. y The daily demand fordﬂights between Chicago and St. Louis at the price p is q = 350 − 0.7 p (in hundreds In Exercises 73–80, compute . x price elasticity of demand when p = $250 and estimate the number of additional of passengers). Calculatedthe passengers if the ticket price is lowered by 1%. 73. x 3 − y 3 = 4 SOLUTION

Consider the equation x 3 − y 3 = 4. Differentiating with respect to x yields 3x 2 − 3y 2

dy = 0. dx

Therefore, x2 dy = 2. dx y 75. y = x2y 2 + 2x2 2 4x − 9y = 36 SOLUTION Consider the equation y = x y 2 + 2x 2 . Differentiating with respect to x yields dy dy = 2x y + y 2 + 4x. dx dx Therefore, dy y 2 + 4x = . dx 1 − 2x y

77. x +yy = 3x 2 + 2y 2 =x+y x SOLUTION Squaring the equation x + y = 3x 2 + 2y 2 and combining like terms leaves 2x y = 2x 2 + y 2 . Differentiating with respect to x yields 2x

dy dy + 2y = 4x + 2y . dx dx

Therefore, y − 2x dy = . dx y−x y = sin(x + y)

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79. tan(x + y) = x y SOLUTION

Consider the equation tan(x + y) = x y. Differentiating with respect to x yields dy dy + y. sec2 (x + y) 1 + =x dx dx

Therefore, dy y − sec2 (x + y) = . dx sec2 (x + y) − x 81. Find the points on the2graph of x 3 − y 3 = 3x y − 3 where the tangent line is horizontal. (sin x)(cos y) = x − y SOLUTION Suppose x 3 − y 3 = 3x y − 3. Differentiating with respect to x leads to 3x 2 − 3y 2

dy dy = 3x + 3y, dx dx

or dy x2 − y . = dx x + y2 Tangents to the curve are therefore horizontal when y = x 2 . Substituting y = x 2 into the equation for the curve yields x 3 − x 6 = 3x 3 − 3 or x 6 + 2x 3 − 3 = (x 3 − 1)(x 3 + 3) = 0. √ √ Thus, x = 1 or x = − 3 3. The corresponding y-coordinates are y = 1 and y = 3 9. Thus, the points along the curve x 3 − y 3 = 3x y − 3 where the tangent line is horizontal are: √ √ 3 3 (1, 1) and (− 3, 9). 83. For which values of α is f (x) = |x|α differentiable at x = 0? Find the points on the graph of x 2/3 + y 2/3 = 1 where the tangent line has slope 1. SOLUTION Let f (x) = |x|α . If α < 0, then f (x) is not continuous at x = 0 and therefore cannot be differentiable at x = 0. If α = 0, then the function reduces to f (x) = 1, which is differentiable at x = 0. Now, suppose α > 0 and consider the limit f (x) − f (0) |x|α = lim . x −0 x→0 x→0 x lim

If 0 < α < 1, then |x|α = −∞ x→0− x lim

while

|x|α =∞ x→0+ x lim

and f (0) does not exist. If α = 1, then lim

|x|

x→0− x

= −1

while

lim

|x|

x→0+ x

=1

and f (0) again does not exist. Finally, if α > 1, then |x|α = 0, x→0 x lim

so f (0) does exist. In summary, f (x) = |x|α is differentiable at x = 0 when α = 0 and when α > 1. is the water level rising when the water level 85. Water pours into the tank−1 in Figure 7 at a rate of 20 m3 /min. How fast 2 sin(x ) for x = 0 and f (0) = 0. Show that f (x) exists for all x (including x = 0) but that Let f (x) = x is h = 4 m? f (x) is not continuous at x = 0 (Figure 6).

36 m 8m 10 m 24 m

FIGURE 7

Chapter Review Exercises

157

SOLUTION When the water level is at height h, the length of the upper surface of the water is 24 + 3 2 h and the volume of water in the trough is 3 15 1 V = h 24 + 24 + h (10) = 240h + h 2 . 2 2 2

Therefore, dh dV = (240 + 15h) = 20 m3 /min. dt dt When h = 4, we have 20 1 dh = = m/min. dt 240 + 15(4) 15 87. A light moving at 3 ft/s approaches a 6-ft man standing 12 ft from a wall (Figure 8). The light is 3 ft above the The minute hand of a clock is 4 in. long and the hour hand is 3 in. long. How fast is the distance between the tips ground. How fast is the tip P of the man’s shadow moving when the light is 24 ft from the wall? of the hands changing at 3 o’clock?

3 ft 12 ft

3 ft/s

FIGURE 8 SOLUTION

Let x denote the distance between the man and the light. Using similar triangles, we ﬁnd P −3 3 = x 12 + x

or

P=

36 + 6. x

Therefore, dP 36 d x =− 2 . dt x dt When the light is 24 feet from the wall, x = 12. With ddtx = −3, we have dP 36 = − 2 (−3) = 0.75 ft/s. dt 12 89. (a) Side x of the triangle in Figure 9 is increasing at 2 cm/s and side y is increasing at 3 cm/s. Assume that θ A bead slides down the curve x y = 10. Find the bead’s horizontal velocity if its height at time t seconds is decreases in such a2 way that the area of the triangle has the constant value 4 cm2 . How fast is θ decreasing when x = 4, y = 80 − 16t cm. y = 4? (b) How fast is the distance between P and Q changing when x = 4, y = 4? P x

y

Q

FIGURE 9 SOLUTION

(a) The area of the triangle is A=

1 x y sin θ = 4. 2

Differentiating with respect to t, we obtain dA 1 1 dθ dx dy = x y cos θ + y sin θ + x sin θ = 0. dt 2 dt 2 dt dt When x = y = 4, we have 12 (4)(4) sin θ = 4, so sin θ = 12 . Thus, θ = π6 and √ 1 1 1 1 1 3 dθ (4)(4) + (4) (2) + (4) (3) = 0. 2 2 dt 2 2 2 2

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Solving for d θ /dt, we ﬁnd dθ 5 = − √ ≈ −0.72 rad/s. dt 4 3 (b) By the Law of Cosines, the distance D between P and Q satisﬁes D 2 = x 2 + y 2 − 2x y cos θ , so 2D

dD dθ dy dx dx dy = 2x + 2y + 2x y sin θ − 2x cos θ − 2y cos θ . dt dt dt dt dt dt

With x = y = 4 and θ = π6 , D=

42 + 42 − 2(4)(4)

√

√ 3 = 4 2 − 3. 2

Therefore, √ √ 20 − 12 3 − 8 3 16 + 24 − √ dD 3 = ≈ −1.50 cm/s. √ dt 8 2− 3

APPLICATIONS OF 4 THE DERIVAT IVE 4.1 Linear Approximation and Applications Preliminary Questions 1. Estimate g(1.2) − g(1) if g (1) = 4. SOLUTION

Using the Linear Approximation, g(1.2) − g(1) ≈ g (1)(1.2 − 1) = 4(0.2) = 0.8.

2. Estimate f (2.1), assuming that f (2) = 1 and f (2) = 3. SOLUTION

Using the Linear Approximation, f (2.1) ≈ f (2) + f (2)(2.1 − 2) = 1 + 3(0.1) = 1.3

3. The velocity of a train at a given instant is 110 ft/s. How far does the train travel during the next half-second (use the Linear Approximation)? SOLUTION Using the Linear Approximation, we estimate that the train travels at the constant velocity of 110 ft/sec over the next half-second; hence, we estimate that the train will travel 55 ft over the next half-second.

4. Discuss how the Linear Approximation makes the following statement more precise: The sensitivity of the output to a small change in input depends on the derivative. SOLUTION The Linear Approximation tells us that the change in the output value of a function is approximately equal to the value of the derivative times the change in the input value.

5. Suppose that the linearization of f (x) at a = 2 is L(x) = 2x + 4. What are f (2) and f (2)? SOLUTION

The linearization of f (x) at a = 2 is L(x) = f (2) + f (2)(x − 2) = f (2) · x + ( f (2) − 2 f (2))

Matching this to the given expression, L(x) = 2x + 4, it follows that f (2) = 2. Moreover, f (2) − 2 f (2) = 4, so f (2) = 8.

Exercises In Exercises 1–4, use the Linear Approximation to estimate f = f (3.02) − f (3) for the given function. 1. f (x) = x 2 SOLUTION

Let f (x) = x 2 . Then f (x) = 2x and f ≈ f (3)x = 6(.02) = 0.12.

3. f (x) = x −1 4 f (x) = x 1 SOLUTION Let f (x) = x −1 . Then f (x) = −x −2 and f ≈ f (3)x = − (.02) = −.00222. 9 In Exercises 5–10, 1estimate f using the Linear Approximation and use a calculator to compute both the error and the f (x)error. = percentage x +1 5. f (x) = x − 2x 2 ,

a = 5,

x = −0.4

Let f (x) = x − 2x 2 , a = 5, and x = −0.4. Then f (x) = 1 − 4x, f (a) = f (5) = −19 and f ≈ f (a)x = (−19)(−0.4) = 7.6. The actual change is SOLUTION

f = f (a + x) − f (a) = f (4.6) − f (5) = −37.72 − (−45) = 7.28. The error in the Linear Approximation is therefore |7.28 − 7.6| = 0.32; in percentage terms, the error is 0.32 × 100% ≈ 4.40% 7.28 f (x) =

√

1 + x,

a = 8, x = 1

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7. f (x) =

1 , 1 + x2

a = 3,

x = 0.5

Let f (x) = 1 2 , a = 3, and x = .5. Then f (x) = − 2x2 2 , f (a) = f (3) = −.06 and f ≈ 1+x (1+x ) f (a)x = −.06(.5) = −0.03. The actual change is

SOLUTION

f = f (a + x) − f (a) = f (3.5) − f (3) ≈ −.0245283. The error in the Linear Approximation is therefore | − .0245283 − (−.03)| = .0054717; in percentage terms, the error is .0054717 −.0245283 × 100% ≈ 22.31% 9. f (x) = tan x, a = π4 , x = 0.013 f (x) = sin x, a = 0, x = 0.02 π π SOLUTION Let f (x) = tan x, a = 4 , and x = .013. Then f (x) = sec2 x, f (a) = f ( 4 ) = 2 and f ≈ f (a)x = 2(.013) = .026. The actual change is

π

π ≈ 1.026344 − 1 = .026344. f = f (a + x) − f (a) = f + .013 − f 4 4 The error in the Linear Approximation is therefore |.026344 − .026| = .000344; in percentage terms, the error is .000344 × 100% ≈ 1.31% .026344 In Exercises estimate using the Linear Approximation and ﬁnd the error using a calculator. π , quantity x = 0.03 f (x) 11–16, = cos x, a = the 4 √ √ 11. 26 − 25 √ 1 1 SOLUTION Let f (x) = x, a = 25, and x = 1. Then f (x) = 2 x −1/2 and f (a) = f (25) = 10 . • The Linear Approximation is f ≈ f (a)x = 1 (1) = .1. 10 • The actual change is f = f (a + x) − f (a) = f (26) − f (25) ≈ .0990195. • The error in this estimate is |.0990195 − .1| = .000980486.

1 1 1/4 − 161/4 13. √ 16.5− 101 10 SOLUTION

−0.0005.

1 )= Let f (x) = √1 , a = 100, and x = 1. Then f (x) = ddx (x −1/2 ) = − 12 x −3/2 and f (a) = − 12 ( 1000 x

• The Linear Approximation is f ≈ f (a)x = −0.0005(1) = −0.0005. • The actual change is

f = f (a + x) − f (a) = √

1 101

−

1 = −0.000496281. 10

• The error in this estimate is |−0.0005 − (−0.000496281)| = 3.71902 × 10−6 .

15. sin(0.023) Hint: Estimate sin(0.023) − sin(0). 1 1 √ − SOLUTION 98 Let10f (x) = sin x, a = 0, and x = .023. Then f (x) = cos x and f (a) = f (0) = 1. • The Linear Approximation is f ≈ f (a)x = 1(.023) = .023. • The actual change is f = f (a + x) − f (a) = f (0.023) − f (0) = 0.02299797. • The error in this estimate is |.023 − .02299797| ≈ 2.03 × 10−6 .

17. The cube root of 27 is 3. How much larger is the cube root of 27.2? Estimate using the Linear Approximation. (15.8)1/4 1 1 SOLUTION Let f (x) = x 1/3 , a = 27, and x = .2. Then f (x) = 3 x −2/3 and f (a) = f (27) = 27 . The Linear Approximation is 1 f ≈ f (a)x = (.2) = .0074074 27 Approximation. Note: You must express θ in radians. 19. Estimate sin 61◦ − sin √ the√Linear √ √ 60◦ using Which is larger: 2.1 − 2 or 9.1 − 9? Explain using the Linear Approximation.

S E C T I O N 4.1

Linear Approximation and Applications

161

π 1 π SOLUTION Let f (x) = sin x, a = π 3 , and x = 180 . Then f (x) = cos x and f (a) = f ( 3 ) = 2 . Finally, the Linear Approximation is

1 π π f ≈ f (a)x = = ≈ .008727 2 180 360 21. Atmospheric pressure P at altitude h = 40,000 ft is 390 lb/ft2 . Estimate P at h = 41,000 if d P/dh = T = 25◦ C A thin silver wire has length L = 18 cm when the temperature is T = 30◦ C. Estimate the length whenh=40,000 3. −5◦ −1 −0.0188 lb/ft if the coefﬁcient of thermal expansion is k = 1.9 × 10 C (see Example 3). SOLUTION

Let P(h) by the pressure P at altitude h. By the Linear Approximation, P(41000) − P(40000) ≈

d P h = −.0188 lbs/ft3 (1000 ft) = −18.8 lbs/ft2 . dh h=40000

Hence, P(41000) ≈ 390 lbs/ft2 − 18.8 lbs/ft2 = 371.2 lbs/ft2 . 23. The side s of a square carpet is measured at 6 ft. Estimate ◦the maximum error in the area A of the carpet if s◦ is resistance a copper wire at temperature T = 20 C is R = 15 . Estimate the resistance at T = 22 C, accurateThe to within half R anof inch. assuming that d R/d T T =20 = 0.06 /◦ C. SOLUTION Let s be the length in feet of the side of the square carpet. Then A(s) = s 2 is the area of the carpet. With 1 (note that 1 inch equals 1 foot), an estimate of the size of the error in the area is given by the Linear a = 6 and s = 24 12 Approximation: 1 A ≈ A (6)s = 12 = .5 ft2 24 25. A stone tossed vertically in the air with velocitythe v ft/s reaches a maximum height of h= v 2 /64 ft. increases A spherical balloon has a radius of 6initial in. Estimate change in volume and surface area if the radius (a) Estimate by 0.3 in.h if v is increased from 25 to 26 ft/s. (b) Estimate h if v is increased from 30 to 31 ft/s. (c) In general, does a 1 ft/s increase in initial velocity cause a greater change in maximum height at low or high initial velocities? Explain. SOLUTION

A stone tossed vertically with initial velocity v ft/s attains a maximum height of h = v 2 /64 ft.

1 (25)(1) = .78125 ft. (a) If v = 25 and v = 1, then h ≈ h (v)v = 32

1 (30)(1) = .9375 ft. (b) If v = 30 and v = 1, then h ≈ h (v)v = 32 (c) A one foot per second increase in initial velocity v increases the maximum height by approximately v/32 ft. Accordingly, there is a bigger effect at higher velocities. 2 ft, 27. TheIfstopping automobile (aftertoapplying the brakes) F(s) = 1.1s the pricedistance of a bus for passanfrom Albuquerque Los Alamos is set atisx approximately dollars, a bus company takes+in0.054s a monthly where s is the in 1.5x mph.−Use the2Linear Approximation to estimate the change in stopping distance per additional (in thousands of dollars). revenue ofspeed R(x) = 0.01x mph when s = 35 and when s = 55. (a) Estimate the change in revenue if the price rises from $50 to $53. 2. SOLUTION Let that F(s)x==1.1s .054swill (b) Suppose 80.+How revenue be affected by a small increase in price? Explain using the Linear •Approximation. The Linear Approximation at s = 35 mph is

F ≈ F (35)s = (1.1 + .108 × 35)s = 4.88s ft The change in stopping distance per additional mph for s = 35 mph is approximately 4.88 ft. • The Linear Approximation at s = 55 mph is

F ≈ F (55)s = (1.1 + .108 × 55)s = 7.04s ft The change in stopping distance per additional mph for s = 55 mph is approximately 7.04 ft. 29. Estimate the weight loss per mile of altitude gained for a 130-lb pilot. At which altitude would she weigh 129.5 lb? Juan measures the circumference C of a spherical ball at 40 cm and computes the ball’s volume V . Estimate the See Example 4. maximum possible error in V if the error in C is at most 2 cm. Recall that C = 2π r and V = 43 π r 3 , where r is the SOLUTION From the discussion in the text, the weight loss W at altitude h (in miles) for a person weighing W0 at ball’s radius. the surface of the earth is approximately W ≈ −

1 W h 1980 0

If W0 = 130 pounds, then W ≈ −0.066h. Accordingly, the pilot loses approximately 0.066 pounds per mile of altitude gained. She will weigh 129.5 pounds at the altitude h such that −0.066h = −0.5, or h = 0.5/0.066 ≈ 7.6 miles. 31. The volume of a certain gas (in liters) is related to pressure P (in atmospheres) by the formula P V = 24. Suppose How much would a 160-lb astronaut weigh in a satellite orbiting the earth at an altitude of 2,000 miles? Estimate that V = 5 with a possible error of ±0.5 L. the astronaut’s weight loss per additional mile of altitude beyond 2,000.

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(a) Compute P and estimate the possible error. (b) Estimate the maximum allowable error in V if P must have an error of at most 0.5 atm. SOLUTION

(a) We have P = 24V −1 . For V = 5 L, P = 24/5 = 4.8 atm. If the error in computing the volume is ±0.5 liters, then the error in computing the pressure is approximately 24 12 |P| ≈ |P (V )V | = | − 24V −2 V | = − (±.5) = = .48 atm 25 25 (b) When the volume is V = 5 L, the error in computing the pressure is approximately 24 24 −2 |V | = .5 atm |P| ≈ |P (V )V | = | − 24V V | = − V = 25 25 Thus, the maximum allowable error in V is approximately V = ±.520833 L. In Exercises 33–38, ﬁnd the linearization at x = a. The dosage D of diphenhydramine for a dog of body mass w kg is D = kw 2/3 mg, where k is a constant. A cocker 33. y = cosspaniel x sin x,hasamass = 0 w = 10 kg according to a veterinarian’s scale. Estimate the maximum allowable error in w if the percentage error in the dosage1D must be less than 5%. SOLUTION Let f (x) = sin x cos x = 2 sin 2x. Then f (x) = cos 2x. The linearization at a = 0 is L(x) = f (a)(x − a) + f (a) = 1(x − 0) + 0 = x. 35. y = (1 + x)−1/2 , a = 0 π y = cos x sin x, a = 1 SOLUTION Let f (x) = (1 +4x)−1/2 . Then f (x) = − 2 (1 + x)−3/2 . The linearization at a = 0 is 1 L(x) = f (a)(x − a) + f (a) = − x + 1. 2 , a=0 37. y = (1 + x 2 )−1/2 y = (1 + x)−1/2 , a = 32 −1/2 SOLUTION Let f (x) = (1 + x ) . Then f (x) = −x(1 + x 2 )−3/2 , f (a) = 1 and f (a) = 0, so the linearization at a is L(x) = f (a)(x − a) + f (a) = 1. √ √ Estimate 16.2 39. π using the linearization L(x) of f (x) = x at a sin x y = determine , a if=the estimate is too large or too small. of axes and x 2 SOLUTION Let f (x) = x 1/2 , a = 16, and x = .2. Then f (x) = linearization to f (x) is

= 16. Plot f (x) and L(x) on the same set 1 x −1/2 and f (a) = f (16) = 1 . The 2 8

1 1 L(x) = f (a)(x − a) + f (a) = (x − 16) + 4 = x + 2. 8 8

√ Thus, we have 16.2 ≈ L(16.2) = 4.025. Graphs of f (x) and L(x) are shown below. Because the graph of L(x) lies above the graph of f (x), we expect that the estimate from the Linear Approximation is too large. y 5 4 3 2 1

L(x)

0

5

f (x) x 10

15

20

25

√ √ to compute the percentage error. In Exercises 41–46, approximate using linearization and use a calculator Estimate 1/ 15 using a suitable linearization of f (x) = 1/ x. Plot f (x) and L(x) on the same set of axes √ 41. and 17determine if the estimate is too large or too small. Use a calculator to compute the percentage error. SOLUTION

to f (x) is

Let f (x) = x 1/2 , a = 16, and x = 1. Then f (x) = 12 x −1/2 , f (a) = f (16) = 18 and the linearization

Thus, we have

√

1 1 L(x) = f (a)(x − a) + f (a) = (x − 16) + 4 = x + 2. 8 8 17 ≈ L(17) = 4.125. The percentage error in this estimate is √ 17 − 4.125 √ × 100% ≈ .046% 17

S E C T I O N 4.1

Linear Approximation and Applications

163

43. (17)1/4 1 √ 1 1 SOLUTION Let f (x) = x 1/4 , a = 16, and x = 1. Then f (x) = 4 x −3/4 , f (a) = f (16) = 32 and the linearization 17 to f (x) is 1 1 3 (x − 16) + 2 = x+ . L(x) = f (a)(x − a) + f (a) = 32 32 2 Thus, we have (17)1/4 ≈ L(17) = 2.03125. The percentage error in this estimate is (17)1/4 − 2.03125 × 100% ≈ .035% (17)1/4 45. (27.001)1/31/3 (27.03) 1 1 SOLUTION Let f (x) = x 1/3 , a = 27, and x = .001. Then f (x) = 3 x −2/3 , f (a) = f (27) = 27 and the linearization to f (x) is 1 1 L(x) = f (a)(x − a) + f (a) = (x − 27) + 3 = x + 2. 27 27 Thus, we have (27.001)1/3 ≈ L(27.001) ≈ 3.0000370370370. The percentage error in this estimate is (27.001)1/3 − 3.0000370370370 × 100% ≈ .000000015% (27.001)1/3

π on the same set of axes. 5/3 f (x) = tan x and its linearization L(x) at a = (1.2)Plot 4 (a) Does the linearization overestimate or underestimate f (x)? (b) Show, by graphing y = f (x) − L(x) and y = 0.1 on the same set of axes, that the error | f (x) − L(x)| is at most 0.1 for 0.55 ≤ x ≤ 0.95. (c) Find an interval of x-values on which the error is at most 0.05. 47.

SOLUTION

Let f (x) = tan x and a = π4 . Then f (x) = sec2 x, f (a) = 2 and

π L(x) = f (a) + f (a)(x − a) = 1 + 2 x − . 4

Graphs of f (x) and L(x) are shown below. y 2 x 1 −2

(a) From the ﬁgure above, we see that the graph of the linearization L(x) lies below the graph of f (x); thus, the linearization underestimates f (x). (b) The graph of y = f (x) − L(x) is shown below at the left. Below at the right, portions of the graphs of y = f (x) − L(x) and y = 0.1 are shown. From the graph on the right, we see that | f (x) − L(x)| ≤ 0.1 for 0.55 ≤ x ≤ 0.95.

y 4 3 2 1 x 0

x

1

0.5 0.6 0.7 0.8 0.9

1

(c) Portions of the graphs of y = f (x) − L(x) and y = 0.05 are shown below. From the graph, we see that | f (x) − L(x)| ≤ 0.05 roughly for 0.62 < x < 0.93.

x 0.6

0.7

0.8

0.9

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In Exercises 49–50, use the following fact derived from2 Newton’s Laws: An object released at an angle θ with initial Compute the linearization L(x) 1of 2f (x) = x − x 3/2 at a = 2. Then plot f (x) − L(x) and ﬁnd an interval velocity v ft/s travels a total distance s = 32 v sin 2θ ft (Figure 8). around a = 1 such that | f (x) − L(x)| ≤ 0.1. 49. A player located 18.1 ft from a basket launches a successful jump shot from a height of 10 ft (level with the rim of the basket), at an angle θ = 34◦ and initial velocity of v = 25 ft/s. y

q

x

FIGURE 8 Trajectory of an object released at an angle θ .

(a) Show that the distance s of the shot changes by approximately 0.255θ ft if the angle changes by an amount θ . Remember to convert the angles to radians in the Linear Approximation. (b) Is it likely that the shot would have been successful if the angle were off by 2◦ ? 1 v 2 sin 2t = Using Newton’s laws and the given initial velocity of v = 25 ft/s, the shot travels s = 32 625 sin 2t ft, where t is in radians. 32

SOLUTION

(a) If θ = 34◦ (i.e., t = 17 90 π ), then

s ≈ s (t)t =

625 π 17 17 625 π t = π θ · cos cos ≈ 0.255θ . 16 45 16 45 180

(b) If θ = 2◦ , this gives s ≈ 0.51 ft, in which case the shot would not have been successful, having been off half a foot. 51. Compute the linearization of f (x) = 3x − 4 at a = 0 and a = 2. Prove more generally that a linear function Estimate change inatthe distance of the shot if the angle changes from 50◦ to 51◦ for v = 25 ft/s and coincides with itsthe linearization x= a for alls a. v = 30 ft/s. Is the shot more sensitive to the angle when the velocity is large or small? Explain. SOLUTION Let f (x) = 3x − 4. Then f (x) = 3. With a = 0, f (a) = −4 and f (a) = 3, so the linearization of f (x) at a = 0 is L(x) = −4 + 3(x − 0) = 3x − 4 = f (x). With a = 2, f (a) = 2 and f (a) = 3, so the linearization of f (x) at a = 2 is L(x) = 2 + 3(x − 2) = 2 + 3x − 6 = 3x − 4 = f (x). More generally, let g(x) = bx + c be any linear function. The linearization L(x) of g(x) at x = a is L(x) = g (a)(x − a) + g(a) = b(x − a) + ba + c = bx + c = g(x); i.e., L(x) = g(x). 53. Show that the Linear Approximation to f (x) = tan x at x = π4 yields the estimate tan( π4 + h) − 1 ≈ 2h. Compute According to (3), error in the Linear Approximation is of “order two” in h. Show that the Linear Approxi√ √,the the error E for h = 10−n 1 ≤ n ≤ 4, and verify that E satisﬁes the Error Bound (3) with K = 6.2. mation to f (x) = x at x = 9 yields the estimate 9 + h − 3 ≈ 16 h. Then compute the error E for h = 10−n , SOLUTION Let = tan x. Then fthat ( π4 )E=≤1,0.006h f (x) 2=. sec2 x and f ( π4 ) = 2. Therefore, by the Linear Approxima1 ≤ n ≤ 4, andf (x) verify numerically tion,

π

π

π = tan f +h − f + h − 1 ≈ 2h. 4 4 4 From the following table, we see that for h = 10−n , 1 ≤ n ≤ 4, E ≤ 6.2h 2 . h

E = | tan( π4 + h) − 1 − 2h|

6.2h 2

10−1 10−2 10−3 10−4

2.305 × 10−2 2.027 × 10−4 2.003 × 10−6 2.000 × 10−8

6.20 × 10−2 6.20 × 10−4 6.20 × 10−6 6.20 × 10−8

S E C T I O N 4.2

Extreme Values

165

Further Insights and Challenges 55. (a) Show that f (x) = sin x and g(x) = tan x have the same linearization at a = 0. Show that forisany real numbermore k, (1accurately? + x)k ≈ 1 Explain + kx forusing smallax. Estimate (1.02) π ].0.7 and (1.02)−0.3 . (b) Which function approximated graph over [0, 6

(c) Calculate the error in these linearizations at x = π6 . Does the answer conﬁrm your conclusion in (b)? SOLUTION Let f (x) = sin x and g(x) = tan x. (a) The Linear Approximation of f (x) at a = 0 is L(x) = f (0)(x − 0) + f (0) = cos 0 · (x − 0) + sin 0 = x, whereas the Linear Approximation of g(x) at a = 0 is L(x) = f (0)(x − 0) + f (0) = sec2 0 · (x − 0) + tan 0 = x. (b) The linearization L(x) = x more closely approximates the function f (x) = sin x than g(x) = tan x, since the graph of x lies closer to that of sin x than to tan x. y

tan x x sin x

0.5 0.4 0.3 0.2 0.1 x 0

0.1 0.2 0.3 0.4 0.5

(c) The error in the Linear Approximation for f at x = π6 is sin( π6 ) − π6 ≈ −.02360; the error in the Linear Approximation for g at x = π6 is tan( π6 ) − π6 ≈ .05375. These results conﬁrm what the graphs of the curves told us in (b). error in the Linear Approximation at a = 1. Show directly that 57. Let f (x) = x −1 and let E = | f − f (1)h| be the 2 , and let E In = this | f case, − f (5)h| the error f (5 + h) −that f (5), where 1 . Hint: 1 ≤ 1be 3 . in the Linear ApproxiE = h 2Let /(1 + fh).=Then prove E≤ 2h 2 iff (x) − 12 = ≤ xh ≤ + h ≤ 2 in h). 2 mation. Verify directly that E satisﬁes (3) with K = 22 (thus E is of order two SOLUTION Let f (x) = x −1 . Then f = f (1 + h) − f (1) =

1 h −1=− 1+h 1+h

and

E = | f − f (1)h| = −

h h2 + h = . 1+h 1+h

1 ≤ 2. Thus, E ≤ 2h 2 for − 1 ≤ h ≤ 1 . If − 12 ≤ h ≤ 12 , then 12 ≤ 1 + h ≤ 32 and 23 ≤ 1+h 2 2

Consider the curve y 4 + x 2 + x y = 13. Calculate d y/d x at P = (3, 1) and ﬁnd the linearization y = L(x) at P. 4.2 (a)Extreme Values (b) Plot the curve and y = L(x) on the same set of axes. (c) Although y is only deﬁned implicitly as a function of x, we can use y = L(x) to approximate y for x near 3. Use Preliminary Questions this idea to ﬁnd an approximate solution of y 4 + 3.12 + 3.1y = 13. Based on your plot in (b), is your approximation 1. If f (x) is continuous on (0, 1), then (choose the correct statement): too large or too small? (a) f (x) has a minimum value on (0, 1). (d) Solve y 4 + 3.12 + 3.1y = 13 numerically using a computer algebra system and use the result to ﬁnd the (b) percentage f (x) has noerror minimum on (0, 1). in yourvalue approximation. (c) f (x) might not have a minimum value on (0, 1). SOLUTION The correct response is (c): f (x) might not have a minimum value on (0, 1). The key is that the interval (0, 1) is not closed. The function might have a minimum value but it does not have to have a minimum value.

2. If f (x) is not continuous on [0, 1], then (choose the correct statement): (a) f (x) has no extreme values on [0, 1]. (b) f (x) might not have any extreme values on [0, 1]. SOLUTION The correct response is (b): f (x) might not have any extreme values on [0, 1]. Although [0, 1] is closed, because f is not continuous, the function is not guaranteed to have any extreme values on [0, 1].

3. What is the deﬁnition of a critical point? A critical point is a value of the independent variable x in the domain of a function f at which either f (x) = 0 or f (x) does not exist.

SOLUTION

4. True or false: If f (x) is differentiable and f (x) = 0 has no solutions, then f (x) has no local minima or maxima. True. If f (x) is differentiable but f (x) = 0 has no solutions, then f (x) has no critical points. As critical points are the only candidates for local minima and local maxima, it follows that f (x) has no local extrema. SOLUTION

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5. Fermat’s Theorem does not claim that if f (c) = 0, then f (c) is a local extreme value (this is false). What does Fermat’s Theorem assert? SOLUTION

Fermat’s Theorem claims: If f (c) is a local extreme value, then either f (c) = 0 or f (c) does not exist.

6. If f (x) is continuous but has no critical points in [0, 1], then (choose the correct statement): (a) f (x) has no min or max on [0, 1]. (b) Either f (0) or f (1) is the minimum value on [0, 1]. SOLUTION The correct response is (b): either f (0) or f (1) is the minimum value on [0, 1]. Remember that extreme values occur either at critical points or endpoints. If a continuous function on a closed interval has no critical points, the extreme values must occur at the endpoints.

Exercises 1. The following questions refer to Figure 15. y 6 5 4 3

f(x)

2 1 x 1

2

3

4

5

6

7

8

FIGURE 15

(a) (b) (c) (d) (e)

How many critical points does f (x) have? What is the maximum value of f (x) on [0, 8]? What are the local maximum values of f (x)? Find a closed interval on which both the minimum and maximum values of f (x) occur at critical points. Find an interval on which the minimum value occurs at an endpoint.

SOLUTION

(a) f (x) has three critical points on the interval [0, 8]: at x = 3, x = 5 and x = 7. Two of these, x = 3 and x = 5, are where the derivative is zero and one, x = 7, is where the derivative does not exist. (b) The maximum value of f (x) on [0, 8] is 6; the function takes this value at x = 0. (c) f (x) achieves a local maximum of 5 at x = 5. (d) Answers may vary. One example is the interval [4, 8]. Another is [2, 6]. (e) Answers may vary. The easiest way to ensure this is to choose an interval on which the graph takes no local minimum. One example is [0, 2]. In Exercises 3–10, ﬁnd all critical points of the function. State whether f (x) = x −1 (Figure 16) has a minimum or maximum value on the following intervals: (b) (1, 2) (c) [1, 2] 3. (a) f (x)(0, =2) x 2 − 2x + 4 SOLUTION

Let f (x) = x 2 − 2x + 4. Then f (x) = 2x − 2 = 0 implies that x = 1 is the lone critical point of f .

9 3− 5. f (x)f (x) = x= 7x −x 22 − 54x + 2 2 Let f (x) = x 3 − 92 x 2 − 54x + 2. Then f (x) = 3x 2 − 9x − 54 = 3(x + 3)(x − 6) = 0 implies that x = −3 and x = 6 are the critical points of f . x 7. f (x) = 2 3 2 f (t) = x 8t + 1− t

SOLUTION

SOLUTION

x 1 − x2 . Then f (x) = 2 Let f (x) = 2 = 0 implies that x = ±1 are the critical points of f . x +1 (x + 1)2

9. f (x) = x 1/3 f (t) = 4t − t 2 + 1 1 SOLUTION Let f (x) = x 1/3 . Then f (x) = 3 x −2/3 . The derivative is never zero but does not exist at x = 0. Thus, x = 0 is the only critical point of f . 11. (a) (b) (c)

Let f (x) = x 2 2− 4x + 1. x = Findf (x) the critical point c of f (x) and compute f (c). x +1 Compute the value of f (x) at the endpoints of the interval [0, 4]. Determine the min and max of f (x) on [0, 4].

S E C T I O N 4.2

Extreme Values

167

(d) Find the extreme values of f (x) on [0, 1]. SOLUTION

Let f (x) = x 2 − 4x + 1.

(a) Then f (c) = 2c − 4 = 0 implies that c = 2 is the sole critical point of f . We have f (2) = −3. (b) f (0) = f (4) = 1. (c) Using the results from (a) and (b), we ﬁnd the maximum value of f on [0, 4] is 1 and the minimum value is −3. (d) We have f (1) = −2. Hence the maximum value of f on [0, 1] is 1 and the minimum value is −2. 13. Find the critical points of f (x) = sin x + cos x and determine the extreme values on [0, π2 ]. Find the extreme values of 2x 3 − 9x 2 + 12x on [0, 3] and [0, 2]. SOLUTION

• Let f (x) = sin x + cos x. Then on the interval 0, π , we have f (x) = cos x − sin x = 0 at x = π , the only 2 4 critical point of f in this interval. √ √ 2 and f (0) = f ( π2 ) = 0, the maximum value of f on 0, π2 is 2, while the minimum value is 0.

• Since f ( π ) = 4

√ Plot f (x) = 2 x − x on [0, 4] and determine the maximum value graphically. Then verify your answer 15. Compute the critical points of h(t) = (t 2 − 1)1/3 . Check that your answer is consistent with Figure 17. Then using calculus. ﬁnd the extreme values of h(t)√on [0, 1] and [0, 2]. SOLUTION The graph of y = 2 x − x over the interval [0, 4] is shown below. From the graph, we see that at x = 1, the function achieves its maximum value of 1. y 1 0.8 0.6 0.4 0.2 x 0

1

2

3

4

√ To verify the information obtained from the plot, let f (x) = 2 x − x. Then f (x) = x −1/2 − 1. Solving f (x) = 0 yields the critical points x = 0 and x = 1. Because f (0) = f (4) = 0 and f (1) = 1, we see that the maximum value of f on [0, 4] is 1. In Exercises 17–45, ﬁnd the maximum and minimum values of the function on the given interval. Plot f (x) = 2x 3 − 9x 2 + 12x on [0, 3] and locate the extreme values graphically. Then verify your answer 2 − 4x + 2, [0, 3] 17. using y = 2xcalculus. SOLUTION Let f (x) = 2x 2 − 4x + 2. Then f (x) = 4x − 4 = 0 implies that x = 1 is a critical point of f . On the interval [0, 3], the minimum value of f is f (1) = 0, whereas the maximum value of f is f (3) = 8. (Note: f (0) = 2.)

− 1, [−2, 2] 19. y = x 2 − 6x y = −x 2 + 10x + 43, [3, 8] SOLUTION Let f (x) = x 2 − 6x − 1. Then f (x) = 2x − 6 = 0 implies that x = 3 is a critical point of f . The minimum of f on the interval [−2, 2] is f (2) = −9, whereas its maximum is f (−2) = 15. (Note: The critical point x = 3 is outside the interval [−2, 2].) 21. y = −4x 22 + 3x + 4, [−1, 1] y = x − 6x − 1, [−2, 0] 3 SOLUTION Let f (x) = −4x 2 + 3x + 4. Then f (x) = −8x + 3 = 0 implies that x = 8 is a critical point of f . The 3 minimum of f on the interval [−1, 1] is f (−1) = −3, whereas its maximum is f ( 8 ) = 4.5625. (Note: f (1) = 3.) 23. y = x 3 −36x + 1, [−1, 1] y = x − 3x + 1, [0, 2] √ SOLUTION Let f (x) = x 3 − 6x + 1. Then f (x) = 3x 2 − 6 = 0 implies that x = ± 2 are the critical points of f . The minimum of f on the interval [−1, 1] is f (1) = −4, whereas its maximum is f (−1) = 6. (Note: The critical points √ x = ± 2 are not in the interval [−1, 1].) 25. y = x 3 +33x 2 − 9x + 2, [−1, 1] y = x − 12x 2 + 21x, [0, 2] SOLUTION Let f (x) = x 3 + 3x 2 − 9x + 2. Then f (x) = 3x 2 + 6x − 9 = 3(x + 3)(x − 1) = 0 implies that x = 1 and x = −3 are the critical points of f . The minimum of f on the interval [−1, 1] is f (1) = −3, whereas its maximum is f (−1) = 13. (Note: The critical point x = −3 is not in the interval [−1, 1].) 27. y = x 3 +33x 2 −29x + 2, [−4, 4] y = x + 3x − 9x + 2, [0, 2] SOLUTION Let f (x) = x 3 + 3x 2 − 9x + 2. Then f (x) = 3x 2 + 6x − 9 = 3(x + 3)(x − 1) = 0 implies that x = 1 and x = −3 are the critical points of f . The minimum of f on the interval [−4, 4] is f (1) = −3, whereas its maximum is f (4) = 78. (Note: f (−4) = 22 and f (−3) = 29.) 29. y = x 5 −53x 2 , [−1, 5] y = x − x, [0, 2]

168

CHAPTER 4

A P P L I C AT I O N S O F T H E D E R I VATI V E 1/3 ≈ 1.06 are SOLUTION Let f (x) = x 5 − 3x 2 . Then f (x) = 5x 4 − 6x = 0 implies that x = 0 and x = 1 5 (150) critical points of f . Over the interval [−1, 5], the minimum value of f is f (−1) = −4, whereas its maximum value is 9 (150)2/3 ≈ −2.03). f (5) = 3050. (Note: f (0) = 0 and f ( 15 (150)1/3 ) = − 125

x2 + 1 2 6] , 3s [5, 31. y =y = 2s 3 − + 3, x −4 SOLUTION

Let f (x) =

[−4, 4]

x2 + 1 . Then x −4 f (x) =

(x − 4) · 2x − (x 2 + 1) · 1 x 2 − 8x − 1 = =0 2 (x − 4) (x − 4)2

√ implies x = 4 ± 17 are critical points of f . x = 4 is not a critical point because x = 4 is not in the domain of f . On the interval [5, 6], the minimum of f is f (6) = 37 2 = 18.5, whereas the maximum of f is f (5) = 26. (Note: The critical √ points x = 4 ± 17 are not in the interval [5, 6].) 4x 33. y = x − 1 − x , [0, 3] y = x2 + 1 , [1, 4] x + 3x 4x SOLUTION Let f (x) = x − . Then x +1 f (x) = 1 −

4 (x − 1)(x + 3) = =0 (x + 1)2 (x + 1)2

implies that x = 1 and x = −3 are critical points of f . x = −1 is not a critical point because x = −1 is not in the domain of f . The minimum of f on the interval [0, 3] is f (1) = −1, whereas the maximum is f (0) = f (3) = 0. (Note: The critical point x = −3 is not in the interval [0, 3].) 35. y = (2 + x) 2 2 + (2 − x)2 , [0, 2] y = 2 x + 1 − x, [0, 2] SOLUTION Let f (x) = (2 + x) 2 + (2 − x)2 . Then

2(x − 1)2 2 + (2 − x)2 − (2 + x)(2 + (2 − x)2 )−1/2 (2 − x) = =0 2 + (2 − x)2 √ implies that x = 1 is the √ critical point of f . On the interval √ [0, 2], the minimum is f (0) = 2 6 ≈ 4.898979 and the maximum is f (2) = 4 2 ≈ 5.656854. (Note: f (1) = 3 3 ≈ 5.196152.) √ + x 2 − 2 x, [0, 4] 37. y = x y = 1 + x 2 − 2x, [3, 6] √ SOLUTION Let f (x) = x + x 2 − 2 x. Then f (x) =

√

1 1 + 2x − 2 f (x) = (x + x 2 )−1/2 (1 + 2x) − x −1/2 = √ √ 2

√

1+x

2 x 1+x

=0 √

implies that x = 0 and x = 23 are the critical points of f . Neither x = −1 nor x = − 23 is a critical point because

√ neither is in the domain of f . On the interval [0, 4], the minimum of f is f 23 ≈ −.589980 and the maximum is f (4) ≈ .472136. (Note: f (0) = 0.) 39. y = sin x cos x, [0, π2 ] y = (t − t 2 )1/3 , [−1, 2] π 1 π SOLUTION Let f (x) = sin x cos x = 2 sin 2x. On the interval 0, 2 , f (x) = cos 2x = 0 when x = 4 . The π π 1 minimum of f on this interval is f (0) = f ( 2 ) = 0, whereas the maximum is f ( 4 ) = 2 . √ π] 41. y =y =2xθ − −sin secx,θ , [0, [0,2π 3] √ √ π π SOLUTION Let f (θ ) = 2θ − sec θ . On the interval [0, 3 ], f (θ ) = 2 − sec θ tan θ = 0 at θ = 4 . The minimum √ value of f on this interval is f (0) = −1, whereas the maximum value over this interval is f ( π4 ) = 2( π4 − 1) ≈ √ −.303493. (Note: f ( π3 ) = 2 π3 − 2 ≈ −.519039.) 43. y = θ − 2 sin θ , [0, 2π ] y = cos θ + sin θ , [0, 2π ] SOLUTION Let g(θ ) = θ − 2 sin θ . On the interval [0, 2π ], g (θ ) = 1 − 2 cos θ = 0 at √ minimum of g on this interval is g( π3 ) = π3 − 3 ≈ −.685 and the maximum is g( 53 π ) = g(0) = 0 and g(2π ) = 2π ≈ 6.283.) 45. y = tan x − 2x, [0, 1] y = sin3 θ − cos2 θ ,

[0, 2π ]

θ = π3 and θ = 53 π . The √ 5 π + 3 ≈ 6.968. (Note: 3

S E C T I O N 4.2

Extreme Values

169

SOLUTION Let f (x) = tan x − 2x. Then on the interval [0, 1], f (x) = sec2 x − 2 = 0 at x = π 4 . The minimum of f is f ( π4 ) = 1 − π2 ≈ −.570796 and the maximum is f (0) = 0. (Note: f (1) = tan 1 − 2 ≈ −.442592.)

Find the critical points of f (x) = 2 cos 3x + 3 cos 2x. Check your answer against a graph of f (x). Let f (θ ) = 2 sin 2θ + sin 4θ . SOLUTION (x) θisisdifferentiable are2θlooking for points where f (x) = 0 only. Setting f (x) = a critical pointforif all cosx, 4θso = we − cos . (a) Show fthat −6 sin 3x − 6 sin 2x, we get sin 3x = − sin 2x. Looking at a unit circle, we ﬁnd the relationship between angles y and x (b) Show, using a unit circle, that cos θ1 = − cos θ2 if and only if θ1 = π ± θ2 + 2π k for an integer k. such that sin y = − sin x. This technique is also used in Exercise 46. (c) Show that cos 4θ = − cos 2θ if and only if θ = π /2 + π k or θ = π /6 + (π /3)k. (d) Find the six critical points of f (θ ) on [0, 2π ] and ﬁnd the extreme values of f (θ ) on this interval. (e) Check your results against a graph of f (θ ). 47.

From the diagram, we see that sin y = − sin x if y is either (i.) the point antipodal to x (y = π + x + 2π k) or (ii.) the point obtained by reﬂecting x through the horizontal axis (y = −x + 2π k). Since sin 3x = − sin 2x, we get either 3x = π + 2x + 2π k or 3x = −2x + 2π k. Solving each of these equations for x yields x = π + 2π k and x = 25π k, respectively. The values of x between 0 and 2π are 0, 25π , 45π , π , 65π , 85π , and 2π . The graph is shown below. As predicted, it has horizontal tangent lines at 25π k and at x = π2 . Each of these points is a local extremum. y 4 2 x 1

−2

2

3

4

5

6

−4

In Exercises 48–51, ﬁnd the critical points and the extreme values on [0, 3]. In Exercises 50 and 51, refer to Figure 18. y

y 1

30 20 10 x

−6

2

−π

π 2

2

π

3π 2

x

y = | cos x |

y = | x 2 + 4x − 12 |

FIGURE 18

49. y = |3x − 9| y = |x − 2| SOLUTION Let f (x) = |3x − 9| = 3|x − 3|. For x < 3, we have f (x) = −3. For x > 3, we have f (x) = 3. Now as f (x) − f (3) 3(3 − x) − 0 f (x) − f (3) 3(x − 3) − 0 x → 3−, we have = → −3; whereas as x → 3+, we have = → 3. x −3 x −3 x −3 x −3 f (x) − f (3) does not exist and the lone critical point of f is x = 3. Alternately, we examine Therefore, f (3) = lim x −3 x→3 the graph of f (x) = |3x − 9| shown below. To ﬁnd the extrema of f (x) on [0, 3], we test the values of f (x) at the critical point and the endpoints. f (0) = 9 and f (3) = 0, so f (x) takes its minimum value of 0 at x = 3, and its maximum value of 9 at x = 0. y 12 8 4 x 2

4

6

51. y = | cos x| y = |x 2 + 4x − 12| SOLUTION Let f (x) = | cos x|. There are two types of critical points: points of the form π n where the derivative is zero and points of the form n π + π /2 where the derivative does not exist.

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To ﬁnd points where f (x) does not exist, we look at points where the inside of the absolute value function is equal to zero, and take derivatives in either direction, to ﬁnd out whether they are the same. cos x = 0 at the points x = − π2 , x = π2 , and x = 32π , etc. Only one of these, x = π2 is in the interval [0, 3], so we check the one critical point and the endpoints. f (0) = 1, f (3) = | cos 3| ≈ .98992, and f ( π2 ) = 0, so f (x) takes its maximum value of 1 at x = 0 and its minimum of 0 at x = π2 . In Exercises 53–56, verify Rolle’s Theorem for the given interval. Let f (x) = 3x − x 3 . Check that f (−2) = f (1). What may we conclude from Rolle’s Theorem? Verify this 1 −1 53. conclusion. f (x) = x + x , [ , 2] 2

SOLUTION

Because f is continuous on [ 12 , 2], differentiable on ( 12 , 2) and 1 5 1 1 1 f = + 1 = = 2 + = f (2), 2 2 2 2 2

we may conclude from Rolle’s Theorem that there exists a c ∈ ( 12 , 2) at which f (c) = 0. Here, f (x) = 1 − x −2 = x 2 −1 , so we may take c = 1. 2 x

x2 π , 5] 3π 55. f (x)f (x) = = sin x, , [[3, 4 4 ] 8x − 15 SOLUTION Because f is continuous on [3, 5], differentiable on (3, 5) and f (3) = f (5) = 1, we may conclude from Rolle’s Theorem that there exists a c ∈ (3, 5) at which f (c) = 0. Here,

f (x) =

(8x − 15)(2x) − 8x 2 2x(4x − 15) = , (8x − 15)2 (8x − 15)2

so we may take c = 15 4 . 57. Use Rolle’s Theorem to prove that f (x) = x 5 + 2x 3 + 4x − 12 has at most one real root. f (x) = sin2 x − cos2 x, [ π4 , 34π ] SOLUTION We use proof by contradiction. Suppose f (x) = x 5 + 2x 3 + 4x − 12 has two real roots, x = a and x = b. Then f (a) = f (b) = 0 and Rolle’s Theorem guarantees that there exists a c ∈ (a, b) at which f (c) = 0. However, f (x) = 5x 4 + 6x 2 + 4 ≥ 4 for all x, so there is no c ∈ (a, b) at which f (c) = 0. Based on this contradiction, we conclude that f (x) = x 5 + 2x 3 + 4x − 12 cannot have more than one real root. bloodstream after t hours is 59. The concentration C(t) (in mg/cm3 ) of a drug in3 a patient’s x x2 Use Rolle’s Theorem to prove that f (x) = + + x + 1 has at most one real root. 6 = 2 0.016t C(t) t 2 + 4t + 4 Find the maximum concentration and the time at which it occurs. SOLUTION

C (t) =

.016(t 2 + 4t + 4) − (.016t (2t + 4)) −t 2 + 4 2−t = .016 2 = .016 . 2 2 (t + 4t + 4) (t + 4t + 4)2 (t + 2)3

C (t) exists for all t ≥ 0, so we are looking for points where C (t) = 0. C (t) = 0 when t = 2, so we check C(2) = .002 mg3 . Since C(0) = 0 and C(t) → 0 as t → ∞, the point (2, .002) is a local maximum of C(t). cm

Hence, the maximum concentration occurs 2 hours after being administered, and that concentration is .002

mg . cm3

61. Bees build honeycomb structures out of cells with a hexagonal base and three rhombus-shaped faces on top as in Maximum of a Resonance Curve size of the of aiscircuit or other oscillatory system to an input Figure 20. Using geometry, we can show thatThe the surface arearesponse of this cell signal of frequency ω (“omega”) is described in suitable units by the function 3 √ A(θ ) = 6hs + s 2 ( 3 csc 1 θ − cot θ ) φ (ω ) = 2 . 2 2 ω02 − ω 2 )2fact + 4D where h, s, and θ are as indicated in the ﬁgure. It is a (remarkable thatωbees “know” which angle θ minimizes the surface area (and therefore requires the least amount of wax). Both ω0 (the natural frequency of the system) and D (the damping factor) are positive constants. The graph of φ is (a) Show this anglecurve, is approximately 54.7◦frequency by ﬁndingω the>critical point of A(its θ )maximum for 0 < θ value, < π /2if(assume h and s called that a resonance and the positive 0 where φ takes such a positive r √ are constant). frequency exists, is the resonant frequency. Show that a resonant √

frequency exists if and only if 0 < D < ω0 / 2 Conﬁrm, by graphing f (θ ) = 3 csc θ − cot θ , that the critical point indeed minimizes the surface area. (b) (Figure 19). Then show that if this condition is satisﬁed, ωr = ω02 − 2D 2 .

S E C T I O N 4.2

Extreme Values

171

q

h

s

FIGURE 20 A cell in a honeycomb constructed by bees. SOLUTION

√ (a) Because h and s are constant relative to θ , we have A (θ ) = 32 s 2 (− 3 csc θ cot θ + csc2 θ ) = 0. From this, we get √ 3 csc θ cot θ = csc2 θ , or cos θ = √1 , whence θ = cos−1 √1 = .955317 radians = 54.736◦ . 3 3 √ (b) The plot of 3 csc θ − cot θ , where θ is given in degrees, is given below. We can see that the minimum occurs just below 55◦ . h 1.5 1.48 1.46 1.44 1.42 1.4 40 45 50 55 60 65

Degrees

63. FindMigrating the maximum of yto=swim x a −at x baon [0, 1] where < a < b.the In particular, ﬁnd theof maximum of According y = x 5 − xto10one on ﬁsh tend velocity v that 0minimizes total expenditure energy E. [0, 1]. 3 v model, E is proportional to f (v) = , where vr is the velocity of the river water. SOLUTION v − vr (a) Find the critical points of f (v). • Let f (x) = x a − x b . Then f (x) = ax a−1 − bx b−1 . Since a < b, f (x) = x a−1 (a − bx b−a ) = 0 implies (say, and plot (v).interval Conﬁrm[0,that a minimum the critical (b) Choose of vxr = a < value r = 10) , which is inf the 1] asf (v) a x b on the interval [0, 1], which gives x = (b) f (x) > 0 and thus the maximum value of f on [0, 1] is a 1/(b−a) a a/(b−a) a b/(b−a) − . f = b b b • Let f (x) = x 5 − x 10 . Then by part (a), the maximum value of f on [0, 1] is

1 1 1 1 2 1 1 1/5 f = = − = . − 2 2 2 2 4 4 In Exercises 64–66, plot the function using a graphing utility and ﬁnd its critical points and extreme values on [−5, 5]. 1 1 y= + 1 y = 1 + |x − 1| 1 + |x − 4| 1 + |x − 1| SOLUTION Let

65.

f (x) =

1 1 + . 1 + |x − 1| 1 + |x − 4|

The plot follows: 1.2 1 0.8 0.6 0.4 0.2 −5 −4 −3 −2 −1

1

2

3

4

5

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We can see on the plot that the critical points of f (x) lie at the cusps at x = 1 and x = 4 and at the location of the 5 5 4 7 horizontal tangent line at x = 52 . With f (−5) = 17 70 , f (1) = f (4) = 4 , f ( 2 ) = 5 and f (5) = 10 , it follows that the maximum value of f (x) on [−5, 5] is f (1) = f (4) = 54 and the minimum value is f (−5) = 17 70 . 67. (a) Use implicit differentiation to ﬁnd the critical points on the curve 27x 2 = (x 2 + y 2 )3 . x y = Plot|xthe 2 −curve (b) 1| + and |x 2 the − 4|horizontal tangent lines on the same set of axes. SOLUTION

(a) Differentiating both sides of the equation 27x 2 = (x 2 + y 2 )3 with respect to x yields dy 54x = 3(x 2 + y 2 )2 2x + 2y . dx Solving for d y/d x we obtain x(9 − (x 2 + y 2 )2 ) dy 27x − 3x(x 2 + y 2 )2 = . = 2 2 2 dx 3y(x + y ) y(x 2 + y 2 )2 Thus, the derivative is zero when x 2 + y 2 = 3. Substituting into the equation for the curve, this yields x 2 = 1, or x = ±1. There are therefore four points at which the derivative is zero: √ √ √ √ (−1, − 2), (−1, 2), (1, − 2), (1, 2). There are also critical points where the derivative does not exist. This occurs when y = 0 and gives the following points with vertical tangents: √ 4 (0, 0), (± 27, 0). (b) The curve 27x 2 = (x 2 + y 2 )3 and its horizontal tangents are plotted below. y 1 −2

x

−1

1

2

−1

69. Sketch the graph of a continuous function on (0, 4) having a local minimum but no absolute minimum. Sketch the graph of a continuous function on (0, 4) with a minimum value but no maximum value. SOLUTION Here is the graph of a function f on (0, 4) with a local minimum value [between x = 2 and x = 4] but no absolute minimum [since f (x) → −∞ as x → 0+]. y 10 x 1

2

3

−10

71. Sketch the graph of a function f (x) on [0, 4] with a discontinuity such that f (x) has an absolute minimum but no Sketch the graph of a function on [0, 4] having absolute maximum. (a) Two local maxima and one local minimum. SOLUTION Here is the graph of a function f on [0, 4] that (a) has a discontinuity [at x = 4] and (b) has an absolute (b) An absolute minimum that occurs at an endpoint, and an absolute maximum that occurs at a critical point. minimum [at x = 0] but no absolute maximum [since f (x) → ∞ as x → 4−]. y 4 3 2 1 x 0

1

2

3

4

The Mean Value Theorem and Monotonicity

S E C T I O N 4.3

173

Further Insights and Challenges on only positive values. More generally, ﬁnd 73. Show, by considering its minimum, that f (x) = x 2 − 2x + 3 takes 2 + b2 . Show that extreme values f (x) = a sinfunction x + b cosfx(x) are=±x 2a+ the conditions on rthe and s under whichofthe quadratic r x + s takes on only positive values. Give examples of r and s for which f takes on both positive and negative values. SOLUTION

• Observe that f (x) = x 2 − 2x + 3 = (x − 1)2 + 2 > 0 for all x. Let f (x) = x 2 + r x + s. Completing the square, we note that f (x) = (x + 12 r )2 + s − 14 r 2 > 0 for all x provided that s > 14 r 2 . • Let f (x) = x 2 − 4x + 3 = (x − 1)(x − 3). Then f takes on both positive and negative values. Here, r = −4 and

s = 3.

75. Generalize Exercise 74: Show that if the horizontal 2line y = c intersects the graph of f (x) = x 2 + r x + s at two Show that if the quadratic polynomial f (x) = x + r x + s takes on both positive and x 1negative + x 2 values, then its points (x1 , f (xvalue (x 2 , at f (x then f (x) takes its (Figure 21). minimum occurs the midpoint between theminimum two roots.value at the midpoint M = 1 )) and 2 )), 2 y f(x) c

y=c

x x1

M

x2

FIGURE 21

Suppose that a horizontal line y = c intersects the graph of a quadratic function f (x) = x 2 + r x + s in two points (x 1 , f (x 1 )) and (x 2 , f (x 2 )). Then of course f (x1 ) = f (x 2 ) = c. Let g(x) = f (x) − c. Then g(x1 ) = g(x 2 ) = 0. By Exercise 74, g takes on its minimum value at x = 12 (x 1 + x2 ). Hence so does f (x) = g(x) + c. SOLUTION

77. Find the minimum and maximum values of f (x) = x p (1 − x)q on [0, 1], where p and q are positive numbers. The graphs in Figure 22 show that a cubic polynomial may have a local min and max, or it may have neither. p (1 − x)q , 0 ≤ x ≤ 1, where p 1and 1 axpositive 2 + bx numbers. SOLUTION Let f (x) q are Then f has neither a local min + c that ensure Find conditions on = thexcoefﬁcients a and b of f (x) = 3 x 3 + 2 nor max. Hint: Apply Exercise 73 to f (x). f (x) = x p q(1 − x)q−1 (−1) + (1 − x)q px p−1 p = x p−1 (1 − x)q−1 ( p(1 − x) − qx) = 0 at x = 0, 1, p+q The minimum value of f on [0, 1] is f (0) = f (1) = 0, whereas its maximum value is p p pqq f . = p+q ( p + q) p+q Prove that if f is continuous and f (a) and f (b) are local minima where a < b, then there exists a value c between a and b such that f (c) is a local maximum. (Hint: Apply Theorem 1 to the interval [a, b].) Show that 4.3continuity The Mean Value Theorem and Monotonicity is a necessary hypothesis by sketching the graph of a function (necessarily discontinuous) with two local minima but no local maximum.

Preliminary Questions 1. Which value of m makes the following statement correct? If f (2) = 3 and f (4) = 9, where f (x) is differentiable, then the graph of f has a tangent line of slope m. SOLUTION

The Mean Value Theorem guarantees that the function must have a tangent line with slope equal to 9−3 f (4) − f (2) = = 3. 4−2 4−2

Hence, m = 3 makes the statement correct. 2. (a) (b) (c) (d)

Which of the following conclusions does not follow from the MVT (assume that f is differentiable)? If f has a secant line of slope 0, then f (c) = 0 for some value of c. If f (5) < f (9), then f (c) > 0 for some c ∈ (5, 9). If f (c) = 0 for some value of c, then there is a secant line whose slope is 0. If f (x) > 0 for all x, then every secant line has positive slope.

Conclusion (c) does not follow from the Mean Value Theorem. As a counterexample, consider the function f (x) = x 3 . Note that f (0) = 0, but no secant line has zero slope.

SOLUTION

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3. Can a function that takes on only negative values have a positive derivative? Sketch an example or explain why no such functions exist. SOLUTION

Yes. The ﬁgure below displays a function that takes on only negative values but has a positive derivative. y x

4. (a) Use the graph of f (x) in Figure 10 to determine whether f (c) is a local minimum or maximum. (b) Can you conclude from Figure 10 that f (x) is a decreasing function? y

x

c

FIGURE 10 Graph of derivative f (x). SOLUTION

(a) To the left of x = c, the derivative is positive, so f is increasing; to the right of x = c, the derivative is negative, so f is decreasing. Consequently, f (c) must be a local maximum. (b) No. The derivative is a decreasing function, but as noted in part (a), f (x) is increasing for x < c and decreasing for x > c.

Exercises In Exercises 1–6, ﬁnd a point c satisfying the conclusion of the MVT for the given function and interval. 1. y = x −1 , SOLUTION

[1, 4] Let f (x) = x −1 , a = 1, b = 4. Then f (x) = −x −2 , and by the MVT, there exists a c ∈ (1, 4) such that 1

1

− f (b) − f (a) 1 1 = 4 1 =− . − 2 = f (c) = b−a 4−1 4 c Thus c2 = 4 and c = ±2. Choose c = 2 ∈ (1, 4). 3. y = (x − √1)(x − 3), [1, 3] y = x, [4, 9] SOLUTION Let f (x) = (x − 1) (x − 3), a = 1, b = 3. Then f (x) = 2x − 4, and by the MVT, there exists a c ∈ (1, 3) such that f (b) − f (a) 0−0 2c − 4 = f (c) = = = 0. b−a 3−1 Thus 2c − 4 = 0 and c = 2 ∈ (1, 3). x 5. y =y = cos, x − [3,sin 6]x, [0, 2π ] x +1 SOLUTION

Let f (x) = x/ (x + 1), a = 3, b = 6. Then f (x) =

such that

1 , and by the MVT, there exists a c ∈ (3, 6) (x+1)2

6 3 (c) = f (b) − f (a) = 7 − 4 = 1 . = f b−a 6−3 28 (c + 1)2 √ √ Thus (c + 1)2 = 28 and c = −1 ± 2 7. Choose c = 2 7 − 1 ≈ 4.29 ∈ (3, 6).

1

7. Let = x 5 + x 2 . Check that the secant line between x = 0 and x = 1 has slope 2. By the MVT, 3 , f (x) [−3, y = x f (c) = 2 for some c ∈−1] (0, 1). Estimate c graphically as follows. Plot f (x) and the secant line on the same axes. Then plot the lines y = 2x + b for different values of b until you ﬁnd a value of b for which it is tangent to y = f (x). Zoom in on the point of tangency to ﬁnd its x-coordinate.

The Mean Value Theorem and Monotonicity

S E C T I O N 4.3 SOLUTION

175

Let f (x) = x 5 + x 2 . The slope of the secant line between x = 0 and x = 1 is 2−0 f (1) − f (0) = = 2. 1−0 1

A plot of f (x), the secant line between x = 0 and x = 1, and the line y = 2x − 0.764 is shown below at the left. The line y = 2x − 0.764 appears to be tangent to the graph of y = f (x). Zooming in on the point of tangency (see below at the right), it appears that the x-coordinate of the point of tangency is approximately 0.62. y

0.6

y = x5 + x 2

y

0.5

4

0.4

y = 2x − .764

2

x

0.3 0.52

1

x 0.56

0.6

0.64

9. Determine the intervals on which f (x) is increasing or decreasing, assuming that Figure 11 is the graph of the Determine derivative f (x). the intervals on which f (x) is positive and negative, assuming that Figure 11 is the graph of f (x). y

x 1

2

3

4

5

6

FIGURE 11

f (x) is increasing on every interval (a, b) over which f (x) > 0, and is decreasing on every interval over which f (x) < 0. If the graph of f (x) is given in Figure 11, then f (x) is increasing on the intervals (0, 2) and (4, 6), SOLUTION

and is decreasing on the interval (2, 4).

In Exercises 11–14, sketch the graph of a function f (x) whose derivative f (x) has the given description. Plot the derivative f (x) of f (x) = 3x 5 − 5x 3 and describe the sign changes of f (x). Use this to determine local f (x). > 0extreme for x >values 3 andoff (x)

Here is the graph of a function f for which f (x) > 0 for x > 3 and f (x) < 0 for x < 3. y 10 8 6 4 2 x 0

1

2

3

4

5

13. f (x) is negative on (1, 3) and positive everywhere else. f (x) > 0 for x < 1 and f (x) < 0 for x > 1. SOLUTION Here is the graph of a function f for which f (x) is negative on (1, 3) and positive elsewhere. y 8 6 4 2 x −2

1

2

3

4

In Exercises the First Derivative f (x)15–18, makes use the sign transitions +, −,Test +, to −.determine whether the function attains a local minimum or local maximum (or neither) at the given critical point. 15. y = 7 + 4x − x 2 ,

c=2

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CHAPTER 4

A P P L I C AT I O N S O F T H E D E R I VATI V E SOLUTION Let f (x) = 7 + 4x − x 2 . Then f (c) = 4 − 2c = 0 implies c = 2 is a critical point of f . Since f makes the sign transition +, − as x increases through c = 2, we conclude that f (2) = 11 is a local maximum of f .

x2 17. y =y = x 3 ,− 27x c =+ 0 2, c = −3 x +1 SOLUTION

x 2 . Then Let f (x) = x+1

f (c) =

2c(c + 1) − c2 c(c + 2) = = 0 at c = 0. (c + 1)2 (c + 1)2

Since f makes the sign transition −, + as x increases through c = 0, we conclude that f (0) = 0 is a local minimum of f. 19. Assuming that Figure 11 πis the graph of the derivative f (x), state whether f (2) and f (4) are local minima or maxima.y = sin x cos x, c = 4 SOLUTION

• f (x) makes a transition from positive to negative at x = 2, so f (2) is a local maximum. • f (x) makes a transition from negative to positive at x = 4, so f (4) is a local minimum.

In Exercises 21–40, ﬁnd the and the intervals which thef function increasing or decreasing, and deterapply Figure 12 shows the critical graph ofpoints the derivative f (x) ofon a function (x). Findisthe critical points of f (x) and the First Testare to local each critical mineDerivative whether they minima,point. maxima, or neither. SOLUTION

Here is a table legend for Exercises 21–40. SYMBOL

MEANING

−

The entity is negative on the given interval.

0

The entity is zero at the speciﬁed point.

+

The entity is positive on the given interval.

U

The entity is undeﬁned at the speciﬁed point.

f is increasing on the given interval.

f is decreasing on the given interval.

M

f has a local maximum at the speciﬁed point.

m

f has a local minimum at the speciﬁed point.

¬

There is no local extremum here.

21. y = −x 2 + 7x − 17 SOLUTION

Let f (x) = −x 2 + 7x − 17. Then f (x) = 7 − 2x = 0 yields the critical point c = 72 . x

−∞, 72

7/2

7,∞ 2

f

+

0

−

f

M

2 23. y = x 3 − 6x y = 5x 2 + 6x − 4 SOLUTION Let f (x) = x 3 − 6x 2 . Then f (x) = 3x 2 − 12x = 3x(x − 4) = 0 yields critical points c = 0, 4.

25. y = 3x 4 + 8x 3 −3 6x 2 − 24x y = x(x + 1)

x

(−∞, 0)

0

(0, 4)

4

(4, ∞)

f

+

0

−

0

+

f

M

m

The Mean Value Theorem and Monotonicity

S E C T I O N 4.3 SOLUTION

177

Let f (x) = 3x 4 + 8x 3 − 6x 2 − 24x. Then f (x) = 12x 3 + 24x 2 − 12x − 24 = 12x 2 (x + 2) − 12(x + 2) = 12(x + 2)(x 2 − 1) = 12 (x − 1) (x + 1) (x + 2) = 0

yields critical points c = −2, −1, 1. x

(−∞, −2)

−2

(−2, −1)

−1

(−1, 1)

1

(1, ∞)

f

−

0

+

0

−

0

+

f

m

M

m

27. y = 13 x 3 2+ 32 x 2 + 2x 2+ 4 y = x + (10 − x) 1 3 SOLUTION Let f (x) = 3 x 3 + 2 x 2 + 2x + 4. Then f (x) = x 2 + 3x + 2 = (x + 1) (x + 2) = 0 yields critical points c = −2, −1. x

(−∞, −2)

−2

(−2, −1)

−1

(−1, ∞)

f

+

0

−

0

+

f

M

m

29. y = x 4 +5x 3 3 y = x +x +1 3 SOLUTION Let f (x) = x 4 + x 3 . Then f (x) = 4x 3 + 3x 2 = x 2 (4x + 3) yields critical points c = 0, − 4 . x

−∞, − 34

− 34

− 34 , 0

0

(0, ∞)

f

−

0

+

0

+

f

m

¬

1 31. y =y =2 x 2 − x 4 x +1 SOLUTION

−1

−2 Let f (x) = x 2 + 1 . Then f (x) = −2x x 2 + 1 = 0 yields critical point c = 0. x

(−∞, 0)

0

(0, ∞)

f

+

0

−

f

M

33. y = x +2x x −1 + 1 (x > 0) y= 2 −1 −2 = 0 yields the critical point c = 1. (Note: c = −1 SOLUTION xLet + 1f (x) = x + x for x > 0. Then f (x) = 1 − x is not in the interval under consideration.) x

(0, 1)

1

(1, ∞)

f

−

0

+

f

m

(x > 0) 35. y = x 5/2 4− x 2 3/2 y = x − 4x (x > 0) 5 5 16 SOLUTION Let f (x) = x 5/2 − x 2 . Then f (x) = 2 x 3/2 − 2x = x( 2 x 1/2 − 2) = 0, so the critical point is c = 25 . (Note: c = 0 is not in the interval under consideration.) x

(0, 16 25 )

16 25

( 16 25 , ∞)

f

−

0

+

f

m

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37. y = sin θ cos θ , [0, 2π ] y = x −2 − 4x −1 (x > 0) SOLUTION Let f (θ ) = sin θ cos θ . Then f (θ ) = − sin2 θ + cos2 θ = 0 implies that tan2 θ = 1. On the interval [0, 2π ], this yields c = π4 , 34π , 54π , 74π . x

(0, π4 )

π 4

( π4 , 34π )

3π 4

( 34π , 54π )

5π 4

( 54π , 74π )

7π 4

( 74π , 2π )

f

+

0

−

0

+

0

−

0

+

f

M

m

M

m

39. y = θ + cos θ , [0, 2π ] y = cos θ + sin θ , [0, 2π ] π SOLUTION Let f (θ ) = θ + cos θ . Then f (θ ) = 1 − sin θ = 0, which yields c = 2 on the interval [0, 2π ]. x

0, π2

π 2

π

2 , 2π

f

+

0

+

f

¬

b b 41. Show =θx, 2 + y =that θ −f (x) 2 cos [0,bx2π+] c is decreasing on (−∞, − 2 ) and increasing on (− 2 , ∞).

Let f (x) = x 2 + bx + c. Then f (x) = 2x + b = 0 yields the critical point c = − b2 .

• For x < − b , we have f (x) < 0, so f is decreasing on −∞, − b . 2 2

• For x > − b , we have f (x) > 0, so f is increasing on − b , ∞ . 2 2

SOLUTION

43. Find conditions on a and3 b that2 ensure that f (x) = x 3 + ax + b is increasing on (−∞, ∞). Show that f (x) = x − 2x + 2x is an increasing function. Hint: Find the minimum value of f (x). SOLUTION Let f (x) = x 3 + ax + b. • If a > 0, then f (x) = 3x 2 + a > 0 and f is increasing for all x. • If a = 0, then

f (x 2 ) − f (x 1 ) = (3x 23 + b) − (3x 13 + b) = 3(x 2 − x 1 )(x 22 + x 2 x 1 + x 12 ) > 0 whenever x2 > x1 . Thus, f is increasing for all x. • If a < 0, then f (x) = 3x 2 + a < 0 and f is decreasing for |x| <

− a3 .

In summary, f (x) = x 3 + ax + b is increasing on (−∞, ∞) whenever a ≥ 0. 45. Sam made two statements that Deborah found dubious. x(x 2 − 1) (a) “Although the average for my is trip 70 mph,ofatano point inf time did my 70describe mph.” the thewas derivative function (x). Plot h(x)speedometer and use the read plot to Suppose that h(x)velocity = x 2 me + 1going 70 mph, my speedometer never read 65 mph.” (b) “Although a policeman clocked local extrema and the increasing/decreasing behavior of f (x). Sketch a plausible graph for f (x) itself. In each case, which theorem did Deborah apply to prove Sam’s statement false: the Intermediate Value Theorem or the Mean Value Theorem? Explain. SOLUTION

(a) Deborah is applying the Mean Value Theorem here. Let s(t) be Sam’s distance, in miles, from his starting point, let a be the start time for Sam’s trip, and let b be the end time of the same trip. Sam is claiming that at no point was s (t) =

s(b) − s(a) . b−a

This violates the MVT. (b) Deborah is applying the Intermediate Value Theorem here. Let v(t) be Sam’s velocity in miles per hour. Sam started out at rest, and reached a velocity of 70 mph. By the IVT, he should have reached a velocity of 65 mph at some point. 47. Show that f (x) = 1 − |x| satisﬁes the conclusion of the MVT on [a, b] if both a and b are positive or negative, but 2 2 2 2 where not if a Determine < 0 and b > 0. f (x) = (1,000 − x) + x is increasing. Use this to decide which is larger: 1,000 or 998 + 22 . SOLUTION Let f (x) = 1 − |x|. • If a and b (where a < b) are both positive (or both negative), then f is continuous on [a, b] and differentiable on

(a, b). Accordingly, the hypotheses of the MVT are met and the theorem does apply. Indeed, in these cases, any point c ∈ (a, b) satisﬁes the conclusion of the MVT (since f is constant on [a, b] in these instances).

S E C T I O N 4.3

The Mean Value Theorem and Monotonicity

179

1 f (b) − f (a) 0 − (−1) = . Yet there is no point c ∈ (−2, 1) such that = b−a 1 − (−2) 3 f (c) = 13 . Indeed, f (x) = 1 for x < 0, f (x) = −1 for x > 0, and f (0) is undeﬁned. The MVT does not apply in this case, since f is not differentiable on the open interval (−2, 1).

• For a = −2 and b = 1, we have

a+b 49. Show that if f is aofquadratic then = intervalsatisﬁes conclusion the MVT on [a, b] Which values c satisfy polynomial, the conclusion ofthe themidpoint MVT on cthe [a, b] ifthe f (x) is a linearoffunction? 2 for any a and b. SOLUTION Let f (x) = px 2 + qx + r with p = 0 and consider the interval [a, b]. Then f (x) = 2 px + q, and by the MVT we have

pb2 + qb + r − pa 2 + qa + r f (b) − f (a) 2 pc + q = f (c) = = b−a b−a

=

(b − a) ( p (b + a) + q) = p (b + a) + q b−a

Thus 2 pc + q = p(a + b) + q, and c =

a+b . 2

51. Suppose that f (2) = −2 and f (x) ≥ 5. Show that f (4) ≥ 8. Suppose that f (0) = 4 and f (x) ≤ 2 for x > 0. Apply the MVT to the interval [0, 3] to prove that f (3) ≤ 10. SOLUTION The MVT, applied to the [2,all 4],xguarantees there exists a c ∈ (2, 4) such that Prove more generally that f (x) ≤ 4interval + 2x for > 0. f (c) =

f (4) − f (2) 4−2

or

f (4) − f (2) = 2 f (c).

Because f (x) ≥ 5, it follows that f (4) − f (2) ≥ 10, or f (4) ≥ f (2) + 10 = 8.

Further Insights and Challenges 53. Prove that if f (0) = g(0) and f (x) ≤ g (x) for x ≥ 0, then f (x) ≤ g(x) for all x ≥ 0. Hint: Show that f (x) − g(x) Show that the cubic function f (x) = x 3 + ax 2 + bx + c is increasing on (−∞, ∞) if b > a 2 /3. is nonincreasing. Let h(x) = f (x) − g(x). By the sum rule, h (x) = f (x) − g (x). Since f (x) ≤ g (x) for all x ≥ 0, h (x) ≤ 0 for all x ≥ 0. This implies that h is nonincreasing. Since h(0) = f (0) − g(0) = 0, h(x) ≤ 0 for all x ≥ 0 (as SOLUTION

h is nonincreasing, it cannot climb above zero). Hence f (x) − g(x) ≤ 0 for all x ≥ 0, and so f (x) ≤ g(x) for x ≥ 0.

55. Use Exercises 53 and 54 to establish the following assertions for all x ≥ 0 (each assertion follows from the previous Use Exercise 53 to prove that sin x ≤ x for x ≥ 0. one): (a) cos x ≥ 1 − 12 x 2 (b) sin x ≥ x − 16 x 3 1 x4 (c) cos x ≤ 1 − 12 x 2 + 24 (d) Can you guess the next inequality in the series? SOLUTION

(a) We prove this using Exercise 53: Let g(x) = cos x and f (x) = 1 − 12 x 2 . Then f (0) = g(0) = 1 and g (x) = − sin x ≥ −x = f (x) for x ≥ 0 by Exercise 54. Now apply Exercise 53 to conclude that cos x ≥ 1 − 12 x 2 for x ≥ 0.

(b) Let g(x) = sin x and f (x) = x − 16 x 3 . Then f (0) = g(0) = 0 and g (x) = cos x ≥ 1 − 12 x 2 = f (x) for x ≥ 0 by part (a). Now apply Exercise 53 to conclude that sin x ≥ x − 16 x 3 for x ≥ 0.

1 x 4 and f (x) = cos x. Then f (0) = g(0) = 1 and g (x) = −x + 1 x 3 ≥ − sin x = f (x) (c) Let g(x) = 1 − 12 x 2 + 24 6 1 x 4 for x ≥ 0. for x ≥ 0 by part (b). Now apply Exercise 53 to conclude that cos x ≤ 1 − 12 x 2 + 24

1 x 5 , valid for x ≥ 0. To construct (d) from (c), we note (d) The next inequality in the series is sin x ≤ x − 16 x 3 + 120 that the derivative of sin x is cos x, and look for a polynomial (which we currently must do by educated guess) whose 1 x 4 . We know the derivative of x is 1, and that a term whose derivative is − 1 x 2 should be of derivative is 1 − 12 x 2 + 24 2 1 x 4 should be of the form Dx 5 . the form C x 3 . ddx C x 3 = 3C x 2 = − 12 x 2 , so C = − 16 . A term whose derivative is 24 1 x 4 , so that 5D = 1 , or D = 1 . From this, ddx Dx 5 = 5Dx 4 = 24 24 120 exists Deﬁne = x 3and sin( f1x )(x) for = x = 0 and f (0) = 0.that f (x) = mx + b, where m = f (0) and b = f (0). 57. Assume that ff(x) 0 for all x. Prove (a) Show that f (x) is continuous at x = 0 and x = 0 is a critical point of f . Examine the graphs of f (x) and f (x). Can the First Derivative Test be applied? (b) (c) Show that f (0) is neither a local min nor max. SOLUTION

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A P P L I C AT I O N S O F T H E D E R I VATI V E

(a) Let f (x) = x 3 sin( 1x ). Then f (x) = 3x 2 sin

1 1 1 1 + x 3 cos (−x −2 ) = x 3x sin − cos . x x x x

This formula is not deﬁned at x = 0, but its limit is. Since −1 ≤ sin x ≤ 1 and −1 ≤ cos x ≤ 1 for all x, 1 1 3x sin 1 + cos 1 ≤ |x|(3|x| + 1) | f (x)| = |x| 3x sin − cos ≤ |x| x x x x so, by the Squeeze Theorem, lim | f (x)| = 0. But does f (0) = 0? We check using the limit deﬁnition of the derivative: x→0

1 f (x) − f (0) = lim x 2 sin = 0. x −0 x x→0 x→0

f (0) = lim

Thus f (x) is continuous at x = 0, and x = 0 is a critical point of f . (b) The ﬁgure below at the left shows f (x), and the ﬁgure below at the right shows f (x). Note how the two functions oscillate near x = 0, which implies that the First Derivative Test cannot be applied. y

−0.2

y

x 0.2

−0.2

x 0.2

(c) As x approaches 0 from either direction, f (x) alternates between positive and negative arbitrarily close to x = 0. This means that f (0) cannot be a local minimum (since f (x) gets lower than f (0) arbitrarily close to 0), nor can f (0) be a local maximum (since f (x) takes values higher than f (0) arbitrarily close to x = 0). Therefore f (0) is neither a local minimum nor a local maximum of f .

4.4 The Shape of a Graph Preliminary Questions 1. Choose the correct answer: If f is concave up, then f is: (a) increasing (b) decreasing The correct response is (a): increasing. If the function is concave up, then f is positive. Since f is the derivative of f , it follows that the derivative of f is positive and f must therefore be increasing. SOLUTION

2. If x 0 is a critical point and f is concave down, then f (x 0 ) is a local: (a) min (b) max (c) undetermined SOLUTION

By the Second Derivative Test, the correct response is (b): maximum.

In Questions 3–8, state whether true or false and explain. Assume that f (x) exists for all x. 3. If f (c) = 0 and f (c) < 0, then f (c) is a local minimum. SOLUTION

False. By the Second Derivative Test, the correct conclusion would be that f (c) is a local maximum.

4. A function that is concave down on (−∞, ∞) can have no minimum value. SOLUTION

True.

5. If f (c) = 0, then f must have a point of inﬂection at x = c. False. f has an inﬂection point at x = c provided concavity changes at x = c. Points where f (c) = 0 are simply candidates for inﬂection points. The function f (x) = x 4 provides a counterexample. Here, f (x) = 12x 2 . Thus, f (0) = 0 but f (x) does not change sign at x = 0. Therefore, f (x) = x 4 does not have an inﬂection point at x = 0. SOLUTION

6. If f has a point of inﬂection at x = c, then f (c) = 0. SOLUTION True. In general, if f has an inﬂection point at x = c, then either f (c) = 0 or f (c) does not exist. Because we are assuming that f (x) exists for all x, we must have f (c) = 0.

7. If f is concave up and f changes sign at x = c, then f changes sign from negative to positive at x = c. True. If f is concave up, then f is positive and f is increasing; therefore, f must go from negative to positive at x = c. SOLUTION

S E C T I O N 4.4

The Shape of a Graph

181

8. If f (c) is a local maximum, then f (c) must be negative. SOLUTION True. In general, it could be that either f (c) does not exist or f (c) is negative. Because we are assuming that f (x) exists for all x, we must have f (c) < 0.

9. Suppose that f (c) = 0 and f (x) changes sign from + to − at x = c. Which of the following statements are correct? (a) f (x) has a local maximum at x = a. (b) f (x) has a local minimum at x = a. (c) f (x) has a local maximum at x = a. (d) f (x) has a point of inﬂection at x = a. Statements (c) and (d) are correct. Because f (x) goes from positive to negative, it follows that f (x) goes from increasing to decreasing and so must have a local maximum at x = c. Also, because f (c) = 0 and f (x) changes sign at x = c, it follows that f has a point of inﬂection at x = c. SOLUTION

Exercises 1. Match the graphs in Figure 12 with the description: (a) f (x) < 0 for all x. (c) f (x) > 0 for all x.

(A)

(b) f (x) goes from + to −. (d) f (x) goes from − to +.

(B)

(C)

(D)

FIGURE 12 SOLUTION

(a) (b) (c) (d)

In C, we have f (x) < 0 for all x. In A, f (x) goes from + to −. In B, we have f (x) > 0 for all x. In D, f (x) goes from − to +.

3. Sketch the graph of an increasing function such that f (x) changes from + to − at x = 2 and from − to + at x = 4. Match each statement with a graph in Figure 13 that represents company proﬁts as a function of time. Do the same for a decreasing function. (a) The outlook is great: The growth rate keeps increasing. SOLUTION The graph shown below at the left is an increasing function which changes from concave up to concave (b) We’re losing money, but not as quickly as before. down at x = 2 and from concave down to concave up at x = 4. The graph shown below at the right is a decreasing (c) We’re money, it’s getting worse asdown time at goes function which losing changes from and concave up to concave x =on. 2 and from concave down to concave up at x = 4. (d) We’re doing well, but our growth rate is leveling off. y y (e) Business had been cooling off, but now it’s picking up. 2 6 (f) Business had been picking up, but now it’s cooling off. 4 1 2 x 2

4

x 2

4

5. If Figure 14 is the graph of the derivative f (x), where do the points of inﬂection of f (x) occur, and on which If Figure 14 is the graph of a function f (x), where do the points of inﬂection of f (x) occur, and on which interval interval is f (x) concave down? is f (x) concave down? SOLUTION Points of inﬂection occur when f (x) changes sign. Consequently, points of inﬂection occur when f (x) changes from increasing to decreasing or from decreasing to increasing. In Figure 14, this occurs at x = b and at x = e; therefore, f (x) has an inﬂection point at x = b and another at x = e. The function f (x) will be concave down when f (x) < 0 or when f (x) is decreasing. Thus, f (x) is concave down for b < x < e. In Exercises 7–14,14determine theof intervals on which the function concave uppoints or down and ﬁnd the of inﬂection. If Figure is the graph the second derivative f (x), is where do the of inﬂection of points f (x) occur, and on + 7x +is10f (x) concave down? 7. which y = x 2interval

SOLUTION Let f (x) = x 2 + 7x + 10. Then f (x) = 2x + 7 and f (x) = 2 > 0 for all x. Therefore, f is concave up everywhere, and there are no points of inﬂection.

9. y = x − 2 cos x y = t 3 − 3t 2 + 1

182

CHAPTER 4

A P P L I C AT I O N S O F T H E D E R I VATI V E SOLUTION Let f (x) = x − 2 cos x. Then f (x) = 1 + 2 sin x and f (x) = 2 cos x = 0 at x = 1 2 (2n + 1) π , where n is an integer. Now, f is concave up on the intervals

π π − + 2n π , + 2n π 2 2

where n is any integer since f (x) > 0 there. Moreover, f is concave down on the intervals 3π π + 2n π , + 2n π 2 2 where n is any integer since f (x) < 0 there. Finally, because f (x) changes sign at each x = 12 (2n + 1)π , there is a point of inﬂection at each of these locations. √ 11. y = x(x − 8 x) 5 y = 4x − 5x 4 √ SOLUTION Let f (x) = x(x − 8 x) = x 2 − 8x 3/2 . Then f (x) = 2x − 12x 1/2 and f (x) = 2 − 6x −1/2 . Now, f is concave down for 0 < x < 9 since f (x) < 0 there. Moreover, f is concave up for x > 9 since f (x) > 0 there. Finally, because f (x) changes sign at x = 9, f (x) has a point of inﬂection at x = 9. 1 13. y =y =2 (x − 2)(1 − x 3 ) x +3 SOLUTION

1 2x . Then f (x) = − 2 Let f (x) = 2 and x +3 (x + 3)2 f (x) = −

2(x 2 + 3)2 − 8x 2 (x 2 + 3) 6x 2 − 6 = . (x 2 + 3)4 (x 2 + 3)3

Now, f is concave up for |x| > 1 since f (x) > 0 there. Moreover, f is concave down for |x| < 1 since f (x) < 0 there. Finally, because f (x) changes sign at both x = −1 and x = 1, f (x) has a point of inﬂection at both x = −1 and x = 1. 15. Sketch the7/5 graph of f (x) = x 4 and state whether f has any points of inﬂection. Verify your conclusion by showing y = x that f (x) does not change sign. SOLUTION

From the plot of f (x) = x 4 below, it appears that f has no points of inﬂection. Indeed, f (x) = 4x 3 and

f (x) = 12x 2 = 0 at x = 0, but f (x) does not change sign as x increases through 0 because 12x 2 ≥ 0 for all x. y 1

x

−1

1

The her growth of a sunﬂower its ﬁrst bean 100 days is modeled by the logistic curve ybelow. = h(t)Inshown in 17. Through website, Leticia hasduring been selling bag chairs with well monthly sales as recorded a report Figure 15. Estimate growth ratereached at the point ofofinﬂection explain its signiﬁcance. Then make a rough sketch of to investors, she the states, “Sales a point inﬂectionand when I started using pay-per-click advertising.” In which the ﬁrst anddid second derivatives of h(t). month that occur? Explain. Height (cm)

Month Sales

300

1

250 2

2

3

4

5

6

7

8

20

30

35

38

44

60

90

200 150 100 50 t (days) 20

40

60

80

100

FIGURE 15 SOLUTION The point of inﬂection in Figure 15 appears to occur at t = 40 days. The graph below shows the logistic curve with an approximate tangent line drawn at t = 40. The approximate tangent line passes roughly through the points (20, 20) and (60, 240). The growth rate at the point of inﬂection is thus

220 240 − 20 = = 5.5 cm/day. 60 − 20 40 Because the logistic curve changes from concave up to concave down at t = 40, the growth rate at this point is the maximum growth rate for the sunﬂower plant.

The Shape of a Graph

S E C T I O N 4.4

183

Height (cm) 300 250 200 150 100 50 t (days) 20

40

60

80

100

Sketches of the ﬁrst and second derivative of h(t) are shown below at the left and at the right, respectively. h′

h′′

6 5

0.1

4

t

3

20

2

40

60

80

100

−0.1

1 t 20

40

60

80

100

In Exercises 19–28, ﬁnd the the graph critical of f (x) and Derivative Testof (ifinﬂection possible)of to fdetermine whether Figure 16 shows ofpoints the derivative f (x)use onthe [0, Second 1.2]. Locate the points (x) and the points eachwhere corresponds to a local minimum or maximum. the local minima and maxima occur. Determine the intervals on which f (x) has the following properties: (b) Decreasing 19. (a) f (x)Increasing = x 3 − 12x 2 + 45x (c) Concave up (d) Concave down SOLUTION Let f (x) = x 3 − 12x 2 + 45x. Then f (x) = 3x 2 − 24x + 45 = 3(x − 3)(x − 5), and the critical points are x = 3 and x = 5. Moreover, f (x) = 6x − 24, so f (3) = −6 < 0 and f (5) = 6 > 0. Therefore, by the Second Derivative Test, f (3) = 54 is a local maximum, and f (5) = 50 is a local minimum.

21. f (x) = 3x 4 − 8x 3 + 6x 2 f (x) = x 4 − 8x 2 + 1 SOLUTION Let f (x) = 3x 4 − 8x 3 + 6x 2 . Then f (x) = 12x 3 − 24x 2 + 12x = 12x(x − 1)2 = 0 at x = 0, 1 and f (x) = 36x 2 − 48x + 12. Thus, f (0) > 0, which implies f (0) is a local minimum; however, f (1) = 0, which is inconclusive. 23. f (x) = x 5 −5x 3 2 f (x) = x − x

3 SOLUTION Let f (x) = x 5 − x 3 . Then f (x) = 5x 4 − 3x 2 = x 2 (5x 2 − 3) = 0 at x = 0, x = ± 5 and f (x) = 3 3 3 2 20x − 6x = x(20x − 6). Thus, f − 35 < 0, 5 > 0, which implies f 5 is a local minimum, and f which implies that f − 35 is a local maximum; however, f (0) = 0, which is inconclusive. 1 25. f (x)f (x) = = sin2 x + cos x, [0, π ] cos x + 2 1 sin x SOLUTION Let f (x) = = 0 at x = 0, ±π , ±2π , ±3π , . . . , and . Then f (x) = cos x + 2 (cos x + 2)2 f (x) =

cos2 x + 2 cos x + 2 sin2 x . (cos x + 2)3

Thus, f (0) > 0, f (±2π ) > 0 and f (±4π ) > 0, which implies that f (x) has local minima when x is an even multiple of π . Similarly, f (±π ) < 0, f (±3π ) < 0, and f (±5π ) < 0, which implies that f (x) has local maxima when x is an odd multiple of π . 27. f (x) = 3x 3/2 − 1x 1/2 f (x) = 3/2 − x 1/2 . Then f (x) = 9 x 1/2 − 1 x −1/2 = 1 x −1/2 (9x − 1), so there are two critical SOLUTION Letx 2f − (x)x = + 23x 2 2 2 1 points: x = 0 and x = 9 . Now, f (x) =

9 −1/2 1 −3/2 1 + x = x −3/2 (9x + 1). x 4 4 4

Thus, f 19 > 0, which implies f 19 is a local minimum. f (x) is undeﬁned at x = 0, so the Second Derivative Test cannot be applied there. In Exercises 29–40, ﬁnd the intervals on which f is concave up or down, the points of inﬂection, and the critical points, f (x) = x 7/4 − x and determine whether each critical point corresponds to a local minimum or maximum (or neither).

184

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A P P L I C AT I O N S O F T H E D E R I VATI V E SOLUTION

Here is a table legend for Exercises 29–40. SYMBOL

MEANING

−

The entity is negative on the given interval.

0

The entity is zero at the speciﬁed point.

+

The entity is positive on the given interval.

U

The entity is undeﬁned at the speciﬁed point.

The function ( f , g, etc.) is increasing on the given interval.

The function ( f , g, etc.) is decreasing on the given interval.

The function ( f , g, etc.) is concave up on the given interval.

The function ( f , g, etc.) is concave down on the given interval.

M

The function ( f , g, etc.) has a local maximum at the speciﬁed point.

m

The function ( f , g, etc.) has a local minimum at the speciﬁed point.

I

The function ( f , g, etc.) has an inﬂection point here.

¬

There is no local extremum or inﬂection point here.

29. f (x) = x 3 − 2x 2 + x SOLUTION

Let f (x) = x 3 − 2x 2 + x.

• Then f (x) = 3x 2 − 4x + 1 = (x − 1)(3x − 1) = 0 yields x = 1 and x = 1 as candidates for extrema. 3 • Moreover, f (x) = 6x − 4 = 0 gives a candidate for a point of inﬂection at x = 2 . 3

x

(−∞, 13 )

1 3

( 13 , 1)

1

(1, ∞)

x

(−∞, 23 )

2 3

( 23 , ∞)

f

+

0

−

0

+

f

−

0

+

f

M

m

f

I

31. f (t) = t 2 − t23 f (x) = x (x − 4) SOLUTION Let f (t) = t 2 − t 3 . • Then f (t) = 2t − 3t 2 = t (2 − 3t) = 0 yields t = 0 and t = 2 as candidates for extrema. 3 • Moreover, f (t) = 2 − 6t = 0 gives a candidate for a point of inﬂection at t = 1 . 3

t

(−∞, 0)

0

(0, 23 )

2 3

( 23 , ∞)

t

(−∞, 13 )

1 3

( 13 , ∞)

f

−

0

+

0

−

f

+

0

−

f

m

M

f

I

33. f (x) = x 2 − x41/2 2 f (x) = 2x − 3x + 2 SOLUTION Let f (x) = x 2 − x 1/2 . Note that the domain of f is x ≥ 0.

2/3 • Then f (x) = 2x − 1 x −1/2 = 1 x −1/2 4x 3/2 − 1 = 0 yields x = 0 and x = 1 as candidates for extrema. 2 2 4 • Moreover, f (x) = 2 + 1 x −3/2 > 0 for all x ≥ 0, which means there are no inﬂection points. 4

1 x 35. f (t) = 2 f (x)t =+ 12 x +2

x

0

2/3 0, 14

2/3

f

U

−

0

+

f

M

m

1 4

2/3 1 4

,∞

The Shape of a Graph

S E C T I O N 4.4

185

1 . Let f (t) = 2 t +1 2t • Then f (t) = − 2 = 0 yields t = 0 as a candidate for an extremum. 2 t +1 2

2 − 2t 2 − 2 t 2 + 1 (2) − 2t · 2 t 2 + 1 (2t) 2 3t 2 − 1 8t • Moreover, f (t) = − = 4 3 = 3 = 0 gives candidates t2 + 1 t2 + 1 t2 + 1

for a point of inﬂection at t = ± 13 .

SOLUTION

t

(−∞, 0)

0

(0, ∞)

f

+

0

−

f

M

−∞, − 13

− 13

− 13 , 13

f

+

0

−

0

+

f

I

I

t

1 3

1,∞ 3

37. f (θ ) = θ + sin θ for 0 ≤ θ ≤ 2π 1 f (x) = SOLUTION Let 4f (θ ) = θ + sin θ on [0, 2π ]. x +1 • Then f (θ ) = 1 + cos θ = 0 yields θ = π as a candidate for an extremum. • Moreover, f (θ ) = − sin θ = 0 gives candidates for a point of inﬂection at θ = 0, at θ = π , and at θ = 2π .

θ

(0, π )

f

+

f

π

(π , 2π )

θ

0

(0, π )

π

(π , 2π )

2π

0

+

f

0

−

0

+

0

¬

f

¬

I

¬

39. f (x) = x − sin x for 0 ≤ x ≤ 2π f (t) = sin2 t for 0 ≤ t ≤ π SOLUTION Let f (x) = x − sin x on [0, 2π ]. • Then f (x) = 1 − cos x > 0 on (0, 2π ). • Moreover, f (x) = sin x = 0 gives a candidate for a point of inﬂection at x =

π.

(0, 2π )

x

(0, π )

π

(π , 2π )

f

+

f

+

0

−

f

f

I

x

π <x < π 41. infectious slowly at the beginning of an epidemic. The infection process accelerates until a f (x)An = tan x for ﬂu−spreads 2 2 majority of the susceptible individuals are infected, at which point the process slows down. (a) If R(t) is the number of individuals infected at time t, describe the concavity of the graph of R near the beginning and end of the epidemic. (b) Write a one-sentence news bulletin describing the status of the epidemic on the day that R(t) has a point of inﬂection. SOLUTION

(a) Near the beginning of the epidemic, the graph of R is concave up. Near the epidemic’s end, R is concave down. (b) “Epidemic subsiding: number of new cases declining.” 43. Water is pumped a sphere a variable in such wayh(t) that be thethe water level rises at a constant Water is pumped intointo a sphere at aatconstant raterate (Figure 17).a Let water level at time t. Sketchrate the c (Figure 17). Let V (t) be the volume of water at time t. Sketch the graph of V (t) (approximately, but with graph of h(t) (approximately, but with the correct concavity). Where does the point of inﬂection occur? the correct concavity). Where does the point of inﬂection occur?

h

FIGURE 17

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A P P L I C AT I O N S O F T H E D E R I VATI V E SOLUTION Because water is entering the sphere in such a way that the water level rises at a constant rate, we expect the volume to increase more slowly near the bottom and top of the sphere where the sphere is not as “wide” and to increase more rapidly near the middle of the sphere. The graph of V (t) should therefore start concave up and change to concave down when the sphere is half full; that is, the point of inﬂection should occur when the water level is equal to the radius of the sphere. A possible graph of V (t) is shown below. V

t

In Exercises 45–47, sketch the graph of a function f (x) satisfying all of the given conditions. (Continuation of Exercise 43) When the water level in a sphere of radius R is h, the volume of water is 2 − 1 3 ). Assume the level rises at a constant rate c = 1 (i.e., h = t). = π> (Rh 0 and < 0 for all x. 45. Vf (x) 3f h(x) (a) Find the inﬂection point of V (t). Does this agree with your conclusion in Exercise 43? SOLUTION Here is the graph of a function f (x) satisfying f (x) > 0 for all x and f (x) < 0 for all x. (b) Plot V (t) for R = 1. y

0.5 x 1

2

−0.5

47. (i) f (x) < 0 for x < 0 and f (x) > 0 for x > 0, and f (x) for > all2,x,and and f (x) > 0 for |x| < 2. (x) < 0>for0 |x| (ii) f(i) (ii) f (x) < 0 for x < 0 and f (x) > 0 for x > 0. SOLUTION

Interval

(−∞, −2)

(−2, 0)

(0, 2)

(2, ∞)

Direction

Concavity

One potential graph with this shape is the following: y

−10

x 10

Further Insights and Challenges In Exercises 48–50, assume that f (x) is differentiable. (x) > 0. Prove that the graph of f (x) “sits 49. that f Derivative (x) exists and above” its tangent ProofAssume of the Second Testf Let c be a critical point in (a, b) such that f (c) > 0 (the case f lines (c) 0 (b) Show thatpart G(c) (c) that = 0 there and Gexists (x) > for all x. Use(a, thisb)tocontaining conclude that G (x) for< x< (b) Use (a)= to G show an0open interval c such that c. Then the MVT,that thatf G(x) G(c)minimum. for x = c. > 0 ifdeduce, c < x <using b. Conclude (c) is > a local SOLUTION

(a) Let c be any number. Then y = f (c)(x − c) + f (c) is the equation of the line tangent to the graph of f (x) at x = c and G(x) = f (x) − f (c)(x − c) − f (c) measures the amount by which the value of the function exceeds the value of the tangent line (see the ﬁgure below). Thus, to prove that the graph of f (x) “sits above” its tangent lines, it is sufﬁcient to prove that G(x) ≥ 0 for all c.

S E C T I O N 4.4

The Shape of a Graph

187

y

x

(b) Note that G(c) = f (c) − f (c)(c − c) − f (c) = 0, G (x) = f (x) − f (c) and G (c) = f (c) − f (c) = 0. Moreover, G (x) = f (x) > 0 for all x. Now, because G (c) = 0 and G (x) is increasing, it must be true that G (x) < 0 for x < c and that G (x) > 0 for x > c. Therefore, G(x) is decreasing for x < c and increasing for x > c. This implies that G(c) = 0 is a minimum; consequently G(x) > G(c) = 0 for x = c. 51. Let C(x) be the cost of producing x units of a certain good. Assume that the graph of C(x) is concave up. Assume that f (x) exists and let c be a point of inﬂection of f (x). (a) Show that the average cost A(x) = C(x)/x is minimized at that production level x 0 for which average cost equals (a) Use marginal cost.the method of Exercise 49 to prove that the tangent line at x = c crosses the graph (Figure 18). Hint: Show that G(x) sign at x(0, = 0) c. and (x , C(x )) is tangent to the graph of C(x). (b) Show that changes the line through 0 0 Let C(x) be the cost of producing x units of a commodity. Assume the graph of C is concave up. (a) Let A(x) = C(x)/x be the average cost and let x 0 be the production level at which average cost is minimized. x x C (x 0 ) − C(x 0for ) f (x) = graphing f (x) and the tangent line at each inﬂection point on ) = 0this conclusion = 0 implies x C 1(xby Then(b) A (x 0Verify 20+ 0 ) − C(x 0 ) = 0, whence C (x 0 ) = C(x 0 )/x 0 = A(x 0 ). In other 3x 2 x the same set of axes. 0 words, A(x0 ) = C (x 0 ) or average cost equals marginal cost at production level x 0 . To conﬁrm that x 0 corresponds to a local minimum of A, we use the Second Derivative Test. We ﬁnd SOLUTION

x 2 C (x 0 ) − 2(x 0 C (x 0 ) − C(x 0 )) C (x 0 ) A (x 0 ) = 0 = >0 3 x0 x0 because C is concave up. Hence, x0 corresponds to a local minimum. (b) The line between (0, 0) and (x 0 , C(x 0 )) is C(x 0 ) C(x 0 ) − 0 (x − x0 ) + C(x 0 ) = A(x 0 )(x − x 0 ) + C(x 0 ) (x − x 0 ) + C(x 0 ) = x0 − 0 x0 = C (x0 )(x − x 0 ) + C(x 0 ) which is the tangent line to C at x0 . 53. Critical Points and Inﬂection Points If f (c) = 0 and f (c) is neither a local min or max, must x = c be a point Let f (x) be a polynomial of degree n. of inﬂection? This is true of most “reasonable” examples (including the examples in this text), but it is not true in general. (a) Show that if n is odd and n > 1, then f (x) has at least one point of inﬂection. Let have a point of inﬂection if n is even. (b) Show by giving an example that f (x) need not x 2 sin 1x for x = 0 f (x) = 0 for x = 0 (a) Use the limit deﬁnition of the derivative to show that f (0) exists and f (0) = 0. (b) Show that f (0) is neither a local min nor max. (c) Show that f (x) changes sign inﬁnitely often near x = 0 and conclude that f (x) does not have a point of inﬂection at x = 0. x 2 sin (1/x) for x = 0 . SOLUTION Let f (x) = 0 for x = 0 1 f (x) − f (0) x 2 sin (1/x) (a) Now f (0) = lim = lim = lim x sin = 0 by the Squeeze Theorem: as x → 0 x −0 x x x→0 x→0 x→0 we have x sin 1 − 0 = |x| sin 1 → 0, x x since | sin u| ≤ 1. (b) Since sin( 1x ) oscillates through every value between −1 and 1 with increasing frequency as x → 0, in any open interval (−δ , δ ) there are points a and b such that f (a) = a 2 sin( a1 ) < 0 and f (b) = b2 sin( b1 ) > 0. Accordingly, f (0) = 0 can neither be a local minimum value nor a local maximum value of f . (c) In part (a) it was shown that f (0) = 0. For x = 0, we have 1 1 1 1 1 f (x) = x 2 cos − 2 + 2x sin = 2x sin − cos . x x x x x As x → 0, f (x) oscillates increasingly rapidly; consequently, f (x) changes sign inﬁnitely often near x = 0. From this we conclude that f (x) does not have a point of inﬂection at x = 0.

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A P P L I C AT I O N S O F T H E D E R I VATI V E

4.5 Graph Sketching and Asymptotes Preliminary Questions 1. Sketch an arc where f and f have the sign combination ++. Do the same for −+. SOLUTION An arc with the sign combination ++ (increasing, concave up) is shown below at the left. An arc with the sign combination −+ (decreasing, concave up) is shown below at the right. y

y

x

x

2. If the sign combination of f and f changes from ++ to +− at x = c, then (choose the correct answer): (a) f (c) is a local min (b) f (c) is a local max (c) c is a point of inﬂection SOLUTION

Because the sign of the second derivative changes at x = c, the correct response is (c): c is a point of

inﬂection. 3. What are the following limits? (a) lim x 3 x→∞

(b)

lim x 3

(c)

x→−∞

lim x 4

x→−∞

SOLUTION

(a) limx→∞ x 3 = ∞ (b) limx→−∞ x 3 = −∞ (c) limx→−∞ x 4 = ∞ 4. What is the sign of a2 if f (x) = a2 x 2 + a1 x + a0 satisﬁes

lim

x→−∞

f (x) = −∞?

The behavior of f (x) = a2 x 2 + a1 x + a0 as x → −∞ is controlled by the leading term; that is, limx→−∞ f (x) = limx→−∞ a2 x 2 . Because x 2 → +∞ as x → −∞, a2 must be negative to have limx→−∞ f (x) = −∞. SOLUTION

5. What is the sign of the leading coefﬁcient a7 if f (x) is a polynomial of degree 7 such that

lim

x→−∞

f (x) = ∞?

SOLUTION The behavior of f (x) as x → −∞ is controlled by the leading term; that is, lim x→−∞ f (x) = limx→−∞ a7 x 7 . Because x 7 → −∞ as x → −∞, a7 must be negative to have limx→−∞ f (x) = ∞.

6. The second derivative of the function f (x) = (x − 4)−1 is f (x) = 2(x − 4)−3 . Although f (x) changes sign at x = 4, f (x) does not have a point of inﬂection at x = 4. Why not? SOLUTION

The function f does not have a point of inﬂection at x = 4 because x = 4 is not in the domain of f.

Exercises 1. Determine the sign combinations of f and f for each interval A–G in Figure 18. y

y = f(x)

A

B C

D

E F

G

FIGURE 18 SOLUTION

• In A, f is decreasing and concave up, so f < 0 and f > 0. • In B, f is increasing and concave up, so f > 0 and f > 0.

• In C, f is increasing and concave down, so f > 0 and f < 0. • In D, f is decreasing and concave down, so f < 0 and f < 0. • In E, f is decreasing and concave up, so f < 0 and f > 0.

x

S E C T I O N 4.5

Graph Sketching and Asymptotes

189

• In F, f is increasing and concave up, so f > 0 and f > 0. • In G, f is increasing and concave down, so f > 0 and f < 0. f 19. takeExample: on the given sign combinations. In Exercises drawchange the graph of atransition function for which State 3–6, the sign at each point A–Gfinand Figure f (x) goes from + to − at A.

3. ++,

+−,

SOLUTION

x = 0.

−−

This function changes from concave up to concave down at x = −1 and from increasing to decreasing at −1

0

1

x

−1

y

5. −+, −−, −+ +−, −−, −+ SOLUTION The function is decreasing everywhere and changes from concave up to concave down at x = −1 and from concave down to concave up at x = − 12 . y

0.05

x

−1

0

7. Sketch graph +− of y = x 2 − 2x + 3. −+,the++, SOLUTION Let f (x) = x 2 − 2x + 3. Then f (x) = 2x − 2 and f (x) = 2. Hence f is decreasing for x < 1, is increasing for x > 1, has a local minimum at x = 1 and is concave up everywhere. y 5 4 3 2 1 x

−1

1

2

3

9. Sketch graph of the cubic f (x) = x 3 2− 3x 2 + 2. For extra accuracy, plot the zeros of f (x), which are x = 1 and √ the the Sketch of y2.73. = 3 + 5x − 2x . x = 1 ± 3 or x ≈graph −0.73, Let f (x) = x 3 − 3x 2 + 2. Then f (x) = 3x 2 − 6x = 3x(x − 2) = 0 yields x = 0, 2 and f (x) = 6x − 6. Thus f is concave down for x < 1, is concave up for x > 1, has an inﬂection point at x = 1, is increasing for x < 0 and for x > 2, is decreasing for 0 < x < 2, has a local maximum at x = 0, and has a local minimum at x = 2. SOLUTION

y 2 1 −1

x −1

1

2

3

−2

11. Extend the sketch of the graph of f (x) = cos x + 1 x over [0, π ] in Example 4 to the interval [0, 5π ]. Show that the cubic x 3 − 3x 2 + 6x has a point 2of inﬂection but no local extreme values. Sketch the graph. 1 1 π 5π 13π 17π SOLUTION Let f (x) = cos x + 2 x. Then f (x) = − sin x + 2 = 0 yields critical points at x = 6 , 6 , 6 , 6 , 25π , and 29π . Moreover, f (x) = − cos x so there are points of inﬂection at x = π , 3π , 5π , 7π , and 9π . 6 6 2 2 2 2 2

190

CHAPTER 4

A P P L I C AT I O N S O F T H E D E R I VATI V E y 6 4 2 x 0

2

4

6

8 10 12 14

In Exercises 13–30, sketch the graph of the function. Indicate the transition points (local extrema and points of inﬂection). Sketch the graphs of y = x 2/3 and y = x 4/3 . 13. y = x 3 + 32 x 2 Let f (x) = x 3 + 32 x 2 . Then f (x) = 3x 2 + 3x = 3x (x + 1) and f (x) = 6x + 3. This shows that f has critical points at x = 0 and x = −1 and a candidate for an inﬂection point at x = − 12 . SOLUTION

Interval

(−∞, −1)

(−1, − 12 )

(− 12 , 0)

(0, ∞)

+−

−−

−+

++

Signs of f and f

Thus, there is a local maximum at x = −1, a local minimum at x = 0, and an inﬂection point at x = − 12 . Here is a graph of f with these transition points highlighted as in the graphs in the textbook. y 2 1 −2

x

−1

−1

15. y = x 2 −34x 3 y = x − 3x + 5 SOLUTION Let f (x) = x 2 − 4x 3 . Then f (x) = 2x − 12x 2 = 2x(1 − 6x) and f (x) = 2 − 24x. Critical points are 1 . at x = 0 and x = 16 , and the sole candidate point of inﬂection is at x = 12 Interval Signs of f and f

(−∞, 0)

1 ) (0, 12

1 , 1) ( 12 6

( 16 , ∞)

−+

++

+−

−−

1 . Here is the graph Thus, f (0) is a local minimum, f ( 16 ) is a local maximum, and there is a point of inﬂection at x = 12 of f with transition points highlighted as in the textbook: y 0.04 x

−0.2

0.2 −0.04

17. y = 4 −12x32 + 162x 4 y = 3 x + x + 3x

1 2 2 SOLUTION Let f (x) = 6 x 4 − 2x 2 + 4. Then f (x) = 3 x 3 − 4x = 3 x x 2 − 6 and f (x) = 2x 2 − 4. This shows √ √ that f has critical points at x = 0 and x = ± 6 and has candidates for points of inﬂection at x = ± 2. Interval Signs of f and f

√ (−∞, − 6)

√ √ (− 6, − 2)

√ (− 2, 0)

−+

++

+−

(0,

√

2)

−−

√ √ ( 2, 6)

√ ( 6, ∞)

−+

++

√ √ Thus, f has local minima at x = ± 6, a local maximum at x = 0, and inﬂection points at x = ± 2. Here is a graph of f with transition points highlighted.

S E C T I O N 4.5

Graph Sketching and Asymptotes

191

y 10 5 −2

2

x

19. y = x 5 + 5x y = 7x 4 − 6x 2 + 1 SOLUTION Let f (x) = x 5 + 5x. Then f (x) = 5x 4 + 5 = 5(x 4 + 1) and f (x) = 20x 3 . f (x) > 0 for all x, so the graph has no critical points and is always increasing. f (x) = 0 at x = 0. Sign analyses reveal that f (x) changes from negative to positive at x = 0, so that the graph of f (x) has an inﬂection point at (0, 0). Here is a graph of f with transition points highlighted. y 40 20 −2

−1

x 1

−20

2

−40

21. y = x 4 −53x 3 +34x y = x − 5x SOLUTION Let f (x) = x 4 − 3x 3 + 4x. Then f (x) = 4x 3 − 9x 2 + 4 = (4x 2 − x − 2)(x − 2) and f (x) = √ 1 ± 33 2 and candidate points of 12x − 18x = 6x(2x − 3). This shows that f has critical points at x = 2 and x = 8 √ 3 inﬂection at x = 0 and x = 2 . Sign analyses reveal that f (x) changes from negative to positive at x = 1−8 33 , from √

√

positive to negative at x = 1+8 33 , and again from negative to positive at x = 2. Therefore, f ( 1−8 33 ) and f (2) are √ local minima of f (x), and f ( 1+ 8 33 ) is a local maximum. Further sign analyses reveal that f (x) changes from positive to negative at x = 0 and from negative to positive at x = 32 , so that there are points of inﬂection both at x = 0 and x = 32 . Here is a graph of f (x) with transition points highlighted. y 6 4 2 −1

x 1

−2

2

23. y = 6x 7 − 7x 6 y = x 2 (x − 4)2 SOLUTION Let f (x) = 6x 7 − 7x 6 . Then f (x) = 42x 6 − 42x 5 = 42x 5 (x − 1) and f (x) = 252x 5 − 210x 4 = 42x 4 (6x − 5). Critical points are at x = 0 and x = 1, and candidate inﬂection points are at x = 0 and x = 56 . Sign analyses reveal that f (x) changes from positive to negative at x = 0 and from negative to positive at x = 1. Therefore f (0) is a local maximum and f (1) is a local minimum. Also, f (x) changes from negative to positive at x = 56 . Therefore, there is a point of inﬂection at x = 56 . Here is a graph of f with transition points highlighted. y 2 1 −0.5

x −1

0.5

1

√ 25. y = x − x y = x 6 − 3x 4 √ 1 SOLUTION Let f (x) = x − x = x − x 1/2 . Then f (x) = 1 − 2 x −1/2 . This shows that f has critical points at x = 0 1 (where the derivative does not exist) and at x = 4 (where the derivative is zero). Because f (x) < 0 for 0 < x < 14

and f (x) > 0 for x > 14 , f 14 is a local minimum. Now f (x) = 14 x −3/2 > 0 for all x > 0, so the graph is always concave up. Here is a graph of f with transition points highlighted.

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A P P L I C AT I O N S O F T H E D E R I VATI V E y 0.8 0.6 0.4 0.2 x 0.5

−0.2

1

1.5

2

√ √ + 9−x 27. y = x√ y = x +2 √ √ SOLUTION Let f (x) = x + 9 − x = x 1/2 + (9 − x)1/2 . Note that the domain of f is [0, 9]. Now, f (x) = 1 x −1/2 − 1 (9 − x)−1/2 and f (x) = − 1 x −3/2 − 1 (9 − x)−3/2 . Thus, the critical points are x = 0, x = 9 and 2 2 4 4 2 x = 9. Sign analysis reveals that f (x) > 0 for 0 < x < 92 and f (x) < 0 for 92 < x < 9, so f has a local maximum at x = 92 . Further, f (x) < 0 on (0, 9), so the graph is always concave down. Here is a graph of f with the transition point highlighted. y 4 3 2 1 x 0

1 2 3 4 5 6 7 8 9

29. y = (x 2 − x)1/3 1/3 y = x(1 − x) SOLUTION Let f (x) = (x 2 − x)1/3 . Then f (x) = and 1 f (x) =

3

1 (2x − 1)(x 2 − x)−2/3 3

2 2(x 2 − x)−2/3 − (2x − 1)(2x − 1)(x 2 − x)−5/3

3

2 2 = (x 2 − x)−5/3 3(x 2 − x) − (2x − 1)2 = − (x 2 − x)−5/3 (x 2 − x + 1). 9 9

Critical points of f (x) are points where the numerator 2x − 1 = 0 or where f (x) doesn’t exist, that is, at x = 12 and points where x 2 − x = 0 so that x = 1 or x = 0. Candidate points of inﬂection lie at points where f (x) = 0 (of which there are none), and points where f (x) does not exist (at x = 0 and x = 1). Sign analyses reveal that f (x) < 0 for x < 0 and for x > 1, while f (x) > 0 for 0 < x < 1. Therefore, the graph of f (x) has points of inﬂection at x = 0 and x = 1. Since (x 2 − x)−2/3 is positive wherever it is deﬁned, the sign of f (x) depends solely on the sign of 2x − 1. Hence, f (x) does not change sign at x = 0 or x = 1, and goes from negative to positive at x = 12 . f ( 12 ) is, in that case, a local minimum. Here is a graph of f (x) with the transition points indicated. y 1.5 1 0.5 −1

x −0.5

1

2

31. Sketch the3graph of f (x) = 18(x − 3)(x − 1)2/3 using the following formulas: y = (x − 4x)1/3 3 30(x − 95 ) (x) = 20(x − 5 ) , f f (x) = (x − 1)1/3 (x − 1)4/3 SOLUTION

f (x) =

30(x − 95 ) (x − 1)1/3

S E C T I O N 4.5

Graph Sketching and Asymptotes

193

yields critical points at x = 95 , x = 1. f (x) =

20(x − 35 ) (x − 1)4/3

yields potential inﬂection points at x = 35 , x = 1. Interval

signs of f and f

(−∞, 35 )

+−

( 35 , 1) (1, 95 ) ( 95 , ∞)

++ −+ ++

The graph has an inﬂection point at x = 35 , a local maximum at x = 1 (at which the graph has a cusp), and a local minimum at x = 95 . The sketch looks something like this. y −2

40 −1 20

1

2

3

−20 −40 −60 −80

x

In Exercises 32–37, sketch the graph over the given interval. Indicate the transition points. 33. y = sin x + cos x, [0, 2π ] y = x + sin x, [0, 2π ] SOLUTION Let f (x) = sin x + cos x. Setting f (x) = cos x − sin x = 0 yields sin x = cos x, so that tan x = 1, and x = π4 , 54π . Setting f (x) = − sin x − cos x = 0 yields sin x = − cos x, so that − tan x = 1, and x = 34π , x = 74π . Interval

signs of f and f

(0, π4 )

+−

( π4 , 34π )

−−

( 34π , 54π )

−+

( 54π , 74π )

++

( 74π , 2π )

+−

The graph has a local maximum at x = π4 , a local minimum at x = 54π , and inﬂection points at x = 34π and x = 74π . Here is a sketch of the graph of f (x): y 1 x 1

2

3

4

5

6

−1

35. y = sin x + 12 x, [0,2 2π ] y = 2 sin x − cos x, [0, 2π ] 1 1 2π 4π SOLUTION Let f (x) = sin x + 2 x. Setting f (x) = cos x + 2 = 0 yields x = 3 or 3 . Setting f (x) = − sin x = 0 yields potential points of inﬂection at x = 0, π , 2π .

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A P P L I C AT I O N S O F T H E D E R I VATI V E

Interval

signs of f and f

(0, 23π )

+−

( 23π , π )

−−

(π , 43π )

−+

( 43π , 2π )

++

The graph has a local maximum at x = 23π , a local minimum at x = 43π , and an inﬂection point at x = π . Here is a graph of f without transition points highlighted. y 3 2 1 x 0

1

2

3

4

5

6

1 √ ] π] [0, π[0, 37. y =y sin x −x +sin 32x, = sin cos x, 2 Hint for Exercise 37: We ﬁnd numerically that there is one point of inﬂection in the interval, occurring at x ≈ 1.3182. Let f (x) = sin x − 12 sin 2x. Setting f (x) = cos x − cos 2x = 0 yields cos 2x = cos x. Using the double angle formula for cosine, this gives 2 cos2 x − 1 = cos x or (2 cos x + 1)(cos x − 1) = 0. Solving for x ∈ [0, π ], we ﬁnd x = 0 or 23π . Setting f (x) = − sin x + 2 sin 2x = 0 yields 4 sin x cos x = sin x, so sin x = 0 or cos x = 14 . Hence, there are potential points of inﬂection at x = 0, x = π and x = cos−1 41 ≈ 1.31812. SOLUTION

Interval

Sign of f and f

(0, cos−1 41 )

++

(cos−1 14 , 23π )

+−

( 23π , π )

−−

The graph of f (x) has a local maximum at x = 23π and a point of inﬂection at x = cos−1 41 . y

1

x 0

1

2

3

39. Suppose that f is twice differentiable satisfying (i) f (0) = 1, (ii) f (x) > 0 for all x = 0, and (iii) f (x) < 0 for all > sign transitions possible? x < 0 andAre f (x) 0 for x > 0. Let g(x) =Explain f (x 2 ). with a sketch why the transitions ++ → −+ and −− → +− do not occur if the function is differentiable. (See Exercise 83 for a proof.) (a) Sketch a possible graph of f (x). (b) Prove that g(x) has no points of inﬂection and a unique local extreme value at x = 0. Sketch a possible graph of g(x). SOLUTION

(a) To produce a possible sketch, we give the direction and concavity of the graph over every interval. Interval

(−∞, 0)

(0, ∞)

Direction

Concavity

A sketch of one possible such function appears here:

Graph Sketching and Asymptotes

S E C T I O N 4.5

195

y

2 −2

x

−1

1

2

(b) Let g(x) = f (x 2 ). Then g (x) = 2x f (x 2 ). If g (x) = 0, either x = 0 or f (x 2 ) = 0, which implies that x = 0 as well. Since f (x 2 ) > 0 for all x = 0, g (x) < 0 for x < 0 and g (x) > 0 for x > 0. This gives g(x) a unique local extreme value at x = 0, a minimum. g (x) = 2 f (x 2 ) + 4x 2 f (x 2 ). For all x = 0, x 2 > 0, and so f (x 2 ) > 0 and f (x 2 ) > 0. Thus g (x) > 0, and so g (x) does not change sign, and can have no inﬂection points. A sketch of g(x) based on the sketch we made for f (x) follows: indeed, this sketch shows a unique local minimum at x = 0. y

2 −1

x 1

In Exercises following limits theofnumerator and Explain. denominator by the highest power of x Which41–50, of the calculate graphs in the Figure 20 cannot be (divide the graph a polynomial? appearing in the denominator). 41. lim

x

x→∞ x + 9

SOLUTION

1 x x −1 (x) 1 = = lim −1 = 1. = lim x→∞ x + 9 x→∞ x (x + 9) x→∞ 1 + 9 1+0 x lim

3x 22 + 20x 43. lim 3x + 20x 4 x→∞ lim 2x + 3x 3 − 29 x→∞ 4x 2 + 9 SOLUTION

3 + 20 0 3x 2 + 20x x −4 (3x 2 + 20x) x2 x3 = = 0. = lim = lim x→∞ 2x 4 + 3x 3 − 29 x→∞ x −4 (2x 4 + 3x 3 − 29) x→∞ 2 + 3 − 29 2 4 x

lim

x

7x − 9 45. lim 4 x→∞ 4x + 3 lim x→∞ x + 5 SOLUTION

7− 7x − 9 x −1 (7x − 9) = lim −1 = lim x→∞ 4x + 3 x→∞ x (4x + 3) x→∞ 4 + lim

47.

9 7 x = . 3 4 x

7x 2 2− 9 lim 7x − 9 x→−∞ lim 4x + 3 x→∞ 4x + 3

SOLUTION

7x − 9x 7x 2 − 9 x −1 (7x 2 − 9) = −∞. = lim = lim x→−∞ 4x + 3 x→−∞ x −1 (4x + 3) x→−∞ 4 + 3 x lim

49.

x2 − 1 limlim 5x − 9 x→−∞ +34 x→∞x4x +1

SOLUTION

x− x2 − 1 x −1 (x 2 − 1) = lim = lim x→−∞ x + 4 x→−∞ x −1 (x + 4) x→−∞ 1 + lim

1 x = −∞. 4 x

196

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A P P L I C AT I O N S O F T H E D E R I VATI V E

In Exercises 51–56, calculate the limit. 2x 5 + 3x 4 − 31x lim 41 x→∞x 8x 2+ − 31x 2 + 12 51. lim x→∞ x + 1 SOLUTION

√

√

Looking at only the highest powers in the top and bottom of

x 2 +1 x+1 , it seems this quotient behaves like

x2 x −1 yields x = x . Multiplying top and bottom by x

x2 + 1 = x +1

1 + 12 x

. 1 + 1x

From this we get: lim 1 + 1/x 2 1 x2 + 1 x→∞ lim = = = 1. x→∞ x + 1 lim 1 + 1/x 1

x→∞

x +1 53. lim 4 x→∞ 3 x 2 +x1 + 1 lim x→−∞ x 3 + 1 SOLUTION We can see that the denominator behaves like x 2/3 . Dividing top and bottom by x 2/3 yields: lim x 1/3 + 1/x 2/3 x +1 ∞ x→∞ = = = ∞. lim 3 x→∞ 3 x 2 + 1 2 1 lim 1 + 1/x x→∞

55.

x

lim 4/3 +1/3 4x 1/3 x→−∞ (xx6 + 1) lim x→∞ x 5/4 − 2x

SOLUTION

Looking at highest exponents of x, we see the denominator behaves like x 2 . Dividing top and bottom by

x 2 yields: x = x→−∞ (x 6 + 1)1/3

lim 1/x

x→−∞

lim

lim (1 + 1/x 6 )1/3

=

x→−∞

0 = 0. 1

4 4xFigure − 9 21 is the graph of f (x) = 2x − 1 ? Explain on the basis of horizontal asymptotes. 57. Which curve in lim 1 + x4 x→−∞ (3x 4 − 2)1/3 y

y

2

−4

2

x

−2

2

4

−4

−1.5

x

−2

2

4

−1.5

(A)

(B)

FIGURE 21 SOLUTION

Since 2 2x 4 − 1 = · lim 1 = 2 x→±∞ 1 + x 4 1 x→±∞ lim

the graph has left and right horizontal asymptotes at y = 2, so the left curve is the graph of f (x) =

2x 4 − 1 . 1 + x4

59. Match the functions with their graphs in Figure 23. 3x 2 3x 2 1 the graphs in Figure 22 with the two functions y = x and y = Match . Explain. (a) y = 2 (b) yx 2=− 12 x2 − 1 x −1 x +1 x 1 (d) y = 2 (c) y = 2 x +1 x −1

S E C T I O N 4.5 y

Graph Sketching and Asymptotes

197

y

x

x (A)

(B)

y

y x x

(C)

(D)

FIGURE 23 SOLUTION

1 should have a horizontal asymptote at y = 0 and vertical asymptotes at x = ±1. Further, the x 2 −1 graph should consist of positive values for |x| > 1 and negative values for |x| < 1. Hence, the graph of 21 is (D). x −1 2 (b) The graph of 2x should have a horizontal asymptote at y = 1 and no vertical asymptotes. Hence, the graph of x +1 x 2 is (A). 2 x +1 (c) The graph of 21 should have a horizontal asymptote at y = 0 and no vertical asymptotes. Hence, the graph of x +1 1 is (B). 2 x +1 (d) The graph of 2x should have a horizontal asymptote at y = 0 and vertical asymptotes at x = ±1. Further, the x −1

(a) The graph of

graph should consist of positive values for −1 < x < 0 and x > 1 and negative values for x < 1 and 0 < x < 1. Hence, the graph of 2x is (C). x −1

x 61. Sketch the graph of f (x) = 2 2x −using 1 the formulas x Sketch the graph of f (x) = + 1 . x +1 1 − x2 f (x) = , (1 + x 2 )2 SOLUTION

• Because

f (x) =

2x(x 2 − 3) . (x 2 + 1)3

x . Let f (x) = 2 x +1 lim

x→±∞

f (x) = 11 ·

lim x −1 = 0, y = 0 is a horizontal asymptote for f .

x→±∞

1 − x2 2 is negative for x < −1 and x > 1, positive for −1 < x < 1, and 0 at x = ±1. x2 + 1 Accordingly, f is decreasing for x < −1 and x > 1, is increasing for −1 < x < 1, has a local minimum value at x = −1 and a local maximum value at x = 1. • Moreover,

2x x 2 − 3 f (x) = 3 . x2 + 1 • Now f (x) =

Here is a sign chart for the second derivative, similar to those constructed in various exercises in Section 4.4. (The legend is on page 184.)

√

√ √

√ √ √ −∞, − 3 − 3, 0 0, 3 3 3, ∞ x − 3 0 f

−

0

+

0

−

0

+

f

I

I

I

• Here is a graph of f (x) =

x x2 + 1

.

198

CHAPTER 4

A P P L I C AT I O N S O F T H E D E R I VATI V E y 0.5 −10

−5

x 5

10

−0.5

In Exercises 62–77, sketch the graph of the function. Indicate the asymptotes, local extrema, and points of inﬂection. x 63. y = 1 y 2x = −1 2x − 1 SOLUTION Let f (x) =

x −1 , so that f is decreasing for all x = 12 . Moreover, f (x) = . Then f (x) = 2x − 1 (2x − 1)2 4 x 1 , so that f is concave up for x > 12 and concave down for x < 12 . Because lim = , f has a x→±∞ 2x − 1 2 (2x − 1)3 horizontal asymptote at y = 12 . Finally, f has a vertical asymptote at x = 12 with lim

x→ 12 −

x = −∞ 2x − 1

and

lim

x→ 12 +

x = ∞. 2x − 1

y 4 2 −1

x 1

−2

2

3

−4

x +3 65. y = x +1 y x=− 2 2x − 1 x +3 −5 SOLUTION Let f (x) = , so that f is decreasing for all x = 2. Moreover, f (x) = . Then f (x) = x −2 (x − 2)2 x +3 10 , so that f is concave up for x > 2 and concave down for x < 2. Because lim = 1, f has a horizontal x→±∞ x − 2 (x − 2)3 asymptote at y = 1. Finally, f has a vertical asymptote at x = 2 with lim

x +3

x→2− x − 2

= −∞

and

lim

x +3

x→2+ x − 2

= ∞.

y 10 5 −10

−5

x 5

−5

10

−10

1 1 67. y = + 1 x −1 y x= x + x 1 2x 2 − 2x + 1 1 SOLUTION Let f (x) = , so that f is decreasing for all x = 0, 1. Moreover, + . Then f (x) = − x x− x 2 (x − 1)2 1

2 2x 3 − 3x 2 + 3x − 1 , so that f is concave up for 0 < x < 12 and x > 1 and concave down for x < 0 f (x) = x 3 (x − 1)3 1 1 and 12 < x < 1. Because lim + = 0, f has a horizontal asymptote at y = 0. Finally, f has vertical x→±∞ x x −1 asymptotes at x = 0 and x = 1 with 1 1 1 1 lim + = −∞ and lim + =∞ x −1 x −1 x→0− x x→0+ x and

1 1 + x −1 x→1− x lim

= −∞

and

1 1 + x −1 x→1+ x lim

= ∞.

S E C T I O N 4.5

Graph Sketching and Asymptotes

199

y 5 x

−1

1

−5

2

4 3 69. y = 1 −1 + 13 y = x− x x x −1 3 4 SOLUTION Let f (x) = 1 − + 3 . Then x x 3 12 3(x − 2)(x + 2) , f (x) = 2 − 4 = x x x4 so that f is increasing for |x| > 2 and decreasing for −2 < x < 0 and for 0 < x < 2. Moreover, 6 48 6(8 − x 2 ) , f (x) = − 3 + 5 = x x x5 √ √ √ so that f is √ concave down for −2 2 < x < 0 and for x > 2 2, while f is concave up for x < −2 2 and for 0 < x < 2 2. Because 4 3 lim 1 − + 3 = 1, x→±∞ x x f has a horizontal asymptote at y = 1. Finally, f has a vertical asymptote at x = 0 with 4 4 3 3 and lim 1 − + 3 = ∞. lim 1 − + 3 = −∞ x x x→0− x→0+ x x y 6 4 2 x

−6 −4 −2

−2

2

4

6

−4 −6

1 1 71. y = 2 + 1 2 y x= 2 (x − 2) x − 6x + 8 1 1 SOLUTION Let f (x) = + . Then 2 x (x − 2)2 f (x) = −2x −3 − 2 (x − 2)−3 = −

4(x − 1)(x 2 − 2x + 4) , x 3 (x − 2)3

so that f is increasing for x < 0 and for 1 < x < 2, is decreasing for 0 < x < 1 and for x > 2, and has a local minimum at x = 1. Moreover, f (x) = 6x −4 + 6 (x − 2)−4 , so that f is concave up for all x = 0, 2. Because 1 1 + lim = 0, f has a horizontal asymptote at y = 0. Finally, f has vertical asymptotes at x = 0 and x→±∞ x 2 (x − 2)2 x = 2 with 1 1 1 1 + + lim = ∞ and lim =∞ x→0− x 2 x→0+ x 2 (x − 2)2 (x − 2)2 and

1 1 + x→2− x 2 (x − 2)2 lim

=∞

1 1 + x→2+ x 2 (x − 2)2

and

lim

y

2 −2 −1

x 1

2

3

4

= ∞.

200

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A P P L I C AT I O N S O F T H E D E R I VATI V E

4 73. y = 2 1 1 y x= −29− x (x − 2)2 4 8x . Then f (x) = − SOLUTION Let f (x) = 2 , so that f is increasing for x < −3 and for −3 < x < 0, 2 x2 − 9 x −9

24 x 2 + 3 is decreasing for 0 < x < 3 and for x > 3, and has a local maximum at x = 0. Moreover, f (x) = 3 , so x2 − 9 4 = 0, f that f is concave up for x < −3 and for x > 3 and is concave down for −3 < x < 3. Because lim 2 x→±∞ x − 9 has a horizontal asymptote at y = 0. Finally, f has vertical asymptotes at x = −3 and x = 3, with 4 4 lim = ∞ and lim = −∞ x→−3− x 2 − 9 x→−3+ x 2 − 9 and

4 x→3− x 2 − 9

lim

= −∞

4 x→3+ x 2 − 9

and

lim

= ∞.

y 2 1 x

−5

5

−1 −2

x12 75. y =y =2 2 (x (x −21)(x + 1)2+ 1) SOLUTION Let f (x) =

x2 (x 2 − 1)(x 2 + 1)

.

Then f (x) = −

2x(1 + x 4 ) (x − 1)2 (x + 1)2 (x 2 + 1)2

,

so that f is increasing for x < −1 and for −1 < x < 0, is decreasing for 0 < x < 1 and for x > 1, and has a local maximum at x = 0. Moreover, f (x) =

2 + 24x 4 + 6x 8 , (x − 1)3 (x + 1)3 (x 2 + 1)3

so that f is concave up for |x| > 1 and concave down for |x| < 1. Because

lim

x2

x→±∞ (x 2 − 1)(x 2 + 1)

horizontal asymptote at y = 0. Finally, f has vertical asymptotes at x = −1 and x = 1, with lim

x2

x→−1− (x 2 − 1)(x 2 + 1)

=∞

and

x2

lim

x→−1+ (x 2 − 1)(x 2 + 1)

= −∞

and lim

x2

x→1− (x 2 − 1)(x 2 + 1)

= −∞

and

lim

y 2 1 −2

x

−1

1 −1 −2

x2

x→1+ (x 2 − 1)(x 2 + 1)

2

= ∞.

= 0, f has a

S E C T I O N 4.5

Graph Sketching and Asymptotes

201

x 77. y = 1 2 y =x + 1 2 x +1 SOLUTION Let f (x) =

x x2 + 1

.

Then f (x) = (x 2 + 1)−3/2

and

f (x) =

−3x . (x 2 + 1)5/2

Thus, f is increasing for all x, is concave up for x < 0, is concave down for x > 0, and has a point of inﬂection at x = 0. Because x x lim =1 and lim = −1, x→∞ x 2 + 1 x→−∞ x 2 + 1 f has horizontal asymptotes of y = −1 and y = 1. There are no vertical asymptotes. y 1

x

−5

5 −1

Further Insights and Challenges In Exercises 78–82, we explore functions whose graphs approach a nonhorizontal line as x → ∞. A line y = ax + b is called a slant asymptote if lim ( f (x) − (ax + b)) = 0

x→∞

or lim ( f (x) − (ax + b)) = 0.

x→−∞

If f (x) =x 2P(x)/Q(x), where P and Q are polynomials of degrees m + 1 and m, then by long division, we 79. . Verify the following: Let f (x) = can write x −1 (a) f is concave down on (−∞, 1) andf concave up+onb)(1, as in Figure 24. (x) = (ax + ∞) P (x)/Q(x) 1

where P1 is a polynomial of degree < m. Show that y = ax + b is the slant asymptote of f (x). Use this procedure to ﬁnd the slant asymptotes of the functions: (b) f (0)2 is a local max and f (2) a local min. x x3 + x (a) y(c)= lim f (x) = −∞ and lim f (x) = ∞. (b) y = 2 x +2 x +x +1 x→1− x→1+ (d) y = x + 1 is a slant asymptote of f (x) as x → ±∞. SOLUTION Since deg(P1 ) < deg(Q), (e) The slant asymptote lies above the graph of f (x) for x < 1 and below the graph for x > 1. P1 (x) lim = 0. x→±∞ Q(x) Thus lim ( f (x) − (ax + b)) = 0

x→±∞

and y = ax + b is a slant asymptote of f . x2 4 x2 (a) = x −2+ ; hence y = x − 2 is a slant asymptote of . x +2 x +2 x +2 3 x +x x +1 x3 + x (b) 2 = (x − 1) + 2 ; hence, y = x − 1 is a slant asymptote of 2 . x +x +1 x −1 x +x +1 81. Show that y = 3x is a slant asymptote for f (x) = 3x + x −2 . Determine whether f (x) approaches the slant asympx2 tote from abovethe or graph belowof andf (x) make of the graph. . Proceed as in the previous exercise to ﬁnd the slant asymptote. Sketch = a sketch x +1

202

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A P P L I C AT I O N S O F T H E D E R I VATI V E SOLUTION

Let f (x) = 3x + x −2 . Then lim ( f (x) − 3x) =

x→±∞

lim (3x + x −2 − 3x) =

x→±∞

lim x −2 = 0

x→±∞

which implies that 3x is the slant asymptote of f (x). Since f (x) − 3x = x −2 > 0 as x → ±∞, f (x) approaches the slant asymptote from above in both directions. Moreover, f (x) = 3 − 2x −3 and f (x) = 6x −4 . Sign analyses reveal

−1/3 a local minimum at x = 32 ≈ .87358 and that f is concave up for all x = 0. Limit analyses give a vertical asymptote at x = 0. y 5 −4

x

−2

2

4

−5

83. Assume that f (x) and f (x) exist for all x and let c be a critical point of f (x). Show that f (x) cannot make a − x 2Apply the MVT to f (x). transition from the ++graph to −+ x ==c. 1Hint: Sketch ofatf (x) . 2−x SOLUTION Let f (x) be a function such that f (x) > 0 for all x and such that it transitions from ++ to −+ at a critical point c where f (c) is deﬁned. That is, f (c) = 0, f (x) > 0 for x < c and f (x) < 0 for x > c. Let g(x) = f (x). The previous statements indicate that g(c) = 0, g(x 0 ) > 0 for some x 0 < c, and g(x1 ) < 0 for some x 1 > c. By the Mean Value Theorem, g(x 1 ) − g(x 0 ) = g (c0 ), x1 − x0 for some c0 between x 0 and x1 . Because x1 > c > x 0 and g(x 1 ) < 0 < g(x 0 ), g(x 1 ) − g(x 0 ) < 0. x1 − x0 But, on the other hand g (c0 ) = f (c0 ) > 0, so there is a contradiction. This means that our assumption of the existence of such a function f (x) must be in error, so no function can transition from ++ to −+. If we drop the requirement that f (c) exist, such a function can be found. The following is a graph of f (x) = −x 2/3 . f (x) > 0 wherever f (x) is deﬁned, and f (x) transitions from positive to negative at x = 0. y −1

−0.5

x 0.5

1

−0.8

Assume that f (x) exists and f (x) > 0 for all x. Show that f (x) cannot be negative for all x. Hint: Show f (b) = 0 for some b and use the result of Exercise 49 in Section 4.4.

4.6 Applied Optimization Preliminary Questions

1. The problem is to ﬁnd the right triangle of perimeter 10 whose area is as large as possible. What is the constraint equation relating the base b and height h of the triangle? The perimeter of a right triangle is the sum of the lengths of the base, the height and the hypotenuse. If the 2 2 base has length b and the height is h, then the length of the hypotenuse is b + h and the perimeter of the triangle is 2 2 P = b + h + b + h . The requirement that the perimeter be 10 translates to the constraint equation b + h + b2 + h 2 = 10. SOLUTION

2. What are the relevant variables if the problem is to ﬁnd a right circular cone of surface area 20 and maximum volume?

S E C T I O N 4.6 SOLUTION

Applied Optimization

203

The relevant variables are the radius r and the height h of the cone. We are asked to maximize the volume V =

1 2 πr h 3

subject to the constraint S = π r r 2 + h 2 + π r 2 = 20. 3. Does a continuous function on an open interval always have a maximum value? SOLUTION No, it is possible for a continuous function on an open interval to have no maximum value. As an example, consider the function f (x) = x −1 on the open interval (0, 1). Because f is a decreasing function on (0, 1) and

lim f (x) = lim

1

x→0+ x

x→0+

= ∞,

it follows that f does not have a maximum value on (0, 1). 4. Describe a way of showing that a continuous function on an open interval (a, b) has a minimum value. If the function tends to inﬁnity at the endpoints of the interval, then the function must take on a minimum value at a critical point. SOLUTION

Exercises 1. (a) (b) (c) (d)

Find the dimensions of the rectangle of maximum area that can be formed from a 50-in. piece of wire. What is the constraint equation relating the lengths x and y of the sides? Find a formula for the area in terms of x alone. Does this problem require optimization over an open interval or a closed interval? Solve the optimization problem.

SOLUTION

(a) The perimeter of the rectangle is 50 inches, so 50 = 2x + 2y, which is equivalent to y = 25 − x. (b) Using part (a), A = x y = x(25 − x) = 25x − x 2 . (c) This problem requires optimization over the closed interval [0, 25], since both x and y must be non-negative. 25 (d) A (x) = 25 − 2x = 0, which yields x = 25 2 and consequently, y = 2 . Because A(0) = A(25) = 0 and 25 25 2 A( 2 ) = 156.25, the maximum area 156.25 in is achieved with x = y = 2 inches. 3. Find the positive number x such that the sum of x and its reciprocal is as small as possible. Does this problem require A 100-in. piece of wire is divided into two pieces and each piece is bent into a square. How should this be done optimization over an open interval or a closed interval? in order to minimize the sum of the areas of the two squares? SOLUTION Let x > 0 and f (x) = x + x −1 . Here we require optimization over the open interval (0, ∞). Solve (a) Express the sum of the areas of the squares in terms of the lengths x and y of the two pieces. f (x) = 1 − x −2 = 0 for x > 0 to obtain x = 1. Since f (x) → ∞ as x → 0+ and as x → ∞, we conclude that f has (b) What is the constraint equation relating x and y? an absolute minimum of f (1) = 2 at x = 1. (c) Does this problem require optimization over an open or closed interval? 5. Find positive numbers x, y such that x y = 16 and x + y is as small as possible. legs a right triangle have lengths a and b satisfying a + b = 10. Which values of a and b maximize the (d) The Solve theofoptimization problem. −1 SOLUTION x, y > 0. Now x y = 16 implies y = 16 area of theLet triangle? x . Let f (x) = x + y = x + 16x . Solve f (x) = −2 = 0 for x > 0 to obtain x = 4 and, consequently, y = 4. Since f (x) → ∞ as x → 0+ and as x → ∞, we 1 − 16x conclude that f has an absolute minimum of f (4) = 8 at x = y = 4. 7. (a) (b)

Let S be the set of all rectangles with area 100. A 20-in. piece of wire is bent into an L-shape. Where should the bend be made to minimize the distance between What areends? the dimensions of the rectangle in S with the least perimeter? the two Is there a rectangle in S with the greatest perimeter? Explain.

SOLUTION

Consider the set of all rectangles with area 10.

(a) Let x, y > 0 be the lengths of the sides. Now x y = 100, so that y = 100/x. Let p(x) = 2x + 2y = 2x + 200x −1 be the perimeter. Solve p (x) = 2 − 200x −2 = 0 for x > 0 to obtain x = 10. Since p(x) → ∞ as x → 0+ and as x → ∞, the least perimeter is p (10) = 40 when x = 10 and y = 10. (b) There is no rectangle in this set with greatest perimeter. For as x → 0+ or as x → ∞, we have p(x) = 2x + 200x −1 → ∞. 9. Suppose that 600 ft of fencing are used to enclose a corral in the shape of a rectangle with a semicircle whose A box has a square base of side x and height y. diameter is a side of the rectangle as in Figure 10. Find the dimensions of the corral with maximum area. (a) Find the dimensions x, y for which the volume is 12 and the surface area is as small as possible. (b) Find the dimensions for which the surface area is 20 and the volume is as large as possible.

FIGURE 10

204

CHAPTER 4

A P P L I C AT I O N S O F T H E D E R I VATI V E SOLUTION Let x be the width of the corral and therefore the diameter of the semicircle, and let y be the height of the rectangular section. Then the perimeter of the corral can be expressed by the equation 2y + x + π2 x = 2y + (1 + π )x = 600 ft or equivalently, y = 1 600 − (1 + π )x . Since x and y must both be nonnegative, it follows that x must 2 2 2 600 ]. The area of the corral is the sum of the area of the rectangle and semicircle, be restricted to the interval [0, 1+ π /2

A = x y + π8 x 2 . Making the substitution for y from the constraint equation, A(x) =

1

1

π π π 2 π 2 x 600 − (1 + )x + x 2 = 300x − 1+ x + x . 2 2 8 2 2 8

Now, A (x) = 300 − 1 + π2 x + π4 x = 0 implies x = 300π ≈ 168.029746 feet. With A(0) = 0 ft2 , 1+ 4 600 300 and A ≈ 25204.5 ft2 ≈ 21390.8 ft2 , A 1 + π /4 1 + π /2 it follows that the corral of maximum area has dimensions x=

300 ft 1 + π /4

y=

and

150 ft. 1 + π /4

11. A landscape architect wishes to enclose a rectangular garden on one side by a brick wall costing $30/ft and on the Find the rectangle of maximum area that can be inscribed in a right triangle with legs of length 3 and 4 if the other three sides by a metal fence costing $10/ft. If the area of the garden is 1,000 ft2 , ﬁnd the dimensions of the garden sides of the rectangle are parallel to the legs of the triangle, as in Figure 11. that minimize the cost. SOLUTION

Let x be the length of the brick wall and y the length of an adjacent side with x, y > 0. With x y = 1000

or y = 1000 x , the total cost is C(x) = 30x + 10 (x + 2y) = 40x + 20000x −1 . √ −2 = 0 for x > 0 to obtain x = 10 5. Since C(x) → ∞ as x → 0+ and as x → ∞, the Solve C (x) = 40 − 20000x √ √ √ √ minimum cost is C(10 5) = 800 5 ≈ $1788.85 when x = 10 5 ≈ 22.36 ft and y = 20 5 ≈ 44.72 ft. 13. FindFind the the point P on x 2 closest the point point onthe theparabola line y = yx = closest to thetopoint (1, 0).(3, 0) (Figure 12). y

y = x2 P x 3

FIGURE 12 SOLUTION

With y = x 2 , let’s equivalently minimize the square of the distance, f (x) = (x − 3)2 + y 2 = x 4 + x 2 − 6x + 9.

Then f (x) = 4x 3 + 2x − 6 = 2(x − 1)(2x 2 + 2x + 3), so that f (x) = 0 when x = 1 (plus two complex solutions, which we discard). Since f (x) → ∞ as x → ±∞, P = (1, 1) is the point on y = x 2 closest to (3, 0). 15. Find the dimensions of the rectangle of maximum area that can be inscribed in a circle of radius r (Figure 13). A box is constructed out of two different types of metal. The metal for the top and bottom, which are both square, costs $1/ft2 and the metal for the sides costs $2/ft2 . Find the dimensions that minimize cost if the box has a volume of 20 ft3 . r

FIGURE 13

Place the center of the circle at the origin with the sides of the rectangle (of lengths 2x > 0 and 2y > 0) parallel to the coordinate axes. By the Pythagorean Theorem, x 2 + y 2 = r 2 , so that y = r 2 − x 2 . Thus the area of the rectangle is A(x) = 2x · 2y = 4x r 2 − x 2 . To guarantee both x and y are real and nonnegative, we must restrict x to the interval [0, r ]. Solve SOLUTION

4x 2 A (x) = 4 r 2 − x 2 − =0 r2 − x2

S E C T I O N 4.6

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205

√ for x > 0 to obtain x = √r . Since A(0) = A(r ) = 0 and A(r/ 2) = 2r 2 , the rectangle of maximum area has dimensions x = y = √r .

2

2

17. Find the angle θ that maximizes the area of the isosceles triangle whose legs have length (Figure 14). Problem of Tartaglia (1500–1557) Among all positive numbers a, b whose sum is 8, ﬁnd those for which the product of the two numbers and their difference is largest. Hint: Let x = a − b and express abx in terms of x alone. q

FIGURE 14 SOLUTION

The area of the triangle is A(θ ) =

1 2 sin θ , 2

where 0 ≤ θ ≤ π . Setting A (θ ) =

1 2 cos θ = 0 2

yields θ = π2 . Since A(0) = A(π ) = 0 and A( π2 ) = 12 2 , the angle that maximizes the area of the isosceles triangle is θ = π2 . 19. Rice production requires both labor and capital investment in equipment and land. Suppose that if x dollars per acre 2 + h 2 . Find the dimensions of the r 2 h andinitsequipment surface area S = πthen r rthe The volume a right circular is π3invested are invested in laborof and y√dollars per cone acre are andis land, yield P of rice per acre is given √ cone with surface area 1x and maximal by the formula P = 100 + 150 y. If volume a farmer(Figure invests15). $40/acre, how should he divide the $40 between labor and capital investment in order to maximize the amount of rice produced? √ √ √ 50 √ SOLUTION Since x + y = 40, we have P(x) = 100 x + 150 y = 100 x + 150 40 − x. Solve P (x) = √ − x √ √ 75 160 = 0 to obtain x = 13 ≈ $12.31. Since P(0) = 300 10 ≈ 948.68 and P(40) = 200 10 ≈ 632.46, we have √ 40 − x

√ √ 160 360 that the maximum yield is P 160 13 = 100 13 10 ≈ 1140.18 when x = 13 ≈ $12.31 and y = 13 ≈ $27.69. 21. Find the dimensions x and y of the rectangle inscribed in a circle of radius r that maximizes the quantity x y 2 . Kepler’s Wine Barrel Problem The following problem was stated and solved in the work Nova stereometria SOLUTION the center the circle of radius r at the originpublished with the sides of the rectangle (of lengths x >Kepler 0 and doliorum Place vinariorum (NewofSolid Geometry of a Wine Barrel), in 1615 by the astronomer Johannes y inscribed x )2can 2 = r 2 , whence 2 = 4r 2 radius What are the dimensions the cylinderTheorem, of largestwe volume be in the sphere of y > (1571–1630). 0) parallel to the coordinate axes. By theofPythagorean have (that + ( ) y − x 2. 2 2 − x22 ), where 2 2 3 2 2 π x(R x is one-half the height of the R? Hint: Show that the volume of an inscribed cylinder is 2 for Let f (x) = x y = 4xr − x . Allowing for degenerate rectangles, we have 0 ≤ x ≤ 2r . Solve √ f 3(x) = 4r − 3x 2r 2r 16 2r cylinder. x ≥ 0 to obtain x = √ . Since f (0) = f (2r ) = 0, the maximal value of f is f ( √ ) = 9 3r when x = √ and 3 3 3

2 y = 2 3r. 23. Consider a rectangular industrial warehouse consisting of three separate spaces of equal size as in Figure 17. Assume Find the angle θ that maximizes the area of the trapezoid with a base of length 4 and sides of length 2, as in that the wall materials cost $200 per linear ft and the company allocates $2,400,000 for the project. Figure 16. (a) Which dimensions maximize the total area of the warehouse? (b) What is the area of each compartment in this case?

FIGURE 17

Let the dimensions of one compartment be height x and width y. Then the perimeter of the warehouse is given by P = 4x + 6y and the constraint equation is cost = 2,400,000 = 200(4x + 6y), which gives y = 2000 − 23 x.

(a) Area is given by A = 3x y = 3x 2000 − 23 x = 6000x − 2x 2 , where 0 ≤ x ≤ 3000. Then A (x) = 6000 − 4x = 0 yields x = 1500 and consequently y = 1000. Since A(0) = A(3000) = 0 and A(1500) = 4, 500, 000, the area of the warehouse is maximized with a height of 1500 feet and a total width of 3000 feet. (b) The area of one compartment is 1500 · 1000 = 1, 500, 000 square feet. SOLUTION

2 . Suppose that 25. TheSuppose, amount of lightprevious reachingexercise, a point that at a the distance r fromconsists a lightof source A of intensity A is Isize. A /r Find in the warehouse n separate spaces of Iequal a formula a second lightofsource B of intensitypossible I B = 4Iarea located 10 ft from A. Find the point on the segment joining A and B A isof in terms n for the maximum the warehouse. where the total amount of light is at a minimum.

206

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A P P L I C AT I O N S O F T H E D E R I VATI V E SOLUTION Place the segment in the x y-plane with A at the origin and B at (10, 0). Let x be the distance from A. Then 10 − x is the distance from B. The total amount of light is IA IB 4 1 f (x) = 2 + = I + . A x x2 (10 − x)2 (10 − x)2

Solve f (x) = I A

2 − 3 3 x (10 − x) 8

=0

for 0 ≤ x ≤ 10 to obtain (10 − x)3 = 4= x3

3 10 −1 x

or

x=

10 √ ≈ 3.86 ft. 1+ 3 4

Since f (x) → ∞ as x → 0+ and x → 10− we conclude that the minimal amount of light occurs 3.86 feet from A. 27. According to postal regulations, a carton is classiﬁed as “oversized” if the sum of its height and girth (the perimeter 4−x of its base) in.largest Find the dimensions of abecarton withinsquare base bounded that is not Findexceeds the area108 of the rectangle that can inscribed the region byoversized the graphand of yhas = maximum and 2 +x volume. the coordinate axes (Figure 18). SOLUTION Let h denote the height of the carton and s denote the side length of the square base. Clearly the volume will be maximized when the sum of the height and girth equals 108; i.e., 4s + h = 108, whence h = 108 − 4s. Allowing for degenerate cartons, the carton’s volume is V (s) = s 2 h = s 2 (108 − 4s), where 0 ≤ s ≤ 27. Solve V (s) = 216s − 12s 3 = 0 for s to obtain s = 0 or s = 18. Since V (0) = V (27) = 0, the maximum volume is V (18) = 11664 in3 when s = 18 in and h = 36 in. 29. What is the area of the largest rectangle that can be circumscribed around a rectangle of sides L and H ? Hint: Find the maximum area of a triangle formed in the ﬁrst quadrant by the x-axis, y-axis, and a tangent line to the Express the area of the circumscribed rectangle in terms of the angle θ (Figure 19). graph of y = (x + 1)−2 .

q H L

FIGURE 19 SOLUTION Position the L × H rectangle in the ﬁrst quadrant of the x y-plane with its “northwest” corner at the origin. Let θ be the angle the base of the circumscribed rectangle makes with the positive x-axis, where 0 ≤ θ ≤ π2 . Then the area of the circumscribed rectangle is A = L H + 2 · 12 (H sin θ )(H cos θ ) + 2 · 12 (L sin θ )(L cos θ ) = L H + 12 (L 2 + H 2 ) sin 2θ , which has a maximum value of L H + 12 (L 2 + H 2 ) when θ = π4 because sin 2θ achieves its maximum when θ = π4 .

31. An 8-billion-bushel corn crop brings a price of $2.40/bushel. A commodity broker uses the following rule of thumb: Optimal Priceby Let r be thethen monthly rent increases per unit inby an10x apartment building with 100results units. in A maximum survey reveals that If the crop is reduced x percent, the price cents. Which crop size revenue all units canprice be rented when rHint: = $900 and that one unit becomes with each $10 increase in rent. Suppose that and what is the per bushel? Revenue is equal to price timesvacant crop size. the average monthly maintenance per occupied unit is $100/month. SOLUTION Let x denote the percentage reduction in crop size. Then the price for corn is 2.40 + 0.10x, the crop size is (a) Show that the number of units rented is n = 190 − r/10 for 900 ≤ r ≤ 1,900. 8(1 − 0.01x) and the revenue (in billions of dollars) is (b) Find a formula for the net cash intake (revenue minus maintenance) and determine the rent r that maximizes intake. R(x) = (2.4 + .1x)8(1 − .01x) = 8(−.001x 2 + .076x + 2.4), where 0 ≤ x ≤ 100. Solve R (x) = −.002x + .076 = 0 to obtain x = 38 percent. Since R(0) = 19.2, R(38) = 30.752, and R(100) = 0, revenue is maximized when x = 38. So we reduce the crop size to 8(1 − .38) = 4.96 billion bushels. The price would be $2.40 + .10(38) = 2.40 + 3.80 = $6.20. 33. Let P = (a, b) be a point in the ﬁrst quadrant. Given n numbers x1 , . . . , x n , ﬁnd the value of x minimizing the sum of the squares: (a) Find the slope of the line through P such that the triangle bounded by this line and the axes in the ﬁrst quadrant has minimal area. (x − x 1 )2 + (x − x 2 )2 + · · · + (x − xn )2 First solve the problem for n = 2, 3 and then solve it for arbitrary n.

Applied Optimization

S E C T I O N 4.6

207

(b) Show that P is the midpoint of the hypotenuse of this triangle. SOLUTION Let P(a, b) be a point in the ﬁrst quadrant (thus a, b > 0) and y − b = m(x − a), −∞ < m < 0, be a line through P that cuts the positive x- and y-axes. Then y = L(x) = m(x − a) + b.

b , 0 . Hence the area of the triangle (a) The line L(x) intersects the y-axis at H (0, b − am) and the x-axis at W a − m is 1 b 1 1 = ab − a 2 m − b2 m −1 . A(m) = (b − am) a − 2 m 2 2

Solve A (m) = 12 b2 m −2 − 12 a 2 = 0 for m < 0 to obtain m = − ab . Since A → ∞ as m → −∞ or m → 0−, we conclude that the minimal triangular area is obtained when m = − ab . (b) For m = −b/a, we have H (0, 2b) and W (2a, 0). The midpoint of the line segment connecting H and W is thus P(a, b). 35. Figure 20 shows a rectangular plot of size 100 × 200 feet. Pipe is to be laid from A to a point P on side BC and A truck gets 10 mpg (miles per gallon) traveling along an interstate highway at 50 mph, and this is reduced by from there to C. The cost of laying pipe through the lot is $30/ft (since it must be underground) and the cost along the 0.15 mpg for each mile per hour increase above 50 mph. side of the plot is $15/ft. (a) If the truck driver is paid $30/hour and diesel fuel costs P = $3/gal, which speed v between 50 and 70 mph (a) Let f (x) be the total cost, where x is the distance from P to B. Determine f (x), but note that f is discontinuous at will minimize the cost of a trip along the highway? Notice that the actual cost depends on the length of the trip but x = 0 (when x = 0, the cost of the entire pipe is $15/ft). the optimal speed does not. (b) What is the most economical way to lay the pipe? What if the cost along the sides is $24/ft? (b) Plot cost as a function of v (choose the length arbitrarily) and verify your answer to part (a). (c) Do you expect the optimal speed v to Bincrease or decrease P C if fuel costs go down to P = $2/gal? Plot the 200same − x axis and verify your conclusion. graphs of cost as a function of v for P = 2 and P =x 3 on the 100 A

200

FIGURE 20 SOLUTION

(a) Let x be the distance from P to B. If x > 0, then the length of the underground pipe is of the pipe along the side of the plot is 200 − x. The total cost is f (x) = 30 1002 + x 2 + 15(200 − x).

1002 + x 2 and the length

If x = 0, all of the pipe is along the side of the plot and f (0) = 15(200 + 100) = $4500. (b) To locate the critical points of f , solve 30x f (x) = − 15 = 0. 1002 + x 2

√ We ﬁnd√x = ±100/ 3. Note that only the positive value is in the domain of the problem. Because f (0) = $4500, f (100/ 3) = $5598.08 and f (200) = $6708.20, the most economical way to lay the pipe is to place the pipe along the side of the plot. If the cost of laying the pipe along the side of the plot is $24 per foot, then f (x) = 30 1002 + x 2 + 24(200 − x) and 30x f (x) = − 24. 1002 + x 2 The only critical point in the domain of the problem is x = 400/3. Because f (0) = $7200, f (400/3) = $6600 and f (200) = $6708.20, the most economical way to lay the pipe is place the underground pipe from A to a point 133.3 feet to the right of B and continuing to C along the side of the plot. 37. In Example 6 in this section, ﬁnd the x-coordinate of the point P where the light beam strikes the mirror if h 1 = 10, the=dimensions of a cylinder of volume 1 m3 of minimal cost if the top and bottom are made of material that h 2 = 5,Find and L 20. costs twice as much as the material for the side. SOLUTION Substitute h 1 = 10 feet, h 2 = 5 feet, and L = 20 feet into

L−x = x 2 + h 21 (L − x)2 + h 22 x

Solving for x gives x = 40 3 feet.

to obtain

x x 2 + 102

=

20 − x (20 − x)2 + 52

.

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A P P L I C AT I O N S O F T H E D E R I VATI V E

In Exercises 38–40, a box (with no top) is to be constructed from a piece of cardboard of sides A and B by cutting out squares of length h from the corners and folding up the sides (Figure 21).

B h A

FIGURE 21

39. Which value of h maximizes the volume if A = B? Find the value of h that maximizes the volume of the box if A = 15 and B = 24. What are the dimensions of the SOLUTION When A = B, the volume of the box is resulting box? V (h) = hx y = h ( A − 2h)2 = 4h 3 − 4 Ah 2 + A2 h, A 2 2 where 0 ≤ h ≤ A 2 (allowing for degenerate boxes). Solve V (h) = 12h − 8 Ah + A = 0 for h to obtain h = 2 or A A A 2 A 3 h = 6 . Because V (0) = V ( 2 ) = 0 and V ( 6 ) = 27 A , volume is maximized when h = 6 .

41. The monthly output P of a light bulb factory is given by the formula P = 350L K , where L is the amount invested cardboard ABto=produce 144). Which that a box invested of heightinh equipment = 3 in. is constructed using 144 in.2 Ifofthe in laborSuppose and K the amount (in thousands of dollars). company(i.e., needs 10,000values units A and Bhow maximize the investment volume? be divided among labor and equipment to minimize the cost of production? The per month, should the cost of production is L + K . 200 . Accordingly, the cost of production is SOLUTION Since P = 10000 and P = 350L K , we have L = 7K C(K ) = L + K = K + 200 for K ≥ 0 to obtain K = 7K 2 minimum cost of production is achieved for K = L = Solve C (K ) = 1 −

200 . 7K

√

10 14. Since C(K ) → ∞ as K → 0+ and as K → ∞, the 7 √ 10 14 ≈ 5.345 or $5435 invested in both labor and equipment. 7

43. Janice can swim 3 mph and run 8 mph. She is standing at one bank of a river that is 300 ft wide and wants to reach Use calculus to show that among right trianglesaswith hypotenuse ofswim length 1, the isosceles triangle has a point located 200 ft downstream on the otherallside as quickly possible. She will diagonally across the river and maximum area. Can you see more directly why this must be true by reasoning from Figure 22? then jog along the river bank. Find the best route for Janice to take. Let lengths be in feet, times in seconds, and speeds in ft/s. Let x be the distance from the point directly opposite Janice on the other shore to the point where she starts to run after having swum. Then x 2 + 3002 is the 176 distance she swims and 200 − x is the distance she runs. Janice swims at 22 5 ft/s (i.e., 3 mph) and runs at 15 ft/s (i.e., 8 mph). The total time she travels is 375 200 − x 5 2 15 x 2 + 3002 f (x) = x + 90000 + + = − x, 0 ≤ x ≤ 200. 22/5 176/15 22 22 176 SOLUTION

Solve 5 x 15 =0 − 22 x 2 + 90000 176 √ √ 4875 750 to obtain x = 180 11 55. Since f (0) = 22 ≈ 221.59 and f (200) = 11 13 ≈ 245.83, weconclude that the minimum √ √ √ 375 375 480 2 2 amount of time f 180 11 55 = 44 55 + 22 ≈ 80.35 s occurs when Janice swims x + 300 = 11 55 ≈ √ 323.62 ft and runs 200 − x = 200 − 180 11 55 ≈ 78.64 ft. f (x) =

45. (a) Find the radius and height of a cylindrical can of total surface area A whose volume is as large as possible. Delivery Schedule gas station gallons total of gasoline (b) CanOptimal you design a cylinder with totalAsurface area sells A andQminimal volume?per year, which is delivered N times per year in equal shipments of Q/N gallons. The cost of each delivery is d dollars and the yearly storage costs are SOLUTION Let a closed cylindrical can be of radius r and height h. s QT , where T is the length shipments and s is a constant. Show that costs √ of time (a fraction of a year) between A .) Find the optimal number of deliveries are minimized for N = s Q/d. (Hint: Express T in terms of N 2 2 (a) Its total surface area is S = 2π r + 2π r h = A, whence h = − r. Its volume is thus V (r ) = π r 2 h =if 12QAr= − r million gal, d = $8,000, and s = 30 cents/gal-yr. Your answer2πshould be a whole number, so compare costs for the

A integer nearest the) = optimal A .NSolve 1 A −value. A (r 2 for r > 0 to obtain r = π r 3 ,two where 0 < values r ≤ of V 3 π r . Since V (0) = V ( 2π 2 2π ) = 0 and 6π √ A 6 A3/2 = V √ , 6π 18 π the maximum volume is achieved when

r=

A 6π

and

1 h= 3

6A . π

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S E C T I O N 4.6

209

(b) For a can of total surface area A, there are cans of arbitrarily small volume since lim V (r ) = 0. r →0+

47. A billboard of height b is mounted on the side of a building with its bottom edge at a distance h from the street. At Find the area of the largest isosceles triangle that can be inscribed in a circle of radius r . what distance x should an observer stand from the wall to maximize the angle of observation θ (Figure 23)? (a) Find x using calculus. You may wish to use the addition formula cot(a − b) =

1 + cot a cot b . cot b − cot a

(b) Solve the problem again using geometry (without any calculation!). There is a unique circle passing through points B and C which is tangent to the street. Let R be the point of tangency. Show that θ is maximized at the point R. Hint: The two angles labeled ψ are, in fact, equal because they subtend equal arcs on the circle. Let A be the intersection of the circle with PC and show that ψ = θ + P B A > θ . (c) Prove that the two answers in (a) and (b) agree.

C b

P

A

h

q x

y

B y

q

P

R

FIGURE 23 SOLUTION

(a) From the leftmost diagram in Figure 23 and the addition formula for the cotangent function, we see that 1+ x x x 2 + h(b + h) cot θ = x b+hx h = , bx h − b+h where b and h are constant. Now, differentiate with respect to x and solve − csc2 θ

x 2 − h(b + h) dθ = =0 dx bx 2

to obtain x = bh + h 2 . Since this is the only critical point, and since θ → 0 as x → 0+ and θ → 0 as x → ∞, θ (x) reaches its maximum at x = bh + h 2 . (b) Following the directions, and mindful of the diagram in Figure 23, let C be the point at the top of the painting along the wall. For every radius r , there is a unique circle of radius r through B and C with center to the front of the billboard. Only one of these is tangent to the street (all the others are either too big or too small). Draw such a circle (call it D) , as shown in the right half of Figure 23, and let R be the point where the circle touches the street. Let P be any other such point along the street. We will prove that the angle of observation θ at P is less than the angle of observation ψ at R, thus proving that R is the point with maximum angle of observation along the ﬂoor. P, C, and B form a triangle. Let A be the topmost point of intersection of the triangle and circle D as shown. Because the two angles both subtend the arc BC, the measure of angle C AB is equal to the measure of angle C R B. By supplementarity, ψ + P AB = 180◦ . Because they form a triangle, θ + P AB + P B A = 180◦ . Thus, ψ = θ + P B A > θ . (c) To show that the two answers agree, let O be the center of the circle. One observes that if d is the distance from R to the wall, then O has coordinates (−d, b2 + h). This is because the height of the center is equidistant from points B and C and because the center must lie directly above R if the circle is tangent to the ﬂoor. Now we can solve for d. The radius of the circle is clearly b2 + h, by the distance formula:

2

O B = d2 +

2 2 b b +h−h = +h 2 2

This gives d2 = or d =

2 2 b b = bh + h 2 +h − 2 2

bh + h 2 as claimed.

49. Snell’s Law, derived in Exercise 48, explains why it is impossible to see above the water if the angle of vision θ2 is 24 represents the surface of a swimming pool. A light beam travels from point A located such thatSnell’s sin θ2 Law > v2 /vFigure 1. above the pool to point B located underneath the water. Let v1 be the velocity of light in air and let v2 be the velocity (a) Show that if this inequality holds, then there is no angle θ1 for which Snell’s Law holds. of light in water (it is a fact that v1 > v2 ). Prove Snell’s Law of Refraction according to which the path from A to B that takes the least time satisﬁes the relation sin θ

sin θ

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A P P L I C AT I O N S O F T H E D E R I VATI V E

(b) What will you see if you look through the water with an angle of vision θ2 such that sin θ2 > v2 /v1 ? SOLUTION

v sin θ1 sin θ2 v v v (a) Suppose sin θ2 > 2 . By Snell’s Law, = , whence sin θ1 = 1 sin θ2 > 1 · 2 = 1, a contradiction, v1 v1 v2 v2 v2 v1 since sin θ ≤ 1 for all real values of θ . Accordingly, there is no real value θ1 for which Snell’s Law holds. (b) The underwater observer sees what appears to be a silvery mirror due to total internal reﬂection. 51. Vascular Branching A small blood vessel of radius r branches off at an angle θ from a larger vessel of radius R to blank margins oftowidth 6 in. onLaw, the top and bottom andto4 blood in. onﬂow the sides. Find the A poster of aarea ft2 has supply blood along path6 from A to B. According Poiseuille’s the total resistance is proportional to dimensions that maximize the printed area. a − b cot θ b csc θ T = + R4 r4 where a and b are as in Figure 25. Show that the total resistance is minimized when cos θ = (r/R)4 . B r b R

q

A a

FIGURE 25

SOLUTION

a − b cot θ b csc θ + . Set R4 r4 b csc2 θ b csc θ cot θ = 0. − T (θ ) = R4 r4

With a, b, r, R > 0 and R > r , let T (θ ) =

Then

b r 4 − R 4 cos θ R 4 r 4 sin2 θ

so that cos θ =

r 4 cos θ = . R

r 4 R

. Since

lim T (θ ) = ∞ and

θ →0+

= 0,

lim T (θ ) = ∞, the minimum value of T (θ ) occurs when

θ →π −

53. Let (a, b) be a ﬁxed point in the ﬁrst quadrant and let S(d) be the sum of the distances from (d, 0) to the points Find the minimum length of a beam that can clear a fence of height h and touch a wall located b ft behind the (0, 0), (a, b), and (a, −b). √ √ fence (Figure 26). (a) Find the value of √ d for which S(d) is minimal. The answer depends on whether b < 3a or b ≥ 3a. Hint: Show that d = 0 when b ≥ 3a. √ Let a = 1. Plot S(d) for b = 0.5, 3, 3 and describe the position of the minimum. (b) SOLUTION

(a) If d < 0, then the distance from (d, 0) to the other three points can all be reduced by increasing the value of d. Similarly, if d > a, then the distance from (d, 0) to the other three points can all be reduced by decreasing the value of d. It follows that the minimum of S(d) must occur for 0 ≤ d ≤ a. Restricting attention to this interval, we ﬁnd

S(d) = d + 2 (d − a)2 + b2 . Solving 2(d − a) S (d) = 1 + =0 (d − a)2 + b2 √ √ √ yields the critical point d = √ a − b/ 3. If b < 3a, then d = a − b/ 3 > 0 and the minimum occurs at this value of d. On the other hand, if b ≥ 3a, then the minimum occurs at the endpoint d = 0. √ (b) Let a = 1. Plots of S(d) for b = 0.5,√ b = 3 and b = 3 are shown below. For b = 0.5, the results√of (a) indicate the minimum should occur for d = 1 − 0.5/ 3 ≈ 0.711, and this is conﬁrmed in the plot. For both b = 3 and b = 3, the results of (a) indicate that the minimum should occur at d = 0, and both of these conclusions are conﬁrmed in the plots.

S E C T I O N 4.6 y

Applied Optimization

211

y b = 0.5

2.1 2 1.9 1.8 1.7 1.6 1.5

x 0

0.2 0.4 0.6 0.8

b = √3

4.4 4.3 4.2 4.1 4

1

x 0

0.2 0.4 0.6 0.8

1

y b=3 6.8 6.6 6.4 x 0

0.2 0.4 0.6 0.8

1

55. In the of Exercise 54, show any f the minimal force required is proportional to 1/ 1 to + f 2. Thesetting minimum force required to that drivefor a wedge of angle α into a block (Figure 27) is proportional k SOLUTION Let F(α ) = , where k > 0 is a proportionality constant and 0 ≤ α ≤ π2 . Solve 1 sin α + f cos α F(α ) = sin α + f cos α k f sin α − cos α ) ( (α ) = = 0 is required, assuming f = 0.4. where f is a positive constant. Find theFangle α for which the least2 force (sin α + f cos α ) for 0 ≤ α ≤ π2 to obtain tan α = 1f . From the diagram below, we note that when tan α = 1f , 1 sin α = 1+ f2

f cos α = . 1+ f2

and

Therefore, at the critical point the force is k 2 √1 +√f 2 1+ f 2 1+ f

k = . 1+ f2

Since F(0) = 1f k > √ k 2 and F( π2 ) = k > √ k 2 , we conclude that the minimum force is proportional to 1+ f 1+ f 1/ 1 + f 2 .

√1 + f 2 1

α f

57. Find the maximum length of a pole that can be carried horizontally around a corner joining corridors of widths 8 ft The problem is to put a “roof” of side s on an attic room of height h and width b. Find the smallest length s for and 4 ft (Figure 29). which this is possible. See Figure 28. 4

8

FIGURE 29 SOLUTION In order to ﬁnd the length of the longest pole that can be carried around the corridor, we have to ﬁnd the shortest length from the left wall to the top wall touching the corner of the inside wall. Any pole that does not ﬁt in this shortest space cannot be carried around the corner, so an exact ﬁt represents the longest possible pole. Let θ be the angle between the pole and a horizontal line to the right. Let c1 be the length of pole in the 8 ft corridor and let c2 be the length of pole in the 4 foot corridor. By the deﬁnitions of sine and cosine,

4 = sin θ c2

and

8 = cos θ , c1

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so that c1 = cos8 θ , c2 = sin4 θ . What must be minimized is the total length, given by f (θ ) =

8 4 + . cos θ sin θ

Setting f (θ ) = 0 yields 4 cos θ 8 sin θ − =0 cos2 θ sin2 θ 8 sin θ 4 cos θ = cos2 θ sin2 θ 8 sin3 θ = 4 cos3 θ As θ < π2 (the pole is being turned around a corner, after all), we can divide both sides by cos3 θ , getting tan3 θ = 12 .

This implies that tan θ = 12

1/3

(tan θ > 0 as the angle is acute).

1/3 Since f (θ ) → ∞ as θ → 0+ and as θ → π2 −, we can tell that the minimum is attained at θ0 where tan θ0 = 12 . Because tan θ0 =

opposite 1 = 1/3 , adjacent 2

we draw a triangle with opposite side 1 and adjacent side 21/3 .

c 1 q 21/3

By Pythagoras, c =

1 + 22/3 , so 1 sin θ0 = 1 + 22/3

and

21/3 cos θ0 = . 1 + 22/3

From this, we get f (θ0 ) =

8 4 8 + = 1/3 1 + 22/3 + 4 1 + 22/3 = 4 1 + 22/3 22/3 + 1 . cos θ0 sin θ0 2

59. Find the isosceles triangle of smallest area that circumscribes a circle of radius 1 (from Thomas Simpson’s The Redo Exercise 57 for corridors of arbitrary widths a and b. Doctrine and Application of Fluxions, a calculus text that appeared in 1750). See Figure 30.

q

1

FIGURE 30 SOLUTION From the diagram, we see that the height h and base b of the triangle are h = 1 + csc θ and b = 2h tan θ = 2(1 + csc θ ) tan θ . Thus, the area of the triangle is

A(θ ) =

1 hb = (1 + csc θ )2 tan θ , 2

where 0 < θ < π . We now set the derivative equal to zero: A (θ ) = (1 + csc θ )(−2 csc θ + sec2 θ (1 + csc θ )) = 0. The ﬁrst factor gives θ = 3π /2 which is not in the domain of the problem. To ﬁnd the roots of the second factor, multiply through by cos2 θ sin θ to obtain −2 cos2 θ + sin θ + 1 = 0,

S E C T I O N 4.6

Applied Optimization

213

or 2 sin2 θ + sin θ − 1 = 0. This is a quadratic equation in sin θ with roots sin θ = −1 and sin θ = 1/2. Only the second solution is relevant and gives us θ = π /6. Since A(θ ) → ∞ as θ → 0+ and as θ → π −, we see that the minimum area occurs when the triangle is an equilateral triangle. basketballand player stands d feet from the basket. Let h and α be as in Figure 31. Using physics, one can show Further AInsights Challenges

that if the player releases the ball at an angle θ , then the initial velocity required to make the ball go through the 61. Bird Migration Power P is the rate at which energy E is consumed per unit time. Ornithologists have found that basket satisﬁes the power consumed by a certain pigeon ﬂying at velocity v m/s is described well by the function P(v) = 17v −1 + 4 16denergy as body fat. 2 10 10−3 v 3 J/s. Assume that the pigeon can store 5v× usable = J of . 2 θ (tan θ − tan α ) cos (a) Find the velocity vpmin that minimizes power consumption. (b) Show that a why pigeon at velocity v and usingfor allαof

(e) For a given α , the optimal angle for shooting the basket is θ0 because it minimizes v 2 and therefore minimizes Minimum to power Power (J/s) the energy required to make the shot (energy is proportional v 2 ). Show that the velocity vopt at the optimal angle consumption θ0 satisﬁes 4 Maximum

2 = vopt

32d cos α 32 d2 distance = . traveled 1 − sin α −h + d 2 + h 2 5

10

15

Velocity (m/s) 2 is an increasing function (f) Show with a graph that for ﬁxed d (say, d = 15 ft, the distance of a free throw), vopt FIGURE 32 and why it can help to jump while shooting. of h. Use this to explain why taller players have an advantage

SOLUTION

17 1/4 ≈ 8.676247. (a) Let P(v) = 17v −1 + 10−3 v 3 . Then P (v) = −17v −2 + 0.003v 2 = 0 implies vpmin = .003 This critical point is a minimum, because it is the only critical point and P(v) → ∞ both as v → 0+ and as v → ∞. 5 · 104 joules 5 · 104 sec (b) Flying at a velocity v, the birds will exhaust their energy store after T = = . The total P(v) joules/sec P(v) 4 5 · 10 v . distance traveled is then D(v) = vT = P(v) P(v) · 5 · 104 − 5 · 104 v · P (v) P(v) − v P (v) = 5 · 104 = 0 implies P(v) − v P (v) = 0, or P (v) = (c) D (v) = 2 (P(v)) (P(v))2 P(v) . Since D(v) → 0 as v → 0 and as v → ∞, the critical point determined by P (v) = P(v)/v corresponds to a v maximum. Graphically, the expression P(v) P(v) − 0 = v v−0 is the slope of the line passing through the origin and (v, P(v)). The condition P (v) = P(v)/v which deﬁnes vdmax therefore indicates that vdmax is the velocity component of the point where a line through the origin is tangent to the graph of P(v). P(v) (d) Using P (v) = gives v −17v −2 + .003v 2 =

17v −1 + .001v 3 = 17v −2 + .001v 2 , v

which simpliﬁes to .002v 4 = 34 and thus vdmax ≈ 11.418583. The maximum total distance is given by D(vdmax ) = 5 · 104 · vdmax = 191.741 kilometers. P(vdmax )

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Seismic Prospecting Exercises 62–64 are concerned with determining the thickness d of a layer of soil that lies on top of a rock formation. Geologists send two sound pulses from point A to point D separated by a distance s. The ﬁrst pulse travels directly from A to D along the surface of the earth. The second pulse travels down to the rock formation, then along its surface, and then back up to D (path ABCD), as in Figure 33. The pulse travels with velocity v1 in the soil and v2 in the rock. s

A q

D q

Soil B

d

C

Rock

FIGURE 33

63. In this exercise, assume that v2 /v1 ≥ 1 + 4(d/s)2 . (a) Show that the time required for the ﬁrst pulse to travel from A to D is t1 = s/v1 . (a) Show that inequality (1) holds if sin θ = v1 /v2 . (b) Show that the time required for the second pulse is (b) Show that the minimal time for the second pulse is 2d s − 2d tan θ = 2d (1 sec θk 2+)1/2 + s − tt22 = vv2 vv1 1

2

provided that where k = v1 /v 2.

t 2d(1 − k 2 )1/2 (c) Conclude that 2 = + k. t1 s

tan θ ≤

s 2d

SOLUTION

(Note: If this inequality is not satisﬁed, then point B does not lie to the left of C.) v1 (a) If θ = that v2 , then (c)sinShow t2 is minimized when sin θ = v1 /v2 . v 1 . = tan θ = 1 2 2 v2 2 v2 − v1 −1 v1

Because vv2 ≥ 1 + 4( ds )2 , it follows that 1

v2 2 −1≥ v1

1+4

2 d 2d −1= . s s

s as required. Hence, tan θ ≤ 2d v v v and tan θ = 1 . (b) For the time-minimizing choice of θ , we have sin θ = 1 from which sec θ = 2 v2 2 2 2 v2 − v1 v2 − v12 Thus

2d s − 2d tan θ 2d v

2 sec θ + = + t2 = v1 v2 v1 v 2 − v 2 2 1 ⎛ ⎞ v12 v2 2d ⎝ ⎠+ s

= − v1 v2 v2 − v2 v v2 − v2 2

2

1

2

s − 2d v1

v22 −v12

v2

1

⎛ ⎞ 2 2 s s 2d ⎝ v22 − v12 ⎠ 2d ⎝ v2 − v1 ⎠

+ + = = v1 v v 2 − v 2 v2 v1 v 2 2 v2 2 2 1 ⎛

⎞

2d = v1 (c) Recall that t1 =

1−

1/2

2d 1 − k 2 v1 2 s s + = + . v2 v2 v1 v2

s . We therefore have v1 1/2 2d 1−k 2 + vs2 t2 v1 = s t1 v1

=

1/2

2d 1 − k 2 s

v + 1 = v2

1/2

2d 1 − k 2 s

+ k.

S E C T I O N 4.6

Applied Optimization

215

65. Three towns A, B, and C are to be joined by an underground ﬁber cable as illustrated in Figure 34(A). Assume that Continue with the assumption of the previous exercise. C is located directly below the midpoint of AB. Find the junction point P that minimizes the total amount of cable used. (a) Find the thickness of the soil layer, assuming that v1 = 0.7v2 , t2 /t1 = 1.3, and s = 1,500 ft. (a) First show that P must lie directly above C. Show that if the junction is placed at a point Q as in Figure 34(B), then t2 are measured experimentally. equation in Exercise 63(c) shows that tmay is a linear func(b) reduce The times t1 andlength 2 /t1 want we can the cable by moving Q horizontallyThe over to the point P lying above C. You to use the of 1/s. What resulttion of Example 6. might you conclude if experiments were formed for several values of s and the points (1/s, t2 /t1 ) did not lie on a straight line? D A

B

A

B

x Cable

x L

P

Q

C

P C (B)

(A)

FIGURE 34

(b) With x as in Figure 34(A), let f (x) be the total length √ of cable used. Show that f (x) has a unique critical point c. Compute c and show that 0 ≤ c ≤ L if and only if D ≤ 2 3 L. (c) Find the minimum of f (x) on [0, L] in two cases: D = 2, L = 4 and D = 8, L = 2. SOLUTION

(a) Look at diagram 34(B). Let T be the point directly above Q on AB. Let s = AT and D = AB so that T B = D − s. Let be the total length of cable from A to Q and B to Q. By the Pythagorean Theorem applied to AQT and B QT , we get: 2 = s 2 + x 2 + (D − s)2 + x 2 = 2x 2 + D 2 − 2Ds + 2s 2 = 2x 2 + D 2 + 2s 2 − 2Ds = 2x 2 + D 2 + 2(s − D/2)2 − D 2 /2. We wish to ﬁnd the value of s producing the minimum value of 2 . Since D and x are not being changed, this clearly occurs where s = D/2, that is, where Q = P. Since it is obvious that PC ≤ QC (QC is the hypotenuse of the triangle P QC), it follows that total cable length is minimized at Q = P. (b) Let f (x) be the total cable length. From diagram 34(A), we get:

f (x) = (L − x) + 2 x 2 + D 2 /4. Then f (x) = −1 +

2x x 2 + D 2 /4

=0

gives 2x =

x 2 + D 2 /4

or 4x 2 = x 2 + D 2 /4 and the critical point is √ c = D/2 3. This is the only critical point of f . It lies in the interval [0, L] if and only if c ≤ L, or √ D ≤ 2 3L . √ (c) The minimum of f will depend on whether D ≤ 2 3L. √ √ √ √ • D = 2, L = 4; 2 3L = 8 3 > D, so c = D/(2 3) = 3/3 (0) = L + D = 6, f (L) =

∈ [0, L]. f √ √ √ 1 2 2 2 L + D /4 = 2 17 ≈ 8.24621, and f (c) = 4 − ( 3/3) + 2 3 + 1 = 4 + 3 ≈ 5.73204. Therefore, the √ total length is minimized where √ x = c = 3/3. √ √ • D = 8, L = 2; 2 3L = 4 3 < D, so c does not lie in the interval [0, L]. f (0) = 2 + 2 64/4 = 10, and √ √ √ f (L) = 0 + 2 4 + 64/4 = 2 20 = 4 5 ≈ 8.94427. Therefore, the total length is minimized were x = L, or where P = C. A jewelry designer plans to incorporate a component made of gold in the shape of a frustum of a cone of height 1 cm and ﬁxed lower radius r (Figure 35). The upper radius x can take on any value between 0 and r . Note that

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4.7 Newton’s Method Preliminary Questions 1. How many iterations of Newton’s Method are required to compute a root if f (x) is a linear function? SOLUTION Remember that Newton’s Method uses the linear approximation of a function to estimate the location of a root. If the original function is linear, then only one iteration of Newton’s Method will be required to compute the root.

2. What happens in Newton’s Method if your initial guess happens to be a zero of f ? SOLUTION

If x0 happens to be a zero of f, then f (x ) x1 = x0 − 0 = x0 − 0 = x0 ; f (x 0 )

in other words, every term in the Newton’s Method sequence will remain x0 . 3. What happens in Newton’s Method if your initial guess happens to be a local min or max of f ? SOLUTION Assuming that the function is differentiable, then the derivative is zero at a local maximum or a local minimum. If Newton’s Method is started with an initial guess such that f (x 0 ) = 0, then Newton’s Method will fail in the sense that x 1 will not be deﬁned. That is, the tangent line will be parallel to the x-axis and will never intersect it.

4. Is the following a reasonable description of Newton’s Method: “A root of the equation of the tangent line to f (x) is used as an approximation to a root of f (x) itself”? Explain. SOLUTION Yes, that is a reasonable description. The iteration formula for Newton’s Method was derived by solving the equation of the tangent line to y = f (x) at x 0 for its x-intercept.

Exercises In Exercises 1–4, use Newton’s Method with the given function and initial value x 0 to calculate x1 , x 2 , x3 . 1. f (x) = x 2 − 2, SOLUTION

x0 = 1

Let f (x) = x 2 − 2 and deﬁne x n+1 = x n −

f (xn ) x n2 − 2 − . = x n f (x n ) 2xn

With x 0 = 1, we compute n

1

2

3

xn

1.5

1.416666667

1.414215686

3. f (x) = x 3 −25, x 0 = 1.6 f (x) = x − 7, x 0 = 2.5 SOLUTION Let f (x) = x 3 − 5 and deﬁne x n+1 = x n −

f (xn ) x3 − 5 = xn − n 2 . f (x n ) 3x n

With x 0 = 1.6 we compute n

1

2

3

xn

1.717708333

1.710010702

1.709975947

3 5. UsefFigure 6 toxchoose (x) = cos − x, an x0 initial = 0.8guess x0 to the unique real root of x + 2x + 5 = 0. Then compute the ﬁrst three iterates of Newton’s Method. y

−2

−1

x 1

2

FIGURE 6 Graph of y = x 3 + 2x + 5.

S E C T I O N 4.7 SOLUTION

Newton’s Method

217

Let f (x) = x 3 + 2x + 5 and deﬁne f (x n ) x 3 + 2x n + 5 . = xn − n 2 f (x n ) 3xn + 2

x n+1 = x n −

We take x 0 = −1.4, based on the ﬁgure, and then calculate n

1

2

3

xn

−1.330964467

−1.328272820

−1.328268856

In Exercises 7–10, useMethod Newton’s to approximate thecos root decimal[0, places compare the value π ] toand Use Newton’s to Method ﬁnd a solution of sin x = 2xtointhree the interval three decimalwith places. Then 2 obtained from a calculator. guess the exact solution and compare with your approximation. √ 7. 10 SOLUTION

Let f (x) = x 2 − 10, and let x 0 = 3. Newton’s Method yields: n

1

2

3

xn

3.16667

3.162280702

3.16227766

A calculator yields 3.16227766. 9. 51/3 1/4 7 SOLUTION

Let f (x) = x 3 − 5, and let x0 = 2. Here are approximations to the root of f (x), which is 51/3 . n

1

2

3

4

xn

1.75

1.710884354

1.709976429

1.709975947

A calculator yields 1.709975947. 11. Use Newton’s Method to approximate the largest positive root of f (x) = x 4 − 6x 2 + x + 5 to within an error of at −1/2 2 −4 most 10 . Refer to Figure 5. SOLUTION

Figure 5 from the text suggests the largest positive root of f (x) = x 4 − 6x 2 + x + 5 is near 2. So let

f (x) = x 4 − 6x 2 + x + 5 and take x 0 = 2. n

1

2

3

4

xn

2.111111111

2.093568458

2.093064768

2.093064358

The largest positive root of x 4 − 6x 2 + x + 5 is approximately 2.093064358. Use a graphing calculator to3 choose an initial guess for the unique positive root of x 4 + x 2 − 2x − 1 = 0. 13. + 1 and use Newton’s Method to approximate the largest positive root to Sketch graph of f of (x)Newton’s = x − 4x Calculate the ﬁrstthe three iterates Method. −3 . within an error of at most 10 SOLUTION Let f (x) = x 4 + x 2 − 2x − 1. The graph of f (x) shown below suggests taking x 0 = 1. Starting from x 0 = 1, the ﬁrst three iterates of Newton’s Method are: n

1

2

3

xn

1.25

1.189379699

1.184171279

y 30 20 10 −1

x 1

2

3

sin θ the smallest positive solution of π . Use = 0.9 to three decimal places. π Use graphing calculator The Estimate ﬁrst positive solution of sin x = 0 is x = Newton’s Method to calculate to afour decimal places. to θ choose the initial guess.

15.

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Let f (θ ) =

sin θ − .9. θ

We use the following plot to obtain an initial guess: y 1

x

0.5

−0.1 −0.2 −0.3 −0.4

1.5

θ = 0.8 seems a good ﬁrst guess for the location of a zero of the function. A series of approximations based on θ0 = 0.8 follows: n

1

2

3

θn

0.7867796879

0.7866830771

0.7866830718

A three digit approximation for the point where sinθ θ = .9 is 0.787. Plugging sin(0.787)/0.787 into a calculator yields 0.900 to three decimal places. 17. Let x 1 , x 2 be the estimates to a root obtained by applying Newton’s Method with x0 = 1 to the function graphed in In 1535, the mathematician Antonio Fior challenged his rival Niccolo Tartaglia to solve this problem: A tree Figure 7. Estimate the numerical values of x1 and x2 , and draw the tangent lines used to obtain them. stands 12 braccia high; it is broken into two parts at such a point that the height of the part left standing is the cube root of the length of the part cut away. What is ythe height of the part left standing? Show that this is equivalent to solving x 3 + x = 12 and ﬁnd the height to three decimal places. Tartaglia, who had discovered the secrets of cubic equations, was able to determine the exact answer: x −1 1 3

2 √ 3 3 3 2,919 + 54 − 2,919 − 54 9 x= FIGURE 7 SOLUTION The graph with tangent lines drawn on it appears below. The tangent line to the curve at (x 0 , f (x 0 )) has an x-intercept at approximately x 1 = 3.0. The tangent line to the curve at (x 1 , f (x 1 )) has an x-intercept at approximately x 2 = 2.2. y

x

−1

1

2

3

19. Find the x-coordinate to two decimal places of the ﬁrst point in the region x > 0 where y = x intersects y = tan x Find the coordinates to two decimal places of the point P in Figure 8 where the tangent line to y = cos x passes (draw a graph). through the origin. SOLUTION Here is a plot of tan x and x on the same axes: y 5 x 1

2

3

4

−5

The ﬁrst intersection with x > 0 lies on the second “branch” of y = tan x, between x = 54π and x = 32π . Let f (x) = tan x − x. The graph suggests an initial guess x 0 = 54π , from which we get the following table: n

1

2

3

4

xn

6.85398

21.921

4480.8

7456.27

This is clearly leading nowhere, so we need to try a better initial guess. Note: This happens with Newton’s Method—it is sometimes difﬁcult to choose an initial guess. We try the point directly between 54π and 32π , x 0 = 118π :

Newton’s Method

S E C T I O N 4.7

n

1

2

3

4

5

6

7

xn

4.64662

4.60091

4.54662

4.50658

4.49422

4.49341

4.49341

219

The ﬁrst point where y = x and y = tan x cross is at approximately x = 4.49341, which is approximately 1.4303π . Newton’s Method is often used to determine interest rates in ﬁnancial calculations. In Exercises 20–22, r denotes a yearly interest rate expressed as a decimal (rather than as a percent). 21. If you borrow L dollars for N years at a yearly interest rate r , your monthly payment of P dollars is calculated using If P dollars are deposited every month in an account earning interest at the yearly rate r , then the value S of the the equation account after N years is 1 − b−12N r L=P where b = 1 + .r b12N +1 − b b−1 12 . S=P where b = 1 + b−1 12 (a) What is the monthly payment if L = $5,000, N = 3, and r = 0.08 (8%)? decideda to deposit 100 dollars month. (b) You have are offered loan of L P==$5,000 to beper paid back over 3 years with monthly payments of P = $200. Use Newton’s Method tovalue compute ﬁndif the interestrate rateisrrof=this (a) What is the afterb5and years the implied yearly interest 0.07loan. (i.e.,Hint: 7%)?Show that (L/P)b12N +1 − (1 + 12N L/P)b + 1that = 0.to save $10,000 after 5 years, you must earn interest at a rate r determined by the equation b61 − (b) Show SOLUTION 101b +

100 = 0. Use Newton’s Method to solve for b (note that b = 1 is a root, but you want the root satisfying > (1 1).+ Then ﬁnd r=. 1.00667 (a) b = .08/12) b−1 1.00667 − 1 P=L = 5000 ≈ $156.69 1 − b−12N 1 − 1.00667−36

(b) Starting from

L=P

1 − b−12N b−1

,

divide by P, multiply by b − 1, multiply by b12N and collect like terms to arrive at (L/P)b12N +1 − (1 + L/P)b12N + 1 = 0. Since L/P = 5000/200 = 25, we must solve 25b37 − 26b36 + 1 = 0. Newton’s Method gives b ≈ 1.02121 and r = 12(b − 1) = 12(.02121) ≈ .25452 So the interest rate is around 25.45%. 23. Kepler’s Problem Although planetary motion is beautifully described by Kepler’s three laws (see Section 14.6), If you deposit P dollars in a retirement fund every year for N years with the intention of then withdrawing there is no simple formula for the position of a planet P along its elliptical orbit as a function of time. Kepler developed a Q dollars per year for M years, you must earn interest at a rate r satisfying P(b N − 1) = Q(1 − b−M ), where method for locating P at time t by drawing the auxiliary dashed circle in Figure 9 and introducing the angle θ (note that b = 1 + r . Assume that $2,000 is deposited each year for 30 years and the goal is to withdraw $8,000 per year for P determines θ , which is the central angle of the point B on the circle). Let a = O A and e = O S/O A (the eccentricity 20 years. Use Newton’s Method to compute b and then ﬁnd r . of the orbit). (a) Show that sector BSA has area (a 2 /2)(θ − e sin θ ). (b) It follows from Kepler’s Second Law that the area of sector BSA is proportional to the time t elapsed since the planet passed point A. More precisely, since the circle has area π a 2 , BSA has area (π a 2 )(t/T ), where T is the period of the orbit. Deduce that 2π t = θ − e sin θ . T (c) The eccentricity of Mercury’s orbit is approximately e = 0.2. Use Newton’s Method to ﬁnd θ after a quarter of Mercury’s year has elapsed (t = T /4). Convert θ to degrees. Has Mercury covered more than a quarter of its orbit at t = T /4? Auxiliary circle

B P

q O

Elliptical orbit

FIGURE 9

S Sun

A

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A P P L I C AT I O N S O F T H E D E R I VATI V E SOLUTION

(a) The sector S AB is the slice O AB with the triangle O P S removed. O AB is a central sector with arc θ and radius 2 O A = a, and therefore has area a 2θ . O P S is a triangle with height a sin θ and base length O S = ea. Hence, the area of the sector is 1 a2 a2 θ − ea 2 sin θ = (θ − e sin θ ). 2 2 2 (b) Since Kepler’s second law indicates that the area of the sector is proportional to the time t since the planet passed point A, we get

π a 2 (t/T ) = a 2 /2 (θ − e sin θ ) 2π

t = θ − e sin θ . T

(c) If t = T /4, the last equation in (b) gives:

π = θ − e sin θ = θ − .2 sin θ . 2 Let f (θ ) = θ − .2 sin θ − π2 . We will use Newton’s Method to ﬁnd the point where f (θ ) = 0. Since a quarter of the year on Mercury has passed, a good ﬁrst estimate θ0 would be π2 . n

1

2

3

4

xn

1.7708

1.76696

1.76696

1.76696

From the point of view of the Sun, Mercury has traversed an angle of approximately 1.76696 radians = 101.24◦ . Mercury has therefore traveled more than one fourth of the way around (from the point of view of central angle) during this time. 25. What happens when you apply Newton’s Method to the equation x 3 − 20x = 1/3 0 with the unlucky initial guess x 0 = 2?What happens when you apply Newton’s Method to ﬁnd a zero of f (x) = x ? Note that x = 0 is the only zero. SOLUTION Let f (x) = x 3 − 20x. Deﬁne x n+1 = x n −

f (x n ) x n3 − 20x n . − = x n f (x n ) 3x n2 − 20

Take x 0 = 2. Then the sequence of iterates is −2, 2, −2, 2, . . . , which diverges by oscillation.

Further Insights and Challenges 27. The roots of f (x) = 13 x 3 − 4x + 1 to three decimal places are −3.583, 0.251, and 3.332 (Figure 10). Determine Let c be a positive number and let f (x) = x −1 − c. the root to which Newton’s Method converges for the initial choices x0 = 1.85, 1.7, and 1.55. The answer shows that a 2x − on cx 2the . Thus, Newton’s MethodMethod. provides a way of computing reciprocals Showinthat x −have ( f (x)/ f (x)) = effect small(a) change x 0 can a signiﬁcant outcome of Newton’s without performing division. y with c = 10.324 and the two initial guesses x = 0.1 and (b) Calculate the ﬁrst three iterates of Newton’s Method 0 x0 = 0.5. 4 (c) Explain graphically why x0 = 0.5 does not yield a sequence of approximations to the reciprocal 1/10.324. 0.2513 3.332 −3.583

x

−4

FIGURE 10 Graph of f (x) = 13 x 3 − 4x + 1. SOLUTION

Let f (x) = 13 x 3 − 4x + 1, and deﬁne xn+1 = x n −

1 x 3 − 4x + 1 f (x n ) n 3 n − = x . n 2 f (xn ) xn − 4

• Taking x 0 = 1.85, we have

n

1

2

3

4

5

6

7

xn

−5.58

−4.31

−3.73

−3.59

−3.58294362

−3.582918671

−3.58291867

• Taking x 0 = 1.7, we have

Antiderivatives

S E C T I O N 4.8

n

1

2

3

4

5

6

7

8

9

xn

−2.05

−33.40

−22.35

−15.02

−10.20

−7.08

−5.15

−4.09

−3.66

n

10

11

12

13

xn

−3.585312288

−3.582920989

−3.58291867

−3.58291867

221

• Taking x 0 = 1.55, we have

n

1

2

3

4

5

6

xn

−0.928

0.488

0.245

0.251320515

0.251322863

0.251322863

In Exercises 28–29, consider a metal rod of length L inches that is fastened at both ends. If you cut the rod and weld on an additional m inches of rod, leaving the ends ﬁxed, the rod will bow up into a circular arc of radius R (unknown), as indicated in Figure 11. 29. Let L = 3 and m = 1. Apply Newton’s Method to Eq. (2) to estimate θ and use this to estimate h. Let h be the maximum vertical displacement of the rod. SOLUTION andθmand = 1. We want the solution of: conclude that (a) ShowWe thatletL L==2R3 sin h=

sin L(1θ− cos θL) = . θ 2 sin θL + m

L sin θ − =0 θ L +m 3 sin θ − = 0. θ 4 sin θ L = . θ L +m

(b) Show L + m = 2R θ and then prove Let f (θ ) = sinθ θ − 34 . y 0.2 0.1

x 0.5

−0.2 −0.2

1

1.5

The ﬁgure above suggests that θ0 = 1.5 would be a good initial guess. The Newton’s Method approximations for the solution follow: n

1

2

3

4

θn

1.2854388

1.2757223

1.2756981

1.2756981

L is approximately 1.2757. Hence The angle where sinθ θ = L+m

h=L

Quadratic Convergence to Square Roots

1 − cos θ ≈ 1.11181. 2 sin θ √ Let f (x) = x 2 − c and let en = x n − c be the error in x n .

1

Show that xn+1 = 2 (x n + c/xn ) and that en+1 = en2 /2xn . 4.8 (a)Antiderivatives √ √

(b) Show that if x0 > c, then xn > c for all n. Explain this graphically. √ √ (c) Assuming that x0 > c, show that en+1 ≤ en2 /2 c. Preliminary Questions 1. Find an antiderivative of the function f (x) = 0. SOLUTION

Since the derivative of any constant is zero, any constant function is an antiderivative for the function

f (x) = 0. ! 2. What is the difference, if any, between ﬁnding the general antiderivative of a function f (x) and evaluating f (x) d x? SOLUTION

No difference. The indeﬁnite integral is the symbol for denoting the general antiderivative.

3. Jacques happens to know that f (x) and g(x) have the same derivative, and he would like to know if f (x) = g(x). Does Jacques have sufﬁcient information to answer his question?

222

CHAPTER 4

A P P L I C AT I O N S O F T H E D E R I VATI V E SOLUTION No. Knowing that the two functions have the same derivative is only good enough to tell Jacques that the functions may differ by at most an additive constant. To determine whether the functions are equal for all x, Jacques needs to know the value of each function for a single value of x. If the two functions produce the same output value for a single input value, they must take the same value for all input values.

4. Write any two antiderivatives of cos x. Which initial conditions do they satisfy at x = 0? SOLUTION

Antiderivatives for cos x all have the form sin x + C for some constant C. At x = 0, sin x + C has the

value C. 5. (a) (b) (c)

Suppose that F (x) = f (x) and G (x) = g(x). Are the following statements true or false? Explain. If f = g, then F = G. If F and G differ by a constant, then f = g. If f and g differ by a constant, then F = G.

SOLUTION

(a) False. Even if f (x) = g(x), the antiderivatives F and G may differ by an additive constant. (b) True. This follows from the fact that the derivative of any constant is 0. (c) False. If the functions f and g are different, then the antiderivatives F and G differ by a linear function: F(x) − G(x) = ax + b for some constants a and b. 6. Determine if y = x 2 is a solution to the differential equation with initial condition dy = 2x, dx

y(0) = 1

d SOLUTION Although d x x 2 = 2x, x 2 takes the value 0 when x = 0, so y = x 2 is not a solution of the indicated initial value problem.

Exercises In Exercises 1–6, ﬁnd the general antiderivative of f (x) and check your answer by differentiating. 1. f (x) = 12x SOLUTION

"

" 12x d x = 12

x d x = 12 ·

1 2 x + C = 6x 2 + C. 2

As a check, we have d (6x 2 + C) = 12x dx as needed. 3. f (x) = x 2 +23x + 2 f (x) = x

SOLUTION

"

" (x 2 + 3x + 2) d x = =

" x 2d x + 3

" x1 dx + 2

x0 dx =

1 3 1 1 x + 3 · x2 + 2 · x1 + C 3 2 1

1 3 3 2 x + x + 2x + C. 3 2

As a check, we have d dx

1 3 3 2 x + x + 2x + C 3 2

= x 2 + 3x + 2

as needed. 5. f (x) = 8x −42 f (x) = x + 1

SOLUTION

"

8x −4 d x = 8

"

x −4 d x = 8 ·

As a check, we have d dx as needed.

1 −3 8 x + C = − x −3 + C. −3 3

8 − x −3 + C 3

= 8x −4

S E C T I O N 4.8

Antiderivatives

223

In Exercises f (x)7–10, = cosmatch x + 3 the sin xfunction with its antiderivative (a)–(d). (a) F(x) = cos(1 − x) (b) F(x) = − cos x (c) F(x) = − 12 cos(x 2 ) (d) F(x) = sin x − x cos x 7. f (x) = sin x SOLUTION

An antiderivative of sin x is − cos x, which is (b). As a check, we have ddx (− cos x) = − (− sin x) = sin x.

9. f (x) = sin(1 − x) f (x) = x sin(x 2 ) An antiderivative of sin (1 − x) is cos (1 − x) or (a). As a check, we have ddx cos(1 − x) = − sin(1 − x) · (−1) = sin(1 − x). SOLUTION

In Exercises the indeﬁnite integral. f (x)11–36, = x sinevaluate x " 11. (x + 1) d x " 1 (x + 1) d x = x 2 + x + C. SOLUTION 2 " "5 13. (t + 3t + 2) dt (9 − 5x) d x " 1 3 SOLUTION (t 5 + 3t + 2) dt = t 6 + t 2 + 2t + C. 6 2 " "−9/5 15. t dt 8s −4 ds " 5 SOLUTION t −9/5 dt = − t −4/5 + C. 4 " " 17. 2 dx 3 (5x − x −2 − x 3/5 ) d x " 2 d x = 2x + C. SOLUTION " " 19. (5t −19) dt √ dx x" 5 (5t − 9) dt = t 2 − 9t + C. SOLUTION 2 " "−2 21. x d3x (x + 4x −2 ) d x " 1 −1 1 SOLUTION x +C =− +C x −2 d x = −1 x " " 23. (x +√3)−2 d x x dx " 1 1 SOLUTION (x + 3)−1 + C = − + C. (x + 3)−2 d x = −1 x +3 " "3 dz 25. z 5 (4t − 9)−3 dt SOLUTION

" "

3 dz = z5

"

1 3z −5 dz = 3 − z −4 4

3 + C = − z −4 + C. 4

√ "

x(x3− 1) d x dx x 3/2 √ SOLUTION To perform this indeﬁnite integral, we need to distribute x = x 1/2 over (x − 1): √ x(x − 1) = x 1/2 (x − 1) = x 3/2 − x 1/2 .

27.

Written in this form, we can take the indeﬁnite integral: "

" √ 1 1 2 2 x 3/2 − x 1/2 d x = x(x − 1) d x = x 5/2 − x 3/2 + C = x 5/2 − x 3/2 + C. 5/2 3/2 5 3

224

CHAPTER 4

A P P L I C AT I O N S O F T H E D E R I VATI V E

" 29.

t"− 7 √ dt −1 (x t + x )(3x 2 − 5x) d x

SOLUTION

To proceed, we ﬁrst distribute √1 = t −1/2 over (t − 7): t

t −7 √ = (t − 7)t −1/2 = t 1/2 − 7t −1/2 . t From this, we get: " " 1 1/2 2 t −7 1 3/2 1/2 −1/2 t t − 7t ) dt = −7 + C = t 3/2 − 14t 1/2 + C. √ dt = (t 3/2 1/2 3 t "

" sin2x − 3 cos x) d x (4 x + 2x − 3 dx " x4 (4 sin x − 3 cos x) d x = −4 cos x − 3 sin x + C. SOLUTION " " 33. cos(6t + 4) dt sin 9x d x SOLUTION Using the integral formula for cos(kt + b), we get: " 1 cos(6t + 4) dt = sin(6t + 4) + C. 6

31.

"

" cos(3 − 4t) dt (4θ + cos 8θ ) d θ SOLUTION From the integral formula for cos(kt + b) with k = −4, b = 3, we get: " 1 1 cos(3 − 4t) dt = sin(3 − 4t) + C = − sin(3 − 4t) + C. −4 4

35.

37. In Figure " 2, which of (A) or (B) is the graph of an antiderivative of f (x)? 18 sin(3z + 8) dz y y y x x x f(x)

(A)

(B)

FIGURE 2

Let F(x) be an antiderivative of f (x). By deﬁnition, this means F (x) = f (x). In other words, f (x) provides information as to the increasing/decreasing behavior of F(x). Since, moving left to right, f (x) transitions from − to + to − to + to − to +, it follows that F(x) must transition from decreasing to increasing to decreasing to increasing to decreasing to increasing. This describes the graph in (A)! SOLUTION

39. Use the formulas for the derivatives of f (x) = tan x and f (x) = sec x to evaluate the integrals. " In Figure 3, which of (A), (B), (C) is not the graph of an antiderivative " of f (x)? Explain. (a) sec2 (3x) d x (b) sec(x + 3) tan(x + 3) d x Recall that ddx (tan x) = sec2 x and ddx (sec x) = sec x tan x. ! (a) Accordingly, we have sec2 (3x) d x = 13 tan(3x) + C. ! (b) Moreover, we see that sec(x + 3) tan(x + 3) d x = sec(x + 3) + C. SOLUTION

In Exercises 41–52, solvefor thethe differential equation initial condition. Use the formulas derivatives of f (x)with = cot x and f (x) = csc x to evaluate the integrals. " " d y 2 (a) csc x d x (b) csc x cot x d x = cos 2x, y(0) = 3 41. dx SOLUTION

Since dd xy = cos 2x, we have

" y=

cos 2x d x =

1 sin 2x + C. 2

Thus 3 = y(0) = so that C = 3. Therefore, y = 12 sin 2x + 3.

1 sin (2 · 0) + C = C, 2

S E C T I O N 4.8

dy =5 d=y x, y(0) dx = x 3 , y(0) = 2 dx dy SOLUTION Since d x = x, we have

Antiderivatives

225

43.

" y=

x dx =

1 2 x + C. 2

Thus 5 = y(0) = 0 + C, so that C = 5. Therefore, y = 12 x 2 + 5. dy 45. d=y 5 − 2t 2 , y(1) = 2 dt = 0, y(3) = 5 dt dy SOLUTION Since dt = 5 − 2t 2 , we have " 2 y = (5 − 2t 2 ) dt = 5t − t 3 + C. 3 Thus, 2 2 = y(1) = 5(1) − (13 ) + C, 3 so that C = −3 + 23 = − 73 . Therefore y = 5t − 23 t 3 − 73 . dy d=y 4t + 9,3 y(0)2 = 1 dt = 8x + 3x − 3, y(1) = 1 dx dy SOLUTION Since dt = 4t + 9, we have

47.

" (4t + 9) dt = 2t 2 + 9t + C.

y=

Thus 1 = y(0) = 0 + C, so that C = 1. Therefore, y = 2t 2 + 9t + 1.

π dy =1 49. x, y d=y sin √ dx 2 =1 = t, y(1) dt dy SOLUTION Since d x = sin x, we have " y = sin x d x = − cos x + C. Thus 1=y

π 2

= 0 + C,

so that C = 1. Therefore, y = 1 − cos x. dy π 3 d=y cos 5x, y(π ) = dx =4 = sin 2z, y dz 4 dy SOLUTION Since d x = cos 5x, we have

51.

" y=

cos 5x d x =

1 sin 5x + C. 5

Thus 3 = y(π ) = 0 + C, so that C = 3. Therefore, y = 3 + 15 sin 5x.

πf and then ﬁnd f . In Exercises ﬁrst ﬁnd d y 53–58, = sec2 3x, y =2 d x = x, f (0) = 1,4 f (0) = 0 53. f (x) Let g(x) = f (x). g (x) = x and g(0) = 1, so g(x) = 12 x 2 + C with g(0) = 1 = C. Hence f (x) = g(x) = 12 x 2 + 1. f (x) = 12 x 2 + 1 and f (0) = 0, so f (x) = 12 ( 13 x 3 ) + x + C = 16 x 3 + x + C, and f (0) = 0 = C. Therefore f (x) = 16 x 3 + x. SOLUTION

55. f (x) = x 3 −32x + 1, f (1) = 0, f (1) = 4 f (x) = x − 2x + 1, f (0) = 1, f (0) = 0

226

CHAPTER 4

A P P L I C AT I O N S O F T H E D E R I VATI V E SOLUTION

Let g(x) = f (x). The problem statement gives us g (x) = x 3 − 2x + 1, g(0) = 0. From g (x), we get

g(x) = 14 x 4 − x 2 + x + C, and from g(1) = 0, we get 0 = 14 − 1 + 1 + C, so that C = − 14 . This gives f (x) = g(x) = 1 x 4 − x 2 + x − 1 . From f (x), we get f (x) = 1 ( 1 x 5 ) − 1 x 3 + 1 x 2 − 1 x + C = 1 x 5 − 1 x 3 + 1 x 2 − 1 x + C. 4 4 4 5 3 2 4 20 3 2 4

From f (1) = 4, we get

1 1 1 1 − + − + C = 4, 20 3 2 4 so that C = 121 30 . Hence, 1 5 1 3 1 2 1 121 x − x + x − x+ . 20 3 2 4 30

f (x) =

π = 1, f π = 6 = cos θ , f 57. f (fθ)(t) −3/2 =t , f 2(4) = 1, f (4) 2 =4 SOLUTION

Let g(θ ) = f (θ ). The problem statement gives g (θ ) = cos θ ,

g

π 2

= 1.

From g (θ ) we get g(θ ) = sin θ + C. From g( π2 ) = 1 we get 1 + C = 1, so C = 0. Hence f (θ ) = g(θ ) = sin θ . From f (θ ) we get f (θ ) = − cos θ + C. From f ( π2 ) = 6 we get C = 6, so f (θ ) = − cos θ + 6. 59. Show that f (x) = tan2 x and g(x) = sec2 x have the same derivative. What can you conclude about the relation f (t) = t − cos t, f (0) = 2, f (0) = −2 between f and g? Verify this conclusion directly. SOLUTION

Let f (x) = tan2 x and g(x) = sec2 x. Then f (x) = 2 tan x sec2 x and g (x) = 2 sec x · sec x tan x =

2 tan x sec2 x; hence f (x) = g (x). Accordingly, f (x) and g(x) must differ by a constant; i.e., f (x) − g(x) = tan2 x − sec2 x = C for some constant C. To see that this is true directly, divide the identity sin2 x + cos2 x = 1 by cos2 x. This yields tan2 x + 1 = sec2 x, so that tan2 x − sec2 x = −1. 61. A particle located at the origin at t = 0 begins along the x-axis with velocity v(t) = 12 t 2 − t ft/s. Let s(t) 1 cos 2 x moving =− 2x + C for some constant C. Find C bys(t). setting x = 0. Show, by computing that sin 2 initial be its position at time t. Statederivatives, the differential equation with condition satisﬁed by s(t) and ﬁnd SOLUTION

Given that v(t) = ds dt and that the particle starts at the origin, the differential equation is 1 ds = t 2 − t, dt 2

s(0) = 0.

Accordingly, we have " s(t) =

1 2 t −t 2

dt =

1 2

"

" t 2 dt −

t dt =

1 1 3 1 2 1 1 · t − t + C = t 3 − t 2 + C. 2 3 2 6 2

Thus 0 = s(0) = 16 (0)3 − 12 (0)2 + C, so that C = 0. Therefore, the position is s(t) = 16 t 3 − 12 t 2 . 2 63. A particle along velocity v(t) = 25t be the position at time t. Repeat moves Exercise 61, the but x-axis replacewith the initial condition s(0)−=t 0 ft/s. with Let s(2)s(t) = 3. (a) Find s(t), assuming that the particle is located at x = 5 at time t = 0. (b) Find s(t), assuming that the particle is located at x = 5 at time t = 2. 2 The differential equation is v(t) = ds dt = 25t − t . (a) Suppose s(0) = 5. Then " " " s(t) = (25t − t 2 ) dt = 25 t dt − t 2 dt

SOLUTION

1 1 25 2 1 3 t − t + C. = 25 · t 2 − t 3 + C = 2 3 2 3 1 2 3 Thus 5 = s(0) = 25 2 (0) − 3 (0) + C, so that C = 5. Therefore, the position is

s(t) =

25 2 1 3 t − t + 5. 2 3

1 3 25 1 142 2 2 3 (b) From (a), we originally ascertained that s(t) = 25 2 t − 3 t + C. Thus 5 = s(2) = 2 (2) − 3 (2) + C = 3 + C, 127 so that C = 5 − 142 3 = − 3 . Therefore, the position is

s(t) =

25 2 1 3 127 t − t − . 2 3 3

S E C T I O N 4.8

Antiderivatives

227

the car come 65. A car traveling 84 ft/s begins to decelerate at a constant rate of 14 ft/s2 . After how many seconds does particle located at the at t = 0before movesstopping? in a straight line with acceleration a(t) = 4 − 12 t ft/s2 . Let v(t) to a stopAand how far will the carorigin have traveled be the velocity and s(t) the position at time t. SOLUTION Since the acceleration of the car is a constant −14ft/s2 , v is given by the differential equation: (a) State and solve the differential equation for v(t) assuming that the particle is at rest at t = 0. (b) Find s(t). dv = −14, v(0) = 84. dt ! ft From dv dt , we get v(t) = −14 dt = −14t + C. Since v(0) = 84, C = 84. From this, v(t) = −14t + 84 s . To ﬁnd the time until the car stops, we must solve v(t) = 0: −14t + 84 = 0 14t = 84 t = 84/14 = 6 s. Now we have a differential equation for s(t). Since we want to know how far the car has traveled from the beginning of its deceleration at time t = 0, we have s(0) = 0 by deﬁnition, so: ds = v(t) = −14t + 84, dt

s(0) = 0.

! From this, s(t) = (−14t + 84) dt = −7t 2 + 84t + C. Since s(0) = 0, we have C = 0, and s(t) = −7t 2 + 84t. At stopping time t = 6 s, the car has traveled s(6) = −7(36) + 84(6) = 252 ft. 67. A 900-kg rocket is released from a spacecraft. As the rocket burns fuel, its mass decreases and its velocity increases. Beginning at rest, an object moves in a straight line with constant acceleration a, covering 100 ft in 5 s. Find a. Let v(m) be the velocity (in meters per second) as a function of mass m. Find the velocity when m = 500 if dv/dm = −50m −1/2 . Assume that v(900) = 0. ! dv SOLUTION Since dm = −50m −1/2 , we have v(m) = −50m −1/2 dm = −100m 1/2 + C. Thus 0 = v(900) = √ √ −100 900 + C = −3000 + C, and C = 3000. Therefore, v(m) = 3000 − 100 m. Accordingly, √ √ v(500) = 3000 − 100 500 = 3000 − 1000 5 ≈ 764 meters/sec. 69. Find constants c1 and c2 such that F(x) = c1 x sin x + c2 cos x is an antiderivative of f (x) = x cos x. As water ﬂows through a tube of radius R = 10 cm, the velocity of an individual water particle depends on dv SOLUTION Let F(x) = c1 x sin x + c2 cos x. If F(x) is to be an antiderivative of f (x) = x cos x, we must have = −0.06r . Determine v(r ), assuming that its distance r from the center of the tube according to the formula F (x) = f (x) for all x. Hence c1 (x cos x + sin x) − c2 sin x = x cos x for dr all x. Equating coefﬁcients on the left- and particles at the velocity. right-hand sides, wewalls have of c1 the = 1tube (i.e.,have the zero coefﬁcients of x cos x are equal) and c1 − c2 = 0 (i.e., the coefﬁcients of sin x are equal). Thus c1 = c2 = 1 and hence F(x) = x sin x + cos x. As a check, we have F (x) = x cos x + sin x − sin x = x cos x = f (x), as required. 71. Verify the linearity properties of the indeﬁnite integral stated in Theorem 3. Find the general antiderivative of (2x + 9)10 . SOLUTION To verify the Sum Rule, let F(x) and G(x) be any antiderivatives of f (x) and g(x), respectively. Because d d d (F(x) + G(x)) = F(x) + G(x) = f (x) + g(x), dx dx dx it follows that F(x) + G(x) is an antiderivative of f (x) + g(x); i.e., " " " ( f (x) + g(x)) d x = f (x) d x + g(x) d x. To verify the Multiples Rule, again let F(x) be any antiderivative of f (x) and let c be a constant. Because d d (cF(x)) = c F(x) = c f (x), dx dx it follows that cF(x) is and antiderivative of c f (x); i.e., " " (c f (x)) d x = c f (x) d x.

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A P P L I C AT I O N S O F T H E D E R I VATI V E

Further Insights and Challenges 73. (a) (b)

Suppose that F (x) = f (x). Suppose that F (x) = f (x) and G (x) = g(x). Is it true that F(x)G(x) is an antiderivative of f (x)g(x)? Conﬁrm Show that 12 F(2x) is an antiderivative of f (2x). or provide a counterexample. Find the general antiderivative of f (kx) for any constant k.

Let F (x) = f (x). (a) By the Chain Rule, we have

SOLUTION

d dx

1 1 F(2x) = F (2x) · 2 = F (2x) = f (2x). 2 2

Thus 12 F(2x) is an antiderivative of f (2x). (b) For nonzero constant k, the Chain Rules gives d 1 1 F (kx) = F (kx) · k = F (kx) = f (kx). dx k k Thus k1 F(kx) is an antiderivative of f (kx). Hence the general antiderivative of f (kx) is 1k F(kx) + C, where C is a constant. −1 75. TheFind Power for antiderivatives does an Rule antiderivative for f (x) = |x|.not apply to f (x) = x . Which of the graphs in Figure 4 could plausibly −1 represent an antiderivative of f (x) = x ? y

y

y x

x 1

3

5

1

3

5

x 1 (A)

3 (B)

5 (C)

FIGURE 4 SOLUTION Let F(x) be an antiderivative of f (x) = x −1 . Then for x > 0, we have F (x) = f (x) = x −1 > 0. In other words, the graph of F(x) is increasing for x > 0. The only graph for which this is true is (A). Accordingly, neither graph (B) nor graph (C) is the graph of F(x).

CHAPTER REVIEW EXERCISES In Exercises 1–6, estimate using the Linear Approximation or linearization and use a calculator to compute the error. 1. 8.11/3 − 2 1 1 SOLUTION Let f (x) = x 1/3 , a = 8 and x = 0.1. Then f (x) = 3 x −2/3 , f (a) = 12 and, by the Linear Approximation,

1 f = 8.11/3 − 2 ≈ f (a)x = (0.1) = 0.00833333. 12 Using a calculator, 8.11/3 − 2 = 0.00829885. The error in the Linear Approximation is therefore |0.00829885 − 0.00833333| = 3.445 × 10−5 . 3. 6251/41 − 62411/4 √ − 1 −3/4 , f (a) = 1 and, by the Linear 1/4 SOLUTION 4.1 Let2 f (x) = x , a = 625 and x = −1. Then f (x) = 4 x 500 Approximation, 1 f = 6241/4 − 6251/4 ≈ f (a)x = (−1) = −0.002. 500 Thus 6251/4 − 6241/4 ≈ 0.002. Using a calculator, 6251/4 − 6241/4 = 0.00200120. The error in the Linear Approximation is therefore |0.00200120 − (0.002)| = 1.201 × 10−6 .

Chapter Review Exercises

5.

229

1√ 1.02 101

Let f (x) = x −1 and a = 1. Then f (a) = 1, f (x) = −x −2 and f (a) = −1. The linearization of f (x) at a = 1 is therefore SOLUTION

L(x) = f (a) + f (a)(x − a) = 1 − (x − 1) = 2 − x, 1 ≈ L(1.02) = 0.98. Using a calculator, 1 = 0.980392, so the error in the Linear Approximation is and 1.02 1.02

|0.980392 − 0.98| = 3.922 × 10−4 . √ In Exercises 7–10, ﬁnd the linearization at the point indicated. 5 33 √ 7. y = x, a = 25 √ 1 1 SOLUTION Let y = x and a = 25. Then y(a) = 5, y = 2 x −1/2 and y (a) = 10 . The linearization of y at a = 25 is therefore L(x) = y(a) + y (a)(x − 25) = 5 +

1 (x − 25). 10

9. A(r ) = 43 π r 3 , a = 3 v(t) = 32t − 4t 2 , a = 2 4 SOLUTION Let A(r ) = 3 π r 3 and a = 3. Then A(a) = 36π , A (r ) = 4π r 2 and A (a) = 36π . The linearization of A(r ) at a = 3 is therefore L(r ) = A(a) + A (a)(r − a) = 36π + 36π (r − 3) = 36π (r − 2). In Exercises the Linear V (h)11–15, = 4h(2use − h)(4 − 2h),Approximation. a=1 11. The position of an object in linear motion at time t is s(t) = 0.4t 2 + (t + 1)−1 . Estimate the distance traveled over the time interval [4, 4.2]. Let s(t) = 0.4t 2 + (t + 1)−1 , a = 4 and t = 0.2. Then s (t) = 0.8t − (t + 1)−2 and s (a) = 3.16. Using the Linear Approximation, the distance traveled over the time interval [4, 4.2] is approximately SOLUTION

s = s(4.2) − s(4) ≈ s (a)t = 3.16(0.2) = 0.632. 13. A store sells 80 MP3 players per week when the players are priced at P = $75. Estimate the number N sold if P is bondassuming that paysthat $10,000 offered Nforifsale at a price P. Thetopercentage yield Y of the bond is raised toA$80, d N /dinP 6=years −4. is Estimate the price is lowered $69. SOLUTION If P is raised to $80, then P = 5. With the assumption 10,000 1/6that d N /d P = −4, we estimate, using the Linear Y = 100 −1 . Approximation, that P dN Verify that if P = $7,500, then Y = 4.91%. the(−4)(5) drop in = yield if the price rises to $7,700. P = −20; N ≈ Estimate dP therefore, we estimate that only 60 MP3 players will be sold per week when the price is $80. On the other hand, if the price is lowered to $69, then P = −6 and N ≈ (−4)(−6) = 24. We therefore estimate that 104 MP3 players will be sold per week when the price is $69. √ b 15. Show a 2 + b ≈ of a +a sphere is measured small. Useatthis to 100 estimate 26 and the ﬁndmaximum the error using a calculator. Thethat circumference C = cm. Estimate percentage error in V if the 2a if b is error in C is at most 3 cm. 2 SOLUTION Let a > 0 and let f (b) = a + b. Then 1 f (b) = . 2 a2 + b By the Linear Approximation, f (b) ≈ f (0) + f (0)b, so

To estimate

√ 26, let a = 5 and b = 1. Then √

b a2 + b ≈ a + . 2a

26 =

1 52 + 1 ≈ 5 + = 5.1. 10

√ The error in this estimate is | 26 − 5.1| = 9.80 × 10−4 .

Use the Intermediate Value Theorem to prove that sin x − cos x = 3x has a solution and use Rolle’s Theorem to show that this solution is unique.

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x 17. Show that f (x) = x + 2 has precisely one real root. x +1 Let f (x) = x + 2x and note that f (0) = 0. Thus, f (x) has at least one real root. Now, suppose f (x) x +1 has two real roots, say a and b. Because f is continuous on [a, b], differentiable on (a, b) and f (a) = f (b) = 0, Rolle’s Theorem guarantees there exists c ∈ (a, b) such that f (c) = 0. However, SOLUTION

f (x) = 1 +

x 2 + 1 − 2x 2 x4 + x2 + 2 = >0 2 2 (x + 1) (x 2 + 1)2

for all x. We have reached a contradiction. Consequently, f (x) must have precisely one real root. 19. Suppose that f (1) = 5 and f (x) ≥ 2 for x ≥ 1. Use the MVT to show that f (8) ≥ 19. Verify the MVT for f (x) = x −1/2 on [1, 4]. SOLUTION Because f is continuous on [1, 8] and differentiable on (1, 8), the Mean Value Theorem guarantees there exists a c ∈ (1, 8) such that f (8) − f (1) 8−1

f (c) =

f (8) = f (1) + 7 f (c).

or

Now, we are given that f (1) = 5 and that f (x) ≥ 2 for x ≥ 1. Therefore, f (8) ≥ 5 + 7(2) = 19. In Exercises 21–24, the local they arethen minima, neither. Use the MVTﬁnd to prove thatextrema if f (x)and ≤ 2determine for x > 0whether and f (0) = 4, f (x) maxima, ≤ 2x + 4orfor all x ≥ 0. 21. f (x) = x 3 − 4x 2 + 4x Let f (x) = x 3 − 4x 2 + 4x. Then f (x) = 3x 2 − 8x + 4 = (3x − 2)(x − 2), so that x = 23 and x = 2 are critical points. Next, f (x) = 6x − 8, so f ( 23 ) = −4 < 0 and f (2) = 4 > 0. Therefore, by the Second Derivative Test, f ( 23 ) is a local maximum while f (2) is a local minimum. SOLUTION

23. f (x) = x 2 (x + 2)3 s(t) = t 4 − 8t 2 SOLUTION Let f (x) = x 2 (x + 2)3 . Then f (x) = 3x 2 (x + 2)2 + 2x(x + 2)3 = x(x + 2)2 (3x + 2x + 4) = x(x + 2)2 (5x + 4), so that x = 0, x = −2 and x = − 45 are critical points. The sign of the ﬁrst derivative on the intervals surrounding the critical points is indicated in the table below. Based on this information, f (−2) is neither a local maximum nor a local minimum, f (− 45 ) is a local maximum and f (0) is a local minimum. Interval

(−∞, −2)

(−2, − 45 )

(− 45 , 0)

(0, ∞)

+

+

−

+

Sign of f

In Exercises 25–30, ﬁnd the extreme values on the interval. f (x) = x 2/3 (1 − x) 25. f (x) = x(10 − x),

[−1, 3]

Let f (x) = x(10 − x) = 10x − x 2 . Then f (x) = 10 − 2x, so that x = 5 is the only critical point. As this critical point is not in the interval [−1, 3], we only need to check the value of f at the endpoints to determine the extreme values. Because f (−1) = −11 and f (3) = 21, the maximum value of f (x) = x(10 − x) on the interval [−1, 3] is 21 while the minimum value is −11. SOLUTION

27. g(θ ) = sin2 θ 4− cos θ6 , [0, 2π ] f (x) = 6x − 4x , [−2, 2] SOLUTION Let g(θ ) = sin2 θ − cos θ . Then g (θ ) = 2 sin θ cos θ + sin θ = sin θ (2 cos θ + 1) = 0 when θ = 0, 23π , π , 43π , 2π . The table below lists the value of g at each of the critical points and the endpoints of the interval [0, 2π ]. Based on this information, the minimum value of g(θ ) on the interval [0, 2π ] is −1 and the maximum value is 54 .

θ g(θ ) 1/3 , [−1, 3] 29. f (x) = x 2/3 − 2x t , [0, 3] R(t) = 2 t +t +1

0

2π /3

π

4π /3

2π

−1

5/4

1

5/4

−1

Chapter Review Exercises

231

Let f (x) = x 2/3 − 2x 1/3 . Then f (x) = 23 x −1/3 − 23 x −2/3 = 23 x −2/3 (x 1/3 − 1), so that the critical √ √ points are x = 0 and x = 1. With f (−1) = 3, f (0) = 0, f (1) = −1 and f (3) = 3 9 − 2 3 3 ≈ −0.804, it follows that the minimum value of f (x) on the interval [−1, 3] is −1 and the maximum value is 3. SOLUTION

31. Find the critical points and extreme values of f (x) = |x − 1| + |2x − 6| in [0, 8]. f (x) = x − tan x, [−1, 1] SOLUTION Let ⎧ ⎪ ⎨7 − 3x, f (x) = |x − 1| + |2x − 6| = 5 − x, ⎪ ⎩ 3x − 7,

x <1 1≤x <3. x ≥3

The derivative of f (x) is never zero but does not exist at the transition points x = 1 and x = 3. Thus, the critical points of f are x = 1 and x = 3. With f (0) = 7, f (1) = 4, f (3) = 2 and f (8) = 17, it follows that the minimum value of f (x) on the interval [0, 8] is 2 and the maximum value is 17. In Exercises 33–36, ﬁnd the points of inﬂection. 1 . Where on the interval [1, 4] does f (x) take on its maximum A function f (x) has derivative f (x) = 4 3 2 x +1 33. y = x − 4x + 4x value? SOLUTION Let y = x 3 − 4x 2 + 4x. Then y = 3x 2 − 8x + 4 and y = 6x − 8. Thus, y > 0 and y is concave up for x > 43 , while y < 0 and y is concave down for x < 43 . Hence, there is a point of inﬂection at x = 43 . x2 35. y =y =2 x − 2 cos x x +4 SOLUTION

4 x2 8x =1− 2 . Then y = 2 Let y = 2 and x +4 x +4 (x + 4)2 y =

(x 2 + 4)2 (8) − 8x(2)(2x)(x 2 + 4) 8(4 − 3x 2 ) = . (x 2 + 4)4 (x 2 + 4)3

Thus, y > 0 and y is concave up for 2 2 −√ < x < √ , 3 3 while y < 0 and y is concave down for 2 |x| ≥ √ . 3 Hence, there are points of inﬂection at 2 x = ±√ . 3 37. (a) (b) (c)

Match the description of f (x) with the graph of its derivative f (x) in Figure 1. x y = f (x) is increasing and concave up. (x 2 − 4)1/3 f (x) is decreasing and concave up. f (x) is increasing and concave down. y

y

y

x

x

x (i)

(ii)

(iii)

FIGURE 1 Graphs of the derivative. SOLUTION

(a) If f (x) is increasing and concave up, then f (x) is positive and increasing. This matches the graph in (ii). (b) If f (x) is decreasing and concave up, then f (x) is negative and increasing. This matches the graph in (i). (c) If f (x) is increasing and concave down, then f (x) is positive and decreasing. This matches the graph in (iii). In Exercises thefor limit. Draw39–52, a curveevaluate y = f (x) which f and f have signs as indicated in Figure 2. 39. lim (9x 4 − 12x 3 ) x→∞

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Because the leading term in the polynomial has a positive coefﬁcient, lim (9x 4 − 12x 3 ) = ∞.

x→∞

x 3 + 2x 41. lim lim 2 (7x 2 − 9x 5 ) x→∞ x→−∞ 4x − 9 SOLUTION

x 3 + 2x (x 3 + 2x)x −2 x + 2x −1 = lim = lim = ∞. 2 2 −2 x→∞ 4x − 9 x→∞ (4x − 9)x x→∞ 4 − 9x −2 lim

x 3 + 2x x 3 + 2x x→∞lim 4x 3 − 92 x→−∞ 4x − 9

43. lim

SOLUTION

lim

x 3 + 2x

x→∞ 4x 3 − 9

= lim

(x 3 + 2x)x −3

x→∞ (4x 3 − 9)x −3

= lim

1 + 2x −2

x→∞ 4 − 9x −3

=

1 . 4

x 2 − 9x3 45. lim x + 2x x→∞limx 2 + 4x x→−∞ 4x 5 − 40x 3 √ SOLUTION For x > 0, x −2 = |x|−1 = x −1 . Then, x 2 − 9x (x 2 − 9x)x −1 x −9 = lim lim = lim = ∞. √ x→∞ x→∞ x 2 + 4x x 2 + 4x x −2 1 + 4x −1

x→∞

x 1/212x + 1 47. lim lim √ x→∞ 4x 92 x→−∞ −4x + 4x SOLUTION

lim √

x→∞

49.

x 1/2 4x − 9

= lim √ x→∞

1 x 1/2 x −1/2 1 = lim = . √ x→∞ −1 −1 2 4x − 9 x 4 − 9x

1 − 2x

lim x 3/2 x→3+ limx − 3 6 x→∞ (16x − 9x 4 )1/4

SOLUTION

As x → 3+, 1 − 2x → −5 and x − 3 → 0+, so 1 − 2x = −∞. x→3+ x − 3 lim

x −5 lim 1 limx − 2 x→2+ x→2− x 2 − 4 SOLUTION As x → 2+, x − 5 → −3 and x − 2 → 0+, so

51.

lim

x −5

x→2+ x − 2

= −∞.

In Exercises 53–62, 1sketch the graph, noting the transition points and asymptotic behavior. lim (x 2+ 3)3 53. y =x→−3+ 12x − 3x Let y = 12x − 3x 2 . Then y = 12 − 6x and y = −6. It follows that the graph of y = 12x − 3x 2 is increasing for x < 2, decreasing for x > 2, has a local maximum at x = 2 and is concave down for all x. Because SOLUTION

lim (12x − 3x 2 ) = −∞,

x→±∞

the graph has no horizontal asymptotes. There are also no vertical asymptotes. The graph is shown below.

Chapter Review Exercises

233

y 10 5 x

−1 −5

1

2

3

4

5

−10

2+3 55. y = x 3 − 2x y = 8x 2 − x 4 SOLUTION Let y = x 3 − 2x 2 + 3. Then y = 3x 2 − 4x and y = 6x − 4. It follows that the graph of y = x 3 − 2x 2 + 3 is increasing for x < 0 and x > 43 , is decreasing for 0 < x < 43 , has a local maximum at x = 0, has a local minimum at x = 43 , is concave up for x > 23 , is concave down for x < 23 and has a point of inﬂection at x = 23 . Because

lim (x 3 − 2x 2 + 3) = −∞

lim (x 3 − 2x 2 + 3) = ∞,

and

x→−∞

x→∞

the graph has no horizontal asymptotes. There are also no vertical asymptotes. The graph is shown below. y 10 5 −1

x 1

−5

2

−10

x 57. y = 3 3/2 y x= 4x + 1− x SOLUTION

x Let y = 3 . Then x +1 y =

x 3 + 1 − x(3x 2 ) 1 − 2x 3 = 3 3 2 (x + 1) (x + 1)2

and (x 3 + 1)2 (−6x 2 ) − (1 − 2x 3 )(2)(x 3 + 1)(3x 2 ) 6x 2 (2 − x 3 ) =− . 3 4 (x + 1) (x 3 + 1)3

x is increasing for x < −1 and −1 < x < 3 12 , is decreasing for x > 3 12 , has a It follows that the graph of y = 3 x +1

√ √ 3 3 3 1 local maximum at x = 2 , is concave up for x < −1 and x > 2, is concave down for −1 < x < 0 and 0 < x < 2 √ 3 and has a point of inﬂection at x = 2. Note that x = −1 is not an inﬂection point because x = −1 is not in the domain of the function. Now, y =

x

lim

x→±∞ x 3 + 1

= 0,

so y = 0 is a horizontal asymptote. Moreover, lim

x

x→−1− x 3 + 1

=∞

and

x

lim

x→−1+ x 3 + 1

so x = −1 is a vertical asymptote. The graph is shown below. y 4 2 −3 −2 −1

x −2 −4

1 x 59. y = y |x = + 2| + 1 2/3 2 (x − 4)

1

2

3

= −∞,

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SOLUTION

Let y =

1 . Because |x + 2| + 1 1

lim

= 0,

x→±∞ |x + 2| + 1

the graph of this function has a horizontal asymptote of y = 0. The graph has no vertical asymptotes as |x + 2| + 1 ≥ 1 for all x. The graph is shown below. From this graph we see there is a local maximum at x = −2. y 1 0.8 0.6 0.4 0.2 x

−8 −6 − 4 −2

2

4

61. y = 2 sin x − cos x on [0, 2π ] y = 2 −yx 3= √3 sin x − cos x. Then y = √3 cos x + sin x and y = −√3 sin x + cos x. It follows that the SOLUTION Let √ graph of y = 3 sin x − cos x is increasing for 0 < x < 5π /6 and 11π /6 < x < 2π , is decreasing for 5π /6 < x < 11π /6, has a local maximum at x = 5π /6, has a local minimum at x = 11π /6, is concave up for 0 < x < π /3 and 4π /3 < x < 2π , is concave down for π /3 < x < 4π /3 and has points of inﬂection at x = π /3 and x = 4π /3. The graph is shown below. y 1 −1

4 1

2

x

3

5

6

63. Find the maximum volume of a right-circular cone placed upside-down in a right-circular cone of radius R and y = 2x − tan x on [0, 2π ] height H (Figure 3). The volume of a cone of radius r and height h is 43 π r 2 h.

h

r

FIGURE 3 SOLUTION

Let r denote the radius and h the height of the upside down cone. By similar triangles, we obtain the

relation H −h H = r R

so

r h = H 1− R

and the volume of the upside down cone is

4 2 4 r3 2 V (r ) = π r h = π H r − 3 3 R

for 0 ≤ r ≤ R. Thus,

3r 2 4 dV = π H 2r − , dr 3 R

and the critical points are r = 0 and r = 2R/3. Because V (0) = V (R) = 0 and 2R 4R 2 4 8R 2 16 2 V π R H, = πH − = 3 3 9 27 81 the maximum volume of a right-circular cone placed upside down in a right-circular cone of radius R and height H is 16 2 π R H. 81

Chapter Review Exercises

235

65. Show that the maximum area of a parallelogram ADEF inscribed in a triangle ABC, as in Figure 4, is equal to On a certain farm, the corn yield is one-half the area of ABC. Y = −0.118x 2 + 8.5x + B12.9 (bushels per acre) where x is the number of corn plants per acre (in thousands). Assume that corn seed costs $1.25 (per thousand seeds) and that corn can be sold for $1.50/bushel. D E (a) Find the value x 0 that maximizes yield Y . Then compute the proﬁt (revenue minus the cost of seeds) at planting level x 0 . (b) Compute the proﬁt P(x) as a function ofA x and ﬁndF the valueCx 1 that maximizes proﬁt. 4 yield lead to maximum proﬁt? maximum (c) Compare the proﬁt at levels x0 and x 1 . Does aFIGURE SOLUTION Let θ denote the measure of angle B AC. Then the area of the parallelogram is given by AD · AF sin θ . Now, suppose that

B E/BC = x. Then, by similar triangles, AD = (1 − x) AB, AF = D E = x AC, and the area of the parallelogram becomes AB · AC x(1 − x) sin θ . The function x(1 − x) achieves its maximum value of 14 when x = 12 . Thus, the maximum area of a parallelogram inscribed in a triangle ABC is 1 1 1 1 AB · AC sin θ = AB · AC sin θ = (area of ABC) . 4 2 2 2 67. Let f (x) be a function whose graph does not pass through the x-axis and let Q = (a, 0). Let P = (x0 , f (x 0 )) be the 1 r 2 (θ − sin θ ). What is the maximum possible area of this region if of closest the shaded in 6). Figure 5 isthat point onThe the area graph to Qregion (Figure Prove 2 P Q is perpendicular to the tangent line to the graph of x 0 . Hint: the length the circular Let q(x) be theofdistance fromarc (x,isf 1? (x)) to (a, 0) and observe that x 0 is a critical point of q(x). y

y = f(x) P = (x 0 , f(x 0))

Q = (a, 0)

x

FIGURE 6 SOLUTION Let P = (a, 0) and let Q = (x 0 , f (x 0 )) be the point on the graph of y = f (x) closest to P. The slope of the segment joining P and Q is then

f (x 0 ) . x0 − a Now, let q(x) =

(x − a)2 + ( f (x))2 ,

the distance from the arbitrary point (x, f (x)) on the graph of y = f (x) to the point P. As (x 0 , f (x 0 )) is the point closest to P, we must have q (x 0 ) =

2(x 0 − a) + 2 f (x 0 ) f (x 0 ) = 0. (x 0 − a)2 + ( f (x 0 ))2

Thus, x −a f (x 0 ) = − 0 =− f (x 0 )

f (x 0 ) −1 . x0 − a

In other words, the slope of the segment joining P and Q is the negative reciprocal of the slope of the line tangent to the graph of y = f (x) at x = x 0 ; hence; the two lines are perpendicular. √ 3 to four decimalaplaces. 69. UseTake Newton’s Method to of estimate a circular piece paper of 25 radius R, remove sector of angle θ (Figure 7), and fold the remaining piece 3 SOLUTION Let f (x) = x − 25 and deﬁne into a cone-shaped cup. Which angle θ produces the cup of largest volume? x n+1 = x n − With x 0 = 3, we ﬁnd

f (x n ) x 3 − 25 = xn − n 2 . f (x n ) 3xn

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A P P L I C AT I O N S O F T H E D E R I VATI V E

Thus, to four decimal places

√ 3

n

1

2

3

xn

2.925925926

2.924018982

2.924017738

25 = 2.9240.

In Exercises 71–80, calculate the indeﬁnite integral. Use Newton’s Method to ﬁnd a root of f (x) = x 2 − x − 1 to four decimal places. " 3 4x − 2x 2 d x 71. " 2 SOLUTION (4x 3 − 2x 2 ) d x = x 4 − x 3 + C. 3 " " − 8) d θ 73. sin(θ9/4 dx x " SOLUTION sin(θ − 8) d θ = − cos(θ − 8) + C. "

" (4t −3 − 12t −4 ) dt cos(5 − 7θ ) d θ " SOLUTION (4t −3 − 12t −4 ) dt = −2t −2 + 4t −3 + C.

75.

"

" dx sec2 x−2/3 + 4t 7/3 ) dt (9t " SOLUTION sec2 x d x = tan x + C.

77.

"

" (y + 2)4 d y tan 3θ sec 3θ d θ " 1 (y + 2)4 d y = (y + 2)5 + C. SOLUTION 5

79.

" In Exercises 81–84, solve the differential equation with initial condition. 3x 3 − 9 dx 2 dy 81. = 4x x3 , y(1) = 4 dx SOLUTION

Let dd xy = 4x 3 . Then

" y(x) =

4x 3 d x = x 4 + C.

Using the initial condition y(1) = 4, we ﬁnd y(1) = 14 + C = 4, so C = 3. Thus, y(x) = x 4 + 3. dy d=y x −1/22, y(1) = 1 dx = 3t + cos t, y(0) = 12 dt dy SOLUTION Let d x = x −1/2 . Then

83.

" y(x) =

x −1/2 d x = 2x 1/2 + C.

√ Using the initial condition y(1) = 1, we ﬁnd y(1) = 2 1 + C = 1, so C = −1. Thus, y(x) = 2x 1/2 − 1. 85. Find f (t), assuming that f (t) = 1 − 2t, f (0) = 2, and f (0) = −1. dy π4 ) = = 12 − 2t. Then = sec2 x, y( SOLUTION d x Suppose f (t) " " f (t) dt = (1 − 2t) dt = t − t 2 + C. f (t) = Using the initial condition f (0) = −1, we ﬁnd f (0) = 0 − 02 + C = −1, so C = −1. Thus, f (t) = t − t 2 − 1. Now, " " 1 1 f (t) = f (t) dt = (t − t 2 − 1) dt = t 2 − t 3 − t + C. 2 3 Using the initial condition f (0) = 2, we ﬁnd f (0) = 12 02 − 13 03 − 0 + C = 2, so C = 2. Thus, f (t) =

1 2 1 3 t − t − t + 2. 2 3

The driver of an automobile applies the brakes at time t = 0 and comes to a halt after traveling 500 ft. Find the automobile’s velocity at t = 0 assuming that the rate of deceleration was a constant −10 ft/s2 .

5 THE INTEGRAL 5.1 Approximating and Computing Area Preliminary Questions 1. Suppose that [2, 5] is divided into six subintervals. What are the right and left endpoints of the subintervals? 1 If the interval [2, 5] is divided into six subintervals, the length of each subinterval is 5−2 6 = 2 . The right 5 7 9 5 7 9 endpoints of the subintervals are then 2 , 3, 2 , 4, 2 , 5, while the left endpoints are 2, 2 , 3, 2 , 4, 2 . SOLUTION

2. If f (x) = x −2 on [3, 7], which is larger: R2 or L 2 ? SOLUTION On [3, 7], the function f (x) = x −2 is a decreasing function; hence, for any subinterval of [3, 7], the function value at the left endpoint is larger than the function value at the right endpoint. Consequently, L 2 must be larger than R2 . 3. Which of the following pairs of sums are not equal? 4 4 # # (a) i, (c)

i=1 4 #

j,

j=1

=1 5 #

(i − 1)

(b)

(d)

i=2

4 # j=1 4 #

j 2,

5 #

k2

k=2

i(i + 1),

i=1

5 #

( j − 1) j

j=2

SOLUTION

(a) Only the name of the index variable has been changed, so these two sums are the same. (b) These two sums are not the same; the second squares the numbers two through ﬁve while the ﬁrst squares the numbers one through four. (c) These two sums are the same. Note that when i ranges from two through ﬁve, the expression i − 1 ranges from one through four. (d) These two sums are the same. Both sums are 1 · 2 + 2 · 3 + 3 · 4 + 4 · 5. 4. Explain why

100 $

j is equal to

j=1 SOLUTION

100 $ j=0

The ﬁrst term in the sum

j but $100

100 $

1 is not equal to

j=1

1.

j=0

j=0 j is equal to zero, so it may be dropped. More speciﬁcally, 100 #

j =0+

j=0

On the other hand, the ﬁrst term in

100 $

100 #

j=

j=1

100 #

j.

j=1

$100

j=0 1 is not zero, so this term cannot be dropped. In particular, 100 # j=0

1=1+

100 #

1 =

j=1

100 #

1.

j=1

5. We divide the interval [1, 5] into 16 subintervals. (a) What are the left endpoints of the ﬁrst and last subintervals? (b) What are the right endpoints of the ﬁrst two subintervals? 1 Note that each of the 16 subintervals has length 5−1 16 = 4 . (a) The left endpoint of the ﬁrst subinterval is 1, and the left endpoint of the last subinterval is 5 − 14 = 19 4 .

(b) The right endpoints of the ﬁrst two subintervals are 1 + 14 = 54 and 1 + 2 14 = 32 . SOLUTION

6. (a) (b) (c)

Are the following statements true or false? The right-endpoint rectangles lie below the graph if f (x) is increasing. If f (x) is monotonic, then the area under the graph lies between R N and L N . If f (x) is constant, then the right-endpoint rectangles all have the same height.

SOLUTION

(a) False. If f is increasing, then the right-endpoint rectangles lie above the graph. (b) True. If f (x) is increasing, then the area under the graph is larger than L N but smaller than R N ; on the other hand, if f (x) is decreasing, then the area under the graph is larger than R N but smaller than L N . (c) True. The height of the right-endpoint rectangles is given by the value of the function, which, for a constant function, is always the same.

THE INTEGRAL

Exercises 1. An athlete runs with velocity 4 mph for half an hour, 6 mph for the next hour, and 5 mph for another half-hour. Compute the total distance traveled and indicate on a graph how this quantity can be interpreted as an area. SOLUTION The ﬁgure below displays the velocity of the runner as a function of time. The area of the shaded region equals the total distance traveled. Thus, the total distance traveled is (4) (0.5) + (6) (1) + (5) (0.5) = 10.5 miles. 6 5 4 3 2 1

Velocity (mph)

CHAPTER 5

0.5 1 1.5 Time (hours)

2

3. A rainstorm hit Portland, Maine, in October 1996, resulting in record rainfall. The rainfall rate R(t) on October 21 Figure 14 shows the velocity of an object over a 3-min interval. Determine the distance traveled over the intervals is recorded, in inches per hour, in the following table, where t is the number of hours since midnight. Compute the total [0, 3] and [1, 2.5] (remember to convert from miles per hour to miles per minute). rainfall during this 24-hour period and indicate on a graph how this quantity can be interpreted as an area.

SOLUTION

t

0–2

2–4

4–9

9–12

12–20

20–24

R(t)

0.2

0.1

0.4

1.0

0.6

0.25

Over each interval, the total rainfall is the time interval in hours times the rainfall in inches per hour. Thus R = 2(.2) + 2(.1) + 5(.4) + 3(1.0) + 8(.6) + 4(.25) = 11.4 inches.

The ﬁgure below is a graph of the rainfall as a function of time. The area of the shaded region represents the total rainfall. 1 Rainfall (inches per hour)

238

0.8 0.6 0.4 0.2 5

10 15 Time (hours)

20

25

5. Compute R6 , L 6 , and M3 to estimate the distance traveled over [0, 3] if the velocity at half-second intervals is as The velocity of an object is v(t) = 32t ft/s. Use Eq. (2) and geometry to ﬁnd the distance traveled over the time follows: intervals [0, 2] and [2, 5].

SOLUTION

t (s)

0

0.5

1

1.5

2

2.5

3

v (ft/s)

0

12

18

25

20

14

20

3−0 For R6 and L 6 , t = 3−0 6 = .5. For M3 , t = 3 = 1. Then

R6 = 0.5 sec (12 + 18 + 25 + 20 + 14 + 20) ft/sec = .5(109) ft = 54.5 ft, L 6 = 0.5 sec (0 + 12 + 18 + 25 + 20 + 14) ft/sec = .5(89) ft = 44.5 ft, and M3 = 1 sec (12 + 25 + 14) ft/sec = 51 ft. 7. (a) (b)

Consider f (x) = 2x + 3 on [0, 3]. Use the following table of values to estimate the area under the graph of f (x) over [0, 1] by computing the Compute L 6 over [0, 3]. 6 and average ofRR 5 and L 5 . Find the error in these approximations by computing the area exactly using geometry.

SOLUTION

Let f (x) = 2x + 3 on [0, 3]. x

0 0.2 0.4 0.6 0.8 1 % & (a) We partition [0, 3] into 6 equally-spaced subintervals. The left 44 endpoints of40 the subintervals are 0, 12 , 1, 32 , 2, 52 f (x) 50 48 46 42 & % whereas the right endpoints are 12 , 1, 32 , 2, 52 , 3 .

S E C T I O N 5.1

Approximating and Computing Area

239

• Let a = 0, b = 3, n = 6, x = (b − a) /n = 1 , and x k = a + kx, k = 0, 1, . . . , 5 (left endpoints). Then 2

L6 =

5 #

f (x k )x = x

k=0

5 #

f (x k ) =

k=0

1 (3 + 4 + 5 + 6 + 7 + 8) = 16.5. 2

• With x k = a + kx, k = 1, 2, . . . , 6 (right endpoints), we have

R6 =

6 #

f (x k )x = x

k=1

6 #

f (x k ) =

k=1

1 (4 + 5 + 6 + 7 + 8 + 9) = 19.5. 2

(b) Via geometry (see ﬁgure below), the exact area is A = 12 (3) (6) + 32 = 18. Thus, L 6 underestimates the true area (L 6 − A = −1.5), while R6 overestimates the true area (R6 − A = +1.5). y 9 6 3 x 0.5

1

1.5

2

2.5

3

9. Estimate R6 , L 6 , and M6 over [0, 1.5] for the function in Figure 15. Let f (x) = x 2 + x − 2. y (a) Calculate R3 and L 3 over [2, 5]. 3 (b) Sketch the graph of f and the rectangles that make up each approximation. Is the area under the graph larger or smaller than R3 ? Than L 3 ? 2 1 x 0.5

1

1.5

FIGURE 15 SOLUTION

& % Let f (x) on [0, 32 ] be given by Figure 15. For n = 6, x = ( 32 − 0)/6 = 14 , {x k }6k=0 = 0, 14 , 12 , 34 , 1, 54 , 32 .

Therefore L6 =

5 1# 1 f (x k ) = (2.4 + 2.35 + 2.25 + 2 + 1.65 + 1.05) = 2.925, 4 k=0 4

R6 =

6 1# 1 f (x k ) = (2.35 + 2.25 + 2 + 1.65 + 1.05 + 0.65) = 2.4875, 4 k=1 4

6 1# 1 1 f x k − x = (2.4 + 2.3 + 2.2 + 1.85 + 1.45 + 0.8) = 2.75. M6 = 4 k=1 2 4 1 . Sketch the graph of f (x) and draw the rectangles whose area is represented by 11. LetEstimate f (x) = R x,2M + 1, and and Lxfor = the 3 graph in Figure 16. 2 3 6 $6 the sum i=1 f (1 + ix)x. SOLUTION Because the summation index runs from i = 1 through i = 6, we will treat this as a right-endpoint approximation to the area under the graph of y = x 2 + 1. With x = 13 , it follows that the right endpoints of the subintervals are x1 = 43 , x2 = 53 , x3 = 2, x 4 = 73 , x5 = 83 and x 6 = 3. The sketch of the graph with the rectangles $6 represented by the sum i=1 f (1 + ix)x is given below. y 3 2.5 2 1.5 1 0.5 x 0.5

1

1.5

2

2.5

3

In Exercises 13–24, theshaded approximation forinthe given17. function interval. do these rectangles represent? Calculate thecalculate area of the rectangles Figure Whichand approximation

240

CHAPTER 5

THE INTEGRAL

13. R8 ,

f (x) = 7 − x,

SOLUTION

[3, 5]

% & Let f (x) = 7 − x on [3, 5]. For n = 8, x = (5 − 3)/8 = 14 , and {x k }8k=0 = 3, 3 14 , 3 12 , 3 34 , 4, 4 14 , 4 12 , 4 34 , 5 .

Therefore R8 =

8 1# 1 1 (7 − x k ) = (3.75 + 3.5 + 3.25 + 3 + 2.75 + 2.5 + 2.25 + 2) = (23) = 5.75. 4 k=1 4 4

= x=2 , 7 − [0,x,1] [3, 5] 15. M4 ,M ,f (x)f (x) 4 ' (3 1 SOLUTION Let f (x) = x 2 on [0, 1]. For n = 4, x = (1 − 0)/4 = 4 and x k∗ = {.125, .375, .625, .875}. k=0 Therefore M4 =

3 1# 1 (x ∗ )2 = (0.1252 + 0.3752 + 0.6252 + 0.8752 ) = .328125. 4 k=0 k 4

17. R6 , f (x) = 2x 2 √ − x + 2, [1, 4] M6 , f (x) = x, [2, 5] % & 1 1 1 1 SOLUTION Let f (x) = 2x 2 − x + 2 on [1, 4]. For n = 6, x = (4 − 1)/6 = 2 , {x k }6 k=0 = 1, 1 2 , 2, 2 2 , 3, 3 2 , 4 . Therefore R6 =

6 1# 1 (2x 2 − x k + 2) = (5 + 8 + 12 + 17 + 23 + 30) = 47.5. 2 k=1 k 2

19. L 5 , f (x) = x −1 , 2 [1, 2] L 6 , f (x) = 2x − x + 2, [1, 4] % & (2−1) 1 6 7 8 9 5 SOLUTION Let f (x) = x −1 on [1, 2]. For n = 5, x = 5 = 5 , {x k }k=0 = 1, 5 , 5 , 5 , 5 , 2 . Therefore L5 =

4 1# 1 5 5 5 5 (x k )−1 = 1+ + + + ≈ .745635. 5 k=0 5 6 7 8 9

21. L 4 , f (x) = cos x, [ π4 , π2 ] M4 , f (x) = x −2 , [1, 3] π π SOLUTION Let f (x) = cos x on [ 4 , 2 ]. For n = 4, x =

π (π /2 − π /4) = 4 16

and

{x k }4k=0 =

π 5π 3π 7π π , , , , . 4 16 8 16 2

Therefore L4 =

3 π # cos x k ≈ .361372. 16 k=0

1 π ,5]3π ] = =2sin x,, [[1, 23. M4 ,R , f (x) f (x) 5 4 4 x +1 SOLUTION

Let f (x) =

1 . For n = 4, x = 5−1 = 1, and {x ∗ }3 k k=0 = {1.5, 2.5, 3.5, 4.5}. Therefore 4 x 2 +1

M4 = 1

3 #

1 = ∗ (x )2 + 1 k=0 k

4 4 4 4 + + + 13 29 53 85

≈ .568154.

In Exercises 25–28, use2the Graphical Insight on page 254 to obtain bounds on the area. L 5 , f (x) = x + 3|x|, [−3, 2] √ 25. Let A be the area under the graph of f (x) = x over [0, 1]. Prove that 0.51 ≤ A ≤ 0.77 by computing R4 and L 4 . Explain your reasoning. SOLUTION

1 1 1 3 4 For n = 4, x = 1−0 4 = 4 and {xi }i=0 = {0 + ix} = {0, 4 , 2 , 4 , 1}. Therefore, √ √ 4 # 1 1 2 3 f (xi ) = + + + 1 ≈ .768 R4 = x 4 2 2 2 i=1

√ √ 1 2 3 1 f (xi ) = L 4 = x 0+ + + ≈ .518. 4 2 2 2 i=0 3 #

Approximating and Computing Area

S E C T I O N 5.1

241

In the plot below, you can see the rectangles whose area is represented by L 4 under the graph and the top of those whose area is represented by R4 above the graph. The area A under the curve is somewhere between L 4 and R4 , so .518 ≤ A ≤ .768.

L 4 , R4 and the graph of f (x). 27. Use R4 and L 4 to show that the area A under the graph of y = sin x over [0, π /2] satisﬁes 0.79 ≤ A ≤ 1.19. Use R6 and L 6 to show that the area A under y = x −2 over [10, 12] satisﬁes 0.0161 ≤ A ≤ 0.0172. SOLUTION Let f (x) = sin x. f (x) is increasing over the interval [0, π /2], so the Insight on page 254 applies, which

4 4 = π8 and {xi }i=0 = {0 + ix}i=0 = {0, π8 , π4 , 38π , π2 }. From indicates that L 4 ≤ A ≤ R4 . For n = 4, x = π /2−0 4 this,

L4 =

3 π# f (xi ) ≈ .79, 8 i=0

R4 =

4 π# f (xi ) ≈ 1.18. 8 i=1

Hence A is between .79 and 1.19.

Left and Right endpoint approximations to A. 29. Show that the area A in Exercise 25 satisﬁes L−1 N ≤ A ≤ R N for all N . Then use a computer algebra system ShowL that the area A under the graph of f (x) = x over [1, 8] satisﬁes to calculate N and R N for N = 100 and 150. Which of these calculations allows you to conclude that A ≈ 0.66 to two decimal places? 1 1 1 1 1 1 1 1 1 1 1 1 1 + √+ + + + + ≤ A ≤ 1 + + + + + + 2 =3 x 4is an 5increasing 6 7function; 8 7 . Now, SOLUTION On [0, 1], f (x) therefore,2 L N3≤ A4≤ R5N for6 all N L 100 = .6614629

R100 = .6714629

L 150 = .6632220

R150 = .6698887

Using the values obtained with N = 150, it follows that .6632220 ≤ A ≤ .6698887. Thus, to two decimal places, A ≈ .66. 31. Calculate the following sums: Show that the area A in Exercise 265 satisﬁes R N ≤ A ≤ L N for all N . Use 5 4 a computer algebra system to # # # L N and R N for N sufﬁciently (a) calculate 3 (b) large3to determine A to within an error(c)of at most k 3 10−4 . i=1 4 #

π (d) sin j 2 j=3

(e)

i=0 4 #

1 k − 1 k=2

(f)

k=2 3 #

3j

j=0

SOLUTION

(a)

(b)

(c)

5 # i=1 5 # i=0 4 #

3 = 3 + 3 + 3 + 3 + 3 = 15. Alternatively,

5 #

3=3

i=1

3 = 3 + 3 + 3 + 3 + 3 + 3 = 18. Alternatively,

5 #

1 = (3)(5) = 15.

i=1 5 # i=0

3=3

5 #

= (3)(6) = 18.

i=0

k 3 = 23 + 33 + 43 = 99. Alternatively,

k=2 4 # k=2

k3 =

4 #

k=1

k3 −

1 #

k=1

k3

=

44 43 42 + + 4 2 4

−

14 13 12 + + 4 2 4

= 99.

242

CHAPTER 5

THE INTEGRAL

(d)

4 #

sin

j=3 4 #

jπ 2

= sin

3π 2

+ sin

4π 2

= −1 + 0 = −1.

1 1 11 1 =1+ + = . k − 1 2 3 6 k=2 3 # 3 j = 1 + 3 + 32 + 33 = 40. (f)

(e)

j=0 200 # = 1, b3 = it17, b4 = −17. Let b1 = 3, b2j by 33. Calculate writing asand a difference of Calculate two sums the andsums. using formula (3). 4 2 3 # # # j=101 bk b j (a) bi (b) (b j + 2 ) (c) b SOLUTION i=2 j=1 k=1 k+1 200 #

j=

j=101

200 #

j−

j=1

100 #

j=

j=1

2002 200 + 2 2

−

1002 100 + 2 2

= 20100 − 5050 = 15050.

In Exercises 34–39, write the sum in summation notation. 35. (22 + 2) + (32 + 3) + (42 + 4) + (52 + 5) 47 + 57 + 67 + 77 + 87 SOLUTION The ﬁrst term is 22 + 2, and the last term is 52 + 5, so it seems that the sum limits are 2 and 5, and the kth term is k 2 + k. Therefore, the sum is: 5 #

(k 2 + k).

k=2

1 +213 + 2 +323 + · · · +4 n + n 3 5 (2 + 2) + (2 + 2) + (2 + 2) + (2 + 2) SOLUTION The ﬁrst term is 1 + 13 and the last term is n + n 3 , so it seems the summation limits are 1 through n, and the k-th term is k + k 3 . Therefore, the sum is 37.

n #

k + k3.

k=1

π

π π 39. sin(π1) + sin 2 + sin + ·n· · + sin + 2 + ··· + 3 n+1 2·3 3·4 (n + 1)(n + 2)

π π SOLUTION The ﬁrst summand is sin 1 = sin 0+1 hence, sin(π ) + sin

π 2

+ sin

π 3

+ · · · + sin

π n+1

=

n # i=0

sin

π . i +1

In Exercises 40–47, use linearity and formulas (3)–(5) to rewrite and evaluate the sums. 41.

20 # 15 + 1) # (2k 12 j 3 k=1 j=1

20 #

SOLUTION

(2k + 1) = 2

k=1

43.

20 # k=1

k+

20 # k=1

1=2

202 20 + 2 2

+ 20 = 440.

200 # 150k 3 # (2k + 1) k=100 k=51

SOLUTION

By rewriting the sum as a difference of two power sums,

200 #

k3 =

k=100

30 2 # 4 j 10 #6 j + 45. (3 − 322 ) j=2 =1

200 # k=1

k3 −

99 # k=1

k3 =

2004 2003 2002 + + 4 2 4

−

994 993 992 + + 4 2 4

= 379507500.

Approximating and Computing Area

S E C T I O N 5.1 SOLUTION

30 # j=2

4 j2 6j + 3

⎛ ⎞ ⎛ ⎞ 30 30 1 30 1 # # # # 4# 4 2 2 2 j+ j = 6⎝ j− j⎠ + ⎝ j − j ⎠ =6 3 j=2 3 j=1 j=2 j=1 j=1 j=1 30 4 303 302 30 302 + −1 + + + −1 =6 2 2 3 3 2 6 30 #

= 6 (464) +

47.

37816 46168 4 = . (9454) = 2784 + 3 3 3

30 # 50 2 − 4s − 1) # (3s j ( j − 1) s=1 j=0

SOLUTION

30 #

(3s 2 − 4s − 1) = 3

s=1

30 #

s2 − 4

s=1

30 #

s−

s=1

30 #

1=3

s=1

303 302 30 + + 3 2 6

In Exercises 48–51, calculate the sum, assuming that a1 = −1,

10 #

ai = 10, and

i=1

49.

10 #

302 30 + 2 2

− 30 = 26475.

bi = 7.

i=1

10 # 10 − b ) # (a i i 2ai i=1 i=1

10 #

SOLUTION

(ai − bi ) =

i=1

51.

−4

10 #

ai −

i=1

10 #

bi = 10 − 7 = 3.

i=1

10 # # a10i

(3a + 4b ) 10 10 # # SOLUTION ai = ai − a1 = 10 − (−1) = 11. i=2 =1

i=2

i=1

In Exercises 52–55, use formulas (3)–(5) to evaluate the limit. 53.

N 3 # Nj # i N →∞ limj=1 N 4 2 N →∞ N i=1

lim

SOLUTION

Let s N =

N # j3 . Then N4 j=1 N 1 # 1 j3 = 4 sN = 4 N j=1 N

Therefore, lim s N = N →∞

N4 N3 N2 + + 4 2 4

=

1 1 1 . + + 4 2N 4N 2

1 . 4

N # i 32 20 N # 55. lim i −i +1 N N →∞ lim N4 i=1 N →∞ N3 i=1 N # 20 i3 SOLUTION Let s N = − . Then N N4 i=1 N N 1 # 20 # 1 i3 − 1= 4 sN = 4 N i=1 N i=1 N

Therefore, lim s N = N →∞

1 79 − 20 = − . 4 4

N4 N3 N2 + + 4 2 4

− 20 =

1 1 1 − 20. + + 4 2N 4N 2

243

244

CHAPTER 5

THE INTEGRAL

In Exercises 56–59, calculate the limit for the given function and interval. Verify your answer by using geometry. 57.

lim L N , f (x) = 5x, [1, 3] lim R N , f (x) = 5x, [0, 3] N →∞ N →∞

SOLUTION Let f (x) = 5x on [1, 3]. Let N > 0 be an integer, and set a = 1, b = 3, and x = (b − a)/N = 2/N . Also, let x k = a + kx = 1 + 2k N , k = 0, 1, . . . N − 1 be the left endpoints of the N subintervals of [1, 3]. Then

−1 2 N# 2k 10 5 1+ = N N N k=0 k=0 10 20 (N − 1)2 N −1 = N+ 2 + = 20 − N 2 2 N

L N = x

N −1 #

f (xk ) =

N −1 #

1+

k=0

−1 20 N# k N k=0

30 20 + 2. N N

The area under the graph is lim L N = 20.

N →∞

The region under the curve is a trapezoid with base width 2 and heights 5 and 15. Therefore the area is 12 (2)(5 + 15) = 20, which agrees with the value obtained from the limit of the left-endpoint approximations. 59.

lim M N , f (x) = x, [0, 1] lim L N , f (x) = 6 − 2x, N →∞ N →∞

[0, 2]

Let f (x) = x on [0, 1]. Let N > 0 be an integer and set a = 0, b = 1, and x = (b − a)/N = N1 . Also, let x k∗ = 0 + (k − 12 )x = 2k−1 2N , k = 1, 2, . . . N be the midpoints of the N subintervals of [0, 1]. Then SOLUTION

N N 1 # 2k − 1 1 # = (2k − 1) N k=1 2N 2N 2 k=1 k=1 N # 1 1 N 1 1 N2 = k−N = 2 + − = . 2 2 2 2 2N 2 2N N k=1

M N = x

N #

f (x k∗ ) =

The area under the curve over [0, 1] is lim M N =

N →∞

1 . 2

The region under the curve over [0, 1] is a triangle with base and height 1, and thus area 12 , which agrees with the answer obtained from the limit of the midpoint approximations. In Exercises 60–69, ﬁnd a formula for R N for the given function and interval. Then compute the area under the graph as a limit. 61. f (x) = x 3 , 2[0, 1] f (x) = x , [0, 1] SOLUTION

1−0 1 Let f (x) = x 3 on the interval [0, 1]. Then x = = and a = 0. Hence, N N N N 1 # 1 1 # 3 f (0 + jx) = j3 j = R N = x N j=1 N3 N 4 j=1 j=1 N4 1 N3 N2 1 1 1 = 4 + + = + + 4 2 4 4 2N N 4N 2 N #

and

1 1 1 + + 2N N →∞ 4 4N 2

lim R N = lim

N →∞

1] 63. f (x) = 1 − x33 , [0, f (x) = x + 2x 2 , [0, 3] SOLUTION

Let f (x) = 1 − x 3 on the interval [0, 1]. Then x = R N = x

N # j=1

f (0 + jx) =

N 1 1 # 1 − j3 3 N j=1 N

=

1 . 4

1−0 1 = and a = 0. Hence, N N

Approximating and Computing Area

S E C T I O N 5.1

N N 1 # 1 # 1 1 = 1− 4 j3 = N − 4 N j=1 N N j=1 N

and

N4 N3 N2 + + 4 2 4

1 1 3 − − 2N N →∞ 4 4N 2

lim R N = lim

N →∞

=

=1−

1 1 1 − − 4 2N 4N 2

3 . 4

65. f (x) = 3x 2 −2x + 4, [1, 5] f (x) = 3x − x + 4, [0, 1] 5−1 4 SOLUTION Let f (x) = 3x 2 − x + 4 on the interval [1, 5]. Then x = = and a = 1. Hence, N N N N N N N # 4 # 20 80 # 24 # 48 192 # f (1 + jx) = j2 + 2 j+ 1 R N = x +6 = 3 j2 2 + j N j=1 N N j=1 N N j=1 N j=1 j=1 192 = 3 N

N3 N2 N + + 3 2 6

80 + 2 N

and

N2 N + 2 2

+

40 96 32 24 N = 64 + + 2 + 40 + + 24 N N N N

136 32 128 + + 2 = 128. N N →∞ N

lim R N = lim

N →∞

2 , [2, 4] 67. f (x)f (x) = x= 2x + 7, [3, 6] 4−2 2 SOLUTION Let f (x) = x 2 on the interval [2, 4]. Then x = N = N and a = 2. Hence, N N N N N # 2 # 16 # 8 # 4 8 8 # 2 R N = x f (2 + jx) = 1 + j + j2 4+ j + j = N j=1 N N j=1 N2 N 2 j=1 N 3 j=1 j=1

8 16 = N+ 2 N N

N2 N + 2 2

8 + 3 N

N3 N2 N + + 3 2 6

and

56 12 4 + + N N →∞ 3 3N 2

lim R N = lim

N →∞

8 8 4 4 + + + N 3 N 3N 2

=8+8+

=

56 . 3

2 , [a, b] (a, b constants with a < b) 69. f (x)f (x) = x= 2x + 1, [a, b] (a, b constants with a < b)

b−a Let f (x) = x 2 on the interval [a, b]. Then x = . Hence, N N N # (b − a) # (b − a) (b − a)2 2 2 a + 2a j f (a + jx) = R N = x +j N N N2 j=1 j=1

SOLUTION

=

N N N a 2 (b − a) # 2a(b − a)2 # (b − a)3 # 1+ j + j2 2 3 N N N j=1 j=1 j=1

a 2 (b − a) 2a(b − a)2 = N+ N N2 = a 2 (b − a) + a(b − a)2 + and

lim R N = lim

N →∞

N →∞

N N2 + 2 2

N2 N N3 + + 3 2 6

a(b − a)2 (b − a)3 (b − a)3 (b − a)3 + + + N 3 2N 6N 2

(b − a)3 1 1 = b3 − a 3 . 3 3 3

In Exercises 70–73, describe the area represented by the limits.

lim

(b − a)3 + N3

a(b − a)2 (b − a)3 (b − a)3 (b − a)3 a 2 (b − a) + a(b − a)2 + + + + N 3 2N 6N 2

= a 2 (b − a) + a(b − a)2 +

N 4 1 # j

245

246

CHAPTER 5

THE INTEGRAL

71.

N 3 # 3j 4 2+ N N →∞ N j=1 lim

SOLUTION

The limit N 3 # 3 4 2+ j · N N →∞ N j=1

lim R N = lim

N →∞

represents the area between the graph of f (x) = x 4 and the x-axis over the interval [2, 5]. N jπ π # π −1 73. lim sin + j 4 5 N# 3 + 2N N →∞ lim2N −2 5 N N →∞ Nj=1 j=0 SOLUTION The limit N π # π jπ sin + 3 2N N →∞ 2N j=1 lim

represents the area between the graph of f (x) = sin x and the x axis over the interval [ π3 , 56π ]. In Exercises 75–80, use the approximation 2indicated (in summation notation) to express the area under the graph as a N j 1 # limit butEvaluate do not evaluate. lim 1− by interpreting it as the area of part of a familiar geometric ﬁgure. N N →∞ N j=1 [0, π ] 75. R N , f (x) = sin x over SOLUTION

Let f (x) = sin x over [0, π ] and set a = 0, b = π , and x = (b − a) /N = π /N . Then R N = x

N #

f (x k ) =

k=1

N kπ π # sin . N k=1 N

Hence N kπ π # sin N N →∞ N k=1

lim R N = lim

N →∞

is the area between the graph of f (x) = sin x and the x-axis over [0, π ]. 77. M N , f (x) = tan x−1 over [ 1 , 1] R N , f (x) = x over2[1, 7] SOLUTION

1− 1

1 and a = 1 . Hence Let f (x) = tan x over the interval [ 12 , 1]. Then x = N 2 = 2N 2 N N # 1 1 1 1 # 1 1 M N = x f tan + j− x = + j− 2 2 2N j=1 2 2N 2 j=1

and so N 1 1 1 1 # tan + j− 2 2N 2 N →∞ 2N j=1

lim M N = lim

N →∞

is the area between the graph of f (x) = tan x and the x-axis over [ 12 , 1]. 79. L N , f (x) = cos x over [ π8 , π ] M N , f (x) = x −2 over [3, 5] SOLUTION

π − π8 7π Let f (x) = cos x over the interval π8 , π . Then x = = and a = π8 . Hence, N 8N N −1

−1 # π π 7π N# 7π f cos + jx = +j L N = x 8 8N j=0 8 8N j=0

and −1 7π N# π 7π cos +j 8 8N N →∞ 8N j=0

lim L N = lim

N →∞

is the area between the graph of f (x) = cos x and the x-axis over [ π8 , π ]. LN,

f (x) = cos x over [ π8 , π4 ]

S E C T I O N 5.1

Approximating and Computing Area

247

In Exercises 81–83, let f (x) = x 2 and let R N , L N , and M N be the approximations for the interval [0, 1]. 1 1 1 1 1 . Interpret the quantity as the area of a region. + + + 2 3 2N 2N 6N 6N 2 1 Let f (x) = x 2 on [0, 1]. Let N > 0 be an integer and set a = 0, b = 1 and x = 1−0 N = N . Then N N # 1 # 1 1 N2 N 1 1 1 N3 2 R N = x f (0 + jx) = j = 3 . + + = + + 2 N 3 2 6 3 2N N N 6N 2 j=1 j=1

Show that R N =

81. SOLUTION

The quantity 6 1 + 2 2N N

in

RN =

1 1 1 + + 3 2N 6N 2

represents the collective area of the parts of the rectangles that lie above the graph of f (x). It is the error between R N and the true area A = 13 . y 1 0.8 0.6 0.4 0.2 x 0.2

0.4

0.6

0.8

1

83. For each of R N , L N , and M N , ﬁnd the smallest integer N for which the error is less than 0.001. Show that SOLUTION

1

1

1

L Nwhen: = − , + • For R N , the error is less than .001 3 2N 6N 2

MN =

1 1 − 3 12N 2

1 1 Then rank the three approximations R N , L N , and M+N in order of increasing accuracy (use the formula for R N in < .001. 2N 6N 2 Exercise 81). We ﬁnd an adequate solution in N : 1 1 < .001 + 2N 6N 2 3N + 1 < .006(N 2 ) 0 < .006N 2 − 3N − 1,

√ 9.024 = 500.333. Hence R in particular, if N > 3+ .012 501 is within .001 of A. • For L N , the error is less than .001 if

1 1 − + < .001. 2N 6N 2

We solve this equation for N :

1 1 < .001 − 2N 6N 2 3N − 1 6N 2 < .001 3N − 1 < .006N 2 0 < .006N 2 − 3N + 1,

√

9−.024 = 499.666. Therefore, L 500 is within .001 units of A. which is satisﬁed if N > 3+ .012 1 • For M N , the error is given by − 2 , so the error is less than .001 if 12N

1 < .001 12N 2 1000 < 12N 2 9.13 < N Therefore, M10 is within .001 units of the correct answer.

248

CHAPTER 5

THE INTEGRAL

Further Insights and Challenges 85. Draw the graph of a positive continuous function on an interval such that R2 and L 2 are both smaller than the exact Although the accuracy of R N generally improves as N increases, this need not be true for small values of N . area under the graph. Can such a function be monotonic? Draw the graph of a positive continuous function f (x) on an interval such that R1 is closer than R2 to the exact area SOLUTION the plot area under the saw-tooth function f (x) is 3, whereas L 2 = R2 = 2. Thus L 2 and R2 under the In graph. Canbelow, such athe function be monotonic? are both smaller than the exact area. Such a function cannot be monotonic; if f (x) is increasing, then L N underestimates and R N overestimates the area for all N , and, if f (x) is decreasing, then L N overestimates and R N underestimates the area for all N . Left/right-endpoint approximation, n = 2 2

1

1

2

Assume f (x) isstatement monotonic. Prove thatThe M Nendpoint lies between R N and Lare M N iswhen closerf to 87. N and Explain thethat following graphically: approximations lessthat accurate (x)the is actual area under the graph than both R N and L N . Hint: Argue from Figure 18; the part of the error in R N due to the ith large. rectangle is the sum of the areas A + B + D, and for M N it is |B − E|. A B C

D E F

x i − 1 midpoint x i

x

FIGURE 18 SOLUTION

Suppose f (x) is monotonic increasing on the interval [a, b], x =

b−a , N

N = {a, a + x, a + 2x, . . . , a + (N − 1)x, b} {x k }k=0

and ' ∗ ( N −1 xk k=0 =

(a + (N − 1)x) + b a + (a + x) (a + x) + (a + 2x) , ,..., . 2 2 2

Note that xi < xi∗ < xi+1 implies f (xi ) < f (xi∗ ) < f (xi+1 ) for all 0 ≤ i < N because f (x) is monotone increasing. Then −1 −1 N b − a N# b − a N# b−a # ∗ f (x k ) < M N = f (xk ) < R N = f (xk ) LN = N k=0 N k=0 N k=1 Similarly, if f (x) is monotone decreasing, −1 −1 N b − a N# b − a N# b−a # ∗ LN = f (x k ) > M N = f (xk ) > R N = f (xk ) N k=0 N k=0 N k=1 Thus, if f (x) is monotonic, then M N always lies in between R N and L N . Now, as in Figure 18, consider the typical subinterval [xi−1 , xi ] and its midpoint xi∗ . We let A, B, C, D, E, and F be the areas as shown in Figure 18. Note that, by the fact that xi∗ is the midpoint of the interval, A = D + E and F = B + C. Let E R represent the right endpoint approximation error ( = A + B + D), let E L represent the left endpoint approximation error ( = C + F + E) and let E M represent the midpoint approximation error ( = |B − E|). • If B > E, then E M = B − E. In this case,

E R − E M = A + B + D − (B − E) = A + D + E > 0, so E R > E M , while E L − E M = C + F + E − (B − E) = C + (B + C) + E − (B − E) = 2C + 2E > 0, so E L > E M . Therefore, the midpoint approximation is more accurate than either the left or the right endpoint approximation.

Approximating and Computing Area

S E C T I O N 5.1

249

• If B < E, then E M = E − B. In this case,

E R − E M = A + B + D − (E − B) = D + E + D − (E − B) = 2D + B > 0, so that E R > E M while E L − E M = C + F + E − (E − B) = C + F + B > 0, so E L > E M . Therefore, the midpoint approximation is more accurate than either the right or the left endpoint approximation. • If B = E, the midpoint approximation is exactly equal to the area. Hence, for B < E, B > E, or B = E, the midpoint approximation is more accurate than either the left endpoint or the right endpoint approximation. thisfor exercise, we prove that lim R N and lim L N exist and are equal if f (x) is positive and ProveInthat any function f (x) onthe [a,limits b], N →∞ N →∞ increasing [the case of f (x) decreasing is similar]. We usebthe − aconcept of a least upper bound discussed in Appendix B. = 1. ( f (b) − f (a)). N − N ≥ (a) Explain with a graph why L N ≤ R M forRall N ,LM N (b) By part (a), the sequence {L N } is bounded by R M for any M, so it has a least upper bound L. By deﬁnition, L is the smallest number such that L N ≤ L for all N . Show that L ≤ R M for all M. (c) According to part (b), L N ≤ L ≤ R N for all N . Use Eq. (8) to show that lim L N = L and lim R N = L. 89.

N →∞

N →∞

SOLUTION

(a) Let f (x) be positive and increasing, and let N and M be positive integers. From the ﬁgure below at the left, we see that L N underestimates the area under the graph of y = f (x), while from the ﬁgure below at the right, we see that R M overestimates the area under the graph. Thus, for all N , M ≥ 1, L N ≤ R M . y

y

x

x

(b) Because the sequence {L N } is bounded above by R M for any M, each R M is an upper bound for the sequence. Furthermore, the sequence {L N } must have a least upper bound, call it L. By deﬁnition, the least upper bound must be no greater than any other upper bound; consequently, L ≤ R M for all M. (c) Since L N ≤ L ≤ R N , R N − L ≤ R N − L N , so |R N − L| ≤ |R N − L N |. From this, lim |R N − L| ≤ lim |R N − L N |.

N →∞

N →∞

By Eq. (8), lim |R N − L N | = lim

N →∞

1

N →∞ N

|(b − a)( f (b) − f (a))| = 0,

so lim |R N − L| ≤ |R N − L N | = 0, hence lim R N = L. N →∞

N →∞

Similarly, |L N − L| = L − L N ≤ R N − L N , so

|L N − L| ≤ |R N − L N | =

(b − a) ( f (b) − f (a)). N

This gives us that lim |L N − L| ≤ lim

N →∞

1

N →∞ N

|(b − a)( f (b) − f (a))| = 0,

so lim L N = L. N →∞

This proves lim L N = lim R N = L . N →∞

N →∞

− the A| <area 10−4 for its thegraph given over function andUse interval. In Exercises 91–92,that use fEq. to ﬁnd aand value of N suchand thatlet |RA N be Assume (x)(9) is positive monotonic, under [a, b]. Eq. (8) to √ show that 91. f (x) = x, [1, 4] √ b and −a SOLUTION Let f (x) = x on [1, 4]. Then b = 4, a = 1, |R N − A| ≤ | f (b) − f (a)| N 4−1 3 3 |R N − A| ≤ ( f (4) − f (1)) = (2 − 1) = . N N N √ We need N3 < 10−4 , which gives N > 30000. Thus |R30001 − A| < 10−4 for f (x) = x on [1, 4]. f (x) =

9 − x 2,

[0, 3]

250

CHAPTER 5

THE INTEGRAL

5.2 The Definite Integral Preliminary Questions " b

1. What is a

d x [here the function is f (x) = 1]?

" b

SOLUTION

a

dx =

" b a

1 · d x = 1(b − a) = b − a.

2. Are the following statements true or false [assume that f (x) is continuous]? " b f (x) d x is the area between the graph and the x-axis over [a, b]. (a) a " b (b) f (x) d x is the area between the graph and the x-axis over [a, b] if f (x) ≥ 0. a " b f (x) d x is the area between the graph of f (x) and the x-axis over [a, b]. (c) If f (x) ≤ 0, then − a

SOLUTION

! (a) False. ab f (x) d x is the signed area between the graph and the x-axis. (b) True. (c) True. " π cos x d x = 0. 3. Explain graphically why 0

Because cos(π − x) = − cos x, the “negative” area between the graph of y = cos x and the x-axis over [ π2 , π ] exactly cancels the “positive” area between the graph and the x-axis over [0, π2 ]. " −1 8 d x negative? 4. Is SOLUTION

−5

No, the integrand is the positive constant 8, so the value of the integral is 8 times the length of the integration interval (−1 − (−5) = 4), or 32. " 6 5. What is the largest possible value of f (x) d x if f (x) ≤ 13 ? SOLUTION

SOLUTION

Because f (x) ≤ 13 ,

" 6 0

0

f (x) d x ≤

1 (6 − 0) = 2. 3

Exercises In Exercises 1–10, draw a graph of the signed area represented by the integral and compute it using geometry. " 3 1.

−3

2x d x

The region bounded by the graph of y = 2x and the x-axis over the interval [−3, 3] consists of two right triangles. One has area 12 (3)(6) = 9 below the axis, and the other has area 12 (3)(6) = 9 above the axis. Hence, SOLUTION

" 3 −3

2x d x = 9 − 9 = 0. y 6 4 2

−3

3.

" 1 " 3 (3x + 4) d x (2x + 4) d x −2 −2

−2

−1 −2 −4 −6

x 1

2

3

S E C T I O N 5.2

The Deﬁnite Integral

251

SOLUTION The region bounded by the graph of y = 3x + 4 and the x-axis over the interval [−2, 1] consists of two right triangles. One has area 12 ( 23 )(2) = 23 below the axis, and the other has area 12 ( 73 )(7) = 49 6 above the axis. Hence,

" 1 −2

49 2 15 − = . 6 3 2

(3x + 4) d x =

y 8 6 4 2 −2

x

−1

1

−2

" 8 " 1 5. (7 − x) d x 4 dx 6 −2

The region bounded by the graph of y = 7 − x and the x-axis over the interval [6, 8] consists of two right triangles. One triangle has area 12 (1)(1) = 12 above the axis, and the other has area 12 (1)(1) = 12 below the axis. Hence, SOLUTION

" 8 6

(7 − x) d x =

1 1 − = 0. 2 2

y 1 0.5 x 2

−0.5

4

6

8

−1

7.

" 5 " 3π /2 2 25 − x d x sin x d x 0

The region bounded by the graph of y = 25 − x 2 and the x-axis over the interval [0, 5] is one-quarter of a circle of radius 5. Hence, " 5 1 25π 25 − x 2 d x = π (5)2 = . 4 4 0 π /2

SOLUTION

y 5 4 3 2 1 x 1

9.

2

3

4

5

" 2 " 3 (2 − |x|) d x |x| d x −2 −2

The region bounded by the graph of y = 2 − |x| and the x-axis over the interval [−2, 2] is a triangle above the axis with base 4 and height 2. Consequently, " 2 1 (2 − |x|) d x = (2)(4) = 4. 2 −2 SOLUTION

y 2 1

−2

" 1

(2x − |x|) d x

−1

x 1

2

252

CHAPTER 5

THE INTEGRAL

" 6 11. Calculate 0

(4 − x) d x in two ways:

(a) As the limit lim R N N →∞

(b) By sketching the relevant signed area and using geometry ! ! Let f (x) = 4 − x over [0, 6]. Consider the integral 06 f (x) d x = 06 (4 − x) d x. (a) Let N be a positive integer and set a = 0, b = 6, x = (b − a) /N = 6/N . Also, let x k = a + kx = 6k/N , k = 1, 2, . . . , N be the right endpoints of the N subintervals of [0, 6]. Then N N N N # # 6 # 6 # 6 6k f (x k ) = 1 − k = 4 4− R N = x N k=1 N N N k=1 k=1 k=1 6 6 N2 N 18 = 4N − + =6− . N N 2 2 N SOLUTION

18 = 6. N N →∞ N →∞ (b) The region bounded by the graph of y = 4 − x and the x-axis over the interval [0, 6] consists of two right triangles. One triangle has area 12 (4)(4) = 8 above the axis, and the other has area 12 (2)(2) = 2 below the axis. Hence, Hence lim R N = lim

6−

" 6 0

(4 − x) d x = 8 − 2 = 6.

y 4 2 x 1

2

3

4

5

6

−2

13. Evaluate the integrals for f (x) shown in Figure 13. " 5 " " 2 Calculate (2x + 1) d x in two ways: As the limit lim R 6N and using geometry. f (x) d x 2 (b) f (x) d x (a) N →∞ 0 0 " 6 " 4 f (x) d x (d) | f (x)| d x (c) 1

1

y

y = f(x)

x 2

4

6

FIGURE 13 The two parts of the graph are semicircles.

Let f (x) be given by Figure 13. ! (a) The deﬁnite integral 02 f (x) d x is the signed area of a semicircle of radius 1 which lies below the x-axis. Therefore,

SOLUTION

" 2 0

1 π f (x) d x = − π (1)2 = − . 2 2

! (b) The deﬁnite integral 06 f (x) d x is the signed area of a semicircle of radius 1 which lies below the x-axis and a semicircle of radius 2 which lies above the x-axis. Therefore, " 6 1 1 3π f (x) d x = π (2)2 − π (1)2 = . 2 2 2 0 ! (c) The deﬁnite integral 14 f (x) d x is the signed area of one-quarter of a circle of radius 1 which lies below the x-axis and one-quarter of a circle of radius 2 which lies above the x-axis. Therefore, " 4 1 1 3 f (x) d x = π (2)2 − π (1)2 = π . 4 4 4 1

S E C T I O N 5.2

The Deﬁnite Integral

253

! (d) The deﬁnite integral 16 | f (x)| d x is the signed area of one-quarter of a circle of radius 1 and a semicircle of radius 2, both of which lie above the x-axis. Therefore, " 6 1 9π 1 | f (x)| d x = π (2)2 + π (1)2 = . 2 4 4 1 In Exercises 14–15, refer to Figure 14. y

y = g (t)

2 1

t 1

−1

2

3

4

5

−2

FIGURE 14

" a " c " c3 such that " g(t) 5 dt and 15. Find a, b, and g(t) dt are as large as possible. Evaluate g(t) dt and0 g(t) dt. b 0 3" a SOLUTION To make the value of g(t) dt as large as possible, we want to include as much positive area as possible. 0 " c g(t) dt as large as possibe, we want to make sure to This happens when we take a = 4. Now, to make the value of b

include all of the positive area and only the positive area. This happens when we take b = 1 and c = 4. In Exercises 17–20, the Psigned area by the integral. the regions of positive and negative area. Describe thesketch partition and the setrepresented of intermediate points C forIndicate the Riemann sum shown in Figure 15. Compute " the2 value of the Riemann sum. 17. (x − x 2 ) d x 0

SOLUTION

! Here is a sketch of the signed area represented by the integral 02 (x − x 2 ) d x. y 0.5 +

x 1

−0.5

−

2

−1 −1.5 −2

19.

" 2π " 3 sin x d x 2 (2x − x ) d x π 0

SOLUTION

! Here is a sketch of the signed area represented by the integral π2π sin x d x. y 0.4 x −0.4 −0.8

1

2

3

4

5

6

7

−

−1.2

" 3π 21–24, determine the sign of the integral without calculating it. Draw a graph if necessary. In Exercises sin x d x " 1 21.

04

−2

x dx

SOLUTION The integrand is always positive. The integral must therefore be positive, since the signed area has only positive part. " " 1 2π x sin x d x 23. x03 d x

−2

As you can see from the graph below, the area below the axis is greater than the area above the axis. Thus, the deﬁnite integral is negative. SOLUTION

254

CHAPTER 5

THE INTEGRAL y 0.2

− 0.2

+ 1

x

2

3

4

5

6

7

−

− 0.4 − 0.6

" 225–28, In Exercises π sin xcalculate the Riemann sum R( f, P, C) for the given function, partition, and choice of intermediate points. Also, sketch the d xgraph of f and the rectangles corresponding to R( f, P, C). x 0 25. f (x) = x, P = {1, 1.2, 1.5, 2}, C = {1.1, 1.4, 1.9} SOLUTION

Let f (x) = x. With P = {x 0 = 1, x 1 = 1.2, x3 = 1.5, x 4 = 2}

C = {c1 = 1.1, c2 = 1.4, c3 = 1.9},

and

we get R( f, P, C) = x1 f (c1 ) + x 2 f (c2 ) + x 3 f (c3 ) = (1.2 − 1)(1.1) + (1.5 − 1.2)(1.4) + (2 − 1.5)(1.9) = 1.59. Here is a sketch of the graph of f and the rectangles. y 2 1.5 1 0.5 x 0.5

1

1.5

2

2.5

27. f (x) = x + 1, P = {−2, −1.6, −1.2, −0.8, −0.4, 0}, (x) = −1.3, x 2 + x, P −0.5, = {2, 3, C = f{−1.7, −0.9, 0} 4.5, 5}, C = {2, 3.5, 5} SOLUTION

Let f (x) = x + 1. With P = {x 0 = −2, x1 = −1.6, x3 = −1.2, x4 = −.8, x5 = −.4, x 6 = 0}

and C = {c1 = −1.7, c2 = −1.3, c3 = −.9, c4 = −.5, c5 = 0}, we get R( f, P, C) = x 1 f (c1 ) + x 2 f (c2 ) + x 3 f (c3 ) + x 4 f (c4 ) + x 5 f (c5 ) = (−1.6 − (−2))(−.7) + (−1.2 − (−1.6))(−.3) + (−0.8 − (−1.2))(.1) + (−0.4 − (−0.8))(.5) + (0 − (−0.4))(1) = .24. Here is a sketch of the graph of f and the rectangles. y 1 0.5 −2

−1

x − 0.5 −1

In Exercises the=basic integral f (x) 29–36, = sin x,use P {0, π6properties , π3 , π2 }, of C the = {0.4, 0.7,and 1.2}the formulas in the summary to calculate the integrals. " 4 x2 dx 29. 0

" 4

SOLUTION

By formula (4), 0

" 4

x2 dx

x2 dx =

1 3 64 (4) = . 3 3

The Deﬁnite Integral

S E C T I O N 5.2

" 3 31. 0

255

(3t + 4) dt " 3

SOLUTION

0

(3t + 4) dt = 3

" 3 0

" 1 " 32 (u − 2u) du 33. (3x + 4) d x 0

t dt + 4

" 3 0

1 51 . (3)2 + 4(3 − 0) = 2 2

1 dt = 3 ·

−2

SOLUTION

" 1 0

(u 2 − 2u) du =

" 1 0

u 2 du − 2

" 1 0

u du =

1 2 1 3 1 (1) − 2 (1)2 = − 1 = − . 3 2 3 3

" 1 " 32 (x + 2x) d x −a (6y + 7y + 1) d y 0 !b !b !b 1 1 SOLUTION First, 0 (x 2 + x) d x = 0 x 2 d x + 0 x d x = 3 b3 + 2 b2 . Therefore " 1 " 0 " 1 " 1 " −a (x 2 + x) d x = (x 2 + x) d x + (x 2 + x) d x = (x 2 + x) d x − (x 2 + x) d x

35.

−a

−a

=

0

1 3 1 2 ·1 + ·1 − 3 2

0

1 1 (−a)3 + (−a)2 3 2

0

=

1 3 1 2 5 a − a + . 3 2 6

37. Prove computing the limit of right-endpoint approximations: " aby 2 " b x2 dx b4 a x3 dx = . 4 0

7

Let f (x) = x 3 , a = 0 and x = (b − a)/N = b/N . Then

SOLUTION

N N N3 N2 b4 b4 b # b3 b4 # b4 N 4 b4 3 3 + + + + R N = x f (xk ) = k . k · 3 = 4 = 4 = N k=1 4 2 4 4 2N N N N 4N 2 k=1 k=1 " b b4 b4 b4 b4 Hence x 3 d x = lim R N = lim + + . = 2 2N 4 N →∞ N →∞ 4 4N 0 N #

In Exercises 38–45, use the formulas in the summary and Eq. (7) to evaluate the integral. 39.

" 2 " 32 (x +2 2x) d x x dx 0 0

Applying the linearity of the deﬁnite integral and the formulas from Examples 4 and 5,

SOLUTION

" 2 0

41.

(x 2 + 2x) d x =

0

x2 dx + 2

" 2 0

x dx =

1 3 1 20 (2) + 2 · (2)2 = . 3 2 3

" 2 " 3 3 (x − 3x ) d x x dx 0 0

Applying the linearity of the deﬁnite integral, the formula from Example 5 and Eq. (7):

SOLUTION

" 2 0

43.

" 2

(x − x 3 ) d x =

" 2 0

x dx −

" 2 0

x3 dx =

1 2 1 4 (2) − (2) = −2. 2 4

" 0 " 1 (2x − 35) d x −3 (2x − x + 4) d x 0

Applying the linearity of the deﬁnite integral, reversing the limits of integration, and using the formulas for the integral of x and of a constant: SOLUTION

" 0 −3

" 3 1

(2x − 5) d x = 2

x3 dx

" 0 −3

x dx −

" 0 −3

5 d x = −2

" −3 0

x dx −

" 0

1 5 d x = −2 · (−3)2 − 15 = −24. 2 −3

256

CHAPTER 5

THE INTEGRAL

" 2 45. 1

(x − x 3 ) d x

SOLUTION

Applying the linearity and the additivity of the deﬁnite integral: " 2 1

(x − x 3 ) d x =

" 2

=

1

x dx −

" 2 1

x3 dx =

1 2 1 (2 ) − (12 ) − 2 2

" 2 0

x dx −

1 4 1 4 (2) − (1) 4 4

In Exercises 46–50, calculate the integral, assuming that " 5 f (x) d x = 5,

" 5

0

0

" 1 0

=

" x dx −

0

2

x3 dx −

" 1

x3 dx

0

9 3 15 − =− . 2 4 4

g(x) d x = 12

" 5 " 5 ( f (x) + 4g(x)) d x ( f (x) + g(x)) d x 0 0 " 5 " 5 " 5 SOLUTION ( f (x) + 4g(x)) d x = f (x) d x + 4 g(x) d x = 5 + 4(12) = 53.

47.

0

0

0

" 5 " 0 49. (3 f (x) − 5g(x)) d x g(x) d x 0 5 " 5 " 5 " 5 SOLUTION (3 f (x) − 5g(x)) d x = 3 f (x) d x − 5 g(x) d x = 3(5) − 5(12) = −45. 0

0

0

In Exercises 51–54, calculate the"integral, assuming that 5 Is it possible to calculate g(x) f (x) d x from " " the information given? " 1

0

" 4 51. 0

2

0

f (x) d x = 1,

0

f (x) d x = 4,

4

1

f (x) d x = 7

f (x) d x " 4

SOLUTION

0

f (x) d x =

" 1 0

f (x) d x +

" 4 1

f (x) d x = 1 + 7 = 8.

" 1 " 2 f (x) d x f (x) d x 4 1 " 1 " 4 SOLUTION f (x) d x = − f (x) d x = −7.

53.

4

1

" 4 55–58, express each integral as a single integral. In Exercises f (x) d x" 7 " 3 2 f (x) d x + f (x) d x 55. 0

" 3

SOLUTION

0

3

f (x) d x +

" 7 3

f (x) d x =

" 7 0

f (x) d x.

" 5 " 9 " 9 " 9 f (x) d x − f (x) d x f (x) d x − f (x) d x 2 2 " " " " 5 " 9 " 9 2 4 9 5 5 SOLUTION f (x) d x − f (x) d x = f (x) d x + f (x) d x − f (x) d x = f (x) d x.

57.

2

2

2

5

2

5

" b " 3 " 9 In Exercises f59–62, calculate the integral, assuming that f is an integrable function such that f (x) d x = 1 − b−1 (x) d x + f (x) d x 7 0. for all b > " 3 59. f (x) d x 1

" 3

SOLUTION

1

61.

f (x) d x = 1 − 3−1 =

" 4 " 4 (4 f (x) − 2) d x f (x) d x 1 2

1

3

2 . 3

The Deﬁnite Integral

S E C T I O N 5.2

" 4 SOLUTION

1

(4 f (x) − 2) d x = 4

" 4

f (x) d x − 2

1

" 4 1

257

1 d x = 4(1 − 4−1 ) − 2(4 − 1) = −3.

" b " b " 1 Explain the difference in graphical interpretation between f (x) d x and | f (x)| d x. f (x) d x a a 1/2 !b SOLUTION When f (x) takes on both positive and negative values on [a, b], a f (x) d x represents the signed area !b between f (x) and the x-axis, whereas a | f (x)| d x represents the total (unsigned) area between f (x) and the x-axis. ! ! Any negatively signed areas that were part of ab f (x) d x are regarded as positive areas in ab | f (x)| d x. Here is a graphical example of this phenomenon. 63.

Graph of | f (x)|

Graph of f (x) 10 −4

−2

30 x 2

−10

20

4

10

−20 −30

−4

x

−2

2

4

In Exercises 65–68," calculate the integral. " 2π 2π sin2 x d x and J = cos2 x d x. Use the following trick to prove that I = J = π : First show " 6 Let I = 0 0 " 2π 65. |3 − x| d x 0 a graph that I = J and then prove I + J = with d x. 0 SOLUTION Over the interval, the region between the curve and the interval [0, 6] consists of two triangles above the x axis, each of which has height 3 and width 3, and so area 92 . The total area, hence the deﬁnite integral, is 9. y 3 2 1 x 1

2

3

4

5

6

Alternately, " 6 0

|3 − x| d x =

" 3

=3

0

(3 − x) d x +

" 3 0

dx −

" 3 0

" 6 3

(x − 3) d x

x dx +

" 6 0

x dx −

" 3 x dx 0

−3

" 6 dx 3

1 1 1 = 9 − 32 + 62 − 32 − 9 = 9. 2 2 2 67.

" 1 " 33 |x | d x |2x − 4| d x −1 1

SOLUTION

|x 3 | =

x3 −x 3

x ≥0 x < 0.

Therefore, " 1 −1

|x 3 | d x =

" 0 −1

−x 3 d x +

" 1 0

x3 dx =

" −1 0

69. Use"the Comparison Theorem to show that 2 " 1 " 1 |x 2 − 1| d x 5 0 x dx ≤ x 4 d x, 0

0

x3 dx +

" 2 1

" 1 0

x3 dx =

x4 dx ≤

" 2 1

1 1 1 (−1)4 + (1)4 = . 4 4 2

x5 dx

258

CHAPTER 5

THE INTEGRAL SOLUTION

On the interval [0, 1], x 5 ≤ x 4 , so, by Theorem 5, " 1 0

x5 dx ≤

" 1

x 4 d x.

0

On the other hand, x 4 ≤ x 5 for x ∈ [1, 2], so, by the same Theorem, " 2 " 2 x4 dx ≤ x 5 d x. 1

1

" " 0.3 71. Prove that 0.0198 1 ≤ 6 1 sin x d1x ≤ 0.0296. Hint: Show that 0.198 ≤ sin x ≤ 0.296 for x in [0.2, 0.3]. Prove that ≤ 0.2 d x ≤ . 3 2 4 x π d SOLUTION For 0 ≤ x ≤ 6 ≈ 0.52, we have d x (sin x) = cos x > 0. Hence sin x is increasing on [0.2, 0.3]. Accordingly, for 0.2 ≤ x ≤ 0.3, we have m = 0.198 ≤ 0.19867 ≈ sin 0.2 ≤ sin x ≤ sin 0.3 ≈ 0.29552 ≤ 0.296 = M Therefore, by the Comparison Theorem, we have " 0.3 " 0.3 " 0.3 m dx ≤ sin x d x ≤ M d x = M(0.3 − 0.2) = 0.0296. 0.0198 = m(0.3 − 0.2) = 0.2

Prove that

73.

Prove that 0.277 ≤

Hint: Graph y = SOLUTION

" π /4 π /8

0.2

0.2

√ sin x 2 dx ≤ x 2 π /4

cos x d x ≤ 0.363. " π /2

sin x and observe that it is decreasing on [ π4 , π2 ]. x

Let f (x) =

sin x . x

As we can see in the sketch √ below, f (x) is decreasing on the interval [π /4, π /2]. Therefore f (x) ≤ f (π /4) for all x in [π /4, π /2]. f (π /4) = 2 π 2 , so:

√ √ " π /2 " π /2 √ sin x 2 2 π2 2 2 dx = = dx ≤ . x π 4 π 2 π /4 π /4 y y = sin x

2 2/p

x

2/p

p /4

p/2

x

" b " b " Suppose that f (x) ≤ g(x) on 1[a, b]. f (x) d x ≤ g(x) d x. Is it also d xBy the Comparison Theorem, Find upper and lower bounds for . a a 3 0 give x a+counterexample. 4 true that f (x) ≤ g (x) for x ∈ [a, b]? If not,

75.

The assertion f (x) ≤ g (x) is false. Consider a = 0, b = 1, f (x) = x, g(x) = 2. f (x) ≤ g(x) for all x in the interval [0, 1], but f (x) = 1 while g (x) = 0 for all x. SOLUTION

whether trueChallenges or false. If false, sketch the graph of a counterexample. Further State Insights and "" ab

(x)ddxx=>00.if f (x) is an odd function. (a) If f (x) > 0, then 77. Explain graphically: f f(x) a −a " b SOLUTION is danx odd then f (−x) = − f (x) for all x. Accordingly, for every positively signed area in the (b) If Iff f(x) > 0,function, then f (x) > 0. a where f is above the x-axis, there is a corresponding negatively signed area in the left half-plane where right half-plane f is below the x-axis. Similarly, for every negatively signed area in the right half-plane where f is below the x-axis, there is a corresponding positively signed area in the left half-plane where f is above the x-axis. We conclude that the net area between the graph of f and the x-axis over [−a, a] is 0, since the positively signed areas and negatively signed areas cancel each other out exactly.

The Fundamental Theorem of Calculus, Part I

S E C T I O N 5.3

259

y 4 2 −1 −2

x 1

−2

2

−4

79. Let k and b be" positive. Show, by comparing the right-endpoint approximations, that 1

Compute

" 1 sin(sin(x))(sin2 (x) + 1)" dbx. x k d x = bk+1 xk dx

−1

0

0

! Let k and b be any positive numbers. Let f (x) = x k on [0, b]. Since f is continuous, both 0b f (x) d x ! and 01 f (x) d x exist. Let N be a positive integer and set x = (b − 0) /N = b/N . Let x j = a + jx = bj/N , j = 1, 2, . . . , N be the right endpoints of the N subintervals of [0, b]. Then the right-endpoint approximation to !b k !b 0 f (x) d x = 0 x d x is ⎛ ⎞ N N k N # # 1 b # bj f (x j ) = = bk+1 ⎝ k+1 jk⎠ . R N = x N N N j=1 j=1 j=1 SOLUTION

! ! In particular, if b = 1 above, then the right-endpoint approximation to 01 f (x) d x = 01 x k d x is N N N k # j 1 # 1 # 1 f (x j ) = = k+1 j k = k+1 R N S N = x N N N b j=1 j=1 j=1 In other words, R N = bk+1 S N . Therefore, " b " 1 x k d x = lim R N = lim bk+1 S N = bk+1 lim S N = bk+1 x k d x. N →∞

0

N →∞

N →∞

0

81. Show that Eq. (4) holds for b ≤ 0. Verify by interpreting the integral as an area: SOLUTION Let c = −b. Since b < 0, c > 0, so by Eq. (4), " b 1 1 1 "− cx 2 d x = b1 1 − b2 + θ 2 3 2 2 0 x dx = c . 3 0 Here, 0 ≤ b ≤ 1 and θ is the angle between 0 and π2 such that sin θ = b. Furthermore, x 2 is an even function, so symmetry of the areas gives " 0 −c

x2 dx =

" c

x 2 d x.

0

Finally, " b 0

x2 dx =

" −c 0

x2 dx = −

" 0 −c

x2 dx = −

" c 0

1 1 x 2 d x = − c3 = b3 . 3 3

Theorem 4 remains true without the assumption a ≤ b ≤ c. Verify this for the cases b < a < c and c < a < b.

5.3 The Fundamental Theorem of Calculus, Part I Preliminary Questions 1. Assume that f (x) ≥ 0. What is the area under the graph of f (x) over [0, 2] if f (x) has an antiderivative F(x) such that F(0) = 3 and F(2) = 7? SOLUTION

Because f (x) ≥ 0, the area under the graph of y = f (x) over the interval [0, 2] is " 2 0

f (x) d x = F(2) − F(0) = 7 − 3 = 4.

2. Suppose that F(x) is an antiderivative of f (x). What is the graphical interpretation of F(4) − F(1) if f (x) takes on both positive and negative values?

260

CHAPTER 5

THE INTEGRAL

! Because F(x) is an antiderivative of f (x), it follows that F(4) − F(1) = 14 f (x) d x. Hence, F(4) − F(1) represents the signed area between the graph of y = f (x) and the x-axis over the interval [1, 4]. " 7 " 7 f (x) d x and f (x) d x, assuming that f (x) has an antiderivative F(x) with values from the follow3. Evaluate SOLUTION

0

2

ing table:

SOLUTION

0

2

7

F(x)

3

7

9

Because F(x) is an antiderivative of f (x), " 7 0

and

" 7 2

4. (a) (b) (c)

x

f (x) d x = F(7) − F(0) = 9 − 3 = 6

f (x) d x = F(7) − F(2) = 9 − 7 = 2.

Are the following statements true or false? Explain. The FTC I is only valid for positive functions. To use the FTC I, you have to choose the right antiderivative. If you cannot ﬁnd an antiderivative of f (x), then the deﬁnite integral does not exist.

SOLUTION

(a) False. The FTC I is valid for continuous functions. (b) False. The FTC I works for any antiderivative of the integrand. (c) False. If you cannot ﬁnd an antiderivative of the integrand, you cannot use the FTC I to evaluate the deﬁnite integral, but the deﬁnite integral may still exist. " 9 5. What is the value of f (x) d x if f (x) is differentiable and f (2) = f (9) = 4? 2

SOLUTION

" 9

Because f is differentiable, 2

f (x) d x = f (9) − f (2) = 4 − 4 = 0.

Exercises In Exercises 1–4, sketch the region under the graph of the function and ﬁnd its area using the FTC I. 1. f (x) = x 2 ,

[0, 1]

SOLUTION y 1 0.8 0.6 0.4 0.2 x 0.2

0.4

0.6

0.8

1

We have the area A=

" 1 0

x2 dx =

1 3 1 1 x = . 3 0 3

3. f (x) = sin x, [0, π /2] f (x) = 2x − x 2 , [0, 2]

SOLUTION

y 1 0.8 0.6 0.4 0.2 x 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

S E C T I O N 5.3

The Fundamental Theorem of Calculus, Part I

Let A be the area indicated. Then A=

" π /2 0

π /2 sin x d x = − cos x = 0 − (−1) = 1. 0

In Exercises f (x) 5–34, = cosevaluate x, [0, πthe /2]integral using the FTC I. " 6 x dx 5. 3

" 6

SOLUTION

3

x dx =

" 2 " 92 u du 7. 2 dx −3 0 " 2 SOLUTION

−3

1 2 6 1 1 27 x = (6)2 − (3)2 = . 2 3 2 2 2

u 2 du =

1 3 2 1 1 35 u = (2)3 − (−3)3 = . 3 −3 3 3 3

" 3 " 31 (t − t 2 ) dt 9. (x − x 2 ) d x 1 0 " 3 1 4 1 3 3 1 4 1 3 34 1 1 SOLUTION (t 3 − t 2 ) dt = t − t = (3) − (3) − − = . 4 3 4 3 4 3 3 1 1 " 4 " 12 (x + 2) d x4 11. −3 (4 − 5u ) du 0

SOLUTION

" 4 −3

4 1 3 1 3 1 133 x + 2x = (4) + 2(4) − (−3)3 + 2(−3) = . 3 3 3 3 −3

(x 2 + 2) d x =

" 2 " 4 9 (10x 5+ 3x 52) d x −2 (3x + x − 2x) d x 0 " 2 1 6 2 1 6 1 6 9 5 10 10 10 SOLUTION (10x + 3x ) d x = x + x = 2 + 2 − 2 + 2 = 0. 2 2 2 −2 −2 " 3 " 13/2 (4t + t 7/2 ) dt 15. (5u 4 − 6u 2 ) du 1 13.

−1

SOLUTION

" 3 1

(4t 3/2 + t 7/2 ) dt =

√ √ √ 8 5/2 2 9/2 3 72 3 8 2 162 3 82 + t t + 18 3 − + = − . = 5 9 5 5 9 5 45 1

" 4 " 12 dt2 −2 1 t 2 (x − x ) d x 1 " 4 " 4 4

1 −2 dt = −t −1 = −(4)−1 − −(1)−1 = 3 . SOLUTION dt = t 4 1 t2 1 1 " 27 " 41/3 19. x √ dx y dy 1 0 27 " 27 3 3 3 SOLUTION x 1/3 d x = x 4/3 = (81) − = 60. 4 4 4 1 1 " 9 " −1/2 t 4 −4dt 21. x dx 1 1 9 " 9 SOLUTION t −1/2 dt = 2t 1/2 = 2(9)1/2 − 2(1)1/2 = 4. 17.

1

" −1 " 91 23. 8d x −2 x 3 3 d x x 4

1

261

262

CHAPTER 5

THE INTEGRAL

" −1 1 −2 −1 1 1 3 1 d x = − = − (−1)−2 + (−2)−2 = − . x 3 2 2 2 8 −2 x −2

SOLUTION

25.

" 27 " t4 + 1 √ 2 dt πt d x 1 2

SOLUTION

27 " 27 " 27 2 3/2 t +1 (t 1/2 + t −1/2 ) dt = + 2t 1/2 t √ dt = 3 t 1 1 1 √ √ √ 2 2 8 = (81 3) + 6 3 − + 2 = 60 3 − . 3 3 3 " π /2 " π /2 cos x d x cos θ d θ −π /2 0 π /2 " π /2 SOLUTION cos x d x = sin x = 1 − (−1) = 2.

27.

−π /2

" 3π /4 " 2π sin θ d θ 29. cos t dt π /4 0 " 3π /4 SOLUTION

π /4

−π /2

√ √ 3π /4 2 2 √ sin θ d θ = − cos θ = + = 2. 2 2 π /4

" π /4 " 4π 2 31. sec t dt sin x d x 0 2π " π /4 π /4 π 2 SOLUTION sec t dt = tan t = tan − tan 0 = 1. 4 0 0 " π /3 " π /4 33. csc x cot x d x sec θ tan θ d θ π /6 0 π /3

" π /3 π

π 2√ − − csc =2− SOLUTION csc x cot x d x = (− csc x) = − csc 3. 3 6 3 π /6 π /6 " π /235–40, write the integral as a sum of integrals without absolute values and evaluate. In Exercises csc2 y d y " 1 35.

π /6

−2

|x| d x

SOLUTION

1 2 0 1 2 1 1 1 5 |x| d x = (−x) d x + x d x = − x + x = 0 − − (4) + = . 2 2 2 2 2 −2 −2 0 −2 0

" 1

37.

" 0

" 1

" 3 " 53 |x | d x |3 − x| d x −2 0

SOLUTION

" 3 −2

|x 3 | d x =

" 0 −2

(−x 3 ) d x +

1 0 1 3 x 3 d x = − x 4 + x 4 4 −2 4 0 0

" 3

1 1 97 = 0 + (−2)4 + 34 − 0 = . 4 4 4 39.

" π "|cos 3 x| d x |x 2 − 1| d x 0 0

SOLUTION

" π 0

" 5

|cos x| d x =

" π /2 0

|x 2 − 4x + 3| d x

cos x d x +

" π π /2

π /2 π (− cos x) d x = sin x − sin x 0

π /2

= 1 − 0 − (−1 − 0) = 2.

The Fundamental Theorem of Calculus, Part I

S E C T I O N 5.3

263

In Exercises 41–44, evaluate the integral in terms of the constants. " b 41. x3 dx 1

" b

SOLUTION

1

" b " 5a 43. x d x4 x dx 1 b " b

x3 dx =

1 4 b 1 1 1 4 b − 1 for any number b. x = b4 − (1)4 = 4 1 4 4 4

1 6 b 1 1 1 x = b6 − (1)6 = (b6 − 1) for any number b. 6 1 6 6 6 1 " 1 " x x n d x = 0 if n is an odd whole number. Explain graphically. 45. Use the FTC I to show that (t 3 + t) dt −1 SOLUTION

x5 dx =

−x

SOLUTION

We have " 1 −1

xn dx =

x n+1 1 (1)n+1 (−1)n+1 = − . n + 1 −1 n+1 n+1

Because n is odd, n + 1 is even, which means that (−1)n+1 = (1)n+1 = 1. Hence (−1)n+1 1 1 (1)n+1 − = − = 0. n+1 n+1 n+1 n+1 Graphically speaking, for an odd function such as x 3 shown here, the positively signed area from x = 0 to x = 1 cancels the negatively signed area from x = −1 to x = 0. y 1 0.5 − 0.5 −1

x − 0.5

0.5

1

−1

47. Show that the area of a parabolic arch (the shaded region in Figure 5) is equal to four-thirds the area of the triangle What is the area (a positive number) between the x-axis and the graph of f (x) on [1, 3] if f (x) is a negative shown. function whose antiderivative F has the values F(1) = 7 and F(3) = 4? y

a

a+b 2

x b

FIGURE 5 Graph of y = (x − a)(b − x). SOLUTION

We ﬁrst calculate the area of the parabolic arch: " b a

(x − a)(b − x) d x = −

" b a

(x − a)(x − b) d x = −

" b a

(x 2 − ax − bx + ab) d x

b 1 3 a 2 b 2 x − x − x + abx 3 2 2 a

b 1 2x 3 − 3ax 2 − 3bx 2 + 6abx =− a 6 1 3 = − (2b − 3ab2 − 3b3 + 6ab2 ) − (2a 3 − 3a 3 − 3ba 2 + 6a 2 b) 6 1

= − (−b3 + 3ab2 ) − (−a 3 + 3a 2 b) 6 1 1

= − a 3 + 3ab2 − 3a 2 b − b3 = (b − a)3 . 6 6

=−

264

CHAPTER 5

THE INTEGRAL

The indicated triangle has a base of length b − a and a height of a+b b−a 2 a+b . −a b− = 2 2 2 Thus, the area of the triangle is 1 b−a 2 1 = (b − a)3 . (b − a) 2 2 8 Finally, we note that 4 1 1 (b − a)3 = · (b − a)3 , 6 3 8 as required.

" 3 " 1 f (x) d x, where 49. Calculate Does −2 x n d x get larger or smaller as n increases? Explain graphically. 0 12 − x 2 for x ≤ 2 f (x) = for x > 2 x3 SOLUTION

" 3 −2

f (x) d x =

" 2 −2

f (x) d x +

" 3 2

f (x) d x =

" 2 −2

(12 − x 2 ) d x +

" 3

x3 dx

2

2 1 1 3 12x − x 3 + x 4 3 4 2 −2 1 1 1 1 = 12(2) − (2)3 − 12(−2) − (−2)3 + 34 − 24 3 3 4 4 =

=

128 65 707 + = . 3 4 12

the function f (x) = sin 3x − x. Find the positive root of Further Plot Insights and Challenges

f (x) to three places and use it to ﬁnd the area under the graph of f (x) in the ﬁrst quadrant. 51. In this exercise, we generalize the result of Exercise 47 by proving the famous result of Archimedes: For r < s, the area of the shaded region in Figure 6 is equal to four-thirds the area of triangle AC E, where C is the point on the parabola at which the tangent line is parallel to secant line AE. (a) Show that C has x-coordinate (r + s)/2. (b) Show that AB D E has area (s − r )3 /4 by viewing it as a parallelogram of height s − r and base of length C F. (c) Show that AC E has area (s − r )3 /8 by observing that it has the same base and height as the parallelogram. (d) Compute the shaded area as the area under the graph minus the area of a trapezoid and prove Archimedes’s result. y B

C

D

A

F

E

r

r+s 2

x

s

FIGURE 6 Graph of f (x) = (x − a)(b − x). SOLUTION

(a) The slope of the secant line AE is (s − a)(b − s) − (r − a)(b − r ) f (s) − f (r ) = = a + b − (r + s) s −r s −r and the slope of the tangent line along the parabola is f (x) = a + b − 2x. If C is the point on the parabola at which the tangent line is parallel to the secant line AE, then its x-coordinate must satisfy a + b − 2x = a + b − (r + s)

or

x=

r +s . 2

S E C T I O N 5.3

The Fundamental Theorem of Calculus, Part I

265

(b) Parallelogram AB D E has height s − r and base of length C F. Since the equation of the secant line AE is y = [a + b − (r + s)] (x − r ) + (r − a)(b − r ), the length of the segment C F is r +s r +s r +s (s − r )2 −a b− − [a + b − (r + s)] − r − (r − a)(b − r ) = . 2 2 2 4 )3 Thus, the area of AB D E is (s−r 4 .

(c) Triangle AC E is comprised of AC F and C E F. Each of these smaller triangles has height s−r 2 and base of

)2 length (s−r 4 . Thus, the area of AC E is

1 s − r (s − r )2 1 s − r (s − r )2 (s − r )3 · + · = . 2 2 4 2 2 4 8 (d) The area under the graph of the parabola between x = r and x = s is " s 1 1 3 s 2 (x − a)(b − x) d x = −abx + (a + b)x − x 2 3 r r 1 1 1 1 = −abs + (a + b)s 2 − s 3 + abr − (a + b)r 2 + r 3 2 3 2 3 1 1 = ab(r − s) + (a + b)(s − r )(s + r ) + (r − s)(r 2 + r s + s 2 ), 2 3 while the area of the trapezoid under the shaded region is 1 (s − r ) [(s − a)(b − s) + (r − a)(b − r )] 2 1 = (s − r ) −2ab + (a + b)(r + s) − r 2 − s 2 2 1 1 = ab(r − s) + (a + b)(s − r )(r + s) + (r − s)(r 2 + s 2 ). 2 2 Thus, the area of the shaded region is 1 1 1 2 1 1 1 1 2 1 1 r + r s + s 2 − r 2 − s 2 = (s − r ) r − r s + s 2 = (s − r )3 , (r − s) 3 3 3 2 2 6 3 6 6 which is four-thirds the area of the triangle AC E. 53. Use the method of Exercise 52 to prove that (a) Apply the Comparison Theorem (Theorem 5 in Section 5.2) to the inequality sin x ≤ x (valid for x ≥ 0) to x2 x4 x2 prove ≤ cos x ≤ 1 − + 1− 2 2 24 x2 3 3 1 − ≤ cos x x x x 5≤ 1. x− ≤ sin x ≤ x 2− + (for x ≥ 0) 6 6 120 (b) Apply it again to prove Verify these inequalities for x = 0.1. Why have we speciﬁed x ≥ 0 for sin x but not cos x? 3 3 SOLUTION By Exercise 52, t − 1 ≤ tx for≤t sin > 0. Integrating inequality over the interval [0, x], and then 6 t ≤ sin xt − x≤ x (for xthis ≥ 0). 6 solving for cos x, yields: (c) Verify these inequalities for x 1= 20.3. 1 4 x − x ≤ 1 − cos x ≤ 2 24 1 1 − x 2 ≤ cos x ≤ 1 − 2

1 2 x 2 1 2 1 4 x + x . 2 24

2 2 x 4 are all even functions, they also apply for These inequalities apply for x ≥ 0. Since cos x, 1 − x2 , and 1 − x2 + 24 x ≤ 0. Having established that

1−

t2 t2 t4 ≤ cos t ≤ 1 − + , 2 2 24

for all t ≥ 0, we integrate over the interval [0, x], to obtain: x−

x3 x3 x5 ≤ sin x ≤ x − + . 6 6 120

266

CHAPTER 5

THE INTEGRAL 1 x 5 are all odd functions, so the inequalities are reversed for x < 0. The functions sin x, x − 16 x 3 and x − 16 x 3 + 120 Evaluating these inequalities at x = .1 yields

0.995000000 ≤ 0.995004165 ≤ 0.995004167 0.0998333333 ≤ 0.0998334166 ≤ 0.0998334167, both of which are true. 55. Assume that | f (x)| ≤ K for x ∈ [a, b]. Use FTC I to prove that | f (x) − f (a)| ≤ K |x − a| for x ∈ [a, b]. Calculate the next pair of inequalities for sin x and cos x by integrating the results of Exercise 53. Can you guess SOLUTION Let a > b be real numbers, and let f (x) be such that | f (x)| ≤ K for x ∈ [a, b]. By FTC, the general pattern? " x f (t) dt = f (x) − f (a). a

Since f (x) ≥ −K for all x ∈ [a, b], we get: f (x) − f (a) =

" x a

f (t) dt ≥ −K (x − a).

Since f (x) ≤ K for all x ∈ [a, b], we get: f (x) − f (a) =

" x a

f (t) dt ≤ K (x − a).

Combining these two inequalities yields −K (x − a) ≤ f (x) − f (a) ≤ K (x − a), so that, by deﬁnition, | f (x) − f (a)| ≤ K |x − a|. (a) Prove that | sin a − sin b| ≤ |a − b| for all a, b (use Exercise 55). (b) Let f (x) = sin(x + a) − sin x. Use part (a) to show that the graph of f (x) lies between the horizontal lines 5.4 y The Fundamental Theorem of Calculus, Part II = ±a. (c) Produce a graph of f (x) and verify part (b) for a = 0.5 and a = 0.2. Preliminary Questions " x 1. What is A(−2), where A(x) = f (t) dt? −2

SOLUTION

By deﬁnition, A(−2) =

2. Let G(x) =

" −2

" x t 3 + 1 dt.

−2

f (t) dt = 0.

4

(a) Is the FTC needed to calculate G(4)? (b) Is the FTC needed to calculate G (4)? SOLUTION

! (a) No. G(4) = 44 t 3 + 1 dt = 0. √ (b) Yes. By the FTC II, G (x) = x 3 + 1, so G (4) = 65. 3. Which of the following deﬁnes an antiderivative F(x) of f (x) = x 2 satisfying F(2) = 0? " x " 2 " x (a) 2t dt (b) t 2 dt (c) t 2 dt 2 SOLUTION

" x The correct answer is (c):

0

2

t 2 dt.

2

4. True or false? Some continuous functions do not have antiderivatives. Explain. " x SOLUTION False. All continuous functions have an antiderivative, namely f (t) dt. 5. Let G(x) =

a

" x3 sin t dt. Which of the following statements are correct? 4

(a) G(x) is the composite function sin(x 3 ).

S E C T I O N 5.4

(b) G(x) is the composite function A(x 3 ), where A(x) = (c) (d) (e) (f)

The Fundamental Theorem of Calculus, Part II

267

" x sin(t) dt. 4

G(x) is too complicated to differentiate. The Product Rule is used to differentiate G(x). The Chain Rule is used to differentiate G(x). G (x) = 3x 2 sin(x 3 ).

Statements (b), (e), and (f) are correct. " 3 t 3 dt at x = 2. 6. Trick question: Find the derivative of

SOLUTION

1

! Note that the deﬁnite integral 13 t 3 dt does not depend on x; hence the derivative with respect to x is 0 for any value of x. SOLUTION

Exercises 1. Write the area function of f (x) = 2x + 4 with lower limit a = −2 as an integral and ﬁnd a formula for it. SOLUTION

Let f (x) = 2x + 4. The area function with lower limit a = −2 is " x " x A(x) = f (t) dt = (2t + 4) dt. −2

a

Carrying out the integration, we ﬁnd x " x (2t + 4) dt = (t 2 + 4t) −2

−2

= (x 2 + 4x) − ((−2)2 + 4(−2)) = x 2 + 4x + 4

or (x + 2)2 . Therefore, A(x) = (x + 2)2 . " x (t 2for − the 2) dt. 3. LetFind G(x)a = formula area function of f (x) = 2x + 4 with lower limit a = 0. 1

(a) What is G(1)? (b) Use FTC II to ﬁnd G (1) and G (2). (c) Find a formula for G(x) and use it to verify your answers to (a) and (b). !x SOLUTION Let G(x) = 1 (t 2 − 2) dt. ! (a) Then G(1) = 11 (t 2 − 2) dt = 0.

(b) Now G (x) = x 2 − 2, so that G (1) = −1 and G (2) = 2. (c) We have x " x 1 3 1 3 1 3 1 5 (t 2 − 2) dt = t − 2t = x − 2x − (1) − 2(1) = x 3 − 2x + . 3 3 3 3 3 1 1 Thus G(x) = 13 x 3 − 2x + 53 and G (x) = x 2 − 2. Moreover, G(1) = 13 (1)3 − 2(1) + 53 = 0, as in (a), and G (1) = −1 and G (2) = 2, as in (b). " x (π /4), where G(x) = " x tan t dt. 5. Find G(1), G (0), and G Find F(0), F (0), and F (3), where F(x) = 1 t 2 + t dt. 0 !1 π SOLUTION By deﬁnition, G(1) = 1 tan t dt = 0. By FTC, G (x) = tan x, so that G (0) = tan 0 = 0 and G ( 4 ) = π tan 4 = 1. " x In Exercises 7–14, ﬁnd formulas for the functions represented du by the integrals. (−2), where H (x) = . Find H (−2) and H " x 2 −2 u + 1 3 u du 7. 2 SOLUTION

F(x) =

" x 2

u 3 du =

1 4 x 1 u = x 4 − 4. 4 2 4

" x"2 x 9. t dtsin u du 1

0

SOLUTION

" x

F(x) =

(t 2 − t) dt

" x2 1

t dt =

2 1 2 x 1 1 t = x4 − . 2 1 2 2

268

CHAPTER 5

THE INTEGRAL

" 5 11. x

(4t − 1) dt

SOLUTION

F(x) =

" x " x 2 sec θ d θ 13. −π /4 cos u du π /4

SOLUTION

F(x) =

" 5 x

5 (4t − 1) dt = (2t 2 − t) = 45 + x − 2x 2 . x

" x −π /4

x sec2 θ d θ = tan θ

−π /4

= tan x − tan(−π /4) = tan x + 1.

In Exercises " √x15–18, express the antiderivative F(x) of f (x) satisfying the given initial condition as an integral. t 3 dt 15. f (x)2= x 4 + 1, F(3) = 0 " x SOLUTION The antiderivative F(x) of f (x) = x 4 + 1 satisfying F(3) = 0 is F(x) = t 4 + 1 dt. 3

17. f (x) = sec x, F(0) = 0 x +1 " x , F(7) = 0 f (x) = 2 x antiderivative +9 SOLUTION The F(x) of f (x) = sec x satisfying F(0) = 0 is F(x) = sec t dt. 0

In Exercises 19–22, calculate the derivative. f (x) = sin(x 3 ), F(−π ) = 0 " x d 19. (t 3 − t) dt dx 0 " x d SOLUTION By FTC II, (t 3 − t) dt = x 3 − x. dx 0 " t d " x d cos 21. 5x d x dt 100 sin(t 2 ) dt dx 1 " t d SOLUTION By FTC II, cos(5x) d x = cos 5t. dt 100 " x " 23. Sketch f (t) dt for each of the functions shown in Figure 10. d thes graph of 1A(x) = du 0 tan ds −2 1 + u2 y

y 2 1 0 −1

2 1 0 −1

x 1

2

3

4

x 1

2

3

4

(B)

(A)

FIGURE 10 SOLUTION

• Remember that A (x) = f (x). It follows from Figure 10(A) that A (x) is constant and consequently A(x) is linear

on the intervals [0, 1], [1, 2], [2, 3] and [3, 4]. With A(0) = 0, A(1) = 2, A(2) = 3, A(3) = 2 and A(4) = 2, we obtain the graph shown below at the left. • Since the graph of y = f (x) in Figure 10(B) lies above the x-axis for x ∈ [0, 4], it follows that A(x) is increasing over [0, 4]. For x ∈ [0, 2], area accumulates more rapidly with increasing x, while for x ∈ [2, 4], area accumulates more slowly. This suggests A(x) should be concave up over [0, 2] and concave down over [2, 4]. A sketch of A(x) is shown below at the right. y

y

3 2.5 2 1.5 1 0.5

4 3 2 1 x 1

Let A(x) =

" x 0

2

3

4

x 1

2

3

4

f (t) dt for f (x) shown in Figure 11. Calculate A(2), A(3), A (2), and A (3). Then ﬁnd a

formula for A(x) (actually two formulas, one for 0 ≤ x ≤ 2 and one for 2 ≤ x ≤ 4) and sketch the graph of A(x).

The Fundamental Theorem of Calculus, Part II

S E C T I O N 5.4

269

25. Make a rough sketch of the graph of the area function of g(x) shown in Figure 12. y y = g(x)

x 1

2

3

4

FIGURE 12 SOLUTION The graph of y = g(x) lies above the x-axis over the interval [0, 1], below the x-axis over [1, 3], and above the x-axis over [3, 4]. The corresponding area function should therefore be increasing on (0, 1), decreasing on (1, 3) and increasing on (3, 4). Further, it appears from Figure 12 that the local minimum of the area function at x = 3 should be negative. One possible graph of the area function is the following. y 4 3 2 1 x 1

−1 −2 −3

2

3

4

" x3 " x 27. FindShow G (x), where G(x) = dt. Hint: Consider x ≥ 0 and x ≤ 0 separately. that |t| dt is equal totan12 tx|x|. 3

0

SOLUTION

By combining the FTC and the chain rule, we have G (x) = tan(x 3 ) · 3x 2 = 3x 2 tan(x 3 ).

In Exercises 29–36, calculate the derivative. " x2 t 3 + 3 dt. Find " x 2 G (1), where G(x) = d 0 2 29. sin t dt dx 0 " x2 SOLUTION Let G(x) = sin2 t dt. By applying the Chain Rule and FTC, we have 0

G (x) = sin2 (x 2 ) · 2x = 2x sin2 (x 2 ). " cos s d " 1/x (u 4 − 3u) du ds d−6 sin(t 2 ) dt dx 1 " s SOLUTION Let G(s) = (u 4 − 3u) du. Then, by the chain rule,

31.

−6

" cos s d d (u 4 − 3u) du = G(cos s) = − sin s(cos4 s − 3 cos s). ds −6 ds 33.

" 0 d " d sin0 2 t dt d x x3 sin2 t dt dx x

SOLUTION

Let F(x) =

" x 0

sin2 t dt. Then

" 0 x3

sin2 t dt = −

" x3 0

sin2 t dt = −F(x 3 ). From this,

" 0 d d sin2 t dt = (−F(x 3 )) = −3x 2 F (x 3 ) = −3x 2 sin2 x 3 . d x x3 dx 35.

" x2 d " tan4 t dt d x d√x x √ t dt d x x2 Hint for Exercise 34:F(x) = A(x 4 ) − A(x 2 ), where A(x) =

" x √

t dt

270

CHAPTER 5

THE INTEGRAL SOLUTION

Let " x2

G(x) = √ tan t dt = x

" x2 0

" √x

tan t dt −

tan t dt. 0

Applying the Chain Rule combined with FTC twice, we have √ √ 1 tan( x) G (x) = tan(x 2 ) · 2x − tan( x) · x −1/2 = 2x tan(x 2 ) − √ . 2 2 x " x " x " 3u+9 d 37–38, In Exercises let 2A(x) = f (t) dt and B(x) = f (t) dt, with f (x) as in Figure 13. x + 1 dx 0 2 du −u y 2 1 0 −1 −2

y = f(x) x 1

2

3

4

5

6

FIGURE 13

37. Find the min and max of A(x) on [0, 6]. SOLUTION The minimum values of A(x) on [0, 6] occur where A (x) = f (x) goes from negative to positive. This occurs at one place, where x = 1.5. The minimum value of A(x) is therefore A(1.5) = −1.25. The maximum values of A(x) on [0, 6] occur where A (x) = f (x) goes from positive to negative. This occurs at one place, where x = 4.5. The maximum value of A(x) is therefore A(4.5) = 1.25. " x ffor (t)dt, with (x) asvalid in Figure 39. LetFind A(x)formulas = A(x) andf B(x) on [2,14. 4].

0

(a) (b) (c) (d)

Does A(x) have a local maximum at P? Where does A(x) have a local minimum? Where does A(x) have a local maximum? True or false? A(x) < 0 for all x in the interval shown. y R P

S

x

y = f(x) Q

FIGURE 14 Graph of f (x). SOLUTION

(a) In order for A(x) to have a local maximum, A (x) = f (x) must transition from positive to negative. As this does not happen at P, A(x) does not have a local maximum at P. (b) A(x) will have a local minimum when A (x) = f (x) transitions from negative to positive. This happens at R, so A(x) has a local minimum at R. (c) A(x) will have a local maximum when A (x) = f (x) transitions from positive to negative. This happens at S, so A(x) has a local maximum at S. (d) It is true that A(x) < 0 on I since the signed area from 0 to x is clearly always negative from the ﬁgure. " x Find 41–42, the smallest positive point of f (x) is continuous. In Exercises let A(x) = critical f (t) dt, where a " x cos(t 3/2 ) dt F(x) = 41. Area Functions and Concavity Explain why0 the following statements are true. Assume f (x) is differentiable. and determine whether it is a local min or max. (a) If c is an inﬂection point of A(x), then f (c) = 0. (b) A(x) is concave up if f (x) is increasing. (c) A(x) is concave down if f (x) is decreasing. SOLUTION

(a) If x = c is an inﬂection point of A(x), then A (c) = f (c) = 0. (b) If A(x) is concave up, then A (x) > 0. Since A(x) is the area function associated with f (x), A (x) = f (x) by FTC II, so A (x) = f (x). Therefore f (x) > 0, so f (x) is increasing.

S E C T I O N 5.4

The Fundamental Theorem of Calculus, Part II

271

(c) If A(x) is concave down, then A (x) < 0. Since A(x) is the area function associated with f (x), A (x) = f (x) by FTC II, so A (x) = f (x). Therefore, f (x) < 0 and so f (x) is decreasing. " x f (t) dt, (x) as Figure 15. Determine: 43. LetMatch A(x) = the property of with A(x) fwith theincorresponding property of the graph of f (x). Assume f (x) is differentiable. 0

(a) (b) (c) (d)

The intervals on which A(x) is increasing and decreasing Areavalues function A(x) A(x) has a local min or max The x where (a) A(x) is decreasing. The inﬂection points of A(x) (b) intervals A(x) has where a localA(x) maximum. The is concave up or concave down (c) A(x) is concave up. y (d) A(x) goes from concave up to concave down. y = f(x) Graph of f(x) (i) Lies below the x-axis. (ii) Crosses the x-axis from positive to negative. 2 4 6 8 (iii) Has a local maximum. FIGURE 15 (iv) f (x) is increasing.

x 10

12

SOLUTION

(a) A(x) is increasing when A (x) = f (x) > 0, which corresponds to the intervals (0, 4) and (8, 12). A(x) is decreasing when A (x) = f (x) < 0, which corresponds to the intervals (4, 8) and (12, ∞). (b) A(x) has a local minimum when A (x) = f (x) changes from − to +, corresponding to x = 8. A(x) has a local maximum when A (x) = f (x) changes from + to −, corresponding to x = 4 and x = 12. (c) Inﬂection points of A(x) occur where A (x) = f (x) changes sign, or where f changes from increasing to decreasing or vice versa. Consequently, A(x) has inﬂection points at x = 2, x = 6, and x = 10. (d) A(x) is concave up when A (x) = f (x) is positive or f (x) is increasing, which corresponds to the intervals (0, 2) and (6, 10). Similarly, A(x) is concave down when f (x) is decreasing, which corresponds to the intervals (2, 6) and (10, ∞). " x " f (t) dt are decreasing. 45. Sketch the graph 2of an increasing function fx(x) such that both f (x) and A(x) = f (t) dt. Let f (x) = x − 5x − 6 and F(x) = 0 " x 0 (x) is points (x) must whether (a) Find the of F(x) andf determine they are local minima or maxima. SOLUTION If fcritical decreasing, then be negative. Furthermore, if A(x) = f (t) dt is decreasing, 0 (b) Find the points of inﬂection of F(x) and determine whether the concavity changes from up to down or vice then A (x) = f (x) must also be negative. Thus, we need a function which is negative but increasing and concave down. versa. The graph of one such function is shown below. (c) Plot f (x) and F(x) on the same set of axes and conﬁrm your answers to (a) and (b). y x

47.

Find the smallest positive inﬂection point of " x Figure 16 shows the graph of f (x) = x sin x. "Letx F(x) = t sin t dt. 0 cos(t 3/2 ) dt F(x) = 0

3/2 ) to determine whether the concavity changes from up to down or vice versa at this point of Use a(a)graph of ythe = local cos(xmaxima Locate and absolute maximum of F(x) on [0, 3π ]. inﬂection. (b) Justify graphically that F(x) has precisely one zero in the interval [π , 2π ]. SOLUTION points F(x) where F (x) = 0. By FTC, F (x) = cos(x 3/2 ), so F (x) = ]? (c) How Candidate many zerosinﬂection does F(x) have of in [0, 3πoccur 1/2 3/2 −(3/2)x ). Finding theof smallest positive F (x) 0, we get: the concavity changes from up to and, forofeach one,=state whether (d) Findsin(x the inﬂection points F(x) on [0, 3π ]solution down or vice versa. −(3/2)x 1/2 sin(x 3/2 ) = 0

sin(x 3/2 ) = 0 x 3/2 =

(since x > 0)

π

x = π 2/3 ≈ 2.14503. From the plot below, we see that F (x) = cos(x 3/2 ) changes from decreasing to increasing at π 2/3 , so F(x) changes from concave down to concave up at that point.

272

CHAPTER 5

THE INTEGRAL y 1 0.5 x 1

− 0.5

2

3

−1

49. Determine the function g(x) and all"values of c such that x 2 " Determine f (x), assuming that f (t) dt x is equal to x + x. 2 0 g(t) dt = x + x − 6 c

SOLUTION

By the FTC II we have g(x) =

d 2 (x + x − 6) = 2x + 1 dx

and therefore, " x c

g(t) dt = x 2 + x − (c2 + c)

We must choose c so that c2 + c = 6. We can take c = 2 or c = −3.

Further Insights and Challenges

" b Proof of FTC proof that in the textf (t) assumes that f−(x) is increasing. it for allFTC continuous functions, 51. Proof of FTC I II FTCThe I asserts dt = F(b) F(a) if F (x) =Tof prove (x). Assume II and give a new a " x and maximum of f (x) on [x, x + h] (Figure 17). The continuity of f (x) let m(h) and M(h) denote the minimum m(h)Set = lim (x). dt.Show that for h > 0, proofimplies of FTCthat I aslim follows. A(x)M(h) = = f f(t) h→0

h→0

a

(a) Show that F(x) = A(x) + C for some constant. hm(h) " ≤ b A(x + h) − A(x) ≤ h M(h) f (t) dt. (b) Show that F(b) − F(a) = A(b) − A(a) = For h < 0, the inequalities are reversed. Provea that A (x) = f (x). ! x SOLUTION Let F (x) = f (x) and A(x) = a f (t) dt. (a) Then by the FTC, Part II, A (x) = f (x) and thus A(x) and F(x) are both antiderivatives of f (x). Hence F(x) = A(x) + C for some constant C. (b) F(b) − F(a) = ( A(b) + C) − ( A(a) + C) = A(b) − A(a) " b " a " b " b = f (t) dt − f (t) dt = f (t) dt − 0 = f (t) dt a

a

a

a

which proves the FTC, Part I. " b " x 53. Find the values a ≤ b such that (x 2 − 9) d x has minimal value. f (t) dt is an antiderivative Can Every Antiderivative Bea Expressed as an Integral? The area function a !x 2 2 of f (x) for every value of a. However, not all antiderivatives are obtained in this way. The general antiderivative of SOLUTION Let a be given, and let Fa (x) = a (t − 9) dt. Then Fa (x) = x − 9, and the critical points are x = ±3. 1 x 2 +C. Show that F(x) is an area function if C ≤ 0 but not if C > 0. f (x) = x is F(x) = Because Fa (−3) = −6 2and Fa (3) = 6, we see that Fa (x) has a minimum at x = 3. Now, we ﬁnd a minimizing !3 2 !3 2 2 a (x − 9) d x. Let G(x) = x (x − 9) d x. Then G (x) = −(x − 9), yielding critical points x = 3 or x = −3. With x = −3, 3 " 3 1 3 G(−3) = (x 2 − 9) d x = x − 9x = −36. 3 −3 −3 With x = 3, G(3) = Hence a = −3 and b = 3 are the values minimizing

" 3 3

" b a

(x 2 − 9) d x = 0.

(x 2 − 9) d x.

S E C T I O N 5.5

Net or Total Change as the Integral of a Rate

273

5.5 Net or Total Change as the Integral of a Rate Preliminary Questions 1. An airplane makes the 350-mile trip from Los Angeles to San Francisco in 1 hour. Assuming that the plane’s velocity " 1 at time t is v(t) mph, what is the value of the integral v(t) dt? 0

!1

SOLUTION The deﬁnite integral 0 v(t) dt represents the total distance traveled by the airplane during the one hour ! ﬂight from Los Angeles to San Francisco. Therefore the value of 01 v(t) dt is 350 miles.

2. A hot metal object is submerged in cold water. The rate at which the object cools (in degrees per minute) is a function " T f (t) dt? f (t) of time. Which quantity is represented by the integral 0

! The deﬁnite integral 0T f (t) dt represents the total drop in temperature of the metal object in the ﬁrst T minutes after being submerged in the cold water. SOLUTION

3. (a) (b) (c) (d)

Which of the following quantities would be naturally represented as derivatives and which as integrals? Velocity of a train Rainfall during a 6-month period Mileage per gallon of an automobile Increase in the population of Los Angeles from 1970 to 1990

SOLUTION Quantities (a) and (c) involve rates of change, so these would naturally be represented as derivatives. Quantities (b) and (d) involve an accumulation, so these would naturally be represented as integrals.

4. Two airplanes take off at t = 0 from the same place and in the same direction. Their velocities are v1 (t) and v2 (t), respectively. What is the physical interpretation of the area between the graphs of v1 (t) and v2 (t) over an interval [0, T ]? SOLUTION The area between the graphs of v1 (t) and v2 (t) over an interval [0, T ] represents the difference in distance traveled by the two airplanes in the ﬁrst T hours after take off.

Exercises 1. Water ﬂows into an empty reservoir at a rate of 3,000 + 5t gal/hour. What is the quantity of water in the reservoir after 5 hours? SOLUTION

The quantity of water in the reservoir after ﬁve hours is 5 2 5 30125 (3000 + 5t) dt = 3000t + t = = 15,062.5 gallons. 2 2 0 0

" 5

2 perv(t) day.=Find insect after 3 3. A population of insects increases at a moving rate of 200 10t +line 0.25t Find the displacement of a particle in a+ straight withinsects velocity 4t −the 3 ft/s overpopulation the time interval days,[2, assuming that there are 35 insects at t = 0. 5]. SOLUTION

The increase in the insect population over three days is " 3

1 200 + 10t + t 2 dt = 4 0

200t + 5t 2 +

1 3 3 2589 t = = 647.25. 12 4 0

Accordingly, the population after 3 days is 35 + 647.25 = 682.25 or 682 insects. 2 − t bicycles per week (t in weeks). How many bicycles were 5. A factory produces bicycles at a rate of 95 +is0.1t A survey shows that a mayoral candidate gaining votes at a rate of 2,000t + 1,000 votes per day, where t is produced from day 8 to 21? the number of days since she announced her candidacy. How many supporters will the candidate have after 60 days,

assuming The that rate she had no supporters at t = = 95 0? + 10 t 2 SOLUTION of production is r (t) 1

− t bicycles per week and the period between days 8 and 21 corresponds to the second and third weeks of production. Accordingly, the number of bikes produced between days 8 and 21 is " 3 " 3 1 3 1 2 3 2803 1 2 r (t) dt = ≈ 186.87 95 + t − t dt = 95t + t − t = 10 30 2 15 1 1 1 or 187 bicycles. 7. A cat falls from a tree (with zero initial velocity) at time t = 0. How far does the cat fall between t = 0.5 and Find the displacement over the time interval [1, 6] of a helicopter whose (vertical) velocity at time t is v(t) = t = 1 s? Use Galileo’s formula v(t) = −32t ft/s. 0.02t 2 + t ft/s.

274

CHAPTER 5

THE INTEGRAL

Given v(t) = −32 ft/s, the total distance the cat falls during the interval [ 12 , 1] is

SOLUTION

" 1

|v(t)| dt =

1/2

" 1 1/2

1 32t dt = 16t 2

= 16 − 4 = 12 ft.

1/2

In Exercises 9–12, assume that with a particle in avelocity straight100 linem/s. withUse given total−displacement and A projectile is released initial moves (vertical) thevelocity. formulaFind v(t) the = 100 9.8t for velocity totaltodistance traveled over the time interval, and draw a motion diagram like Figure 3 (with distance and time labels). determine the distance traveled during the ﬁrst 15 s. 9. 12 − 4t ft/s,

[0, 5]

" 5

Total displacement is given by

SOLUTION

0

" 5 0

|12 − 4t| dt =

" 3 0

5 (12 − 4t) dt = (12t − 2t 2 ) = 10 ft, while total distance is given by 0

(12 − 4t) dt +

" 5 3

3 5 (4t − 12) dt = (12t − 2t 2 ) + (2t 2 − 12t) = 26 ft. 0

3

The displacement diagram is given here. t =5 t =3 t =0 Distance 0

11. t −2 − 1 m/s, [0.5, 2] 32 − 2t 2 ft/s, [0, 6] SOLUTION

10

" 2

Total displacement is given by

.5

18

2 (t −2 − 1) dt = (−t −1 − t) = 0 m, while total distance is given by .5

1 2 " 2 " 2 " 1 −2 −2 −2 −1 −1 (t − 1) dt + (1 − t ) dt = (−t − t) + (t + t ) = 1 m. t − 1 dt = .5

.5

.5

1

1

The displacement diagram is given here. t =2 t =1 t =0 Distance 0

0.5

13. The rate (in liters per minute) at which water drains from a tank is recorded at half-minute intervals. Use the average cos t m/s, [0, 4π ] of the left- and right-endpoint approximations to estimate the total amount of water drained during the ﬁrst 3 min.

SOLUTION

t (min)

0

0.5

1

1.5

2

2.5

3

l/min

50

48

46

44

42

40

38

Let t = .5. Then R N = .5(48 + 46 + 44 + 42 + 40 + 38) = 129.0 liters L N = .5(50 + 48 + 46 + 44 + 42 + 40) = 135.0 liters

The average of R N and L N is 12 (129 + 135) = 132 liters.

" t2 of a car is recorded at half-second intervals (in feet per second). the average thechange left- and 15. LetThe a(t)velocity be the acceleration of an object in linear motion at time t. Explain why Use a(t) dt is theofnet in right-endpoint approximations to estimate the total distance traveled during the ﬁrst 4t1s. velocity over [t1 , t2 ]. Find the net change in velocity over [1, 6] if a(t) = 24t − 3t 2 ft/s2 . t 0 of 0.5 1 in 1.5 2.5at time 3 t. 3.5 4 be the velocity of the object. SOLUTION Let a(t) be the acceleration an object linear 2motion Let v(t) We know that v (t) = a(t). By FTC, v(t) 0 12 20 29 38 44 32 35 30 t2 " t2 a(t) dt = (v(t) + C) = v(t2 ) + C − (vt1 + C) = v(t2 ) − v(t1 ), t1

t1

which is the net change in velocity over [t1 , t2 ]. Let a(t) = 24t − 3t 2 . The net change in velocity over [1, 6] is 6 " 6 (24t − 3t 2 ) dt = (12t 2 − t 3 ) = 205 ft/s. 1

1

Show that if acceleration a is constant, then the change in velocity is proportional to the length of the time interval.

Net or Total Change as the Integral of a Rate

S E C T I O N 5.5

275

17. The trafﬁc ﬂow rate past a certain point on a highway is q(t) = 3,000 + 2,000t − 300t 2 , where t is in hours and t = 0 is 8 AM. How many cars pass by during the time interval from 8 to 10 AM? SOLUTION

The number of cars is given by " 2 0

q(t) dt =

" 2 0

2 (3000 + 2000t − 300t 2 ) dt = 3000t + 1000t 2 − 100t 3 0

= 3000(2) + 1000(4) − 100(8) = 9200 cars. 19. Carbon Tax To encourage manufacturers to reduce pollution, a carbon tax on each ton of CO2 released into the 0.6x the + 350 dollars. What is the cost Suppose that the marginal of producing video is 0.001x 2 −study atmosphere has been proposed. Tocost model the effectsx of suchrecorders a tax, policymakers marginal cost of abatement ofdeﬁned producing 300cost unitsofifincreasing the setup cost $20,000 (see 4)? production 300 units,tons—Figure what is the cost fromExample x to x + 1 If tons (in units isofset tenatthousand 4). B(x), as the CO2isreduction " 3 of producing 20 additional units? B(t) dt? Which quantity is represented by 0 y Dollars/ton

100 75 50 25 x 1 2 3 Tons reduced (in ten thousands)

FIGURE 4 Marginal cost of abatement B(x).

" 3 SOLUTION

The quantity 0

sphere by 3 tons.

B(t) dt represents the total cost of reducing the amount of CO2 released into the atmo-

Figure shows the migration rate per M(t) of time. Ireland during theofperiod This×is10the rate at Figure which 21. 9 J/hour. Power is the 6rate of energy consumption unit A megawatt power1988–1998. is 106 W or 3.6 people (in thousands persupplied year) move in California or out of the country. 5 shows the"power by the power grid over a typical 1-day period. Which quantity is represented 1991 by the area under the graph? (a) What does M(t) dt represent? 1988

Migration (in thousands)

(b) Did migration over the 11-year period 1988–1998 result in a net inﬂux or outﬂow of people from Ireland? Base your answer on a rough estimate of the positive and negative areas involved. (c) During which year could the Irish prime minister announce, “We are still losing population but we’ve hit an inﬂection point—the trend is now improving.” 30 20 10 0 −10 −20 −30 −40 −50

1994 1988

1990

1992

1996

1998

2000

FIGURE 6 Irish migration rate (in thousands per year). SOLUTION

" 1991

(a) The amount

M(t) dt represents the net migration in thousands of people during the period from 1988–1991. 1988

(b) Via linear interpolation and using the midpoint approximation with n = 10, the migration (in thousands of people) over the period 1988 – 1998 is estimated to be 1 · (−43 − 33.5 − 12 + 0.5 − 2.5 − 6 − 3.5 + 3 + 11.5 + 19) = −66.5 That is, there was a net outﬂow of 66,500 people from Ireland during this period. (c) “The trend is now improving” implies that the population is decreasing, but that the rate of decrease is approaching zero. The population is decreasing with an improving trend in part of the years 1989, 1990, 1991, 1993, and 1994. “We’ve hit an inﬂection point” implies that the rate of population has changed from decreasing to increasing. There are two years in which the trend improves after it was getting worse: 1989 and 1993. During only one of these, 1989, was the population declining for the entire previous year. 23. Heat Capacity The heat capacity C(T ) of a substance is the amount of energy (in joules) required to raise the Figure 7 shows the graph of Q(t), the rate of retail truck sales in the United States (in thousands sold per year). temperature of 1 g by 1◦ C at temperature T . (a) What does the area under the graph over the interval [1995, 1997] represent? (b) Express the total number of trucks sold in the period 1994–1997 as an integral (but do not compute it). (c) Use the following data to compute the average of the right- and left-endpoint approximations as an estimate for

276

CHAPTER 5

THE INTEGRAL

(a) Explain why the energy required to raise the temperature from T1 to T2 is the area under the graph of C(T ) over [T1 , T2 ]. √ (b) How much energy is required to raise the temperature from 50 to 100◦ C if C(T ) = 6 + 0.2 T ? SOLUTION

(a) Since C(T ) is the energy required to raise the temperature of one gram of a substance by one degree when its temperature is T , the total energy required to raise the temperature from T1 to T2 is given by the deﬁnite integral " T2 C(T ) d T . As C(T ) > 0, the deﬁnite integral also represents the area under the graph of C(T ). T1 √ (b) If C(T ) = 6 + .2 T = 6 + 15 T 1/2 , then the energy required to raise the temperature from 50◦ C to 100◦ C is ! 100 50 C(T ) d T or " 100 50

6+

1 1/2 T 5

2 2 3/2 100 3/2 − 6(50) + 2 (50)3/2 = 6(100) + T (100) 15 15 15 50 √ 1300 − 100 2 = ≈ 386.19 Joules 3

dT =

6T +

In Exercises 24 and 25, consider the following. Paleobiologists have studied the extinction of marine animal families during the phanerozoic period, which began 544 million years ago. A recent study suggests that the extinction rate r (t) may be modeled by the function r (t) = 3,130/(t + 262) for 0 ≤ t ≤ 544. Here, t is time elapsed (in millions of years) since the beginning of the phanerozoic period. Thus, t = 544 refers to the present time, t = 540 is 4 million years ago, etc. the total number of extinct families from t = 0 to the present, using M N with N = 544. Use REstimate N or L N with N = 10 (or their average) to estimate the total number of families that became extinct in the periods 100 t≤ 150 and 350 ≤ t ≤ 400. SOLUTION We≤are estimating

25.

" 544 0

using M N with N = 544. If N = 544, t = M N = t

3130 dt (t + 262)

544 − 0 = 1 and {ti∗ }i=1,...N = it − (t/2) = i − 12 . 544

N #

r (ti∗ ) = 1 ·

i=1

544 #

3130 = 3517.3021. 261.5 +i i=1

Thus, we estimate that 3517 families have become extinct over the past 544 million years. output is the rate R of volume of blood pumped by the heart per unit time (in liters per minute). Doctors Further Cardiac Insights and Challenges measure R by injecting A mg of dye into a vein leading into the heart at t = 0 and recording the concentration c(t) 27. A particle located at the origin at t = 0 moves along the x-axis with velocity v(t) = (t + 1)−2 . Show that the of dye (in milligrams per liter) pumped out at short regular time intervals (Figure 8). particle will never pass the point x = 1.

SOLUTION The particle’s velocity is v(t) = s (t) = (t + 1)−2 , an antiderivative for which is F(t) = −(t + 1)−1 . Hence its position at time t is " tout in a small timetinterval [t, t + t] is approximately Rc(t)t. Explain why. (a) The quantity of dye pumped 1 " T s (u) du = F(u) = F(t) − F(0) = 1 − <1 s(t) = t + c(t) dt, (b) Show that A = R 0 where T is large 0enough that all of the dye is 1pumped through the heart but not

0

sotlarge returnswill by never recirculation. for all ≥ 0.that Thusthethedye particle pass the point x = 1. (c) Use the following data to estimate R, assuming that A = 5 mg: A particle located at the origin at t = 0 moves along the x-axis with velocity v(t) = (t + 1)−1/2 . Will the particle be at the point x = 1 at any time t? If so, ﬁnd t. t (s) 0 1 2 3 4 5 5.6 Substitution Method c(t)

0

0.4

2.8

6.5

9.8

8.9

Preliminary Questions

t (s) 6 7 8 9 10 1. Which of the following integrals is a candidate for the Substitution Method? " " " c(t) 6.15 4 2.3 1.1 0 (b) sin x cos x d x (c) x 5 sin x d x (a) 5x 4 sin(x 5 ) d x

The function in (c): x 5 sin x is not of the form g(u(x))u (x). The function in (a) meets the prescribed pattern with g(u) = sin u and u(x) = x 5 . Similarly, the function in (b) meets the prescribed pattern with g(u) = u 5 and u(x) = sin x. SOLUTION

2. Write each of the following functions in the form cg(u(x))u (x), where c is a constant. (a) x(x 2 + 9)4 (b) x 2 sin(x 3 ) (c) sin x cos2 x

S E C T I O N 5.6

Substitution Method

277

SOLUTION

(a) x(x 2 + 9)4 = 12 (2x)(x 2 + 9)4 ; hence, c = 12 , g(u) = u 4 , and u(x) = x 2 + 9. (b) x 2 sin(x 3 ) = 13 (3x 2 ) sin(x 3 ); hence, c = 13 , g(u) = sin u, and u(x) = x 3 . (c) sin x cos2 x = −(− sin x) cos2 x; hence, c = −1, g(u) = u 2 , and u(x) = cos x. " 2 x 2 (x 3 + 1) d x for a suitable substitution? 3. Which of the following is equal to 0 " " " 9 1 2 1 9 (a) u du (b) u du (c) u du 3 0 3 1 0 ! 2 1 !9 SOLUTION With the substitution u = x 3 + 1, the deﬁnite integral 0 x 2 (x 3 + 1) d x becomes 3 1 u du. The correct answer is (c).

Exercises In Exercises 1–6, calculate du for the given function. 1. u = 1 − x 2 SOLUTION

Let u = 1 − x 2 . Then du = −2x d x.

3. u =u x=3 − sin2x SOLUTION Let u = x 3 − 2. Then du = 3x 2 d x. 5. u = cos(x 24) u = 2x + 8x SOLUTION Let u = cos(x 2 ). Then du = − sin(x 2 ) · 2x d x = −2x sin(x 2 ) d x. In Exercises 7–20, u = tan x write the integral in terms of u and du. Then evaluate. " 7. (x − 7)3 d x, u = x − 7 SOLUTION

" 9.

" (x + 1)−22d x, u = x + 1 2 2x x + 1 d x, u = x + 1

SOLUTION

" 11.

Let u = x − 7. Then du = d x. Hence " " 1 1 (x − 7)3 d x = u 3 du = u 4 + C = (x − 7)4 + C. 4 4

Let u = x + 1. Then du = d x. Hence " " (x + 1)−2 d x = u −2 du = −u −1 + C = −(x + 1)−1 + C = −

1 + C. x +1

" sin(2x − 4) d9 x, u = 2x − 4 x(x + 1) d x, u = x + 1

SOLUTION

Let u = 2x − 4. Then du = 2 d x or 12 du = d x. Hence " " 1 1 1 sin u du = − cos u + C = − cos(2x − 4) + C. sin(2x − 4) d x = 2 2 2

"

" x + 13 d x, u = x 2 + 2x x 3 (x 2 + 2x) d x, u = x 4 + 1 (x 4 + 1)4 2 1 SOLUTION Let u = x + 2x. Then du = (2x + 2) d x or 2 du = (x + 1) d x. Hence " " 1 1 −1 1 1 −2 1 x +1 d x = du = + C. − u + C = − (x 2 + 2x)−2 + C = 2 2 2 4 (x 2 + 2x)3 u3 4(x 2 + 2x)2

13.

" √ "

u = 4x − 1 d x, u = 8x + 5 (8x + 5)3 1 SOLUTION Let u = 4x − 1. Then du = 4 d x or 4 du = d x. Hence " " √ 1 1 2 3/2 1 u 1/2 du = 4u − 1 d x = u + C = (4x − 1)3/2 + C. 4 4 3 6

15.

"

4x − 1x d x,

√ x 4x − 1 d x,

u = 4x − 1

278

CHAPTER 5

THE INTEGRAL

" 17.

√ x 2 4x − 1 d x,

SOLUTION

u = 4x − 1

Let u = 4x − 1. Then x = 14 (u + 1) and du = 4 d x or 14 du = d x. Hence 2 " " " √ 1 1 1 (u 5/2 + 2u 3/2 + u 1/2 ) du x 2 4x − 1 d x = (u + 1) u 1/2 du = 4 4 64 1 2 7/2 1 2 5/2 1 2 3/2 = u u u + + +C 64 7 64 5 64 3 =

" 19.

1 1 1 (4x − 1)7/2 + (4x − 1)5/2 + (4x − 1)3/2 + C. 224 160 96

" sin2 x cos x2d x, u = sin2x x cos(x ) d x, u = x

SOLUTION

Let u = sin x. Then du = cos x d x. Hence " " 1 1 sin2 x cos x d x = u 2 du = u 3 + C = sin3 x + C. 3 3

" In Exercises 21–24, show that each of the following integrals is equal to a multiple of sin(u(x)) + C for an appropriate sec2 x tan x d x, u = tan x choice of u(x). " 21. x 3 cos(x 4 ) d x SOLUTION

Let u = x 4 . Then du = 4x 3 d x or 14 du = x 3 d x. Hence " " 1 1 cos u du = sin u + C, x 3 cos(x 4 ) d x = 4 4

which is a multiple of sin(u(x)). " " ) dx 23. x 1/2 2cos(x 3/2 x cos(x 3 + 1) d x SOLUTION

Let u = x 3/2 . Then du = 32 x 1/2 d x or 23 du = x 1/2 d x. Hence " " 2 2 x 1/2 cos(x 3/2 ) d x = cos u du = sin u + C, 3 3

which is a multiple of sin(u(x)). " In Exercises 25–51, evaluate the indeﬁnite integral. cos x cos(sin x) d x " (4x + 3)4 d x

25.

SOLUTION

" 27.

" 1 √ 2 3d x 3 xx−(x7 + 1) d x

SOLUTION

" 29.

Let u = 4x + 3. Then du = 4 d x or 14 du = d x. Hence " " 1 1 1 5 1 (4x + 3)4 d x = u 4 du = u +C = (4x + 3)5 + C. 4 4 5 20

Let u = x − 7. Then du = d x. Hence " " √ (x − 7)−1/2 d x = u −1/2 du = 2u 1/2 + C = 2 x − 7 + C.

" x x2 − 4 dx sin(x − 7) d x

SOLUTION

Let u = x 2 − 4. Then du = 2x d x or 12 du = x d x. Hence " " 1 √ 1 2 3/2 1 x x2 − 4 dx = u du = u + C = (x 2 − 4)3/2 + C. 2 2 3 3

" (2x + 1)(x 2 + x)3 d x

S E C T I O N 5.6

" 31.

Substitution Method

dx (x + 9)2

SOLUTION

Let u = x + 9, then du = d x. Hence " " 1 1 dx du = =− +C =− + C. 2 2 u x + 9 (x + 9) u

" " 2x 2 + x x dx 3 + 3x 2 )2d x (4x x2 + 9 1 SOLUTION Let u = 4x 3 + 3x 2 . Then du = (12x 2 + 6x) d x or 6 du = (2x 2 + x) d x. Hence " " 1 1 1 u −2 du = − u −1 + C = − (4x 3 + 3x 2 )−1 + C. (4x 3 + 3x 2 )−2 (2x 2 + x) d x = 6 6 6

33.

35.

" " 4 5x + 2x dx3 2 2 2 )31)(x + x) d x (x 5(3x + x+

SOLUTION

Let u = x 5 + x 2 . Then du = (5x 4 + 2x) d x. Hence "

" 37.

"

"

1 1 1 1 1 du = − 2 + C = − + C. 3 5 2u 2 (x + x 2 )2 u

" (3x +2 9)310 d x 4 x (x + 1) d x

SOLUTION

39.

5x 4 + 2x dx = (x 5 + x 2 )3

Let u = 3x + 9. Then du = 3 d x or 13 du = d x. Hence " " 1 1 1 11 1 u 10 du = (3x + 9)10 d x = +C = u (3x + 9)11 + C. 3 3 11 33

" x x(x + 1)1/4 d10 x(3x + 9) d x

SOLUTION

Let u = x + 1. Then u − 1 = x and du = d x. Hence " " x(x + 1)1/4 d x = (u − 1)u 1/4 du " = =

" 41.

1 2

1 = 2

"

" (u + 1)u 3/2 du =

1 2

" (u 5/2 + u 3/2 ) du

1 2 7/2 1 2 5/2 1 u u + + C = (x 2 − 1)7/2 + (x 2 − 1)5/2 + C. 7 2 5 7 5

" sin5 x2 cos x 3d x x sin(x ) d x

SOLUTION

45.

4 4 (x + 1)9/4 − (x + 1)5/4 + C. 9 5

Let u = x 2 − 1. Then u + 1 = x 2 and du = 2x d x or 12 du = x d x. Hence " " x 3 (x 2 − 1)3/2 d x = x 2 · x(x 2 − 1)3/2 d x =

43.

4 9/4 4 5/4 − u +C u 9 5

" x 3 (x22 − 1)3/27 d x x (x + 1) d x

SOLUTION

"

(u 5/4 − u 1/4 ) du =

Let u = sin x. Then du = cos x d x. Hence " " 1 1 sin5 x cos x d x = u 5 du = u 6 + C = sin6 x + C. 6 6

" + 9) d x sec2 (4x x 2 sin(x 3 + 1) d x

279

280

CHAPTER 5

THE INTEGRAL SOLUTION

" 47.

" cos 2x 2 x tan24 dx xd x secsin (1 + 2x)

SOLUTION

" 49.

Let u = 4x + 9. Then du = 4 d x or 14 du = d x. Hence " " 1 1 1 sec2 (4x + 9) d x = sec2 u du = tan u + C = tan(4x + 9) + C. 4 4 4

Let u = 1 + sin 2x. Then du = 2 cos 2x or 12 du = cos 2x d x. Hence " " 1 1 1 (1 + sin 2x)−2 cos 2x d x = u −2 du = − u −1 + C = − (1 + sin 2x)−1 + C. 2 2 2

" √x − 1) d x cos x(3 sin sin 4x cos 4x + 1 d x

SOLUTION

Let u = 3 sin x − 1. Then du = 3 cos x d x or 13 du = cos x d x. Hence " " 1 1 2 1 1 u du = (3 sin x − 1) cos x d x = u + C = (3 sin x − 1)2 + C. 3 3 2 6

"

" 2 x(4√tan3 x − 3 tan2 x) d x sec cos x dx √ x SOLUTION Let u = tan x. Then du = sec2 x d x. Hence " " sec2 x(4 tan3 x − 3 tan2 x) d x = (4u 3 − 3u 2 ) du = u 4 − u 3 + C = tan4 x − tan3 x + C.

51.

" " 1)1/4 x 5 d x. 53. Evaluate (x 3 +5 Evaluate x x 3 + 1 d x using u = x 3 + 1. Hint: x 5 d x = x 3 · x 2 d x and x 3 = u − 1. SOLUTION

Let u = x 3 + 1. Then x 3 = u − 1 and du = 3x 2 d x or 13 du = x 2 d x. Hence " " " 1 1 x 5 (x 3 + 1)1/4 d x = u 1/4 (u − 1) du = (u 5/4 − u 1/4 ) du 3 3 4 4 3 1 4 9/4 4 5/4 − u +C = = u (x + 1)9/4 − (x 3 + 1)5/4 + C. 3 9 5 27 15 "

55. Evaluate sin x cos x d x using substitution in two different ways: ﬁrst using u = sin x and then u = cos x. Recon" Can They Both Be Right? Hannah uses the substitution u = tan x and Akiva uses u = sec x to evaluate cile thetan twox different 2 sec x d x.answers. Show that they obtain different answers and explain the apparent contradiction. SOLUTION

First, let u = sin x. Then du = cos x d x and " " 1 1 sin x cos x d x = u du = u 2 + C1 = sin2 x + C1 . 2 2

Next, let u = cos x. Then du = − sin x d x or −du = sin x d x. Hence, " " 1 1 sin x cos x d x = − u du = − u 2 + C2 = − cos2 x + C2 . 2 2 To reconcile these two seemingly different answers, recall that any two antiderivatives of a speciﬁed function differ by a constant. To show that this is true here, note that ( 12 sin2 x + C1 ) − (− 12 cos2 x + C2 ) = 12 + C1 − C2 , a constant. Here we used the trigonometric identity sin2 x + cos2 x = 1. " π π to the integral sin(3x + π ) d x? 57. What are the new limits of integration if we apply the substitution u = 3x + Some Choices Are Better Than Others Evaluate SOLUTION

" The new limits of integration are u(0) = 3sin · 0x+cos π= 2 xπdand x u(π ) = 3π + π = 4π .

0

" 8 In Exercises 59–70, use the Change of Variables Formula to evaluate the deﬁnite integral. twice. First use u = sin x to show that (4x − 9)20 d x? " 3Which of the following is the result of applying the substitution u = 4x − 9 to the integral " " 2 3 " 8 2 "+ (x 59. 8 2) d x sin x cos2 x d x = u1 1 − u du 1 u 20 du (b) u 20 du (a) 4 2 2 " 23 that u = cos x is a better choice. " 23 the integral on the right by a further substitution. Then and evaluate 1 show 20 (c) 4 u du (d) u 20 du 4 −1 −1

S E C T I O N 5.6 SOLUTION

Let u = x + 2. Then du = d x. Hence " 3 1

61.

(x + 2)3 d x =

34 1 4 5 54 3 − = 136. u du = u = 4 3 4 4 3

" 5

" 1 " 6 x √ dx +33 d x 0 (x 2 +x 1) 1

SOLUTION

63.

Substitution Method

Let u = x 2 + 1. Then du = 2x d x or 12 du = x d x. Hence " 1 " 1 2 1 1 1 1 −2 2 1 3 x d x = du = − u = − 16 + 4 = 16 = 0.1875. 2 1 u3 2 2 0 (x 2 + 1)3 1

" 4 " 2 2 x x√ + 9 d x 5x + 6 d x 0 −1

SOLUTION

Let u = x 2 + 9. Then du = 2x d x or 12 du = x d x. Hence " 4

1 x2 + 9 dx =

" 25 √

2 9

0

1 u du = 2

2 3/2 25 98 1 u = 3 (125 − 27) = 3 . 3 9

" 2 " 2 (x + 1)(xx2+ +32x)3 d x dx 1 0 (x 2 + 6x + 1)3 1 2 SOLUTION Let u = x + 2x. Then du = (2x + 2)d x and so 2 du = (x + 1)d x. Hence

65.

" 2 1

67.

(x + 1)(x 2 + 2x)3 d x =

" 1 8 3 1 1 4 8 1 4015 u du = u = (84 − 34 ) = . 2 3 2 4 8 8 3

" π /2 " 17 cos 3x d x−2/3 (x − 9) dx 0 10

SOLUTION

Let u = 3x. Then du = 3 d x or 13 du = d x. Hence " π /2 0

3π /2 " 1 1 3π /2 1 1 cos 3x d x = cos u du = sin u =− −0=− . 3 0 3 3 3 0

" π /2 " π /2 3

cos x sin x d xπ dx cos 3x + 0 2 0 SOLUTION Let u = cos x. Then du = − sin x d x. Hence

69.

" π /2 0

" π /4 71. Evaluate 0 SOLUTION

"

dx √

cos3 x sin x d x = −

using u = 2 +

2 x+sec2x) tan(2 x3d x

Let u = 2 +

√

√

" 0 1

u 3 du =

" 1 0

u 3 du =

1 4 1 1 1 u = −0= . 4 0 4 4

x.

1 d x, so that x. Then du = √ 2 x

√ 2 x du = d x 2(u − 2) du = d x. From this, we get: " " "

dx u−2 u −2 − 2u −3 du = 2 −u −1 + u −2 + C √ 3 = 2 3 du = 2 (2 + x) u √ √ 1 1 −2 − x + 1 1+ x =2 − √ + + C = −2 √ 2 +C =2 √ 2 √ 2 + C. 2+ x (2 + x) (2 + x) (2 + x)

substitution " 2use In Exercises 73–74, to evaluate the integral in terms of f (x). Evaluate r 5 − 4 − r 2 dr . 0

281

282

CHAPTER 5

THE INTEGRAL

" 73.

f (x)3 f (x) d x

SOLUTION

Let u = f (x). Then du = f (x) d x. Hence " " 1 1 f (x)3 f (x) d x = u 3 du = u 4 + C = f (x)4 + C. 4 4

" π /6 " 1/2 " 1 f (x) 75. Show that f (sin θ ) d θ = f (u) du. d x 0 0 1 − u2 f (x)2 SOLUTION Let u = sin θ . Then u(π /6) = 1/2 and u(0) = 0, as required. Furthermore, du = cos θ d θ , so that dθ = If sin θ = u, then u 2 + cos2 θ = 1, so that cos θ = " π /6 0

du . cos θ

1 − u 2 . Therefore d θ = du/ 1 − u 2 . This gives

f (sin θ ) d θ =

" 1/2 0

f (u)

1 1 − u2

du.

" π /2

Further Insights and sinn xChallenges cos x d x, where n is an integer, n = −1. Evaluate

0 77. Use the substitution u = 1 + x 1/n to show that " " 1 + x 1/n d x = n u 1/2 (u − 1)n−1 du

Evaluate for n = 2, 3. SOLUTION

"

n

Let u = 1 + x 1/n . Then x = (u − 1)n and d x = n(u − 1)n−1 du. Accordingly,

" 1 + x 1/n d x =

u 1/2 (u − 1)n−1 du. For n = 2, we have " " " 1 + x 1/2 d x = 2 u 1/2 (u − 1)1 du = 2 (u 3/2 − u 1/2 ) du =2

For n = 3, we have

"

4 2 5/2 2 3/2 4 − u u + C = (1 + x 1/2 )5/2 − (1 + x 1/2 )3/2 + C. 5 3 5 3 "

=3 =

" u 1/2 (u − 1)2 du = 3

1 + x 1/3 d x = 3

(u 5/2 − 2u 3/2 + u 1/2 ) du

2 2 2 7/2 − (2) u u 5/2 + u 3/2 + C 7 5 3

12 6 (1 + x 1/3 )7/2 − (1 + x 1/3 )5/2 + 2(1 + x 1/3 )3/2 + C. 7 5

" a

" π /2 79. Show that f "(x)π /2 d x = 0 ifd θf is an odd function. dθ Evaluate−a I = . Hint: Use substitution to show that I is equal to J = 6,000 6,000 θ 1 + tan θ 1 + cot 0 0 " continuous. SOLUTION We assume that f is If f (x) is an odd function, then f (−x) = − f (x). Let u = −x. Then π /2 x = and −u then and du = −d check thatx or I +−du J == d x. Accordingly, dθ. 0 " a " 0 " a " 0 " a f (x) d x = f (x) d x + f (x) d x = − f (−u) du + f (x) d x −a

−a

=

" a 0

0

f (x) d x −

" a 0

a

0

f (u) du = 0.

81. Show that the two regions in Figure 4 have the same area. Then use the identity cos2 u = 12 (1 + cos 2u) to compute (a) Use the substitution u = x/a to prove that the hyperbola y = x −1 (Figure 3) has the following special property: the second area. " b/a " b 1 1 dx = d x. If a, b > 0, then x a x 1 (b) Show that the areas under the hyperbola over the intervals [1, 2], [2, 4], [4, 8], . . . are all equal.

Chapter Review Exercises y

283

y

1

1

y = 1 − x2

y = cos2 u

x

p 2

1 (A)

u

(B)

FIGURE 4

!1 SOLUTION The area of the region in Figure 4(A) is given by 0 1 − x 2 d x. Let x = sin u. Then d x = cos u du and 1 − x 2 = 1 − sin2 u = cos u. Hence, " 1 " π /2 " π /2 1 − x2 dx = cos u · cos u du = cos2 u du. 0

0

0

This last integral represents the area of the region in Figure 4(B). The two regions in Figure 4 therefore have the same area. ! π /2 Let’s now focus on the deﬁnite integral 0 cos2 u du. Using the trigonometric identity cos2 u = 12 (1 + cos 2u), we have π /2 " " π /2 1 π /2 1 1 π 1 π cos2 u du = 1 + cos 2u du = = · −0= . u + sin 2u 2 0 2 2 2 2 4 0 0 83. Area of an Ellipse Prove the formula A = π ab for the area of the ellipse with equation Area of a Circle The number π is deﬁned as one-half the circumference of the unit circle. Prove that the area of a circle of radius r is A = π r 2 . The case r = 1x 2follows y 2 from Exercise 81. Prove it for all r > 0 by showing that + =1 " r a2 b2 " 1 " a r2 − x2 dx = r2 1 − x 2 d x. 2 0 0 use the formula for the area of a circle (Figure 5). 1 − (x/a) d x, change variables, and Hint: Show that A = 2b −a

y b

−a

a

x

−b

FIGURE 5 Graph of 2

x2 y2 + 2 = 1. 2 a b

2

Consider the ellipse with equation x 2 + y2 = 1; here a, b > 0. The area between the part of the ellipse in a b

!a 2 f (x) d x. By symmetry, the part of the elliptical the upper half-plane, y = f (x) = b2 1 − x 2 , and the x-axis is −a SOLUTION

a

region in the lower half-plane has the same area. Accordingly, the area enclosed by the ellipse is " a " a " a x2 2 f (x) d x = 2 b2 1 − 2 d x = 2b 1 − (x/a)2 d x a −a −a −a Now, let u = x/a. Then x = au and a du = d x. Accordingly, " a " 1

x 2

π = π ab 2b 1− d x = 2ab 1 − u 2 du = 2ab a 2 −a −1 !1 Here we recognized that −1 1 − u 2 du represents the area of the upper unit semicircular disk, which by Exercise 81 is 2( π4 ) = π2 .

CHAPTER REVIEW EXERCISES In Exercises 1–4, refer to the function f (x) whose graph is shown in Figure 1.

284

CHAPTER 5

THE INTEGRAL y 3 2 1 x 1

2

3

4

FIGURE 1

1. Estimate L 4 and M4 on [0, 4]. SOLUTION

and

With n = 4 and an interval of [0, 4], x = 4−0 4 = 1. Then, 1 5 23 L 4 = x( f (0) + f (1) + f (2) + f (3)) = 1 +1+ +2 = 4 2 4 1 3 5 7 1 9 9 M4 = x f + f + f + f =1 +2+ + = 7. 2 2 2 2 2 4 4

Estimate R4 , [a, L 4 ,b]and is larger than 3. Find an interval on M which R4 3]. 4 on [1,

" b a

f (x) d x. Do the same for L 4 .

! In general, R N is larger than ab f (x) d x on any interval [a, b] over which f (x) is increasing. Given the ! graph of f (x), we may take [a, b] = [0, 2]. In order for L 4 to be larger than ab f (x) d x, f (x) must be decreasing over the interval [a, b]. We may therefore take [a, b] = [2, 3]. SOLUTION

In Exercises 5–8, let f (x) = x 2 + 4x. Justify " 2 Sketch the graph of f (x) and the corresponding rectangles 5. Calculate R6 , M6 , and L 6 for f (x) on the interval 9 3 [1, 4]. f (x) d x ≤ ≤ for each approximation. 2 4 1

SOLUTION

Let f (x) = x 2 + 4x. A uniform partition of [1, 4] with N = 6 subintervals has x =

4−1 1 = , 6 2

and x ∗j = a +

x j = a + jx = 1 +

j−

j , 2

j 1 3 x = + . 2 4 2

Now, R6 = x 1 = 2

6 #

f (x j ) =

j=1

1 2

3 5 7 f + f (2) + f + f (3) + f + f (4) 2 2 2

33 65 105 463 + 12 + + 21 + + 32 = . 4 4 4 8

The rectangles corresponding to this approximation are shown below. y 35 30 25 20 15 10 5 x 1

2

3

4

Next, M6 = x 1 = 2

6 #

j=1

f (x ∗j ) =

1 2

5 7 9 11 13 15 f + f + f + f + f + f 4 4 4 4 4 4

105 161 225 297 377 465 + + + + + 16 16 16 16 16 16

=

The rectangles corresponding to this approximation are shown below.

1630 815 = . 32 16

Chapter Review Exercises y 35 30 25 20 15 10 5 x 1

2

3

4

Finally, L 6 = x

5 #

=

f (x j ) =

j=0

1 2

f (1) + f

3 5 7 + f (2) + f + f (3) + f 2 2 2

1 33 65 105 5+ + 12 + + 21 + 2 4 4 4

=

355 . 8

The rectangles corresponding to this approximation are shown below. y 35 30 25 20 15 10 5 x 1

2

3

4

" 2 " 4 7. Find a formula for L N for f (x) on [0, 2] and compute f (x) d x by taking the limit. Find a formula for R N for f (x) on [1, 4] and compute f (x) d x by taking the limit. 0 SOLUTION

1 Let f (x) = x 2 + 4x and N be a positive integer. Then

x =

2 2−0 = N N

and x j = a + jx = 0 +

2j 2j = N N

for 0 ≤ j ≤ N . Thus, L N = x

N −1 #

f (x j ) =

j=0

=

−1 N −1 −1 # 4 j2 2 N# 8j 8 N# 2 + 16 + j j = N j=0 N 2 N N 3 j=0 N 2 j=0

32 12 4 4(N − 1)(2N − 1) 8(N − 1) + . = + + N 3 N 3N 2 3N 2

Finally, " 2 0

4 32 12 + + N N →∞ 3 3N 2

f (x) d x = lim

=

32 . 3

9. Calculate R6 , M6 , and L 6 for f (x)" =x (x 2 + 1)−1 on the interval [0, 1]. Use FTC I to evaluate A(x) = f (t) dt. SOLUTION Let f (x) = (x 2 + 1)−1 . A uniform partition of [0, 1] with N = 6 subintervals has −2 x =

1 1−0 = , 6 6

and x ∗j = a +

j−

x j = a + jx =

j , 6

1 2j − 1 x = . 2 12

Now, R6 = x

6 # j=1

f (x j ) =

1 6

1 1 1 2 5 f + f + f + f + f + f (1) 6 3 2 3 6

285

286

CHAPTER 5

THE INTEGRAL

=

1 6

36 9 4 9 36 1 + + + + + 37 10 5 13 61 2

≈ 0.742574.

Next, M6 = x =

1 6

6 #

f (x ∗j ) =

j=1

1 6

1 1 5 7 3 11 f + f + f + f + f + f 12 4 12 12 4 12

144 16 144 144 16 144 + + + + + 145 17 169 193 25 265

≈ 0.785977.

Finally, L 6 = x

5 #

=

f (x j ) =

j=0

1 6

1 1 1 2 5 + f + f + f + f 6 3 2 3 6

f (0) + f

1 36 9 4 9 36 1+ + + + + 6 37 10 5 13 61

≈ 0.825907.

11. Which approximation to the area is represented by the shaded rectangles in Figure 3? Compute R5 and L 5 . Let R N be the N th right-endpoint approximation for f (x) = x 3 on [0, 4] (Figure 2). y 64(N + 1)2 . (a) Prove that R N = 30 N2 (b) Prove that the area of the region below the right-endpoint rectangles and above the graph is equal to 18

64(2N + 1) . N2

6

x 1

2

3

4

5

FIGURE 3 SOLUTION There are ﬁve rectangles and the height of each is given by the function value at the right endpoint of the subinterval. Thus, the area represented by the shaded rectangles is R5 . From the ﬁgure, we see that x = 1. Then

R5 = 1(30 + 18 + 6 + 6 + 30) = 90

and

L 5 = 1(30 + 30 + 18 + 6 + 6) = 90.

In Exercises 13–34, evaluate the integral. Calculate any two Riemann sums for f (x) = x 2 on the interval [2, 5], but choose partitions with at least ﬁve " subintervals of unequal widths and intermediate points that are neither endpoints nor midpoints. 13. (6x 3 − 9x 2 + 4x) d x " 3 (6x 3 − 9x 2 + 4x) d x = x 4 − 3x 3 + 2x 2 + C. SOLUTION 2 " " 13 − 1)2 d x 15. (2x (4x 3 − 2x 5 ) d x " " 0 4 (2x 3 − 1)2 d x = (4x 6 − 4x 3 + 1) d x = x 7 − x 4 + x + C. SOLUTION 7 " 4 x" 4+ 1 17. dx x 2(x 5/2 − 2x −1/2 ) d x 1 " 4 " 1 x +1 SOLUTION d x = (x 2 + x −2 ) d x = x 3 − x −1 + C. 2 3 x " 4 " 42 |x − 9| d x 19. r −2 dr −1 1

SOLUTION

" 4 −1

|x 2 − 9| d x =

" 3 −1

(9 − x 2 ) d x +

= (27 − 9) − −9 + " 21.

" 32 θ d θ csc [t] dt 1

" 4 3

1 3

(x 2 − 9) d x =

+

9x −

4 1 3 3 1 3 x + x − 9x 3 3 −1 3

64 − 36 − (9 − 27) = 30. 3

Chapter Review Exercises

" csc2 θ d θ = − cot θ + C.

SOLUTION

"

" π2/4 (9t − 4) dt sec sec t tan t dt 0 SOLUTION Let u = 9t − 4. Then du = 9dt and " " 1 1 1 sec2 u du = tan u + C = tan(9t − 4) + C. sec2 (9t − 4) dt = 9 9 9

23.

"

" π−/34)11 dt (9t sin 4θ d θ 0 SOLUTION Let u = 9t − 4. Then du = 9dt and " " 1 1 12 1 u 11 du = (9t − 4)11 dt = u +C = (9t − 4)12 + C. 9 108 108

25.

"

" 22 (3 sin θ ) cos(3θ ) d θ 4y + 1 d y 6 SOLUTION Let u = sin(3θ ). Then du = 3 cos(3θ )d θ and " " 1 1 1 u 2 du = u 3 + C = sin3 (3θ ) + C. sin2 (3θ ) cos(3θ ) d θ = 3 9 9

27.

" 29.

(2x " π3/2+ 3x) d x 2 )5 θ ) sin θ d θ (3x 4 +sec 9x2 (cos 0

SOLUTION

Let u = 3x 4 + 9x 2 . Then du = (12x 3 + 18x) d x = 6(2x 3 + 3x) d x and "

" 31.

1 (2x 3 + 3x) d x = 6 (3x 4 + 9x 2 )5

"

u −5 du = −

1 −4 1 u + C = − (3x 4 + 9x 2 )−4 + C. 24 24

√ " −2 sin θ 412x − cos d xθ dθ −4 (x 2 + 2)3

SOLUTION

" 33.

Let u = 4 − cos θ . Then du = sin θ d θ and " " √ 2 2 sin θ 4 − cos θ d θ = u 1/2 du = u 3/2 + C = (4 − cos θ )3/2 + C. 3 3

/3 + sin y" π2y 3 dθ y 0

SOLUTION

dθ cos2/3 θ Let u = 2y + 3. Then du = 2d y, y = 12 (u − 3) and " " " √ 1 1 (u − 3) u du = (u 3/2 − 3u 1/2 ) du y 2y + 3 d y = 4 4 1 1 2 5/2 1 − 2u 3/2 + C = = u (2y + 3)5/2 − (2y + 3)3/2 + C. 4 5 10 2

35. Combine to write as a single integral " 8 √ " 8 t 2 t + 8 dt 1

0 SOLUTION

f (x) d x +

" 0 −2

f (x) d x +

" 6 8

f (x) d x

First, rewrite " 8 0

f (x) d x =

" 6 0

f (x) d x +

" 8 6

f (x) d x

and observe that " 6 8

f (x) d x = −

" 8 6

f (x) d x.

287

288

CHAPTER 5

THE INTEGRAL

Thus, " 8 0

f (x) d x +

" 6 8

f (x) d x =

" 6 0

f (x) d x.

Finally, " 8 0

f (x) d x +

" 0 −2

f (x) d x +

" 6 8

f (x) d x =

" 6 0

f (x) d x +

" 0 −2

f (x) d x =

" 6 −2

f (x) d x.

" x t dt " x . 37. Find inﬂection points of A(x) = Let A(x) = f (x) d x, where + is 1 the function shown in Figure 4. Indicate on the graph of f where the 3 ft 2(x) SOLUTION Let local minima,

0

maxima, and points of inﬂection of A(x) occur and identify the intervals where A(x) is increasing, decreasing, concave up, or concave down. " x t dt . A(x) = 3 t2 + 1

Then x A (x) = 2 x +1 and A (x) =

(x 2 + 1)(1) − x(2x) 1 − x2 = 2 . 2 2 (x + 1) (x + 1)2

Thus, A(x) is concave down for |x| > 1 and concave up for |x| < 1. A(x) therefore has inﬂection points at x = ±1. 2 (in thousands of gallons per hour), 39. On A a typical city watert at ratemoves of r (t) = velocity 100 + 72t − as 3t shown particleday, startsa at theconsumes origin at time = the 0 and with v(t) in Figure 5. where t is the number of hours past midnight. What is the daily water consumption? How much water is consumed (a) How many times does the particle return to the origin in the ﬁrst 12 s? between 6 PM and midnight? (b) Where is the particle located at time t = 12? SOLUTION With a consumption rate of r (t) = 100 + 72t − 3t 2 thousand gallons per hour, the daily consumption of (c) At which time t is the particle’s distance to the origin at a maximum? water is " 24 24 (100 + 72t − 3t 2 ) dt = 100t + 36t 2 − t 3 = 100(24) + 36(24)2 − (24)3 = 9312, 0

0

or 9.312 million gallons. From 6 PM to midnight, the water consumption is " 24 18

24

(100 + 72t − 3t 2 ) dt = 100t + 36t 2 − t 3 18

= 100(24) + 36(24)2 − (24)3 − 100(18) + 36(18)2 − (18)3 = 9312 − 7632 = 1680,

or 1.68 million gallons. 41. Cost engineers at NASA have the task of projecting the cost P of major space projects. It has been found that the per bicycle), which The learning curve for producing bicycles certain = 12x, −1/5 cost C of developing a projection increases with Pinata the rate factory dC/d Pis≈L(x) 21P −0.65 where(in C hours is in thousands of dollars means that it takes a bike mechanic L(n) hours to assemble the nth bicycle. If 24 bicycles are produced, and P in millions of dollars. What is the cost of developing a projection for a project whose cost turns out to be how P =long $35 does it take to produce the second batch of 12? million? SOLUTION Assuming it costs nothing to develop a projection for a project with a cost of $0, the cost of developing a projection for a project whose cost turns out to be $35 million is

" 35 0

35 21P −0.65 d P = 60P 0.35 = 60(35)0.35 ≈ 208.245, 0

or $208,245. 43. Let fof(x) a positive increasing continuous function b], where 0 ≤ a

Chapter Review Exercises

289

y f (b)

y = f (x) f (a) x a

b

FIGURE 7 SOLUTION We can construct the shaded region in Figure 7 by taking a rectangle of length b and height f (b) and removing a rectangle of length a and height f (a) as well as the region between the graph of y = f (x) and the x-axis over the interval [a, b]. The area of the resulting region is then the area of the large rectangle minus the area of the small rectangle and minus the area under the curve y = f (x); that is, " b f (x) d x. I = b f (b) − a f (a) −

a

In Exercises 45–49, express the limit as an integral (or multiple of an integral) and evaluate. How can we interpret the quantity I in Eq. (1) if a < b ≤ 0? Explain with a graph. N 2j 2 # 45. lim sin N N →∞ N j=1 Let f (x) = sin x and N be a positive integer. A uniform partition of the interval [0, 2] with N subintervals

SOLUTION

has x =

2 N

and

xj =

2j N

for 0 ≤ j ≤ N . Then N N # 2 # 2j sin f (x j ) = R N ; = x N j=1 N j=1 consequently, 2 " 2 N 2j 2 # sin sin x d x = − cos x = 1 − cos 2. = N N →∞ N 0 0 j=1 lim

−1 π N# π j N π # +4k 47. lim 4 sin limN j=0 32+ N N →∞ N N →∞ N k=1

SOLUTION Let f (x) = sin x and N be a positive integer. A uniform partition of the interval [π /2, 3π /2] with N subintervals has

x =

π N

and

xj =

π πj + 2 N

for 0 ≤ j ≤ N . Then −1 N −1 # π N# π πj sin f (x j ) = L N ; + = x N j=0 2 N j=0 consequently, 3π /2 " 3π /2 −1 π N# π πj sin sin x d x = − cos x = 0. + = 2 N N →∞ N π /2 π /2 j=0 lim

1k + 2kN+ · · · N k (k > 0) 4 # 1 N →∞ N k+1 lim 4k 2 N N →∞ + ) k=1 (3 SOLUTION Observe that N

49.

lim

1 1k + 2k + 3k + · · · + N k = N N k+1

)

k k k * N k 2 3 N 1 # j 1 k + + + ··· . = N N N N N j=1 N

290

CHAPTER 5

THE INTEGRAL

Now, let f (x) = x k and N be a positive integer. A uniform partition of the interval [0, 1] with N subintervals has x =

1 N

xj =

and

j N

for 0 ≤ j ≤ N . Then N k N # 1 # j = x f (x j ) = R N ; N j=1 N j=1

consequently, 1 " 1 N k 1 1 1 # j = xk dx = x k+1 = . N k + 1 k + 1 N →∞ N 0 0 j=1 lim

" 1 " π /4 9 f (x) dxx, dassuming that f (x) is an even continuous function such that 51. Evaluate x , using the properties of odd functions. Evaluate 0 2 cos x −π /4 " 2 " 1 f (x) d x = 5, f (x) d x = 8 −2

1

Using the given information

SOLUTION

" 2 −2

f (x) d x =

" 1 −2

f (x) d x +

" 2 1

f (x) d x = 13.

Because f (x) is an even function, it follows that " 0 −2

f (x) d x =

" 2 0

f (x) d x,

so " 2 0

f (x) d x =

13 . 2

Finally, " 1 0

f (x) d x =

" 2 0

f (x) d x −

" 2 1

f (x) d x =

13 3 −5= . 2 2

53. Show that Plot the graph of f (x) = sin mx sin " nx on [0, π ] for the pairs (m, n) = (2, 4), (3, 5) and in each case guess " π f (x) d x =more x F(x) − G(x) f (x) d x. Experiment xwith a few values (including two cases with m = n) and formulate the value of I = 0

" a conjecture for when I is zero. where F (x) = f (x) and G (x) = F(x). Use this to evaluate x cos x d x. Suppose F (x) = f (x) and G (x) = F(x). Then

SOLUTION

d (x F(x) − G(x)) = x F (x) + F(x) − G (x) = x f (x) + F(x) − F(x) = x f (x). dx Therefore, x F(x) − G(x) is an antiderivative of x f (x) and " x f (x) d x = x F(x) − G(x) + C. To evaluate

55.

!

x cos x d x, note that f (x) = cos x. Thus, we may take F(x) = sin x and G(x) = − cos x. Finally, " x cos x d x = x sin x + cos x + C.

" 2 Prove Plot the graph of f (x) = x −2 sin x and show that 0.2 ≤ f (x) d x ≤ 0.9. 1 " " 2 2 1 1 2x d x ≤ 4 and 3−x d x ≤ . 2≤ ≤ 9 3 1 1

Chapter Review Exercises SOLUTION

291

Let f (x) = x −2 sin x. From the ﬁgure below, we see that 0.2 ≤ f (x) ≤ 0.9

for 1 ≤ x ≤ 2. Therefore, 0.2 =

" 1 0

0.2 d x ≤

" 1 0

f (x) d x ≤

" 1 0

0.9 d x = 0.9.

y 1 0.8 0.6

x−2sin x

0.4 0.2 x 0.5

1

1.5

2

In Exercises 57–62, ﬁnd the derivative. " 1 " xbounds for Find upper and lower f (x) d x, where f (x) has the graph shown in Figure 8. 57. A (x), where A(x) = sin(t 3 ) dt 0 3

SOLUTION

Let A(x) =

" x 3

sin(t 3 ) dt. Then A (x) = sin(x 3 ).

" y d " x 3x d x cos t (π ), d y A−2 where A(x) = dt 2 1+t " y d SOLUTION 3x d x = 3 y . d y −2 " x3 "√ sin x 61. G (2), where G(x) = t + 13 dt t dt G (x), where G(x) = 0 −2 " x3 √ SOLUTION Let G(x) = t + 1 dt. Then 59.

0

G (x) =

x3 + 1

d 3 x = 3x 2 x 3 + 1 dx

√ and G (2) = 3(2)2 8 + 1 = 36. " 9 If f (x) is increasing and concave up on [a, b], then L N is more accurate than R N . Explain with a graph: 63. 1 (1), where H (x) = dt Which isHmore accurate if f (x) is increasing and concave down? 4x 2 t SOLUTION Consider the ﬁgure below, which displays a portion of the graph of an increasing, concave up function. y

x

The shaded rectangles represent the differences between the right-endpoint approximation R N and the left-endpoint approximation L N . In particular, the portion of each rectangle that lies below the graph of y = f (x) is the amount by which L N underestimates the area under the graph, whereas the portion of each rectangle that lies above the graph of y = f (x) is the amount by which R N overestimates the area. Because the graph of y = f (x) is increasing and concave up, the lower portion of each shaded rectangle is smaller than the upper portion. Therefore, L N is more accurate (introduces less error) than R N . By similar reasoning, if f (x) is increasing and concave down, then R N is more accurate than L N . Explain with a graph: If f (x) is linear on [a, b], then the

" b a

f (x) d x =

1 (R N + L N ) for all N . 2

APPLICATIONS OF 6 THE INTEGRAL 6.1 Area Between Two Curves Preliminary Questions 1. What is the area interpretation of

" b a

f (x) − g(x) d x if f (x) ≥ g(x)?

!b

SOLUTION Because f (x) ≥ g(x), a ( f (x) − g(x)) d x represents the area of the region bounded between the graphs of y = f (x) and y = g(x), bounded on the left by the vertical line x = a and on the right by the vertical line x = b. " b 2. Is f (x) − g(x) d x still equal to the area between the graphs of f and g if f (x) ≥ 0 but g(x) ≤ 0?

a

SOLUTION

Yes. Since f (x) ≥ 0 and g(x) ≤ 0, it follows that f (x) − g(x) ≥ 0.

3. Suppose that f (x) ≥ g(x) on [0, 3] and g(x) ≥ f (x) on [3, 5]. Express the area between the graphs over [0, 5] as a sum of integrals. SOLUTION Remember that to calculate an area between two curves, one must subtract the equation for the lower curve from the equation for the upper curve. Over the interval [0, 3], y = f (x) is the upper curve. On the other hand, over the interval [3, 5], y = g(x) is the upper curve. The area between the graphs over the interval [0, 5] is therefore given by " 3 " 5 ( f (x) − g(x)) d x + (g(x) − f (x)) d x.

0

3

4. Suppose that the graph of x = f (y) lies to the left of the y-axis. Is

" b a

f (y) d y positive or negative?

If the graph of x = f (y) lies to the left of the y-axis, then for each value of y, the corresponding value of x ! is less than zero. Hence, the value of ab f (y) d y is negative. SOLUTION

Exercises 1. Find the area of the region between y = 3x 2 + 12 and y = 4x + 4 over [−3, 3] (Figure 8). y y = 3x 2 + 12

50

25 y = 4x + 4

−2 −3

−1

x 1

2

3

FIGURE 8

As the graph of y = 3x 2 + 12 lies above the graph of y = 4x + 4 over the interval [−3, 3], the area between the graphs is " 3 " 3

3 (3x 2 + 12) − (4x + 4) d x = (3x 2 − 4x + 8) d x = x 3 − 2x 2 + 8x = 102. SOLUTION

−3

−3

−3

3. Let f (x) = x and g(x) = 2 − x 2 [Figure 9(B)]. Compute the area of the region in Figure 9(A), which lies between y = 2 − x 2 and y = −2 over [−2, 2]. (a) Find the points of intersection of the graphs. (b) Find the area enclosed by the graphs of f and g. SOLUTION

(a) Setting f (x) = g(x) yields 2 − x 2 = x, which simpliﬁes to 0 = x 2 + x − 2 = (x + 2)(x − 1). Thus, the graphs of y = f (x) and y = g(x) intersect at x = −2 and x = 1.

S E C T I O N 6.1

Area Between Two Curves

293

(b) As the graph of y = x lies below the graph of y = 2 − x 2 over the interval [−2, 1], the area between the graphs is 1 " 1

1 1 9 (2 − x 2 ) − x d x = 2x − x 3 − x 2 = . 3 2 2 −2 −2 In Exercises 5–7, ﬁnd the area between y = sin x and y = cos x over the interval. Sketch the curves if necessary. Let f (x) = 8x − 10 and g(x) = x 2 − 4x + 10. π 5. (a) 0, Find the points of intersection of the graphs. (b) 4Compute the area of the region below the graph of f and above the graph of g. π SOLUTION Over the interval [0, 4 ], the graph of y = sin x lies below that of y = cos x. Hence, the area between the two curves is √ √ " π /4 π /4 √ 2 2 = + − (0 + 1) = 2 − 1. (cos x − sin x) d x = (sin x + cos x) 2 2 0 0 7. [0, π ] π π , π π SOLUTION 4 2Over the interval [0, 4 ], the graph of y = sin x lies below that of y = cos x, while over the interval [ 4 , π ], the orientation of the graphs is reversed. The area between the graphs over [0, π ] is then " π /4 0

(cos x − sin x) d x +

" π π /4

(sin x − cos x) d x

π π /4 + (− cos x − sin x) = (sin x + cos x) 0

√

π /4

√ √ 2 2 2 2 = + − (0 + 1) + (1 − 0) − − − = 2 2. 2 2 2 2 √

√

In Exercises 8–10, let f (x) = 20 + x − x 2 and g(x) = x 2 − 5x. 9. Find the area of the region enclosed by the two graphs. Find the area between the graphs of f and g over [1, 3]. SOLUTION Setting f (x) = g(x) gives 20 + x − x 2 = x 2 − 5x, which simpliﬁes to 0 = 2x 2 − 6x − 20 = 2(x − 5)(x + 2). Thus, the curves intersect at x = −2 and x = 5. With y = 20 + x − x 2 being the upper curve, the area between the two curves is 5 " 5

" 5

2 343 (20 + x − x 2 ) − (x 2 − 5x) d x = 20 + 6x − 2x 2 d x = 20x + 3x 2 − x 3 = . 3 3 −2 −2 −2 In Exercises 11–14, of the between shaded region ingraphs the ﬁgure. Compute theﬁnd areathe of area the region the two over [4, 8] as a sum of two integrals. 11.

y

y = x 3 − 2x 2 + 10 −2

x y=

3x 2

2 + 4x − 10

FIGURE 10 SOLUTION

As the graph of y = x 3 − 2x 2 + 10 lies above the graph of y = 3x 2 + 4x − 10, the area of the shaded

region is " 2

" 2

(x 3 − 2x 2 + 10) − (3x 2 + 4x − 10) d x = x 3 − 5x 2 − 4x + 20 d x −2

−2

=

2 1 4 5 3 160 x − x − 2x 2 + 20x = . 4 3 3 −2

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CHAPTER 6

A P P L I C AT I O N S O F T H E I N T E G R A L

13.

y

1 y=− x 2

−1

x

1 y = x 1 − x2

FIGURE 12 √ √ 3 3 1 1 SOLUTION Setting 2 x = x 1 − x 2 yields x = 0 or 2 = 1 − x 2 , so that x = ± 2 . Over the interval [− 2 , 0], √ y = 12 x is the upper curve but over the interval [0, 23 ], y = x 1 − x 2 is the upper curve. The area of the shaded region is then " 0 " √3/2 1 2 dx + 2 − 1 x dx 1 − x 1 − x x − x x √ 2 − 3/2 2 0

=

0 √ 5 5 1 1 2 3/2 5 1 2 1 2 3/2 2 3/2 x + (1 − x ) + = . + − (1 − x ) − x = √ 4 3 3 4 48 48 24 − 3/2 0

15. Find the area of the region enclosed by the curves y = x 3 − 6x and y = 8 − 3x 2 . Setting x 3 − 6x = 8 − 3x 2 yields (x + 1)(x + 4)(x − 2) = 0, so the two curves intersect at x = −4, x = −1 and x = 2. Over the interval [−4, −1], y = x 3 − 6x is the upper curve, while y = 8 − 3x 2 is the upper curve over the interval [−1, 2]. The area of the region enclosed by the two curves is then SOLUTION

" 2

" −1

(x 3 − 6x) − (8 − 3x 2 ) d x + (8 − 3x 2 ) − (x 3 − 6x) d x −4

=

−1

−1 2 1 4 81 1 81 81 x − 3x 2 − 8x + x 3 + 8x − x 3 − x 4 + 3x 2 = + = . 4 4 4 4 2 −4 −1

In Exercises 17–18, ﬁnd the area between the graphs of x = sin y and x = 21 − cos y over the given interval (Figure 14). Find the area of the region enclosed by the semicubical parabola y = x 3 and the line x = 1. y x = sin y x = 1 − cos y 2

x −

2

FIGURE 14

π 17. 0 ≤ y ≤ 2 SOLUTION As shown in the ﬁgure, the graph on the right is x = sin y and the graph on the left is x = 1 − cos y. Therefore, the area between the two curves is given by

" π /2 0

π /2 π π = − + 1 − (−1) = 2 − . (sin y − (1 − cos y)) d y = (− cos y − y + sin y) 2 2 0

2 19. Find the π area ofπthe region lying to the right of x = y + 4y − 22 and the left of x = 3y + 8. − ≤y≤ 2 Setting2 y 2 + 4y − 22 = 3y + 8 yields SOLUTION

0 = y 2 + y − 30 = (y + 6)(y − 5), so the two curves intersect at y = −6 and y = 5. The area in question is then given by " 5

−6

(3y + 8) − (y 2 + 4y − 22)

5 " 5

y3 1331 y2 2 −y − y + 30 d y = − dy = − + 30y = . 3 2 6 −6 −6

y-axis and as an integral 21. Calculate the area enclosed by x = 9 − y 2 and x = 5 in 2two ways: as an integral along the Find the area of the region lying to the right of x = y − 5 and the left of x = 3 − y 2 . along the x-axis.

S E C T I O N 6.1 SOLUTION

Area Between Two Curves

295

Along the y-axis, we have points of intersection at y = ±2. Therefore, the area enclosed by the two curves

is 2 " 2

" 2

32 1 . 9 − y2 − 5 d y = 4 − y 2 d y = 4y − y 3 = 3 3 −2 −2 −2 Along the x-axis, we have integration limits of x = 5 and x = 9. Therefore, the area enclosed by the two curves is 9 " 9 √ 4 32 32 2 9 − x d x = − (9 − x)3/2 = 0 − − = . 3 3 3 5 5 In Exercises 23–24, ﬁnd the area of the region using the method (integration along either the x- or y-axis) that requires Figure 15 shows the graphs of x = y 3 − 26y + 10 and x = 40 − 6y 2 − y 3 . Match the equations with the curve you to evaluate just one integral. and compute the area of the shaded region. 23. Region between y 2 = x + 5 and y 2 = 3 − x From the ﬁgure below, we see that integration along the x-axis would require two integrals, but integration along the y-axis requires only one integral. Setting y 2 − 5 = 3 − y 2 yields points of intersection at y = ±2. Thus, the area is given by SOLUTION

2 " 2

" 2

64 2 . (3 − y 2 ) − (y 2 + 5) d y = 8 − 2y 2 d y = 8y − y 3 = 3 3 −2 −2 −2 y 2

x = 3 − y2

1 −4

x

−2

−1

x = y2 − 5

2

−2

In Exercises 25–41, sketch thex region enclosed by the and compute its area as an integral along the x- or y-axis. Region between y= and x + y = 8 over [2,curves 3] 25. y = 4 − x 2 ,

y = x2 − 4

Setting 4 − x 2 = x 2 − 4 yields 2x 2 = 8 or x 2 = 4. Thus, the curves y = 4 − x 2 and y = x 2 − 4 intersect at x = ±2. From the ﬁgure below, we see that y = 4 − x 2 lies above y = x 2 − 4 over the interval [−2, 2]; hence, the area of the region enclosed by the curves is SOLUTION

2 " 2 2 64 (8 − 2x 2 ) d x = 8x − x 3 = . (4 − x 2 ) − (x 2 − 4) d x = 3 3 −2 −2 −2

" 2

y y = 4 − x2

4 2 −2

−1

x −2 −4

2 27. x =y sin = xy,2 −x6,= πy y= 6 − x 3 , SOLUTION

1

2

y = x2 − 4

y-axis

Here, integration along the y-axis will require less work than integration along the x-axis. The curves

π intersect when 2y π = sin y or when y = 0, ± 2 . From the graph below, we see that both curves are symmetric with respect to the origin. It follows that the portion of the region enclosed by the curves in the ﬁrst quadrant is identical to the region enclosed in the third quadrant. We can therefore determine the total area enclosed by the two curves by doubling the area enclosed in the ﬁrst quadrant. In the ﬁrst quadrant, x = sin y lies to the right of x = 2y π , so the total area enclosed by the two curves is

2

π /2 " π /2

1 π π 2 =2 0− − (−1 − 0) = 2 − . sin y − y d y = 2 − cos y − y 2 π π 4 2 0 0

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CHAPTER 6

A P P L I C AT I O N S O F T H E I N T E G R A L y x=

2

y

1 x = sin y x

−1

1 −1

−3 , y = 4 − x, y = x 29. y =x 3x +y= 4, x − y = 0, y 3+ 3x = 4

The curves y = 3x −3 and y = 4 − x intersect at x = 1, the curves y = 3x −3 and y = x/3 intersect at x = 3 and the curves y = 4 − x and y = x/3 intersect at x = 3. From the graph below, we see that the top of the −3 region enclosed √ by the three curves is√always bounded by y = 4 − x. The bottom of the region is bounded by y = 3x for 1 ≤ x ≤ 3 and by y = x/3 for 3 ≤ x ≤ 3. The total area of the region is then SOLUTION

√

√ " √3

" 3 2 2 3 1 1 2 3 −2 3 −3 (4 − x) − 3x d x + √ (4 − x) − x d x = 4x − x + x + 4x − x √ = 2. 3 2 2 3 1 3 3 1 y 3 y= 3 x3

2

1 y=

y=4−x

x 3 x 1

0

2

3

√ √ 2, y = −x√ x − 2, x = 4 31. y = x x − √ y = 2 − x, y = x, √ x =0 √ SOLUTION Note that y = x x − 2 and y = −x x − 2 are the upper and lower branches, respectively, of the curve y 2 = x 2 (x − 2). The area enclosed by this curve and the vertical line x = 4 is " 4 " 4

√ √ √ x x − 2 − (−x x − 2) d x = 2x x − 2 d x. 2

2

Substitute u = x − 2. Then du = d x, x = u + 2 and √ " 2

4 5/2 8 3/2 2 128 2 3/2 1/2 u 2u du = 2x x − 2 d x = 2(u + 2) u du = + 4u + u = 15 . 5 3 2 0 0 0

" 4

" 2

√

√

y 4

y 2 = x 2 (x − 2)

2 x −2

1

2

3

4

−4

33. x = |y|, x = 6 − y22 y = |x|, y = x − 6 SOLUTION From the graph below, we see that integration along the y-axis will require less work than integration along the x-axis. Moreover, the region is symmetric with respect to the x-axis, so the total area can be determined by doubling the area of the upper portion of the region. For y > 0, setting y = 6 − y 2 yields 0 = y 2 + y − 6 = (y − 2)(y + 3), so the curves intersect at y = 2. Because x = 6 − y 2 lies to the right of x = |y| = y in the ﬁrst quadrant, we ﬁnd the total area of the region is 2

2 " 2 1 1 8 44 6 − y 2 − y d y = 2 6y − y 3 − y 2 = 2 12 − − 2 = . 3 2 3 3 0 0

S E C T I O N 6.1

Area Between Two Curves

297

y 2 x = |y| 1 x 1

−1

2

3

4

5

6

x = 6 − y2

−2

35. x = 12 − y, x = y, x = 2y x = |y|, x = 1 − |y| SOLUTION From the graph below, we see that the bottom of the region enclosed by the three curves is always bounded by y = x2 . On the other hand, the top of the region is bounded by y = x for 0 ≤ x ≤ 6 and by y = 12 − x for 6 ≤ x ≤ 8. The area of the region is then 8 " 6

" 8

x x 3 1 6 x− (12 − x) − dx + d x = x 2 + 12x − x 2 = 9 + (96 − 48) − (72 − 27) = 12. 2 2 4 0 4 0 6 6 y 6

y = 12 − x

4

y=x

2

y=

x 2 x

0

2

4

6

8

37. x = 2y, 3x + 1 = (y − 1)2 x = y − 18y, y + 2x = 0 SOLUTION Setting 2y = (y − 1)2 − 1 yields 0 = y 2 − 4y = y(y − 4), so the two curves intersect at y = 0 and at y = 4. From the graph below, we see that x = 2y lies to the right of x + 1 = (y − 1)2 over the interval [0, 4] along the y-axis. Thus, the area of the region enclosed by the two curves is 4 " 4

" 4

1 32 . 2y − ((y − 1)2 − 1) d y = 4y − y 2 d y = 2y 2 − y 3 = 3 3 0 0 0 y 4

x + 1 = ( y − 1) 2

3 x = 2y

2 1

x 2

4

6

8

+ x 2 (in1/2 the region x > 0) 39. y = 6, y = x −2 1/2 x + y = 1, x +y =1 −2 SOLUTION Setting 6 = x + x 2 yields 0 = x 4 − 6x 2 + 1, which is a quadratic equation in the variable x 2 . By the quadratic formula, √ √ 6 ± 36 − 4 = 3 ± 2 2. x2 = 2 Now, √ √ √ √ and 3 − 2 2 = ( 2 − 1)2 , 3 + 2 2 = ( 2 + 1)2 √ √ so the two curves intersect at x = 2 − 1 and x = 2 + 1. Note there are also two points of intersection with x < 0, but as the problem speciﬁes the region is for x > 0, we neglect these other two values. From the graph below, we see that y = 6 lies above y = x −2 + x 2 , so the area of the region enclosed by the two curves for x > 0 is √ " √2+1

1 3 2+1 16 −2 2 −1 6 − (x + x ) d x = 6x + x − x √ = . √ 3 3 2−1 2−1

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CHAPTER 6

A P P L I C AT I O N S O F T H E I N T E G R A L y 6

4 y = x −2 + x 2 2

x 0

1

2

3

π 3π 41. y = sin x, y = csc2 x, x = , x = 2π y = cos x, y = cos(2x), 4x = 0, x 4= 3 π 3π SOLUTION Over the interval [ 4 , 4 ], y = csc2 x lies above y = sin x. The area of the region enclosed by the two curves is then √ √ " 3π /4 √ 2 3π /4 2 2 csc x − sin x d x = − cot x + cos x = 1− − −1 + = 2 − 2. 2 2 π /4 π /4 y 2 1.5

y = csc 2 x

1

y = sin x

0.5 x 0

0.5

1

1.5

2

43. Sketch a region whose x area is represented2by Plot y = the same set of axes. Use a computer algebra system to ﬁnd the points and y = (x − 1) on " √2/2 x2 + 1 2 −curves. of intersection numerically and compute the √ area between 1 − x the |x| d x − 2/2

and evaluate using geometry.

1 − x 2 − |x| with the yTOP − yBOT template for calculating area, we see that the region in question is bounded along the top by the curve y = 1 − x 2 (the upper half of the unit circle) and is bounded along the bottom by the curve y = |x|. Hence, the region is 14 of the unit circle (see the ﬁgure below). The area of the region must then be SOLUTION

Matching the integrand

1 π π (1)2 = . 4 4 y

0.8

0.4 x

−0.4

0.4

45. Express the area (not signed) of the shaded region in Figure 16 as a sum of three integrals involving the functions f Two athletes run in the same direction along a straight track with velocities v1 (t) and v2 (t) (in ft/s). Assume and g. that y " 5 " 20 f(x)(v1 (t) − v2 (t)) dt = 5 (v1 (t) − v2 (t)) dt = 2, 0

" 35 30

0

x 3

5

9

(v1 (t) − v2 (t)) dt = −2 g(x)

" t2 16v2 (t) dt. v1 (t) − (a) Give a verbal interpretation of the integral FIGURE t1

(b) Is enough information given to determine the distance between the two runners at time t = 5 s? (c) If the runners begin at the same time and place, how far ahead is runner 1 at time t = 20 s? (d) Suppose that runner 1 is 8 ft ahead at t = 30 s. How far is she ahead or behind at t = 35 s?

S E C T I O N 6.1

Area Between Two Curves

299

SOLUTION Because either the curve bounding the top of the region or the curve bounding the bottom of the region or both change at x = 3 and at x = 5, the area is calculated using three integrals. Speciﬁcally, the area is " 3 " 5 " 9 ( f (x) − g(x)) d x + ( f (x) − 0) d x + (0 − f (x)) d x

0

=

" 3 0

3

( f (x) − g(x)) d x +

" 5 3

5

f (x) d x −

" 9 5

f (x) d x.

47. Set up (but do not evaluate) an integral that expresses the 2area between the circles x 2 + y 2 = 2 and x 2 + 2 (y − 1)2Find = 1.the area enclosed by the curves y = c − x and y = x − c as a function of c. Find the value of c for which this area is equal to 1. SOLUTION Setting 2 − y 2 = 1 − (y − 1)2 yields y = 1. The two circles therefore intersect at the points (1, 1) and (−1, 1). From the graph below, we see that over the interval [−1, 1], the upper half of the circle x 2 + y 2 = 2 lies above the lower half of the circle x 2 + (y − 1)2 = 1. The area enclosed by the two circles is therefore given by the integral " 1 2 − x 2 − (1 − 1 − x 2 ) d x. −1

y x2 + y2 = 2 1 x 2 + (y − 1)2 = 1 x 1

−1

49. Find a numerical approximation to the area above y = 1 − (x/π ) and below y = sin x (ﬁnd the points of Set up (but do not evaluate) an integral that expresses the area between the graphs of y = (1 + x 2 )−1 and y = x 2 . intersection numerically). SOLUTION The region in question is shown in the ﬁgure below. Using a computer algebra system, we ﬁnd that y = 1 − x/π and y = sin x intersect on the left at x = 0.8278585215. Analytically, we determine the two curves intersect on the right at x = π . The area above y = 1 − x/π and below y = sin x is then " π

x sin x − 1 − d x = 0.8244398727, π 0.8278585215 where the deﬁnite integral was evaluated using a computer algebra system. y 1 y = sin x

y=1−

x x

1

0

2

3

51. Use a computer algebra system to ﬁnd a numerical approximation to the number c (besides zero) in [0, π2 ], Find a numerical approximation to the area above y = |x| and below y = cos x. where the curves y = sin x and y = tan2 x intersect. Then ﬁnd the area enclosed by the graphs over [0, c]. SOLUTION The region in question is shown in the ﬁgure below. Using a computer algebra system, we ﬁnd that y = sin x and y = tan2 x intersect at x = 0.6662394325. The area of the region enclosed by the two curves is then " 0.6662394325

sin x − tan2 x d x = 0.09393667698, 0

where the deﬁnite integral was evaluated using a computer algebra system. y

0.8 0.6 0.4

y = sin x y = tan 2 x

0.2 x 0

0.2

0.4

0.6

The back of Jon’s guitar (Figure 17) has a length 19 in. He measured the widths at 1-in. intervals, beginning and ending 12 in. from the ends, obtaining the results

300

CHAPTER 6

A P P L I C AT I O N S O F T H E I N T E G R A L

Further Insights and Challenges 53. Find the line y = mx that divides the area under the curve y = x(1 − x) over [0, 1] into two regions of equal area. SOLUTION

First note that " 1 0

x(1 − x) d x =

" 1

0

1 2 1 3 1 1 x − x2 dx = x − x = . 2 3 6 0

Now, the line y = mx and the curve y = x(1 − x) intersect when mx = x(1 − x), or at x = 0 and at x = 1 − m. The area of the region enclosed by the two curves is then " 1−m 0

1−m " 1−m

x2 1 1 3 2 (1 − m)x − x d x = (1 − m) = (1 − m)3 . − x (x(1 − x) − mx) d x = 2 3 6 0 0

To have 16 (1 − m)3 = 12 · 16 requires m =1−

55.

1/3 1 ≈ 0.206299. 2

calculation) following for any n >by 0: the line y = cx (Figure LetExplain c be thegeometrically number such (without that the area under y why = sinthe x over [0, π ] holds is divided in half 18). Find an equation for c and solve this"equation numerically using a computer algebra system. " 1 1 xn dx + x 1/n d x = 1 0

SOLUTION

0

Let A1 denote the area of region 1 in the ﬁgure below. Deﬁne A2 and A3 similarly. It is clear from the

ﬁgure that A1 + A2 + A3 = 1. Now, note that x n and x 1/n are inverses of each other. Therefore, the graphs of y = x n and y = x 1/n are symmetric about the line y = x, so regions 1 and 3 are also symmetric about y = x. This guarantees that A1 = A3 . Finally, " 1 0

xn dx +

" 1 0

x 1/n d x = A3 + ( A2 + A3 ) = A1 + A2 + A3 = 1. y 1 1

2 3 x 0

1

6.2 Setting Up Integrals: Volume, Density, Average Value Preliminary Questions 1. What is the average value of f (x) on [1, 4] if the area between the graph of f (x) and the x-axis is equal to 9? Assuming that f (x) ≥ 0 over the interval [1, 4], the fact that the area between the graph of f and the x-axis ! is equal to 9 indicates that 14 f (x) d x = 9. The average value of f over the interval [1, 4] is then SOLUTION

!4

1 f (x) d x = 9 = 3.

4−1

3

2. Find the volume of a solid extending from y = 2 to y = 5 if the cross section at y has area A(y) = 5 for all y. SOLUTION Because the cross-sectional area of the solid is constant, the volume is simply the cross-sectional area times the length, or 5 × 3 = 15.

3. Describe the horizontal cross sections of an ice cream cone and the vertical cross sections of a football (when it is held horizontally).

Setting Up Integrals: Volume, Density, Average Value

S E C T I O N 6.2

301

SOLUTION The horizontal cross sections of an ice cream cone, as well as the vertical cross sections of a football (when held horizontally), are circles.

4. What is the formula for the total population within a circle of radius R around a city center if the population has a radial function? SOLUTION

Because the population density is a radial function, the total population within a circle of radius R is 2π

" R 0

r ρ(r ) dr,

where ρ(r ) is the radial population density function. 5. What is the deﬁnition of ﬂow rate? SOLUTION

The ﬂow rate of a ﬂuid is the volume of ﬂuid that passes through a cross-sectional area at a given point per

unit time. 6. Which assumption about ﬂuid velocity did we use to compute the ﬂow rate as an integral? SOLUTION To express ﬂow rate as an integral, we assumed that the ﬂuid velocity depended only on the radial distance from the center of the tube.

Exercises 1. Let V be the volume of a pyramid of height 20 whose base is a square of side 8. (a) Use similar triangles as in Example 1 to ﬁnd the area of the horizontal cross section at a height y. (b) Calculate V by integrating the cross-sectional area. SOLUTION

(a) We can use similar triangles to determine the side length, s, of the square cross section at height y. Using the diagram below, we ﬁnd 8 s = 20 20 − y

s=

or

2 (20 − y). 5

4 (20 − y)2 . The area of the cross section at height y is then given by 25

20 − y 20

s

8

(b) The volume of the pyramid is 20 " 20 4 1280 4 (20 − y)2 d y = − (20 − y)3 = . 75 3 0 25 0 3. Use the method of Exercise 2 to ﬁnd the formula for the volume of a right circular cone of height h whose base is a Let V be the volume of a right circular cone of height 10 whose base is a circle of radius 4 (Figure 16). circle of radius r (Figure 16). SOLUTION

(a) From similar triangles (see Figure 16), (a) Use similar triangles to ﬁnd the area of a horizontal cross section at a height y. r h (b) Calculate V by integrating the cross-sectional area. = , h−y r0 where r0 is the radius of the cone at a height of y. Thus, r0 = r − rhy . (b) The volume of the cone is h " h

r y 3 r y 2 −h π r − h hπ r 3 πr 2h r− π dy = = = . h r 3 r 3 3 0 0

Calculate the volume of the ramp in Figure 17 in three ways by integrating the area of the cross sections: (a) Perpendicular to the x-axis (rectangles) (b) Perpendicular to the y-axis (triangles)

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5. Find the volume of liquid needed to ﬁll a sphere of radius R to height h (Figure 18). y

R h

FIGURE 18 Sphere ﬁlled with liquid to height h. SOLUTION

The radius r at any height y is given by r =

R 2 − (R − y)2 . Thus, the volume of the ﬁlled portion of

the sphere is

π

" h 0

r2 dy =

π

" h

0

R 2 − (R − y)2

dy = π

" h 0

(2Ry − y 2 ) d y =

π

h h3 2 . = π Rh − 3 3

y3 Ry 2 −

0

7. Derive a formula for the volume of the wedge in Figure 19(B) in terms of the constants a, b, and c. Find the volume of the wedge in Figure 19(A) by integrating the area of vertical cross sections. The line from c to a is given by the equation (z/c) + (x/a) = 1 and the line from b to a is given by (y/b) + (x/a) = 1. The cross sections perpendicular to the x-axis are right triangles with height c(1 − x/a) and base b(1 − x/a). Thus we have " a

1 1 x 3 a 1 2 bc (1 − x/a) d x = − abc 1 − = 6 abc. 2 6 a 0 0 SOLUTION

In Exercises 9–14, ﬁnd the volume of the solid with given2base 2and cross sections. Let B be the solid whose base is the unit circle x + y = 1 and whose vertical cross √ sections perpendicular to thatthe thecross vertical crossperpendicular sections haveto area =are3(1 − x 2 ) whose and compute 9. the Thex-axis base isare theequilateral unit circletriangles. x 2 + y 2 Show = 1 and sections theA(x) x-axis triangles height the volume of B. and base are equal. SOLUTION At each location x, the side of the triangular cross section that lies in the base of the solid extends from the 2 2 top half of the unit circle (with y = 1 − x ) to the bottom half (with y = − 1 − x ). The triangle therefore has base 2 and height equal to 2 1 − x 2 and area 2(1 − x ). The volume of the solid is then 1 " 1 1 8 2(1 − x 2 ) d x = 2 x − x 3 = . 3 3 −1 −1

2 where −3 ≤ x ≤ 3. The cross sections perpendicular to the x-axis are 9− 11. TheThe basebase is the semicircle y = is the triangle enclosed byx x, + y = 1, the x-axis, and the y-axis. The cross sections perpendicular to squares. the y-axis are semicircles. SOLUTION For each x, the base of the square cross section extends from the semicircle y = 9 − x 2 to the x-axis.

2 9 − x 2 = 9 − x 2 . The volume of the solid is The square therefore has a base with length 9 − x 2 and an area of then 3 " 3

1 9 − x 2 d x = 9x − x 3 = 36. 3 −3 −3 y =interval 3. The cross to the y-axis are squares. 13. TheThe basebase is the enclosed y =sides x 2 and is aregion square, one of by whose is the [0, ] sections along theperpendicular √x-axis. The cross sections perpendicular 2. SOLUTION At any location y,of theheight distance to = thexparabola from the y-axis is y. Thus the base of the square will have to the x-axis are rectangles f (x) √ length 2 y. Therefore the volume is " 3 " 3 3 √ √ 4y d y = 2y 2 = 18. 2 y 2 y dy = 0

0

0

15. Find the volume of the solid whose base is the region |x| + |y| ≤ 1 and whose vertical cross sections perpendicular The base is the region(with enclosed by y = x 2 and y = 3. The cross sections perpendicular to the y-axis are rectangles to the y-axis are semicircles diameter along the base). of height y 3 . SOLUTION The region R in question is a diamond shape connecting the points (1, 0), (0, −1), (−1, 0), and (0, 1). Thus, in the lower half of the x y-plane, the radius of the circles is y + 1 and in the upper half, the radius is 1 − y. Therefore, the volume is " " π 0 π 1 π 1 1 π + = . (y + 1)2 d y + (1 − y)2 d y = 2 −1 2 0 2 3 3 3

Setting Up Integrals: Volume, Density, Average Value

S E C T I O N 6.2

17. Find the volume V of a regular tetrahedron whose face is an equilateral triangle of side s (Figure 20). Show that the volume of a pyramid of height h whose base is an equilateral triangle of side s is equal to

303

√

3 2 hs . 12

s

s

FIGURE 20 Regular tetrahedron. SOLUTION Our ﬁrst task is to determine the relationship between the height of the tetrahedron, h, and the side length of the equilateral triangles, s. Let B be the orthocenter of the tetrahedron (the point directly below the apex), and let b denote the distance from B to each corner of the base triangle. By the Law of Cosines, we have

s 2 = b2 + b2 − 2b2 cos 120◦ = 3b2 , so b2 = 13 s 2 . Thus 2 h 2 = s 2 − b2 = s 2 3

or

h=s

2 . 3

Therefore, using similar triangles, the side length of the equilateral triangle at height z above the base is h−z z s =s−√ . h 2/3 The volume of the tetrahedron is then given by √ √ √ 2 3 s 2/3 " s √2/3 √ s3 2 z z 3 2 dz = − = s−√ s−√ . 4 12 12 2/3 2/3 0 0

19. A frustum of a pyramid is a pyramid with its top cut off [Figure 22(A)]. Let V be the volume of a frustum of height The area of an ellipse is π ab, where a and b are the lengths of the semimajor and semiminor axes (Figure 21). h whose base is a square of side a and top is a square of side b with a > b ≥ 0. Compute the volume of a cone of height 12 whose base is an ellipse with semimajor ha axis a = 6 and semiminor axis (a) Show [Figure 22(B)]. b = 4.that if the frustum were continued to a full pyramid, it would have height a−b (b) Show that the cross section at height x is a square of side (1/ h)(a(h − x) + bx). (c) Show that V = 13 h(a 2 + ab + b2 ). A papyrus dating to the year 1850 BCE indicates that Egyptian mathematicians had discovered this formula almost 4,000 years ago.

b h

a (A)

(B)

FIGURE 22 SOLUTION

(a) Let H be the height of the full pyramid. Using similar triangles, we have the proportion H H −h = a b which gives H=

ha . a−b

(b) Let w denote the side length of the square cross section at height x. By similar triangles, we have a w = . H H −x Substituting the value for H from part (a) gives w=

a(h − x) + bx . h

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(c) The volume of the frustrum is " h 1 0

h

(a(h − x) + bx)

2

" h

1 a 2 (h − x)2 + 2ab(h − x)x + b2 x 2 d x dx = 2 h 0 h 1 a2 2 1 h 2 a + ab + b2 . = 2 − (h − x)3 + abhx 2 − abx 3 + b2 x 3 = 3 3 3 3 h 0

21. Figure 24 shows the solid S obtained by intersecting two cylinders of radius r whose axes are perpendicular. A plane inclined at an angle of 45◦ passes through a diameter of the base of a cylinder of radius r . Find the (a) The horizontal cross section of each cylinder at distance y from the central axis is a rectangular strip. Find the strip’s volume of the region within the cylinder and below the plane (Figure 23). width. (b) Find the area of the horizontal cross section of S at distance y. (c) Find the volume of S as a function of r .

S y

FIGURE 24 Intersection of two cylinders intersecting at right angles. SOLUTION

(a) The horizontal cross section at distance y from the central axis (for −r ≤ y ≤ r ) is a square of width w = 2 r 2 − y2. (b) The area of the horizontal cross section of S at distance y from the central axis is w2 = 4(r 2 − y 2 ). (c) The volume of the solid S is then r " r

1 16 3 r . r 2 − y 2 d y = 4 r 2 y − y 3 = 4 3 3 −r −r 23. Calculate the volume of a cylinder inclined at an angle θ = 30◦ whose height is 10 and whose base is a circle of Let S be the solid obtained by intersecting two cylinders of radius r whose axes intersect at an angle θ . Find the radius 4 (Figure 25). volume of S as a function of r and θ . 4

10 30°

FIGURE 25 Cylinder inclined at an angle θ = 30◦ . SOLUTION

The area of each circular cross section is π (4)2 = 16π , hence the volume of the cylinder is " 10 0

10 16π d x = (16π x) = 160π 0

25. Find the total mass of a 2-m rod whose linear density function is ρ(x) = 1 + 0.5 sin(π x) kg/m for 0 ≤ x ≤ 2. Find the total mass of a 1-m rod whose linear density function is ρ(x) = 10(x + 1)−2 kg/m for 0 ≤ x ≤ 1. SOLUTION The total mass of the rod is cos π x 2 ρ(x) d x = (1 + .5 sin π x) d x = x − .5 = 2 kg, π 0 0 0

" 2

" 2

27. Calculate the population within a 10-mile radius of the city center if the radial population density is ρ(r ) = 4(1 + A mineral deposit along a strip of length 6 cm has density s(x) = 0.01x(6 − x) g/cm for 0 ≤ x ≤ 6. Calculate r 2 )1/3 (in thousands per square mile). the total mass of the deposit.

Setting Up Integrals: Volume, Density, Average Value

S E C T I O N 6.2 SOLUTION

305

The total population is 2π

" 10 0

r · ρ(r ) dr = 2π

" 10 0

10

4r (1 + r 2 )1/3 dr = 3π (1 + r 2 )4/3

0

≈ 4423.59 thousand ≈ 4.4 million. 29. Table 1 lists the population density (in people per squared kilometer) as a function of distance r (in kilometers) from Odzala National Park in the Congo has a high density of gorillas. Suppose that the radial population density is the center of a rural town. Estimate the total population within a 2-km radius of the center by taking the average of the ρ(r ) = 52(1 + r 2 )−2 gorillas per square kilometer, where r is the distance from a large grassy clearing with a source left- and right-endpoint approximations. of food and water. Calculate the number of gorillas within a 5-km radius of the clearing. TABLE 1

SOLUTION

Population Density

r

ρ(r )

r

ρ(r )

0.0

125.0

1.2

37.6

0.2

102.3

1.4

30.8

0.4

83.8

1.6

25.2

0.6

68.6

1.8

20.7

0.8

56.2

2.0

16.9

1.0

46.0

The total population is given by 2π

" 2 0

r · ρ(r ) dr.

With r = 0.2, the left- and right-endpoint approximations to the required deﬁnite integral are L 10 = .2(2π )[0(125) + (.2)(102.3) + (.4)(83.8) + (.6)(68.6) + (.8)(56.2) + (1)(46) + (1.2)(37.6) + (1.4)(30.8) + (1.6)(25.2) + (1.8)(20.7)] = 442.24; R10 = .2(2π )[(.2)(102.3) + (.4)(83.8) + (.6)(68.6) + (0.8)(56.2) + (1)(46) + (1.2)(37.6) + (1.4)(30.8) + (1.6)(25.2) + (1.8)(20.7) + (2)(16.9)] = 484.71. This gives an average of 463.475. Thus, there are roughly 463 people within a 2-km radius of the town center. 2 + 2)−2 deer per km2 , where r is the distance (in ρ(rcm ) =whose 150(r mass 31. TheFind density deer in aofforest is the plate radialoffunction the of total mass a circular radius 20 density is the radial function ρ(r ) = 0.03 + kilometers) to a small meadow. Calculate the number of deer in the region 2 ≤ r ≤ 5 km. 0.01 cos(π r 2 ) g/cm2 . SOLUTION The number of deer in the region 2 ≤ r ≤ 5 km is

2π

5

−2 1 = −150π 1 − 1 ≈ 61 deer. r (150) r 2 + 2 dr = −150π 27 6 r 2 + 2 2 2

" 5

33. Find the ﬂow rate through a tube of radius 4 cm, assuming that the velocity of ﬂuid particles at a distance r cm from 4 the center is v(r ) =a 16 − r 2 cm/s. g/cm has ﬁnite total mass, even Show that circular plate of radius 2 cm with radial mass density ρ(r ) = r though theThe density SOLUTION ﬂow becomes rate is inﬁnite at the origin. 2π

" R 0

r v(r ) dr = 2π

4 " 4

1 cm3 r 16 − r 2 dr = 2π 8r 2 − r 4 = 128π . 4 s 0 0

35. A solid rod of radius 1 cm is placed in a pipe of radius 3 cm so that their axes are aligned. Water ﬂows through −5 Use function Poiseuille’s ) be the of the blood in rate an arterial capillaryofofthe radius 4 × 10 the pipeLet andv(raround thevelocity rod. Find ﬂow if the velocity waterR is=given by them.radial v(r Law ) = 6 (m-s)−1 to determine the velocity at the center of the capillary and the ﬂow rate (use (Example 6) with k = 10 0.5(r − 1)(3 − r ) cm/s. correct units). SOLUTION The ﬂow rate is 2π

" 3 1

r (.5)(r − 1)(3 − r ) dr = π

" 3

1

3 4 3 8π cm3 1 −r 3 + 4r 2 − 3r dr = π − r 4 + r 3 − r 2 = . 4 3 2 3 s 1

In Exercises 37–44,the calculate over the given interval. Bureau of Fisheries created the depth contour map in To estimate volumethe V average of Lake Nogebow, the Minnesota Figure 26 and determined the area of the cross section of the lake at the depths recorded in the table below. Estimate V by taking the average of the right- and left-endpoint approximations to the integral of cross-sectional area.

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37. f (x) = x 3 , SOLUTION

[0, 1]

The average is " 1 " 1 1 1 1 1 x3 dx = x 3 d x = x 4 = . 1−0 0 4 0 4 0

39. f (x) = cos x, [0, π2 ] f (x) = x 3 , [−1, 1] SOLUTION The average is " π /2 π /2 2 1 2

sin t 0 = . cos x d x = π /2 − 0 0 π π 41. f (s) = s −2 , 2[2, 5] f (x) = sec x, [0, π4 ] SOLUTION The average is " 5 1 −1 5 1 1 −2 s ds = − s = . 5−2 2 3 10 2 2 , [−1, 3] − 3x 43. f (x) = 2x 3sin( π /x) , [1, 2] f (x) = SOLUTION The average is x2

3 " 3

1 1 4 1 2x 3 − 3x 2 d x = x − x 3 = 3. 3 − (−1) −1 4 2 −1 45. Let M be thenaverage value of f (x) = x 3 on [0, A], where A > 0. Which theorem guarantees that f (c) = M has a f (x) = x , [0, 1] solution c in [0, A]? Find c. SOLUTION

The Mean Value Theorem for Integrals guarantees that f (c) = M has a solution c in [0, A]. With f (x) =

x 3 on [0, A], M=

" A 1 1 4 A A3 1 x3 dx = x = . A−0 0 A 4 0 4

3

Solving f (c) = c3 = A4 for c yields A c= √ . 3 4 47. Which of f (x) = x sin2 x and g(x) = x 2 sin2 x has a larger average value over [0, 1]? Over [1, 2]? Let f (x) = 2 sin x − x. Use a computer algebra system to plot f (x) and estimate: 2 SOLUTION The functions f and f (x).g differ only in the power of x multiplying sin x. It is also important to note that (a) The positive root α of 2 2 x ≥ The 0 foraverage all x. Now, x ∈ on (0,[0, 1),αx].> x so sin (b) valuefor M each of f (x) (c) A value c ∈ [0, α ] such that f (c)f = (x)M. = x sin2 x > x 2 sin2 x = g(x). Thus, over [0, 1], f (x) will have a larger average value than g(x). On the other hand, for each x ∈ (1, 2), x 2 > x, so g(x) = x 2 sin2 x > x sin2 x = f (x). Thus, over [1, 2], g(x) will have the larger average value. 49. Sketch the graph of a function f (x)sin such x that fπ(x) ≥ 0 on [0, 1] and f (x) ≤ 0 on [1, 2], whose average on that the average value of f (x) = over [ 2 , π ] is less than 0.41. Sketch the graph if necessary. [0, 2] isShow negative. x SOLUTION

Many solutions will exist. One could be y 1 x 1 −1 −2

2

S E C T I O N 6.2

Setting Up Integrals: Volume, Density, Average Value

307

π 51. TheFind temperature T (t)ofatf time (in + hours) in the an art museum varies T (t)M=are 70arbitrary + 5 cos constants. t . Find the the average (x) =t ax b over interval [−M, M], according where a, b,toand 12 average over the time periods [0, 24] and [2, 6]. SOLUTION

• The average temperature over the 24-hour period is

" 24

π

π 24 1 1 60 70 + 5 cos sin t dt = 70t + t = 70◦ F. 24 − 0 0 12 24 π 12 0 • The average temperature over the 4-hour period is

" 6

π

π 6 1 1 60 70 + 5 cos sin t dt = 70t + t = 72.4◦ F. 6−2 2 12 4 π 12 2 53. What is the average area of the circles whose radii vary from 0 to 1? A ball is thrown in the air vertically from ground level with initial velocity 64 ft/s. Find the average height of the SOLUTION Thetime average area extending is ball over the interval from the time of the ball’s release to its return to ground level. Recall that the height at time t is h(t) = 64t − 16t 2 . " 1 π 1 π 1 π r 2 dr = r 3 = . 1−0 0 3 0 3 55. The acceleration of a particle is a(t) = t − t 3 m/s2 for 0 ≤ t ≤ 1. Compute2the average acceleration and average . Find its average velocity during the object withinterval zero initial velocity accelerates a constant ratevelocity of 10 m/s velocityAn over the time [0, 1], assuming that theatparticle’s initial is zero. ﬁrst 15 s. SOLUTION The average acceleration is " 1

1 1 2 1 4 1 1 t − t = m/s2 . t − t 3 dt = 1−0 0 2 4 4 0 An acceleration a(t) = t − t 3 with zero initial velocity gives v(t) =

1 2 1 4 t − t . 2 4

Thus the average velocity is given by 1 " 1 1 7 1 2 1 4 1 3 1 t − t t − t 5 = m/s. dt = 1−0 0 2 4 6 20 60 0 √ 57. Let f (x) = x. Find a value of c in [4, 9] such that f (c) is equal to the average of f on [4, 9]. Let M be the average value of f (x) = x 4 on [0, 3]. Find a value of c in [0, 3] such that f (c) = M. SOLUTION The average value is " 9 " √ 1 9√ 2 3/2 9 38 1 x dx = x dx = x = . 9−4 4 5 4 15 15 4 Then f (c) =

√

c = 38 15 implies c=

38 2 1444 = ≈ 6.417778. 15 225

Give an example of a function (necessarily discontinuous) that does not satisfy the conclusion of the MVT for Further Insights and Challenges Integrals.

59. An object is tossed in the air vertically from ground level with initial velocity v0 ft/s at time t = 0. Find the average speed of the object over the time interval [0, T ], where T is the time the object returns to earth. The height is given by h(t) = v0 t − 16t 2 . The ball is at ground level at time t = 0 and T = v0 /16. The velocity is given by v(t) = v0 − 32t and thus the speed is given by s(t) = |v0 − 32t|. The average speed is SOLUTION

" v0 /16 " " 1 16 v0 /32 16 v0 /16 |v0 − 32t| dt = (v0 − 32t) dt + (32t − v0 ) dt v0 /16 − 0 0 v0 0 v0 v0 /32 v0 /32 16

v0 /16 16

v0 t − 16t 2 16t 2 − v0 t + = v0 /2. = 0 v0 /32 v0 v0 Review the MVT stated in Section 4.3 (Theorem 1, p. 192) and show how it can be used, together with the Fundamental Theorem of Calculus, to prove the MVT for integrals.

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6.3 Volumes of Revolution Preliminary Questions 1. Which of the following is a solid of revolution? (a) Sphere (b) Pyramid

(c) Cylinder

(d) Cube

SOLUTION The sphere and the cylinder have circular cross sections; hence, these are solids of revolution. The pyramid and cube do not have circular cross sections, so these are not solids of revolution.

2. True or false? When a solid is formed by rotating the region under a graph about the x-axis, the cross sections perpendicular to the x-axis are circular disks. SOLUTION

True. The cross sections will be disks with radius equal to the value of the function.

3. True or false? When a solid is formed by rotating the region between two graphs about the x-axis, the cross sections perpendicular to the x-axis are circular disks. SOLUTION

False. The cross sections may be washers.

4. Which of the following integrals expresses the volume of the solid obtained by rotating the area between y = f (x) and y = g(x) over [a, b] around the x-axis [assume f (x) ≥ g(x) ≥ 0]? " b 2 (a) π f (x) − g(x) d x a " b (b) π f (x)2 − g(x)2 d x a

SOLUTION The correct answer is (b). Cross sections of the solid will be washers with outer radius f (x) and inner radius g(x). The area of the washer is then π f (x)2 − π g(x)2 = π ( f (x)2 − g(x)2 ).

Exercises In Exercises 1–4, (a) sketch the solid obtained by revolving the region under the graph of f (x) about the x-axis over the given interval, (b) describe the cross section perpendicular to the x-axis located at x, and (c) calculate the volume of the solid. 1. f (x) = x + 1,

[0, 3]

SOLUTION

(a) A sketch of the solid of revolution is shown below: y 2 x 1

2

3

−2

(b) Each cross section is a disk with radius x + 1. (c) The volume of the solid of revolution is

π

" 3 0

(x + 1)2 d x = π

" 3 0

(x 2 + 2x + 1) d x = π

3 1 3 x + x 2 + x = 21π . 3 0

√ 3. f (x) = x + 1, [1, 4] f (x) = x 2 , [1, 3] SOLUTION

(a) A sketch of the solid of revolution is shown below: y 2 1 x −1 −2

(b) Each cross section is a disk with radius

√

x + 1.

1

2

3

4

S E C T I O N 6.3

Volumes of Revolution

309

(c) The volume of the solid of revolution is 4 " 4 " 4 √ 21π 1 2 ( x + 1)2 d x = π (x + 1) d x = π x + x = . 2 2 1 1 1

π

In Exercises 5–12,−1ﬁnd the volume of the solid obtained by rotating the region under the graph of the function about the f (x) = x , [1, 2] x-axis over the given interval. 5. f (x) = x 2 − 3x, SOLUTION

[0, 3]

The volume of the solid of revolution is

π

" 3 0

(x 2 − 3x)2 d x =

π

" 3 0

(x 4 − 6x 3 + 9x 2 ) d x =

π

3 81π 1 5 3 4 3 x − x + 3x = . 5 2 10 0

7. f (x) = x 5/31, [1, 8] f (x) = 2 , [1, 4] SOLUTION The x volume of the solid of revolution is " 8 " 8 3π 13/3 8 3π 13 24573π π (x 5/3 )2 d x = π x 10/3 d x = x = 13 (2 − 1) = 13 . 13 1 1 1 2 9. f (x)f (x) = = 4 −, x 2[1, , 3] [0, 2] x +1 SOLUTION

The volume of the solid of revolution is

π

" 3 1

3 2 " 3 2 d x = 4π (x + 1)−2 d x = −4π (x + 1)−1 = π . x +1 1 1

√ x + 1, [0, π ] 11. f (x) = cos f (x) = x 4 + 1, [1, 3] SOLUTION The volume of the solid of revolution is

π

π " π " π √ ( cos x + 1)2 d x = π (cos x + 1) d x = π (sin x + x) = π 2 . 0

0

0

√ (a) sketch the region enclosed by the curves, (b) describe the cross section perpendicular to the In Exercises 13–18, f (x) = cos x sin x, [0, π /2] x-axis located at x, and (c) ﬁnd the volume of the solid obtained by rotating the region about the x-axis. 13. y = x 2 + 2,

y = 10 − x 2

SOLUTION

(a) Setting x 2 + 2 = 10 − x 2 yields 2x 2 = 8, or x 2 = 4. The two curves therefore intersect at x = ±2. The region enclosed by the two curves is shown in the ﬁgure below. y y = 10 − x 2 8 4 y = x2 + 2 −2

x

−1

1

2

(b) When the region is rotated about the x-axis, each cross section is a washer with outer radius R = 10 − x 2 and inner radius r = x 2 + 2. (c) The volume of the solid of revolution is " 2

" 2

2 (10 − x 2 )2 − (x 2 + 2)2 d x = π π (96 − 24x 2 ) d x = π 96x − 8x 3 = 256π . −2

15. y = 16 − x, y = 3x + 12, y = x 2 , y = 2x + 3

−2

−2

x = −1

SOLUTION

(a) Setting 16 − x = 3x + 12, we ﬁnd that the two lines intersect at x = 1. The region enclosed by the two curves is shown in the ﬁgure below.

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CHAPTER 6

A P P L I C AT I O N S O F T H E I N T E G R A L y

y = 3x + 12

10

−1

y = 16 − x

x

−0.5

0.5

1

(b) When the region is rotated about the x-axis, each cross section is a washer with outer radius R = 16 − x and inner radius r = 3x + 12. (c) The volume of the solid of revolution is

π

1 " 1

" 1 8 656π (16 − x)2 − (3x + 12)2 d x = π (112 − 104x − 8x 2 ) d x = π 112x − 52x 2 − x 3 = . 3 3 −1 −1 −1

π 17. y = sec x, 1 y = 0,5 x = − 4 , y = , y = −x x 2

x=

π 4

SOLUTION

(a) The region in question is shown in the ﬁgure below. y

y = sec x

1.2 0.8 0.4 x

−0.4

0.4

(b) When the region is rotated about the x-axis, each cross section is a circular disk with radius R = sec x. (c) The volume of the solid of revolution is π /4 " π /4 2 π (sec x) d x = π (tan x) = 2π . −π /4

−π /4

In Exercises 19–22, ﬁnd the volume of the solid obtained by rotating the region enclosed by the graphs about the y-axis π = secinterval. x, y = csc x, y = 0, x = 0, and x = . over theygiven 2 √ 19. x = y, x = 0; 1 ≤ y ≤ 4 SOLUTION When the region in question (shown in the ﬁgure below) is rotated about the y-axis, each cross section is a √ disk with radius y. The volume of the solid of revolution is

" 4 √ 2 15π π y 2 4 π y dy = = 2 . 2 1 1 y 4 3 x=

y

2 1 x 0

0.5

1

1.5

2

√ 21. x = y 2 , √ x = y; 0 ≤ y ≤ 1 x = sin y, x = 0; 0 ≤ y ≤ π SOLUTION When the region in question (shown in the ﬁgure below) is rotated about the y-axis, each cross section is a √ washer with outer radius R = y and inner radius r = y 2 . The volume of the solid of revolution is 1 " 1

y2 3π y 5 √ 2 2 2 ( y) − (y ) d y = π π − . = 2 5 10 0 0

S E C T I O N 6.3

Volumes of Revolution

311

y 1 x = y2 x=

y

x 0

1

In Exercises 23–28, ﬁnd the volume of the solid obtained by rotating region A in Figure 10 about the given axis. x = 4 − y, x = 16 − y 2 ; −3 ≤ y ≤ 4 y

y = x2 + 2

6 A 2

B x 1

2

FIGURE 10

23. x-axis Rotating region A about the x-axis produces a solid whose cross sections are washers with outer radius R = 6 and inner radius r = x 2 + 2. The volume of the solid of revolution is 2 " 2

" 2 1 704π 4 (6)2 − (x 2 + 2)2 d x = π π (32 − 4x 2 − x 4 ) d x = π 32x − x 3 − x 5 = . 3 5 15 0 0 0

SOLUTION

25. y = 2 y = −2 SOLUTION Rotating the region A about y = 2 produces a solid whose cross sections are washers with outer radius R = 6 − 2 = 4 and inner radius r = x 2 + 2 − 2 = x 2 . The volume of the solid of revolution is 2 " 2

128π 1 π . 42 − (x 2 )2 d x = π 16x − x 5 = 5 5 0 0 27. x = −3 y-axis

Rotating region A about x = −3 produces a solid whose cross sections are washers with outer radius √ y − 2 − (−3) = y − 2 + 3 and inner radius r = 0 − (−3) = 3. The volume of the solid of revolution is

SOLUTION

R=

√

π

" 6

2

(3 +

6 " 6 1 y − 2)2 − (3)2 d y = π (6 y − 2 + y − 2) d y = π 4(y − 2)3/2 + y 2 − 2y = 40π . 2 2 2

In Exercises x = 229–34, ﬁnd the volume of the solid obtained by rotating region B in Figure 10 about the given axis. 29. x-axis Rotating region B about the x-axis produces a solid whose cross sections are disks with radius R = x 2 + 2. The volume of the solid of revolution is 2 " 2 " 2 376π 1 5 4 3 π (x 2 + 2)2 d x = π (x 4 + 4x 2 + 4) d x = π x + x + 4x = . 5 3 15 0 0 0 SOLUTION

31. y = 6 y = −2 SOLUTION Rotating region B about y = 6 produces a solid whose cross sections are washers with outer radius R = 6 − 0 = 6 and inner radius r = 6 − (x 2 + 2) = 4 − x 2 . The volume of the solid of revolution is " 2

" 2

824π 8 3 1 5 2 2 2 2 2 4 6 − (4 − x ) d y = π 20 + 8x − x d y = π 20x + x − x = π . 3 5 15 0 0 0 33. x = 2 y-axis Hint for Exercise 32: Express the volume as a sum of two integrals along the y-axis, or use Exercise 26.

312

CHAPTER 6

A P P L I C AT I O N S O F T H E I N T E G R A L SOLUTION Rotating region B about x = 2 produces a solid with two different cross sections. For each y ∈ [0, 2], the √ cross section is a disk with radius R = 2; for each y ∈ [2, 6], the cross section is a disk with radius R = 2 − y − 2. The volume of the solid of revolution is " 2 " 6 " 2 " 6 π (2)2 d y + π (2 − y − 2)2 d y = π 4 dy + π (2 + y − 4 y − 2) d y

0

2

0

2

2 6 1 2 8 3/2 = 32π . = π (4y) + π 2y + y − (y − 2) 2 3 3 0 2

In Exercises 35–46, ﬁnd the volume of the solid obtained by rotating the region enclosed by the graphs about the given x = −3 axis. 35. y = x 2 ,

y = 12 − x,

x = 0,

about y = −2

Rotating the region enclosed by y = x 2 , y = 12 − x and the y-axis (shown in the ﬁgure below) about y = −2 produces a solid whose cross sections are washers with outer radius R = 12 − x − (−2) = 14 − x and inner radius r = x 2 − (−2) = x 2 + 2. The volume of the solid of revolution is " 3

" 3 (14 − x)2 − (x 2 + 2)2 d x = π π (192 − 28x − 3x 2 − x 4 ) d x SOLUTION

0

0

3 1 1872π = π 192x − 14x 2 − x 3 − x 5 = . 5 5 0 y y = 12 − x

12

8

4

y = x2 x

0

1

2

3

37. y = 16 − x, y = 3x + 12, x = 0, about y-axis y = x 2 , y = 12 − x, x = 0, about y = 15 SOLUTION Rotating the region enclosed by y = 16 − x, y = 3x + 12 and the y-axis (shown in the ﬁgure below) about the y-axis produces a solid with two different cross sections. For each y ∈ [12, 15], the cross section is a disk with radius R = 13 (y − 12); for each y ∈ [15, 16], the cross section is a disk with radius R = 16 − y. The volume of the solid of revolution is 2 " 15 " 16 1 π (16 − y)2 d y (y − 12) d y + π 3 12 15 " 15 " 16 1 2 (y 2 − 32y + 256) d y (y − 24y + 144) d y + π =π 12 9 15 15 16 4 π 1 3 1 3 2 2 = y − 12y + 144y + π y − 16y + 256y = π . 9 3 3 3 12 15 y 16 14 12 10 8 6 4 2

y = 16 − x y = 3x + 12

x 0

9 y= , +about 39. y =y =2 ,16 − x, 10y−=x 23x 12, x-axis x = 0, x

0.2 0.4 0.6 0.8

1

about x = 2

SOLUTION The region enclosed by the two curves is shown in the ﬁgure below. Note that the region consists of two pieces that are symmetric with respect to the y-axis. We may therefore compute the volume of the solid of revolution by

S E C T I O N 6.3

Volumes of Revolution

313

considering one of the pieces and doubling the result. Rotating the portion of the region in the ﬁrst quadrant about the x-axis produces a solid whose cross sections are washers with outer radius R = 10 − x 2 and inner radius r = 9x −2 . The volume of the solid of revolution is " 3

" 3

(10 − x 2 )2 − (9x −2 )2 d x = 2π 100 − 20x 2 + x 4 − 81x −4 d x 2π 1

1

= 2π

3 20 3 1 5 −3 = 1472π . x + x + 27x 100x − 3 5 15 1

y

y = 10 − x 2

8 6 4 y = 92 x

2 −3 −2 −1

1

x 2

3

1 5 41. y = , 9y = − x, about y-axis y x= 2 , y2 = 10 − x 2 , about y = 12 x 5 SOLUTION Rotating the region enclosed by y = x −1 and y = 2 − x (shown in the ﬁgure below) about the y-axis produces a solid whose cross sections are washers with outer radius R = 52 − y and inner radius r = y −1 . The volume of the solid of revolution is 2 " 2 " 2 25 5 −1 2 dy = π π − y − (y ) − 5y + y 2 − y −2 d y 2 4 1/2 1/2 = π

2 5 1 9π 25 y − y 2 + y 3 + y −1 . = 4 2 3 8 1/2

y 2 1.5

y = 2.5 − x

1 0.5

y=

1 x x

0

0.5

1

1.5

2

43. y = x 3 , y = x 1/3 , about y-axis x = 2, x = 3, y = 16 − x 4 , y = 0, about y-axis SOLUTION Rotating the region enclosed by y = x 3 and y = x 1/3 (shown in the ﬁgure below) about the y-axis produces a solid whose cross sections are washers with outer radius R = y 1/3 and inner radius r = y 3 . The volume of the solid of revolution is " 1

" 1 32π 3 5/3 1 7 1 (y 1/3 )2 − (y 3 )2 d y = π π (y 2/3 − y 6 ) d y = π − y = y . 5 7 35 −1 −1 −1 y 1

y = x 1/3 y = x3 x

−1

1

−1

about x-axis 45. y 2 = 4x, 3 y = x, 1/3 y = x , y = x , about x = −2 SOLUTION Rotating the region enclosed by y 2 = 4x and y = x (shown in the ﬁgure below) about the x-axis produces √ a solid whose cross sections are washers with outer radius R = 2 x and inner radius r = x. The volume of the solid of

314

CHAPTER 6

A P P L I C AT I O N S O F T H E I N T E G R A L

revolution is " 4

√ 2 1 3 4 32π 2 2 (2 x) − x d x = π 2x − x = π . 3 3 0 0 y 4 3

y 2 = 4x

2

y=x

1 x 0

1

2

3

4

47. Sketch the hypocycloid x 2/3 + y 2/3 = 1 and ﬁnd the volume of the solid obtained by revolving it about the 2= y 4x, y = x, about y = 8 x-axis. SOLUTION

A sketch of the hypocycloid is shown below. y 1

x

−1

1

−1

3/2 For the hypocycloid, y = ± 1 − x 2/3 . Rotating this region about the x-axis will produce a solid whose cross 3/2

sections are disks with radius R = 1 − x 2/3 . Thus the volume of the solid of revolution will be 1 " 1

2 −x 3 32π 9 7/3 9 5/3 2/3 3/2 (1 − x ) π dx = π − x +x = + x . 3 7 5 105 −1 −1

49. A “bead” is formed by removing a cylinder of radius r from the center of a sphere of radius R (Figure 12). Find the 2 2 solid generated rotating volume The of the bead with r =by 1 and R =the 2. region between the branches of the hyperbola y − x = 1 about the x-axis is called a hyperboloid (Figure 11). Find the volume of the hyperboloid for −a ≤ x ≤ a. y

y

h r

R

x

x

FIGURE 12 A bead is a sphere with a cylinder removed.

The equation of the outer circle is x 2 + y 2 = 22 , and the inner cylinder intersects the sphere when √ y = ± 3. Each cross section of the bead is a washer with outer radius 4 − y 2 and inner radius 1, so the volume is given by 2 " √3 " √3

√ 2 2 π √ 4− y −1 d y = π √ 3 − y 2 d y = 4π 3. SOLUTION

− 3

Further Insights and Challenges

− 3

x 2 y 2 + = 1 πaround the x-axis is 51. The solid generated by rotating the region inside the ellipse with equation 3 Find the volume V of the bead (Figure 12) in terms of r and R. Thena show that b V = h , where h is the 6 4 2 is theVvolume the ellipseinisterms rotated called an ellipsoid. ShowThis thatformula the ellipsoid volumeconsequence: height of the bead. has a has surprising can beifexpressed of around h alone,the it 3 π ab . WhatSince y-axis? follows that two beads of height 2 in., one formed from a sphere the size of an orange and the other the size of the earth, would have the same volume! Can you explain intuitively how this is possible?

S E C T I O N 6.3

Volumes of Revolution

315

SOLUTION

• Rotating the ellipse about the x-axis produces an ellipsoid whose cross sections are disks with radius R = b 1 − (x/a)2 . The volume of the ellipsoid is then

π

2 a " a " a 4 1 1 d x = b2 π b 1 − (x/a)2 1 − 2 x 2 d x = b2 π x − 2 x 3 = π ab2 . 3 a 3a −a −a −a

• Rotating the ellipse about the y-axis produces an ellipsoid whose cross sections are disks with radius R = a 1 − (y/b)2 . The volume of the ellipsoid is then

2 b " b " b 4 1 1 d y = a2 π a 1 − (y/b)2 1 − 2 y 2 d y = a 2 π y − 2 y 3 = π a 2 b. 3 b 3b −b −b −b 53. The curve y = f (x) in Figure 14, called a tractrix, has the following property: the tangent line at each point (x, y) A doughnut-shaped solid is called a torus (Figure 13). Use the washer method to calculate the volume of the on the curve has slope torus obtained by rotating the region inside the circle with equation (x − a)2 + y 2 = b2 around the y-axis (assume that a > b). Hint: Evaluate the integral by interpreting it as −ythe area of a circle. dy = . dx 1 − y2 y 1

c

R

y = f (x) x a

2

FIGURE 14 The tractrix.

Let R be the shaded region under the graph of 0 ≤ x ≤ a in Figure 14. Compute the volume V of the solid obtained by revolving R around the x-axis in terms of the constant c = f (a). Hint: Use the disk method and the substitution u = f (x) to show that V =π

" 1 u 1 − u 2 du c

SOLUTION Let y = f (x) be the tractrix depicted in Figure 14. Rotating the region R about the x-axis produces a solid whose cross sections are disks with radius f (x). The volume of the resulting solid is then " a V =π [ f (x)]2 d x.

0

Now, let u = f (x). Then − f (x) −u dx = d x; du = f (x) d x = 2 1 − [ f (x)] 1 − u2 hence, 1 − u2 du, dx = − u and V =π

" c

u2

1

1 − u2 − du u

=π

" 1 u 1 − u 2 du. c

Carrying out the integration, we ﬁnd 1 π π V = − (1 − u 2 )3/2 = (1 − c2 )3/2 . 3 3 c Verify the formula " x2 x1

1 (x − x 1 )(x − x 2 ) d x = (x 1 − x2 )3 6

316

CHAPTER 6

A P P L I C AT I O N S O F T H E I N T E G R A L

55. Let R be the region in the unit circle lying above the cut with the line y = mx + b (Figure 16). Assume the points where the line intersects the circle lie above the x-axis. Use the method of Exercise 54 to show that the solid obtained by π rotating R about the x-axis has volume V = hd 2 , with h and d as in the ﬁgure. 6 y = mx + b

y

R

d h

x2 + y2 = 1 x

FIGURE 16

Let x 1 and x 2 denote the x-coordinates of the points of intersection between the circle x 2 + y 2 = 1 and the line y = mx + b with x 1 < x 2 . Rotating the regionenclosed by the two curves about the x-axis produces a solid whose cross sections are washers with outer radius R = 1 − x 2 and inner radius r = mx + b. The volume of the resulting solid is then " x2

V =π (1 − x 2 ) − (mx + b)2 d x SOLUTION

x1

Because x1 and x 2 are roots of the equation (1 − x 2 ) − (mx + b)2 = 0 and (1 − x 2 ) − (mx + b)2 is a quadratic polynomial in x with leading coefﬁcient −(1 + m 2 ), it follows that (1 − x 2 ) − (mx + b)2 = −(1 + m 2 )(x − x 1 )(x − x 2 ). Therefore, " x2 π (x − x 1 )(x − x 2 ) d x = (1 + m 2 )(x 2 − x 1 )3 . V = −π (1 + m 2 ) 6 x1 From the diagram, we see that h = x 2 − x 1 . Moreover, by the Pythagorean theorem, d 2 = h 2 + (mh)2 = (1 + m 2 )h 2 . Thus, π π π V = (1 + m 2 )h 3 = h (1 + m 2 )h 2 = hd 2 . 6 6 6

6.4 The Method of Cylindrical Shells Preliminary Questions 1. Consider the region R under the graph of the constant function f (x) = h over the interval [0, r ]. What are the height and radius of the cylinder generated when R is rotated about: (a) the x-axis (b) the y-axis SOLUTION

(a) When the region is rotated about the x-axis, each shell will have radius h and height r. (b) When the region is rotated about the y-axis, each shell will have radius r and height h. 2. Let V be the volume of a solid of revolution about the y-axis. (a) Does the Shell Method for computing V lead to an integral with respect to x or y? (b) Does the Disk or Washer Method for computing V lead to an integral with respect to x or y? SOLUTION

(a) The Shell method requires slicing the solid parallel to the axis of rotation. In this case, that will mean slicing the solid in the vertical direction, so integration will be with respect to x. (b) The Disk or Washer method requires slicing the solid perpendicular to the axis of rotation. In this case, that means slicing the solid in the horizontal direction, so integration will be with respect to y.

Exercises In Exercises 1–10, sketch the solid obtained by rotating the region underneath the graph of the function over the given interval about the y-axis and ﬁnd its volume. 1. f (x) = x 3 ,

[0, 1]

The Method of Cylindrical Shells

S E C T I O N 6.4 SOLUTION

317

A sketch of the solid is shown below. Each shell has radius x and height x 3 , so the volume of the solid is 2π

" 1 0

x · x 3 d x = 2π

" 1 0

x 4 d x = 2π

2 1 5 1 x = π. 5 5 0

y 1

x

−1

1

3. f (x) = 3x + √ 2, [2, 4] f (x) = x, [0, 4] SOLUTION A sketch of the solid is shown below. Each shell has radius x and height 3x + 2, so the volume of the solid is " 4 " 4

4 2π x(3x + 2) d x = 2π (3x 2 + 2x) d x = 2π x 3 + x 2 = 136π . 2

2

2

y 14 12 10 8 6 4 2 x

−4

4

5. f (x) = 4 − x 2 , 2[0, 2] f (x) = 1 + x , [1, 3] SOLUTION A sketch of the solid is shown below. Each shell has radius x and height 4 − x 2 , so the volume of the solid is 2 " 2 " 2 1 2π x(4 − x 2 ) d x = 2π (4x − x 3 ) d x = 2π 2x 2 − x 4 = 8π . 4 0 0 0 y

2

x

−2

2

√ 2 ), [0, π ] 7. f (x) = sin(x f (x) = x 2 + 9, [0, 3] SOLUTION A sketch of the solid is shown below. Each shell has radius x and height sin x 2 , so the volume of the solid is " √π 2π x sin(x 2 ) d x. 0

Let u = x 2 . Then du = 2x d x and π " √π " π 2π x sin(x 2 ) d x = π sin u du = −π (cos u) = 2π . 0

0

0

y 1 0.5

−2

−1

x 1

2

318

CHAPTER 6

A P P L I C AT I O N S O F T H E I N T E G R A L

9. f (x) = x + 1−1− 2x 2 , [0, 1] f (x) = x , [1, 3] SOLUTION A sketch of the solid is shown below. Each shell has radius x and height x + 1 − 2x 2 , so the volume of the solid is " 1 " 1 1 3 1 2 1 4 1 2 2π x(x + 1 − 2x 2 ) d x = 2π (x 2 + x − 2x 3 ) d x = 2π x + x − x = π. 3 2 2 3 0 0 0 y 1 0.5 x

−1

1

In Exercises 11–14, xuse the Shell Method to compute the volume of the solids obtained by rotating the region enclosed f (x) =of , [1, 4] the y-axis. by the graphs the functions about 1 + x3 11. y = x 2 ,

y = 8 − x 2,

x =0

The region enclosed by y = x 2 , y = 8 − x 2 and the y-axis is shown below. When rotating this region about the y-axis, each shell has radius x and height 8 − x 2 − x 2 = 8 − 2x 2 . The volume of the resulting solid is SOLUTION

2π

" 2 0

x(8 − 2x 2 ) d x = 2π

2 1 (8x − 2x 3 ) d x = 2π 4x 2 − x 4 = 16π . 2 0 0

" 2

y 8 y = 8 − x2 6 4 2

y = x2 x

0

0.5

1

1.5

2

√ x2 13. y = x, y = y = 8 − x 3 , y = 8 − 4x √ SOLUTION The region enclosed by y = x and y = x 2 is shown below. When rotating this region about the y-axis, √ each shell has radius x and height x − x 2 . The volume of the resulting solid is 2π

" 1 0

√ x( x − x 2 ) d x = 2π

" 1 0

(x 3/2 − x 3 ) d x = 2π

3 2 5/2 1 4 1 − x = π. x 5 4 10 0

y 1 y=

x y = x2

x 0

1

In Exercises y= 1 − |x −15–16, 1|, yuse = 0the Shell Method to compute the volume of rotation of the region enclosed by the curves about the y-axis. Use a computer algebra system or graphing utility to ﬁnd the points of intersection numerically. 15. y = 12 x 2 ,

y = sin(x 2 )

The region enclosed by y = 12 x 2 and y = sin x 2 is shown below. When rotating this region about the y-axis, each shell has radius x and height sin x 2 − 12 x 2 . Using a computer algebra system, we ﬁnd that the x-coordinate of the point of intersection on the right is x = 1.376769504. Thus, the volume of the resulting solid of revolution is " 1.376769504 1 x sin x 2 − x 2 d x = 1.321975576. 2π 2 0 SOLUTION

S E C T I O N 6.4

The Method of Cylindrical Shells

319

y 1 y = sin x 2 y=

x2 2 x

0

1

In Exercises 17–22, sketch the solid obtained by rotating the region underneath the graph of the function over the interval y = 1 − x 4 , y = x, x = 0 about the given axis and calculate its volume using the Shell Method. 17. f (x) = x 3 , [0, 1], SOLUTION

x =2

A sketch of the solid is shown below. Each shell has radius 2 − x and height x 3 , so the volume of the solid

is 2π

" 1 0

(2 − x) x 3

d x = 2π

" 1 0

(2x 3 − x 4 ) d x = 2π

x4 x5 − 2 5

1 3π . = 5 0

y 1

x 0

4

19. f (x) = x −4 ,3 [−3, −1], x = 4 f (x) = x , [0, 1], x = −2 SOLUTION A sketch of the solid is shown below. Each shell has radius 4 − x and height x −4 , so the volume of the solid is " −1 " −1

280π 1 −2 4 −3 −1 −4 −4 −3 d x = 2π 2π (4x − x ) d x = 2π x − x (4 − x) x = 81 . 2 3 −3 −3 −3 y

0.8

0.4 x

−2

10

21. f (x) = a − bx, [0, a/b], x = −1, a, b > 0 1 2], isxshown = 0 below. Each shell has radius x − (−1) = x + 1 and height a − bx, so the f (x) = SOLUTION A sketch of,the[0, solid 2 x +1 volume of the solid is " a/b " a/b

a + (a − b)x − bx 2 d x (x + 1) (a − bx) d x = 2π 2π 0

0

a − b 2 b 3 a/b x − x 2 3 0 2 2 3 a a 2 (a + 3b) a a (a − b) = = 2π − π. + b 2b2 3b2 3b2 = 2π ax +

y a

−2 − a /b

x −2 −1

a/b

320

CHAPTER 6

A P P L I C AT I O N S O F T H E I N T E G R A L

In Exercises 23–28, use the Shell Method to calculate the volume of rotation about the x-axis for the region underneath f (x) = 1 − x 2 , [−1, 1], x = c (with c > 1) the graph. 23. y = x,

0≤x ≤1

SOLUTION When the region shown below is rotated about the x-axis, each shell has radius y and height 1 − y. The volume of the resulting solid is

2π

" 1 0

y(1 − y) d y = 2π

" 1 0

(y − y 2 ) d y = 2π

π 1 2 1 3 1 y − y = . 2 3 3 0

y 1 0.8 0.6

y=x

0.4 0.2 x 0

0.2 0.4 0.6 0.8

1

25. y = x 1/3 − 2,2 8 ≤ x ≤ 27 y =4−x , 0≤ x ≤2 SOLUTION When the region shown below is rotated about the x-axis, each shell has radius y and height 27 − (y + 2)3 . The volume of the resulting solid is 2π

" 1 0

" 1

19y − 12y 2 − 6y 3 − y 4 d y y · 27 − (y + 2)3 d y = 2π 0

= 2π

1 3 1 38π 19 2 y − 4y 3 − y 4 − y 5 = . 2 2 5 5 0

y 1 0.8 0.6

y=

3

5

10 15 20 25 30

x−2

0.4 0.2 x 0

27. y = x −2 ,−12 ≤ x ≤ 4 y = x , 1 ≤ x ≤ 4. Sketch the region and express the volume as a sum of two integrals. SOLUTION When the region shown below is rotated about the x-axis, two different shells are generated. For each 1 ], the shell has radius y and height 4 − 2 = 2; for each y ∈ [ 1 , 1 ], the shell has radius y and height √1 − 2. y ∈ [0, 16 16 4 y The volume of the resulting solid is 2π

" 1/16 0

2y d y + 2π

" 1/4 1/16

y(y −1/2 − 2) d y = 2π

" 1/16 0

2y d y + 2π

" 1/4 1/16

(y 1/2 − 2y) d y

1/4

1/16 2 3/2 = 2π y 2 + 2π − y 2 y 0 3 1/16 =

11π 7π π + = . 128 384 192

y 0.25 0.2 y = 12 x

0.15 0.1 0.05

x 0

y=

√

x, 1 ≤ x ≤ 4

1

2

3

4

The Method of Cylindrical Shells

S E C T I O N 6.4

321

29. Use both the Shell and Disk Methods to calculate the volume of the solid obtained by rotating the region under the graph of f (x) = 8 − x 3 for 0 ≤ x ≤ 2 about: (a) the x-axis (b) the y-axis SOLUTION

(a) x-axis: Using the disk method, the cross sections are disks with radius R = 8 − x 3 ; hence the volume of the solid is

π

" 2 0

(8 − x 3 )2 d x = π

64x − 4x 4 +

1 7 2 576π x = . 7 7 0

With the shell method, each shell has radius y and height (8 − y)1/3 . The volume of the solid is 2π

" 8 0

y (8 − y)1/3 d y

Let u = 8 − y. Then d y = −du, y = 8 − u and 2π

" 8 0

y (8 − y)1/3 d y = 2π

" 8 0

= 2π

(8 − u) · u 1/3 du = 2π

" 8 0

(8u 1/3 − u 4/3 ) du

8 3 576π . 6u 4/3 − u 7/3 = 7 7 0

(b) y-axis: With the shell method, each shell has radius x and height 8 − x 3 . The volume of the solid is 2π

" 2 0

2 1 96π . 4x 2 − x 5 = 5 5 0

x(8 − x 3 ) d x = 2π

Using the disk method, the cross sections are disks with radius R = (8 − y)1/3 . The volume is then given by

π

8 3π 96π 2/3 5/3 (8 − y) d y = − (8 − y) = . 5 5 0 0

" 8

31. Assume that the graph in Figure 9(A) can be described by both y = f (x) and x = h(y). Let V be the volume of the Sketch the solid of rotation about the y-axis for the region under the graph of the constant function f (x) = c solid obtained by rotating the region under the curve about the y-axis. (where c > 0) for 0 ≤ x ≤ r . (a) Describe the ﬁgures generated by rotating segments AB and C B about the y-axis. (a) Find the volume without using integration. (b) Set up integrals that compute V by the Shell and Disk Methods. (b) Use the Shell Method to compute the volume. y

y y = f(x) x = h(y)

1.3

y = g(x)

B

A

A'

B'

x C

x

2

C'

(A)

2 (B)

FIGURE 9 SOLUTION

(a) When rotated about the y-axis, the segment AB generates a disk with radius R = h(y) and the segment C B generates a shell with radius x and height f (x). (b) Based on Figure 9(A) and the information from part (a), when using the Shell Method, V = 2π

" 2 0

x f (x) d x;

when using the Disk Method, V =π

" 1.3 0

(h(y))2 d y.

In Exercises 33–38, use the Shell Method to ﬁnd the volume of the solid obtained by rotating region A in Figure 10 about Let W be the volume of the solid obtained by rotating the region under the curve in Figure 9(B) about the the given axis. y-axis. (a) Describe the ﬁgures generated by rotating segments A B and A C about the y-axis. (b) Set up an integral that computes W by the Shell Method. (c) Explain the difﬁculty in computing W by the Washer Method.

322

CHAPTER 6

A P P L I C AT I O N S O F T H E I N T E G R A L y

y = x2 + 2

6 A 2

B x 1

2

FIGURE 10

33. y-axis When rotating region A about the y-axis, each shell has radius x and height 6 − (x 2 + 2) = 4 − x 2 . The volume of the resulting solid is SOLUTION

2π

" 2 0

x(4 − x 2 ) d x = 2π

2 1 (4x − x 3 ) d x = 2π 2x 2 − x 4 = 8π . 4 0 0

" 2

35. x = 2 x = −3 SOLUTION When rotating region A about x = 2, each shell has radius 2 − x and height 6 − (x 2 + 2) = 4 − x 2 . The volume of the resulting solid is " 2

1 4 2 40π 2 3 2 2 3 2 . 8 − 2x − 4x + x d x = 2π 8x − x − 2x + x = 2π (2 − x) 4 − x d x = 2π 3 4 3 0 0 0 " 2

37. y = −2 x-axis

When rotating region A about y = −2, each shell has radius y − (−2) = y + 2 and height volume of the resulting solid is SOLUTION

2π

" 6 2

√

y − 2. The

(y + 2) y − 2 d y

Let u = y − 2. Then du = d y, y + 2 = u + 4 and 2π

" 4 √ 1024π 2 5/2 8 3/2 4 (y + 2) y − 2 d y = 2π (u + 4) u du = 2π + u = u . 5 3 15 2 0 0

" 6

In Exercises y = 639–44, use the Shell Method to ﬁnd the volumes of the solids obtained by rotating region B in Figure 10 about the given axis. 39. y-axis When rotating region B about the y-axis, each shell has radius x and height x 2 + 2. The volume of the resulting solid is SOLUTION

2π

" 2 0

x(x 2 + 2) d x = 2π

" 2 0

(x 3 + 2x) d x = 2π

2 1 4 x + x 2 = 16π . 4 0

41. x = 2 x = −3 SOLUTION When rotating region B about x = 2, each shell has radius 2 − x and height x 2 + 2. The volume of the resulting solid is 2π

" 2 0

(2 − x) x 2 + 2

2 " 2

32π 2 3 1 4 2 3 2 d x = 2π x − x + 4x − x = . 2x − x + 4 − 2x d x = 2π 3 4 3 0 0

43. y = −2 x-axis

SOLUTION When rotating region B about y = −2, two different shells are generated. For each y ∈ [0, 2], the resulting shell has radius √ y − (−2) = y + 2 and height 2; for each y ∈ [2, 6], the resulting shell has radius y − (−2) = y + 2 and height 2 − y − 2. The volume of the solid is then

2π

" 2 0

2(y + 2) d y + 2π

" 6 2

(y + 2)(2 −

y − 2) d y = 2π

" 6 0

2(y + 2) d y − 2π

= 120π − 2π

" 6 2

" 6 2

(y + 2) y − 2 d y

(y + 2) y − 2 d y.

S E C T I O N 6.4

The Method of Cylindrical Shells

323

In the remaining integral, let u = y − 2, so du = d y and y + 2 = u + 4. Then 2π

" 4 √ 2 5/2 8 3/2 4 1024π (y + 2) y − 2 d y = 2π (u + 4) u du = 2π + u u = 15 . 5 3 2 0 0

" 6

Finally, the volume of the solid is 1024π 776π = . 15 15

120π −

45. Use the Shell Method to compute the volume of a sphere of radius r . y=8 SOLUTION A sphere of radius r can be generated by rotating the region under the semicircle y = r 2 − x 2 around the x-axis. Each shell has radius y and height

2 2 2 2 r − y − − r − y = 2 r 2 − y2. Thus, the volume of the sphere is 2π

" r 0

2y r 2 − y 2 d y.

Let u = r 2 − y 2 . Then du = −2y d y and 2π

2 " r2

√ 2 3/2 r 4 3 u 2y r 2 − y 2 d y = 2π u du = 2π = 3 πr . 3 0 0 0

" r

2+ 47. UseUse the the Shell Method to compute the volume of the by rotating the interior of the circle (x −ra)from Shell Method to calculate the volume V torus of theobtained “bead” formed by removing a cylinder of radius y 2 =the r 2center about of thea y-axis, where a > r . Hint: Evaluate the integral by interpreting part of it as the area of a circle. sphere of radius R (compare with Exercise 49 in Section 6.3). SOLUTION When rotating the region enclosed by the circle (x − a)2 + y 2 = r 2 about the y-axis each shell has radius x and height

r 2 − (x − a)2 − − r 2 − (x − a)2 = 2 r 2 − (x − a)2 .

The volume of the resulting torus is then 2π

" a+r a−r

2x r 2 − (x − a)2 d x.

Let u = x − a. Then du = d x, x = u + a and " a+r " r 2 2 2π 2x r − (x − a) d x = 2π 2(u + a) r 2 − u 2 du −r

a−r

= 4π

" r

−r

" r u r 2 − u 2 du + 4a π r 2 − u 2 du. −r

Now, " r −r

u r 2 − u 2 du = 0

because the integrand is an odd function and the integration interval is symmetric with respect to zero. Moreover, the other integral is one-half the area of a circle of radius r ; thus, " r 1 r 2 − u 2 du = π r 2 . 2 −r Finally, the volume of the torus is 4π (0) + 4a π

1 2 πr 2

= 2π 2 ar 2 .

49. Use the most convenient method to compute the volume of the solid obtained by rotating the region in Figure 12 Use the Shell or Disk Method (whichever is easier) to compute the volume of the solid obtained by rotating the about the axis: region in Figure 11 about: (a) x = 4 (b) y = −2 (a) the x-axis (b) the y-axis

324

CHAPTER 6

A P P L I C AT I O N S O F T H E I N T E G R A L y y = x3 + 2 y = 4 − x2 x 1

2

FIGURE 12

Examine Figure 12. If the indicated region is sliced vertically, then the top of the slice lies along the curve y = x 3 + 2 and the bottom lies along the curve y = 4 − x 2 . On the other hand, the left end of a horizontal slice switches from y = 4 − x 2 to y = x 3 + 2 at y = 3. Here, vertical slices will be more convenient. (a) Now, suppose the region in Figure 12 is rotated about x = 4. Because a vertical slice is parallel to x = 4, we will calculate the volume of the resulting solid using the shell method. Each shell has radius 4 − x and height x 3 + 2 − (4 − x 2 ) = x 3 + x 2 − 2, so the volume is SOLUTION

2π

" 2 1

(4 − x)(x 3 + x 2 − 2) d x = 2π

2 3 4 563π 1 . − x 5 + x 4 + x 3 + x 2 − 8x = 5 4 3 30 1

(b) Now suppose the region is rotated about y = −2. Because a vertical slice is perpendicular to y = −2, we will calculate the volume of the resulting solid using the disk method. Each cross section is a washer with outer radius R = x 3 + 2 − (−2) = x 3 + 4 and inner radius r = 4 − x 2 − (−2) = 6 − x 2 , so the volume is

π

2 " 2

1748π 1 7 1 5 (x 3 + 4)2 − (6 − x 2 )2 d x = π x − x + 2x 4 + 4x 3 − 20x = . 7 5 35 1 1

Further Insights and Challenges

x 2 y 2 = 1 to (Figure Show that obtained rotating 51. Let R The be the region bounded by theofellipse surface area of a sphere radius r ais 4π+r 2 . bUse this derive13). the formula forthe thesolid volume V of abysphere of 4 2 radius R in a new way. R about the y-axis (called an ellipsoid) has volume 3 π a b. (a) Show that the volume of a thin spherical shell of inner radius r and thickness x is approximately 4π r 2 x. y (b) Approximate V by decomposing the sphere of radius R into N thin spherical shells of thickness x = R/N . b (c) Show that the approximation is a Riemann sum which converges to an integral. Evaluate the integral. R a

FIGURE 13 The ellipse

x 2 a

+

x

y 2 b

= 1.

SOLUTION Let’s slice the portion of the ellipse in the ﬁrst and fourth quadrants horizontally and rotate the slices about the y-axis. The resulting ellipsoid has cross sections that are disks with radius a2 y2 R = a2 − 2 . b

Thus, the volume of the ellipsoid is b " b a2 y2 a 2 y 3 2 2 a − 2 dy = π a y − π b 3b2 −b

−b

) =π

a2 b a2b − 3

−

a2 b −a 2 b + 3

* =

4 2 π a b. 3

The bell-shaped curve in Figure 14 is the graph of a certain function y = f (x) with the following property: The tangent line at a point (x, y) on the graph has slope d y/d x = −x y. Let R be the shaded region under the graph for 6.50 Work Energy ≤ x ≤ and a in Figure 14. Use the Shell Method and the substitution u = f (x) to show that the solid obtained by revolving R around the y-axis has volume V = 2π (1 − c), where c = f (a). Observe that as c → 0, the region R becomes inﬁnite but the volume V approaches 2π . Preliminary Questions 1. Why is integration needed to compute the work performed in stretching a spring? SOLUTION Recall that the force needed to extend or compress a spring depends on the amount by which the spring has already been extended or compressed from its equilibrium position. In other words, the force needed to move a spring is variable. Whenever the force is variable, work needs to be computed with an integral.

2. Why is integration needed to compute the work performed in pumping water out of a tank but not to compute the work performed in lifting up the tank?

S E C T I O N 6.5

Work and Energy

325

SOLUTION To lift a tank through a vertical distance d, the force needed to move the tank remains constant; hence, no integral is needed to calculate the work done in lifting the tank. On the other hand, pumping water from a tank requires that different layers of the water be moved through different distances, and, depending on the shape of the tank, may require different forces. Thus, pumping water from a tank requires that an integral be evaluated.

3. Which of the following represents the work required to stretch a spring (with spring constant k) a distance x beyond its equilibrium position: kx, −kx, 12 mk 2 , 12 kx 2 , or 12 mx 2 ? SOLUTION

The work required to stretch a spring with spring constant k a distance x beyond its equilibrium position is x " x 1 1 ky d y = ky 2 = kx 2 . 2 2 0 0

Exercises 1. How much work is done raising a 4-kg mass to a height of 16 m above ground? SOLUTION

The force needed to lift a 4-kg object is a constant (4 kg)(9.8 m/s2 ) = 39.2 N.

The work done in lifting the object to a height of 16 m is then (39.2 N)(16 m) = 627.2 J. In Exercises work (in joules) required to stretch or ftcompress a spring as indicated, assuming that the How 3–6, muchcompute work is the done raising a 4-lb mass to a height of 16 above ground? spring constant is k = 150 kg/s2 . 3. Stretching from equilibrium to 12 cm past equilibrium SOLUTION

The work required to stretch the spring 12 cm past equilibrium is " .12

.12 150x d x = 75x 2 = 1.08 J. 0

0

5. Stretching from 5 to 15 cm past equilibrium Compressing from equilibrium to 4 cm past equilibrium SOLUTION The work required to stretch the spring from 5 cm to 15 cm past equilibrium is " .15 .05

.15 150x d x = 75x 2 = 1.5 J. .05

7. If 5 J of work are needed to stretch a spring 10 cm beyond equilibrium, how much work is required to stretch it Compressing the spring 4 more cm when it is already compressed 5 cm 15 cm beyond equilibrium? SOLUTION

First, we determine the value of the spring constant as follows: " 0.1 0

kx d x =

1 2 0.1 kx = 0.005k = 5 J. 2 0

Thus, k = 1000 kg/s2 . Next, we calculate the work required to stretch the spring 15 cm beyond equilibrium: " 0.15 0

0.15 1000x d x = 500x 2 = 11.25 J. 0

9. If 10 ft-lb of work are needed to stretch a spring 1 ft beyond equilibrium, how far will the spring stretch if a 10-lb If 5 J of work are needed to stretch a spring 10 cm beyond equilibrium, how much work is required to compress weight is attached to its end? it 5 cm beyond equilibrium? SOLUTION First, we determine the value of the spring constant as follows: 1 2 1 1 kx d x = kx = k = 10 ft-lb. 2 2 0 0

" 1

Thus k = 20 lb/ft. Balancing the forces acting on the weight, we have 10 lb = kd = 20d, which implies d = 0.5 ft. A 10-lb weight will therefore stretch the spring 6 inches. In Exercises 11–14, calculate the work against gravity required to build the structure out of brick using the method of Show that the work required to stretch a spring from position a to position b is 12 k(b2 − a 2 ), where k is the Examples 2 and 3. Assume that brick has density 80 lb/ft3 . spring constant. How do you interpret the negative work obtained when |b| < |a|?

326

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A P P L I C AT I O N S O F T H E I N T E G R A L

11. A tower of height 20 ft and square base of side 10 ft SOLUTION The volume of one layer is 100y ft3 and so the weight of one layer is 8000y lb. Thus, the work done against gravity to build the tower is

W =

" 20 0

20 8000y d y = 4000y 2 = 1.6 × 106 ft-lb. 0

13. A 20-ft-high tower in the shape of a right circular cone with base of radius 4 ft A cylindrical tower of height 20 ft and radius 10 ft y 2 2 SOLUTION From similar triangles, the area of one layer is π 4 − 5 ft , so the volume of each small layer is y 2 y 2 3 π 4 − 5 y ft . The weight of one layer is then 80π 4 − 5 y lb. Finally, the total work done against gravity to build the tower is " 20

y 2 128,000π ft-lb. 80π 4 − y dy = 5 3 0 15. Built around 2600 BCE, the Great Pyramid of Giza in Egypt is 485 ft high (due to erosion, its current height is slightly A structure in the shape of a hemisphere of radius 4 ft less) and has a square base of side 755.5 ft (Figure 6). Find the work needed to build the pyramid if the density of the stone is estimated at 125 lb/ft3 .

FIGURE 6 The Great Pyramid in Giza, Egypt. SOLUTION

From similar triangles, the area of one layer is 755.5 −

755.5 2 2 ft , y 485

so the volume of each small layer is 755.5 2 755.5 − y y ft3 . 485 The weight of one layer is then 755.5 2 125 755.5 − y y lb. 485 Finally, the total work needed to build the pyramid was " 485 0

755.5 2 125 755.5 − y y d y = 1.399 × 1012 ft-lb. 485

In Exercises 16–20, calculate the work (in joules) required to pump all of the water out of the tank. Assume that the tank is full, distances are measured in meters, and the density of water is 1,000 kg/m3 . 17. The hemisphere in Figure 8; water exits from the spout as shown. The box in Figure 7; water exits from a small hole at the top. 10

2

FIGURE 8

S E C T I O N 6.5

Work and Energy

327

SOLUTION Place the origin at the center of the hemisphere, and let the positive y-axis point downward. The radius of a layer of water at depth y is 100 − y 2 m, so the volume of the layer is π (100 − y 2 )y m3 , and the force needed to lift the layer is 9800π (100 − y 2 )y N. The layer must be lifted y + 2 meters, so the total work needed to empty the tank is

" 10 0

9800π (100 − y 2 )(y + 2) d y =

112700000π J ≈ 1.18 × 108 J. 3

19. The horizontal cylinder in Figure 10; water exits from a small hole at the top. Hint: Evaluate the integral by interThe conical tank in Figure 9; water exits through the spout as shown. preting part of it as the area of a circle. Water exits here

r

FIGURE 10

Place the origin along the axis of the cylinder. At location y, the layer of water is a rectangular slab of length , width 2 r 2 −y 2 and thickness y. Thus, the volume of the layer is 2 r 2 − y 2 y, and the force needed to lift the layer is 19600 r 2 − y 2 y. The layer must be lifted a distance r − y, so the total work needed to empty the tank is given by " r " r " r

19600 r 2 − y 2 (r − y) d y = 19600r r 2 − y 2 d y − 19600 y r 2 − y 2 d y. SOLUTION

−r

−r

−r

Now, " r −r

y r 2 − y 2 du = 0

because the integrand is an odd function and the integration interval is symmetric with respect to zero. Moreover, the other integral is one-half the area of a circle of radius r ; thus, " r 1 r 2 − y2 d y = πr 2. 2 −r Finally, the total work needed to empty the tank is 1 2 π r − 19600(0) = 9800π r 3 J. 19600r 2 21. Find the work W required to empty the tank in Figure 7 if it is half full of water. The trough in Figure 11; water exits by pouring over the sides. SOLUTION Place the origin on the top of the box, and let the positive y-axis point downward. Note that with this coordinate system, the bottom half of the box corresponds to y values from 2.5 to 5. The volume of one layer of water is 32y m3 , so the force needed to lift each layer is (9.8)(1000)32y = 313600y N. Each layer must be lifted y meters, so the total work needed to empty the tank is " 5 2.5

5 313600y d y = 156800y 2 = 2.94 × 106 J. 2.5

23. Find the work required to empty the tank in Figure 9 if it is half full of water. Assume the tank in Figure 7 is full of water and let W be the work required to pump out half of the water. Do SOLUTION origin the vertex of theininverted and let the y-axis point you expectPlace W tothe equal theatwork computed Exercisecone, 21? Explain andpositive then compute W . upward. Consider a layer of water at a height of y meters. From similar triangles, the area of the layer is

π

y 2 2

m2 ,

so the volume is

π

y 2 2

y m3 .

CHAPTER 6

A P P L I C AT I O N S O F T H E I N T E G R A L

Thus the weight of one layer is 9800π

y 2 2

y N.

The layer must be lifted 12 − y meters, so the total work needed to empty the half-full tank is " 5 0

9800π

y 2 2

(12 − y) d y =

1684375π J ≈ 2.65 × 106 J. 2

25. Assume that the tank in Figure 9 is full. Assume the tank in Figure 9 is full of water and ﬁnd the work required to pump out half of the water. (a) Calculate the work F(y) required to pump out water until the water level has reached level y. Plot F(y). (b) (c)

What is the signiﬁcance of F (y) as a rate of change?

(d)

If your goal is to pump out all of the water, at which water level y0 will half of the work be done?

SOLUTION

(a) Place the origin at the vertex of the inverted cone, and let the positive y-axis point upward. Consider a layer of water at a height of y meters. From similar triangles, the area of the layer is

π

y 2 2

m2 ,

so the volume is

π

y 2 2

y m3 .

Thus the weight of one layer is 9800π

y 2 2

y N.

The layer must be lifted 12 − y meters, so the total work needed to pump out water until the water level has reached level y is " 10 y

9800π

y 2 2

(12 − y) d y = 3675000π − 9800π y 3 +

1225π 4 y J. 2

(b) A plot of F(y) is shown below. y 1 × 10 −7 Work, J

328

8 × 10 −6 6 × 10 −6 4 × 10 −6 2 × 10 −6 x 0

2

4 6 8 Depth, m

10

(c) First, note that F (y) < 0; as y increases, less water is being pumped from the tank, so F(y) decreases. Therefore, when the water level in the tank has reached level y, we can interpret −F (y) as the amount of work per meter needed to remove the next layer of water from the tank. In other words, −F (y) is a “marginal work” function. (d) The amount of work needed to empty the tank is 3675000π J. Half of this work will be done when the water level reaches height y0 satisfying 3675000π − 9800π y03 +

1225π 4 y0 = 1837500π . 2

Using a computer algebra system, we ﬁnd y0 = 6.91 m. 27. How much work is done lifting a 3-m chain over the side of a building if the chain has mass density 4 kg/m? How much work is done lifting a 25-ft chain over the side of a building (Figure 12)? Assume that the chain has SOLUTION segment of the of length ysegments, located a estimate distance the y j meters from the top the building. a density Consider of 4 lb/ft.a Hint: Break up chain the chain into N work performed on of a segment, and The compute work needed to liftasthis segment chain to the top of the building is approximately the limit N→ ∞ as of anthe integral. W j ≈ (4y)(9.8)y j J.

S E C T I O N 6.5

Work and Energy

329

Summing over all segments of the chain and passing to the limit as y → 0, it follows that the total work is " 3 0

3 4 · 9.8y d y = 19.6y 2 = 176.4 J. 0

29. A 20-foot chain with mass density 3 lb/ft is initially coiled on the ground. How much work is performed in lifting An 8-ft chain weighs 16 lb. Find the work required to lift the chain over the side of a building. the chain so that it is fully extended (and one end touches the ground)? SOLUTION Consider a segment of the chain of length y that must be lifted y j feet off the ground. The work needed to lift this segment of the chain is approximately

W j ≈ (3y)y j ft-lb. Summing over all segments of the chain and passing to the limit as y → 0, it follows that the total work is " 20 0

3y d y =

3 2 20 y = 600 ft-lb. 2 0

31. A 1,000-lb wrecking ball hangs from a 30-ft cable of density 10 lb/ft attached to a crane. Calculate the work done if How much work is done lifting a 20-ft chain with mass density 3 lb/ft (initially coiled on the ground) so that its the crane lifts the ball from ground level to 30 ft in the air by drawing in the cable. top end is 30 ft above the ground? SOLUTION We will treat the cable and the wrecking ball separately. Consider a segment of the cable of length y that must be lifted y j feet. The work needed to lift the cable segment is approximately W j ≈ (10y)y j ft-lb. Summing over all of the segments of the cable and passing to the limit as y → 0, it follows that lifting the cable requires " 30 0

30 10y d y = 5y 2 = 4500 ft-lb. 0

Lifting the 1000 lb wrecking ball 30 feet requires an additional 30,000 ft-lb. Thus, the total work is 34,500 ft-lb. In Exercises 32–34, use Newton’s Universal Law of Gravity, according to which the gravitational force between two objects of mass m and M separated by a distance r has magnitude G Mm/r 2 , where G = 6.67 × 10−11 m3 kg−1 s−1 . Although the Universal Law refers to point masses, Newton proved that it also holds for uniform spherical objects, where r is the distance between their centers. 33. Use the result of Exercise 32 to calculate the work required to place a 2,000-kg satellite in an orbit 1,200 km above Two spheres of mass M and m are separated by a distance r1 . Show that 24 the work required to increase6 the the surface of the earth. Assume that the earth is a sphere−1 of mass −1 Me = 5.98 × 10 kg and radius re = 6.37 × 10 m. to aasdistance r2 is equal to W = G Mm(r1 − r2 ). Treatseparation the satellite a point mass. The satellite will move from a distance r1 = re to a distance r2 = re + 1200000. Thus, from Exercise 32, 1 1 ≈ 1.99 × 1010 J. − W = (6.67 × 10−11 )(5.98 × 1024 )(2000) 6.37 × 106 6.37 × 106 + 1200000

SOLUTION

35. Assume that the pressure P and volume V of the gas in a 30-in. cylinder of radius 3 in. with a movable piston are Use the result of Exercise 32 to compute the work required to move a 1,500-kg satellite from an orbit 1,000 to related by P V 1.4 = k, where k is a constant (Figure 13). When the cylinder is full, the gas pressure is 200 lb/in.2 . 1,500 km above the surface of the earth. (a) Calculate k. (b) Calculate the force on the piston as a function of the length x of the column of gas (the force is P A, where A is the piston’s area). (c) Calculate the work required to compress the gas column from 30 to 20 in.

3 x

FIGURE 13 Gas in a cylinder with a piston. SOLUTION

(a) We have P = 200 and V = 270π . Thus k = 200(270π )1.4 = 2.517382 × 106 lb-in2.2 .

330

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A P P L I C AT I O N S O F T H E I N T E G R A L

(b) The area of the piston is A = 9π and the volume of the cylinder as a function of x is V = 9π x, which gives P = k/V 1.4 = k/(9π x)1.4 . Thus F = PA =

k 9π = k(9π )−0.4 x −1.4 . (9π x)1.4

(c) Since the force is pushing against the piston, in order to calculate work, we must calculate the integral of the opposite force, i.e., we have " 20 1 −0.4 20 x −1.4 d x = −k(9π )−0.4 x W = −k(9π )−0.4 = 74677.8 in-lb. −0.4 30 30

Further Insights and Challenges 37. Work-Kinetic Energy Theorem The kinetic energy of an object of mass m moving with velocity v is KE = 1 mv 2 . A 20-foot chain with linear mass density 2 (x) = 0.02x(20 − x) lb/ft the time interval [t1 , t2 ] due to a net force F(x) acting along the (a) Suppose that the object moves from x1 to x 2ρduring interval [x 1 , x2 ]. Let x(t) be the position of the object at time t. Use the Change of Variables formula to show that the on the ground. worklies performed is equal to (a) How much work is done lifting the chain extended (and one end touches the ground)? " x2 so that it is "fully t2 (b) How much work is done liftingWthe so that has a height d xits=top end F(x(t))v(t) dt of 30 ft? = chain F(x) x1

t1

(b) By Newton’s Second Law, F(x(t)) = ma(t), where a(t) is the acceleration at time t. Show that d 1 mv(t)2 = F(x(t))v(t) dt 2 (c) Use the FTC to show that the change in kinetic energy during the time interval [t1 , t2 ] is equal to " t2 F(x(t))v(t) dt t1

(d) Prove the Work-Kinetic Energy Theorem: The change in KE is equal to the work W performed. SOLUTION

(a) Let x 1 = x(t1 ) and x2 = x(t2 ), then x = x(t) gives d x = v(t) dt. By substitution we have " x2 " t2 W = F(x) d x = F(x(t))v(t) dt. x1

t1

(b) Knowing F(x(t)) = m · a(t), we have d 1 2 m · v(t) = m · v(t) v (t) dt 2

(Chain Rule)

= m · v(t) a(t) = v(t) · F(x(t))

(Newton’s 2nd law)

(c) From the FTC, 1 m · v(t)2 = 2

" F(x(t)) v(t) dt.

Since K E = 12 m v 2 , K E = K E(t2 ) − K E(t1 ) =

(d)

W =

" x2 x1

F(x) d x =

" t2 1 1 F(x(t)) v(t) dt. m v(t2 )2 − m v(t1 )2 = 2 2 t1 " t2 t1

F(x(t)) v(t) dt

(Part (a))

= K E(t2 ) − K E(t1 )

(Part (c))

= K E

(as required)

39. With what initial velocity v0 must we ﬁre a rocket so it attains a maximum height r above the earth? Hint: Use the A model train of mass 0.5 kg is placed at one end of a straight 3-m electric track. Assume that a force F(x) = results of Exercises 32 and 37. As the rocket reaches its maximum height, its KE decreases from 12 mv02 to zero. (Exercise 37) to 3x − x 2 N acts on the train at distance x along the track. Use the Work-Kinetic Energy Theorem determine the velocity of the train when it reaches the end of the track.

Chapter Review Exercises SOLUTION

331

The work required to move the rocket a distance r from the surface of the earth is 1 1 W (r ) = G Me m − . re r + re

As the rocket climbs to a height r , its kinetic energy is reduced by the amount W (r ). The rocket reaches its maximum height when its kinetic energy is reduced to zero, that is, when 1 2 1 1 − mv = G Me m . 2 0 re r + re Therefore, its initial velocity must be v0 =

2G Me

1 1 − . re r + re

41. Calculate escape velocity, the minimum initial velocity of an object to ensure that it will continue traveling into With what initial velocity must we ﬁre a rocket so it attains a maximum height of r = 20 km above the surface space and never fall back to earth (assuming that no force is applied after takeoff). Hint: Take the limit as r → ∞ in of the earth? Exercise 39. SOLUTION The result of the previous exercise leads to an interesting conclusion. The initial velocity v0 required to reach a height r does not increase beyond all bounds as r tends to inﬁnity; rather, it approaches a ﬁnite limit, called the escape velocity: 1 2G Me 1 − vesc = lim 2G Me = r →∞ re r + re re

In other words, vesc is large enough to insure that the rocket reaches a height r for every value of r ! Therefore, a rocket ﬁred with initial velocity vesc never returns to earth. It continues traveling indeﬁnitely into outer space. Now, let’s see how large escape velocity actually is: vesc =

2 · 6.67 × 10−11 · 5.989 × 1024 6.37 × 106

1/2 ≈ 11,190 m/sec.

Since one meter per second is equal to 2.236 miles per hour, escape velocity is approximately 11,190(2.236) = 25,020 miles per hour.

CHAPTER REVIEW EXERCISES In Exercises 1–4, ﬁnd the area of the region bounded by the graphs of the functions. 1. y = sin x,

y = cos x,

0≤x ≤

5π 4

The region bounded by the graphs of y = sin x and y = cos x over the interval [0, 54π ] is shown below. For π x ∈ [0, 4 ], the graph of y = cos x lies above the graph of y = sin x, whereas, for x ∈ [ π4 , 54π ], the graph of y = sin x SOLUTION

lies above the graph of y = cos x. The area of the region is therefore given by " 5π /4 " π /4 (cos x − sin x) d x + (sin x − cos x) d x π /4

0

5π /4 π /4 + (− cos x − sin x) = (sin x + cos x) π /4

0

√ √ √ √ √ 2 2 2 2 2 2 = + − (0 + 1) + + − − − = 3 2 − 1. 2 2 2 2 2 2 √

√

y 1 0.5

y = sin x x 1

−0.5 −1

2

y = cos x

3

4

332

CHAPTER 6

A P P L I C AT I O N S O F T H E I N T E G R A L

= x 2 − 1, h(x) = x 2 + x − 2 3. f (x) = x 2 +32x, g(x) f (x) = x − 2x 2 + x, g(x) = x 2 − x SOLUTION The region bounded by the graphs of y = x 2 + 2x, y = x 2 − 1 and y = x 2 + x − 2 is shown below. For each x ∈ [−2, − 12 ], the graph of y = x 2 + 2x lies above the graph of y = x 2 + x − 2, whereas, for each x ∈ [− 12 , 1], the graph of y = x 2 − 1 lies above the graph of y = x 2 + x − 2. The area of the region is therefore given by " −1/2

" 1

(x 2 + 2x) − (x 2 + x − 2) d x + (x 2 − 1) − (x 2 + x − 2) d x −2

−1/2

−1/2 1 1 2 1 = + − x 2 + x x + 2x 2 2 −2 −1/2 1 1 1 1 9 = − 1 − (2 − 4) + − + 1 − − − = . 8 2 8 2 4 y = x 2 + 2x −2

y

y = x2 − 1

−1

1

x

−2 y = x2 + x − 2

In Exercises 5–8, sketch the region bounded π by the graphs of the functions and ﬁnd its area. f (x) = sin x, g(x) = sin 2x, ≤ x ≤ π 3 5. f (x) = x 3 − x 2 − x + 1, g(x) = 1 − x 2 , 0 ≤ x ≤ 1 Hint: Use geometry to evaluate the integral. SOLUTION The region bounded by the graphs of y = x 3 − x 2 − x + 1 and y = 1 − x 2 is shown below. As the 3 2 2 graph of y = 1 − x lies above the graph of y = x − x − x + 1, the area of the region is given by " 1 1 − x 2 − (x 3 − x 2 − x + 1) d x. 0

Now, the region below the graph of y = circle; thus,

1 − x 2 but above the x-axis over the interval [0, 1] is one-quarter of the unit " 1 0

1 − x2 dx =

1 π. 4

Moreover, " 1 0

(x 3 − x 2 − x + 1) d x =

1 5 1 4 1 3 1 2 1 1 1 x − x − x + x = − − + 1 = . 4 3 2 4 3 2 12 0

Finally, the area of the region shown below is 1 5 π− . 4 12 y 1 0.8

y=

1 − x2

0.6 0.4 0.2

y = x3 − x2 − x + 1 x

0

0.2 0.4 0.6 0.8

1

7. y = 4 −1x 2 , y = 3x, y=4 x = 1 − y 2 , by0the ≤ ygraphs ≤ 1 of y = 4 − x 2 , y = 3x and y = 4 is shown below. For x ∈ [0, 1], the y, x = y SOLUTION 2The region bounded graph of y = 4 lies above the graph of y = 4 − x 2 , whereas, for x ∈ [1, 43 ], the graph of y = 4 lies above the graph of y = 3x. The area of the region is therefore given by 4/3 " 1 " 4/3 1 1 3 1 16 8 3 1 (4 − (4 − x 2 )) d x + (4 − 3x) d x = x 3 + 4x − x 2 = + − − 4− = . 3 0 2 3 3 3 2 2 0 1 1

Chapter Review Exercises

333

y y=4

4

y = 3x

y = 4 − x2

3 2 1

x 0

0.2 0.4 0.6 0.8 1 1.2

Use graphing utility to locate the points of intersection of y = e−x and y = 1 − x 2 and ﬁnd the area 9. 3 −a 2y 2 = ytwo y, x = y 2 − y betweenx the curves+(approximately). The region bounded by the graphs of y = e−x and y = 1 − x 2 is shown below. One point of intersection clearly occurs at x = 0. Using a computer algebra system, we ﬁnd that the other point of intersection occurs at x = 0.7145563847. As the graph of y = 1 − x 2 lies above the graph of y = e−x , the area of the region is given by " 0.7145563847

1 − x 2 − e−x d x = 0.08235024596 SOLUTION

0

y 1 y = 1 − x2

0.8 y = e −x

0.6 0.4 0.2

x 0

0.2

0.4

0.6

0.8

11. Find the total weight of a 3-ft metal rod of linear density Figure 1 shows a solid whose horizontal cross section at height y is a circle of radius (1 + y)−2 for 0 ≤ y ≤ H . 2 Find the volume of the solid. ρ(x) = 1 + 2x + x 3 lb/ft. 9 SOLUTION

The total weight of the rod is " 3 0

ρ(x) d x =

x + x2 +

1 4 3 9 33 x =3+9+ = lb. 18 2 2 0

In Exercises the(in average valueunits) of the through functionaover Find 13–16, the ﬂowﬁnd rate the correct pipethe ofinterval. diameter 6 cm if the velocity of ﬂuid particles at a distance r from the center of the pipe is v(r ) = (3 − r ) cm/s. 3 13. f (x) = x − 2x + 2, [−1, 2] SOLUTION

The average value is

2 " 2

1 1 4 1 1 9 1 x 3 − 2x + 2 d x = x − x 2 + 2x = (4 − 4 + 4) − −1−2 = . 2 − (−1) −1 3 4 3 4 4 −1 15. f (x) = |x|, [−4, 4] f (x) = 9 − x 2 , [0, 3] Hint: Use geometry to evaluate the integral. SOLUTION The average value is " " 4 " 4 0 1 1 1 1 2 4 1 1 2 0 |x| d x = (−x) d x + x dx = − x + x = [(0 + 8) + (8 − 0)] = 2. 4 − (−4) −4 8 8 2 2 0 8 −4 0 −4 (x) = x[x], [0,g(t) 3] on [2, 5] is 9. Find 17. Thefaverage value of SOLUTION

" 5 g(t) dt. 2

The average value of the function g(t) on [2, 5] is given by " 5 " 1 5 1 g(t) dt = g(t) dt. 5−2 2 3 2

Therefore, " 5 2

g(t) dt = 3(average value) = 3(9) = 27.

334

CHAPTER 6

A P P L I C AT I O N S O F T H E I N T E G R A L

19. UseFor theall Shell to ﬁnd the volume of the obtained the region between y = x 2 and y = mx x ≥Method 0, the average value of R(x) oversolid [0, x] is equalby torevolving x. Find R(x). about the x-axis (Figure 2). y y = x2 y = mx

x

FIGURE 2 SOLUTION Setting x 2 = mx yields x(x − m) = 0, so the two curves intersect at (0, 0) and (m, m 2 ). To use the shell method, we must slice the solid parallel to the axis of rotation; as we are revolving about the x-axis, this implies a √ horizontal slice and integration in y. For each y ∈ [0, m 2 ], the shell has radius y and height y − my . The volume of the solid is therefore given by

2π

" m2 y

√

0

y y− d y = 2π m

2 5/2 y3 − y 5 3m

m 2 2m 5 m5 2π 5 − = m . = 2π 5 3 15 0

21. Let R be the intersection of the circles of radius 1 centered at (1, 0) and (0, 1). Express as an integral (but do not Use(a)the to ﬁnd the volume of the solid byx-axis. revolving the region between y = x 2 and evaluate): thewasher area ofmethod R and (b) the volume of revolution of Robtained about the y = mx about the y-axis (Figure 2). SOLUTION The region R is shown below. y 1

(x − 1)2 + y2 = 1

0.8 0.6 0.4 0.2

x2 + (y − 1)2 = 1 x

0

0.2 0.4 0.6 0.8

1

(a) A vertical slice of R has its top along the upper left arc of the circle (x − 1)2 + y 2 = 1 and its bottom along the lower right arc of the circle x 2 + (y − 1)2 = 1. The area of R is therefore given by " 1 1 − (x − 1)2 − (1 − 1 − x 2 ) d x. 0

(b) and use the washer method, each cross section is a washer with outer radius If we revolve R about the x-axis 1 − (x − 1)2 and inner radius 1 − 1 − x 2 . The volume of the solid is therefore given by

π

" 1 0

(1 − (x − 1)2 ) − (1 −

1 − x 2 )2 d x.

In Exercises 23–31, the volume of the solid obtained theexpressing region enclosed by theofcurves about the given Use the Shellﬁnd Method to set up an integral (but doby notrotating evaluate) the volume the solid obtained by axis.rotating the region under y = cos x over [0, π /2] about the line x = π . 23. y = 2x,

y = 0,

x = 8;

x-axis

SOLUTION The region bounded by the graphs of y = 2x, y = 0 and x = 8 is shown below. Let’s choose to slice the region vertically. Because a vertical slice is perpendicular to the axis of rotation, we will use the washer method to calculate the volume of the solid of revolution. For each x ∈ [0, 8], the cross section is a circular disk with radius R = 2x. The volume of the solid is therefore given by

π

" 8 0

(2x)2 d x =

4π 3 8 2048π x = . 3 3 0

Chapter Review Exercises

335

y 16 12 y = 2x

8 4

x 0

2

4

6

8

1, 8;axis x= −2−3 25. y =y x=2 − 2x,1, yy==0,2x − x= axis x= SOLUTION The region bounded by the graphs of y = x 2 − 1 and y = 2x − 1 is shown below. Let’s choose to slice the region vertically. Because a vertical slice is parallel to the axis of rotation, we will use the shell method to calculate the volume of the solid of revolution. For each x ∈ [0, 2], the shell has radius x − (−2) = x + 2 and height (2x − 1) − (x 2 − 1) = 2x − x 2 . The volume of the solid is therefore given by 2π

2 1 (x + 2)(2x − x 2 ) d x = 2π 2x 2 − x 4 = 2π (8 − 4) = 8π . 4 0 0

" 2

y 3 2 y = 2x − 1

1

x 1 −1

2

y = x2 − 1

27. y 2 = x 3 , 2 y = x, x = 8; axis x = −1 y = x − 1, y = 2x − 1, axis y = 4 SOLUTION The region bounded by the graphs of y 2 = x 3 , y = x and x = 8 is composed of two components, shown below. Let’s choose to slice the region vertically. Because a vertical slice is parallel to the axis of rotation, we will use the shell method to calculate the volume of the solid of revolution. For each x ∈ [0, 1], the shell as radius x − (−1) = x + 1 and height x − x 3/2 ; for each x ∈ [1, 8], the shell also has radius x + 1, but the height is x 3/2 − x. The volume of the solid is therefore given by 2π

" 1 0

(x + 1)(x − x 3/2 ) d x + 2π

" 8 1

(x + 1)(x 3/2 − x) d x =

y

√ 2π (12032 2 − 7083). 35

y

1

20

0.8 15 0.6 0.4

y=x

y2 = x3

10

y2 = x3

5

0.2

y=x x

x 0

0.2 0.4 0.6 0.8

1

0

2

4

6

8

29. y = 2−x 2 +−14x − 3, y = 0; axis y = −1 y = x , x = 1, x = 3; axis y = −3 SOLUTION The region bounded by the graph of y = −x 2 + 4x − 3 and the x-axis is shown below. Let’s choose to slice the region vertically. Because a vertical slice is perpendicular to the axis of rotation, we will use the washer method to calculate the volume of the solid of revolution. For each x ∈ [1, 3], the cross section is a washer with outer radius R = −x 2 + 4x − 3 − (−1) = −x 2 + 4x − 2 and inner radius r = 0 − (−1) = 1. The volume of the solid is therefore given by

π

3 " 3

20 3 1 5 (−x 2 + 4x − 2)2 − 1 d x = π x − 2x 4 + x − 8x 2 + 3x 5 3 1 1 1 20 56π 243 =π − 162 + 180 − 72 + 9 − −2+ −8+3 = . 5 5 3 15

336

CHAPTER 6

A P P L I C AT I O N S O F T H E I N T E G R A L y y = −x 2 + 4x − 3

1 0.8 0.6 0.4 0.2

x 0

0.5 1 1.5 2 2.5 3

31. y 2 = x −1 , x 3= 1, x = 3; axis x = −3 x = 4y − y , y = 0, y = 2; y-axis SOLUTION The region bounded by the graphs of y 2 = x −1 , x = 1 and x = 3 is shown below. Let’s choose to slice the region vertically. Because a vertical slice is parallel to the axis of rotation, we will use the shell method to calculate the volume of the solid of revolution. For each x ∈ [1, 3], the shell has radius x − (−3) = x + 3 and height 1 1 2 = √ . √ − −√ x x x The volume of the solid is therefore given by 2π

" 3

2 (x + 3) √ d x = 2π x 1

3 √ 40 4 3/2 + 12x 1/2 = 2π 16 3 − x . 3 3 1

y 1

x 1

2

3

1 x

y2 =

−1

In Exercises 32–34, the regions refer to the graph of the hyperbola y 2 − x 2 = 1 in Figure 3. Calculate the volume of revolution about both the x- and y-axes. y 3 y=x

2 1 −c

−1 −2

x

c

y2 − x 2 = 1

−3

FIGURE 3

33. The region between the upper branch of the hyperbola and the line y = x for 0 ≤ x ≤ c. The shaded region between the upper branch of the hyperbola and the x-axis for −c ≤ x ≤ c. SOLUTION

• x-axis: Let’s choose to slice the region vertically. Because a vertical slice is perpendicular to the axis of rotation,

we will use the washer method to calculate the volume of the solid of revolution. For each x ∈ [0, c], cross sections are washers with outer radius R = 1 + x 2 and inner radius r = x. The volume of the solid is therefore given by c " c

2 2 (1 + x ) − x d x = π x = cπ . π 0

0

• y-axis: Let’s choose to slice the region vertically. Because a vertical slice is parallel to the axis of rotation, we will

use the shell method to calculate the volume of the solid of revolution. For each x ∈ [0, c], the shell has radius x and height 1 + x 2 − x. The volume of the solid is therefore given by " c c 2π

2π

(1 + x 2 )3/2 − x 3 = (1 + c2 )3/2 − c3 − 1 . 2π x 1 + x2 − x dx = 3 3 0 0 2 35. LetThe a >region 0. Show that when the branch region between y = a and x −y ax between the upper of the hyperbola = 2.and the x-axis is rotated about the x-axis, the resulting volume is independent of the constant a.

Chapter Review Exercises

337

Setting a x − ax 2 = 0 yields x = 0 and x = 1/a. Using the washer method, cross sections are circular disks with radius R = a x − ax 2 . The volume of the solid is therefore given by SOLUTION

π

" 1/a 0

a 2 (x − ax 2 ) d x =

π

π 1 2 2 1 3 3 1/a 1 1 =π a x − a x − = , 2 3 2 3 6 0

which is independent of the constant a. In Exercises 37–38, water is pumpedlength into aisspherical tank aofforce radius a source locatedto2 20 ft below a hole the A spring whose equilibrium 15 cm exerts of 550ftNfrom when it is stretched cm. Find theat work 3. bottom (Figure 4). The density of water is 64.2 lb/ft required to stretch the spring from 22 to 24 cm.

5

2 Water source

FIGURE 4

37. Calculate the work required to ﬁll the tank. SOLUTION Place the origin at the base of the sphere with the positive y-axis pointing upward. The equation for the great is then x 2 + (y − 5)2 = 25. At location y, the horizontal cross section is a circle of radius circle of the sphere 25 − (y − 5)2 = 10y − y 2 ; the volume of the layer is then π (10y − y 2 )y ft3 , and the force needed to lift the layer is 64.2π (10y − y 2 )y lb. The layer of water must be lifted y + 2 feet, so the work required to ﬁll the tank is given by " 10 " 10 (y + 2)(10y − y 2 ) d y = 64.2π (8y 2 + 20y − y 3 ) d y 64.2π

0

0

= 64.2π

10 1 8 3 y + 10y 2 − y 4 3 4 0

= 74900π ≈ 235,305 ft-lb. 3 of water. The container is raised vertically at a constant speed of 39. A container lb required is ﬁlled with Calculateweighing the work 50 F(h) to ﬁll20 thefttank to height h ft from the bottom of the sphere. 2 ft/s for 1 min, during which time it leaks water at a rate of 13 ft3 /s. Calculate the total work performed in raising the container. The density of water is 64.2 lb/ft3 . SOLUTION Let t denote the elapsed time of the ascent of the container, and let y denote the height of the container. Given that the speed of ascent is 2 ft/s, y = 2t; moreover, the volume of water in the container is

1 1 20 − t = 20 − y ft3 . 3 6 The force needed to lift the container and its contents is then 1 50 + 64.2 20 − y = 1334 − 10.7y lb, 6 and the work required to lift the container and its contents is " 120 0

120 (1334 − 10.7y) d y = (1334y − 5.35y 2 ) = 83040 ft-lb. 0

Let W be the work (against the sun’s gravitational force) required to transport an 80-kg person from Earth to Mars when the two planets are aligned with the sun at their minimal distance of 55.7 × 106 km. Use Newton’s Universal Law of Gravity (see Exercises 32–34 in Section 6.5) to express W as an integral and evaluate it. The sun has mass Ms = 1.99 × 1030 kg, and the distance from the sun to the earth is 149.6 × 106 km.

THE EXPONENTIAL 7 FUNCTION 7.1 Derivative of bx and the Number e Preliminary Questions 1. Which of the following equations is incorrect? (a) 32 · 35 = 37

√ (b) ( 5)4/3 = 52/3

(c) 32 · 23 = 1

(d) (2−2 )−2 = 16

SOLUTION

(a) (b) (c) (d)

This equation is correct: 32 · 35 = 32+5 = 37 . √ This equation is correct: ( 5)4/3 = (51/2 )4/3 = 5(1/2)·(4/3) = 52/3 . This equation is incorrect: 32 · 23 = 9 · 8 = 72 = 1. this equation is correct: (2−2 )−2 = 2(−2)·(−2) = 24 = 16.

2. Which of the following functions can be differentiated using the Power Rule? (b) 2e (c) x e (a) x 2

(d) e x

SOLUTION The Power Rule applies when the function has a variable base and a constant exponent. Therefore, the Power Rule applies to (a) x 2 and (c) x e .

3. For which values of b does b x have a negative derivative? SOLUTION

The function b x has a negative derivative when 0 < b < 1.

4. For which values of b is the graph of y = b x concave up? SOLUTION

The graph of y = b x is concave up for all b > 0 except b = 1.

5. Which point lies on the graph of y = b x for all b? SOLUTION

The point (0, 1) lies on the graph of y = b x for all b.

6. Which of the following statements is not true? (a) (e x ) = e x eh − 1 (b) lim =1 h h→0 (c) The tangent line to y = e x at x = 0 has slope e. (d) The tangent line to y = e x at x = 0 has slope 1. SOLUTION

(a) This statement is true: (e x ) = e x . (b) This statement is true: d x eh − 1 = e = e0 = 1. lim h d x x=0 h→0 (c) This statement is false: the tangent line to y = e x at x = 0 has slope e0 = 1. (d) This statement is true: the tangent line to y = e x at x = 0 has slope e0 = 1.

Exercises 1. Rewrite as a whole number (without using a calculator): (a) 70 (c)

(43 )5

(45 )3 (e) 8−1/3 · 85/3

SOLUTION

(a) 70 = 1.

(b) 102 (2−2 + 5−2 ) (d) 274/3 (f) 3 · 41/4 − 12 · 2−3/2

S E C T I O N 7.1

(b) (c) (d) (e) (f)

Derivative of bx and the Number e

339

102 (2−2 + 5−2 ) = 100(1/4 + 1/25) = 25 + 4 = 29. (43 )5 /(45 )3 = 415 /415 = 1. (27)4/3 = (271/3 )4 = 34 = 81. 8−1/3 · 85/3 = (81/3 )5 /81/3 = 25 /2 = 24 = 16. 3 · 41/4 − 12 · 2−3/2 = 3 · 21/2 − 3 · 22 · 2−3/2 = 0.

In Exercises 2–10, solve for the unknown variable. 3. e2x = e x+1 92x = 98 SOLUTION If e2x = e x+1 then 2x = x + 1, and x = 1. x+1 5. 3x =t 2 13 4t−3 e =e 1 SOLUTION Rewrite ( 3 ) x+1 as (3−1 ) x+1 = 3−x−1 . Then 3 x = 3−x−1 , which requires x = −x − 1. Thus, x = −1/2. 7. 4−x √ = 2x+1 ( 5)x = 125 SOLUTION Rewrite 4−x as (22 )−x = 2−2x . Then 2−2x = 2 x+1 , which requires −2x = x + 1. Solving for x gives x = −1/3. 9. k 3/2 4= 27 12 b = 10 SOLUTION Raise both sides of the equation to the two-thirds power. This gives k = (27)2/3 = (271/3 )2 = 32 = 9. In Exercises 11–14, determine the limit. (b2 )x+1 = b−6 11. lim 4x x→∞

SOLUTION

lim 4x = ∞.

x→∞

1 −x 13. limlim 4−x x→∞ 4 x→∞ SOLUTION

lim

x→∞

1 −x 4

= lim 4x = ∞. x→∞

In Exercises 15–18, 2 ﬁnd the equation of the tangent line at the point indicated. lim e x−x x→∞ 15. y = 4e x , x 0 = 0 Let f (x) = 4e x . Then f (x) = 4e x and f (0) = 4. At x0 = 0, f (0) = 4, so the equation of the tangent line is y = 4(x − 0) + 4 = 4x + 4. SOLUTION

17. y = e x+24x , x 0 = −1 y = e , x0 = 0 SOLUTION Let f (x) = e x+2 . Then f (x) = e x+2 and f (−1) = e1 . At x 0 = −1, f (−1) = e, so the equation of the tangent line is y = e(x + 1) + e = ex + 2e. In Exercises 19–41, ﬁnd the derivative. 2 y = e x , x0 = 1 19. f (x) = 7e2x + 3e4x SOLUTION

d (7e2x + 3e4x ) = 14e2x + 12e4x . dx

21. f (x) = eπ x f (x) = e−5x d πx SOLUTION e = π eπ x . dx 23. f (x) = 4e−x−4x+9 + 7e−2x f (x) = e d SOLUTION (4e−x + 7e−2x ) = −4e−x − 14e−2x . dx 25. f (x) = x 2 e2x 2 ex f (x) =d 2 2x 2x 2 2x SOLUTION x e = 2xe + 2x e . dx −2x )4 27. f (x) = (2e3x + 2e f (x) = (1 + e x )4 d SOLUTION (2e3x + 2e−2x )4 = 4(2e3x + 2e−2x )3 (6e3x − 4e−2x ). dx

29. f (x) = e1/x 2 f (x) = e x +2x−3

340

CHAPTER 7

THE EXPONENTIAL FUNCTION

SOLUTION

−e1/x d 1/x = . e dx x2

31. f (x) = esin x3 f (x) = e d sin x SOLUTION = cos xesin x . e dx 33. f (x) = sin(e x )2 2 f (x) =de(x +2x+3) x SOLUTION sin(e ) = e x cos(e x ). dx 1√ 35. f (t)f (t) = = e −3t t 1−e d 1 d SOLUTION = (1 − e−3t )−1 = −(1 − e−3t )−2 (3e−3t ). dt 1 − e−3t dt −2t ) 37. f (t) = cos(te f (t) = et+1/2t−1 d SOLUTION cos(te−2t ) = − sin(te−2t )(t (−2e−2t ) + e−2t ) = (2te−2t − e−2t ) sin(te−2t ). dt

) 39. f (x) = tan(e5−6x ex f (x) =d 3xtan(e + 15−6x ) = −6e5−6x sec2 (e5−6x ). SOLUTION dx x

41. f (x) = ee x+1 e +x f (x) =d exx x 2e − SOLUTION e = 1e x ee . dx In Exercises 42–47, ﬁnd the critical points and determine whether they are local minima, maxima, or neither. 43. f (x) = x + ex −x f (x) = e − x SOLUTION Setting f (x) = 1 − e−x equal to zero and solving for x gives e−x = 1 which is true if and only if x = 0. f (x) = e−x , so f (0) = e0 = 1 > 0. Therefore, x = 0 corresponds to a local minimum. 45. f (x) = x 2 e x x e for x > 0 f (x) = SOLUTION Setting f (x) = (x 2 + 2x)e x equal to zero and solving for x gives (x 2 + 2x)e x = 0 which is true if and x only if x = 0 or x = −2. Now, f (x) = (x 2 + 4x + 2)e x . Because f (0) = 2 > 0, x = 0 corresponds to a local minimum. On the other hand, f (−2) = (4 − 8 + 2)e−2 = −2/e2 < 0, so x = −2 corresponds to a local maximum. t 47. g(t) = (t 3 − 2t)e et g(t) = 2 SOLUTION t +1

g (t) = (t 3 − 2t)(et ) + et (3t 2 − 2) = et (t 3 + 3t 2 − 2t − 2). √ The critical points occur when t 3 + 3t 2 − 2t − 2 = 0. This occurs when t = 1, −2 ± 2. g (t) = et (3t 2 + 6t − 2) + et (t 3 + 3t 2 − 2t − 2) = et (t 3 + 6t 2 + 4t − 4). Thus, • g (1) = 7e > 0, so t = 1 corresponds to a local minimum; √ √ • g (−2 + 2) ≈ −2.497 < 0, so t = −2 + 2 corresponds to a local maximum; and √ √ • g (−2 − 2) ≈ .4108 > 0, so t = −2 − 2 corresponds to a local minimum.

In Exercises 48–53, ﬁnd the critical points and points of inﬂection. Then sketch the graph. 49. y = e−x +−x ex y = xe SOLUTION Let f (x) = e−x + e x . Then f (x) = −e−x + e x =

e2x − 1 . ex

Thus, x = 0 is a critical point, and f (x) is increasing for x > 0 and decreasing for x < 0. Observe that f (x) = e−x + e x > 0 for all x, so f (x) is concave up for all x and there are no points of inﬂection. A graph of y = f (x) is shown below.

S E C T I O N 7.1

Derivative of bx and the Number e

341

y

6 4

−2

x

−1

1

2

−x 51. y =y e= e−x cos x on [−π /2, π /2] 2

Let f (x) = e−x . Then f (x) = −2xe−x and x = 0 is the only critical point. Further, f (x) is increasing for x < 0 and decreasing for x > 0. Now, 2

SOLUTION

2

f (x) = 4x 2 e−x − 2e−x = 2e−x (2x 2 − 1), 2

2

√

2

√

√

so f (x) is concave up for |x| > 22 , is concave down for |x| < 22 and has points of inﬂection at x = ± 22 . A graph of y = f (x) is shown below. y 1

0.4 0.2 −2

x

−1

1

2

on [0, 10] 53. y = x 2 e−x y = ex − x SOLUTION Let f (x) = x 2 e−x . Then f (x) = −x 2 e−x + 2xe−x = x(2 − x)e−x , so x = 0 and x = 2 are critical points. Also, f (x) is increasing for 0 < x < 2 and decreasing for 2 < x < 10. Since f (x) = −(2x − x 2 )e−x + (2 − 2x)e−x = (x 2 − 4x + 2)e−x , √ √ √ √ we ﬁnd f (x) is concave up for 0 < x√< 2 − 2 and for 2 + 2 < x < 10, is concave down for 2 − 2 < x < 2 + 2, and has inﬂection points at x = 2 ± 2. A graph of y = f (x) is shown below. y 0.6 0.4 0.2 x 0

2

4

6

8

10

In Exercises 55–68, evaluate integral. Use Newton’s Methodthe to indeﬁnite ﬁnd the two solutions of e x = 5x to three decimal places (Figure 7). " 55. (e x + 2) d x " SOLUTION

" 57.

" 2 dy ye y 4x e dx

SOLUTION

" 59.

(e x + 2) d x = e x + 2x + C.

Use the substitution u = y 2 , du = 2y d y. Then " " 2 1 1 1 2 eu du = eu + C = e y + C. ye y d y = 2 2 2

" dt e−9t 4x (e + 1) d x

342

CHAPTER 7

THE EXPONENTIAL FUNCTION SOLUTION

" 61.

" dt et ext + 1−x (e + e ) d x

SOLUTION

" 63.

Use the substitution u = −9t, du = −9 dt. Then " " 1 1 1 eu du = − eu + C = − e−9t + C. e−9t dt = − 9 9 9

Use the substitution u = et + 1, du = et dt. Then " " √ 2 2 et et + 1 dt = u du = u 3/2 + C = (et + 1)3/2 + C. 3 3

" 10x ) d x (7 − e−x (e − 4x) d x

SOLUTION

First, observe that " " " " (7 − e10x ) d x = 7 d x − e10x d x = 7x − e10x d x.

In the remaining integral, use the substitution u = 10x, du = 10 d x. Then " " 1 1 u 1 10x eu du = e10x d x = + C. e +C = e 10 10 10 Finally, " (7 − e10x ) d x = 7x − " 65.

2 " −4x d xe4x xe e2x −

SOLUTION

" 67.

1 10x + C. e 10

dx ex Use the substitution u = −4x 2 , du = −8x d x. Then " " 2 2 1 1 1 eu du = − eu + C = − e−4x + C. xe−4x d x = − 8 8 8

" ex d xx √ x eex +cos(e 1 ) dx

SOLUTION

Use the substitution u = e x + 1, du = e x d x. Then " √

ex ex + 1

" dx =

√ √ du √ = 2 u + C = 2 e x + 1 + C. u

69. Find" an approximation to m 4 using the limit deﬁnition and estimate the slope of the tangent line to y = 4x at x = 0 and x = 2.e x (e2x + 1)3 d x SOLUTION

Recall

4h − 1 . h h→0

m 4 = lim Using a table of values, we ﬁnd h

4h − 1 h

.01 .001 .0001 .00001

1.39595 1.38726 1.38639 1.38630

Thus m 4 ≈ 1.386. Knowing that y (x) = m 4 · 4x , it follows that y (0) ≈ 1.386 and y (2) ≈ 1.386 · 16 = 22.176. 71. Find the area between y = e x and y = e−x over [0, 2]. Find the area between y = e x and y = e2x over [0, 1].

S E C T I O N 7.1 SOLUTION

Derivative of bx and the Number e

343

Over [0, 2], the graph of y = e x lies above the graph of y = e−x . Hence, the area between the graphs is 2 1 1 (e x − e−x ) d x = (e x + e−x ) = e2 + 2 − (1 + 1) = e2 − 2 + 2 . e e 0 0

" 2

73. Find the volume obtained by revolving y = e x about the x-axis for 0 ≤ x ≤ 1. Find the area bounded by y = e2 , y = e x , and x = 0. SOLUTION Each cross section of the solid is a disk with radius R = e x . The volume is then

π

" 1 0

e2x d x =

π 2x 1 π e = (e2 − 1). 2 2 0

x f (x). Show that if g(x) is another function satisfying g (x) = g(x), then g(x) = Ce forx some constant C. Hint: Compute the derivative of g(x)e−x . 75. Prove that f (x) = e is not a polynomial function. Hint: Differentiation lowers the degree of a polynomial by 1.

TheInsights function fand (x) =Challenges e satisﬁes f (x) = Further x

Assume f (x) = e x is a polynomial function of degree n. Then f (n+1) (x) = 0. But we know that any derivative of e x is e x and e x = 0. Hence, e x cannot be a polynomial function. SOLUTION

77. Generalize Exercise 76; that is, use induction (if you are familiar with this method of proof) to prove that for all Recall the following property of integrals: If f (t) ≥ g(t) for all t ≥ 0, then for all x ≥ 0, n ≥ 0, " x " x 1 2 f (t)1 dt3 ≥ 1 x e ≥ 1 + x + x + x + · · · +g(t) xdtn (x ≥ 0) 0 20 6 n! t

x ≥ 1 + x for x ≥ 0. Then prove, by e76. > Assume 1. Use (5) prove thatis etrue thetostatement for n = k. We need to prove the successive the1.following inequalities (for x ≥ 0): statement is trueintegration, for n = k + By the Induction Hypothesis, x ≥ 1for The inequality because SOLUTION For n e=≥1,1eholds + tx ≥by0 Exercise

x 2 k /k!. 1 1 2 1 3 x ≥ · · ·1+ x + x +xx + e x ≥ 1e +≥x 1++ xx 2+, x /2e+ 2 2 6 Integrating both sides of this inequality yields " x et dt = e x − 1 ≥ x + x 2 /2 + · · · + x k+1 /(k + 1)! 0

or e x ≥ 1 + x + x 2 /2 + · · · + x k+1 /(k + 1)! as required. 79. Calculate the ﬁrst three derivatives xof f (x) = xe x . Then guess the formula for f (n) (x) (use induction to prove it if x ex e you are Use familiar with this of proof). ≥ = ∞. Then use Exercise 77 to prove more and conclude that lim Exercise 76 tomethod show that x→∞ x 2 6 x2 x x x x x x x x SOLUTION f (x) = e e + xe , f (x) = e + e + xe = 2e + xe , f (x) = 2e x + e x + xe x = 3e x + xe x . So all n. generally that lim(n) n = ∞ for x→∞ one would guess that f x(x) = ne x + xe x . Assuming this is true for f (n) (x), we verify that f (n+1) (x) = ( f (n) (x)) = x x x x x ne + e + xe = (n + 1)e + xe . 81. Prove in two ways that the numbers m a satisfy Consider the equation e x = λ x, where λ is a constant. m a + mdraw intuition, a graph of y = e x and the line y = λ x. (a) For which λ does it have a unique solution?mFor ab = b (b) For which λ does it have at least one solution? (a) First method: Use the limit deﬁnition of m b and ah − 1 (ab)h − 1 bh − 1 h =b + h h h (b) Second method: Apply the Product Rule to a x b x = (ab)x . SOLUTION

(ab)h − 1 bh (a h − 1) bh − 1 ah − 1 bh − 1 = lim + = lim bh lim + lim h h h h h h→0 h→0 h→0 h→0 h→0

(a) m ab = lim

ah − 1 bh − 1 + lim = ma + mb. h h h→0 h→0

= lim

So, m ab = m a + m b . (b) m ab (ab)x = ((ab)x ) = (a x b x ) = (a x ) b x + (b x ) a x = m a a x b x + m b a x b x = a x b x (m a + m b ) = (ab)x (m a + m b ). Therefore, we have m ab (ab)x = (ab)x (m a + m b ). Dividing both sides by (ab)x , we see that m ab = m a + m b .

344

CHAPTER 7

THE EXPONENTIAL FUNCTION

7.2 Inverse Functions Preliminary Questions 1. (a) (c) (e)

Which of the following satisfy f −1 (x) = f (x)? f (x) = x f (x) = 1 f (x) = |x|

SOLUTION

(b) f (x) = 1 − x √ (d) f (x) = x (f) f (x) = x −1

The functions (a) f (x) = x, (b) f (x) = 1 − x and (f) f (x) = x −1 satisfy f −1 (x) = f (x).

2. The graph of a function looks like the track of a roller coaster. Is the function one-to-one? Because the graph looks like the track of a roller coaster, there will be several locations at which the graph has the same height. The graph will therefore fail the horizontal line test, meaning that the function is not one-to-one. SOLUTION

3. Consider the function f that maps teenagers in the United States to their last names. Explain why the inverse of f does not exist. SOLUTION Many different teenagers will have the same last name, so this function will not be one-to-one. Consequently, the function does not have an inverse.

4. View the following fragment of a train schedule for the New Jersey Transit System as deﬁning a function f from towns to times. Is f one-to-one? What is f −1 (6:27)?

SOLUTION

Trenton

6:21

Hamilton Township

6:27

Princeton Junction

6:34

New Brunswick

6:38

This function is one-to-one, and f −1 (6:27) = Hamilton Township.

5. A homework problem asks for a sketch of the graph of the inverse of f (x) = x + cos x. Frank, after trying but failing to ﬁnd a formula for f −1 (x), says it’s impossible to graph the inverse. Bianca hands in an accurate sketch without solving for f −1 . How did Bianca complete the problem? SOLUTION

The graph of the inverse function is the reﬂection of the graph of y = f (x) through the line y = x.

Exercises 1. Show that f (x) = 7x − 4 is invertible by ﬁnding its inverse. y+4 x +4 SOLUTION Solving y = 7x − 4 for x yields x = . Thus, f −1 (x) = . 7 7 3. What is the largest interval containing zero on which f (x) = sin x is one-to-one? Is f (x) = x 2 + 2 one-to-one? If not, describe a domain on which it is one-to-one. SOLUTION Looking at the graph of sin x, the function is one-to-one on the interval [−π /2, π /2]. 5. Verify that f (x) = x 3 x+− 3 2and g(x) = (x − 3)1/3 are inverses by showing that f (g(x)) = x and g( f (x)) = x. Show that f (x) = is invertible by ﬁnding its inverse. SOLUTION x +3

(a) What is the domain of3 f (x)? The range of f −1 (x)? • f (g(x)) = (x − 3)1/3 +−13 = x − 3 + 3 = x. (b) What is the

domain of f (x)?

The range of f (x)? • g( f (x)) =

x3 + 3 − 3

1/3

= x3

1/3

= x.

t + 1R is v(R) = t + 1 M and radius 7. TheRepeat escapeExercise velocity 5from and g(t) = . for af planet (t) = of mass t − 1 constant. Find the inverse of v(R) as a function of R. t − 1 SOLUTION

To ﬁnd the inverse, we solve y=

2G M R

R=

2G M . y2

for R. This yields

Therefore, v −1 (R) =

2G M . R2

2G M , where G is the universal gravitational R

S E C T I O N 7.2

Inverse Functions

345

In Exercises ﬁnd avertically domain in onthe which f islands one-to-one and alater. formula forofthe of f functions restrictedare to one-to-one? this domain. A ball9–16, is tossed air and T seconds Which theinverse following Sketch the graphs of f and f −1 . (a) The ball’s height s(t) for 0 ≤ t ≤ T ball’s 9. (b) f (x)The = 3x − 2velocity v(t) for 0 ≤ t ≤ T (c) The ball’s maximum height as a function of initial velocity SOLUTION The linear function f (x) = 3x − 2 is one-to-one for all real numbers. Solving y = 3x − 2 for x gives x = (y + 2)/3. Thus,

f −1 (x) =

x +2 . 3 y

f −1(x) =

x+2 3

−2

1 x

−1

1 −1 −2

f(x) = 3x − 2

1 11. f (x)f (x) = =4−x x +1 The graph of f (x) = 1/(x + 1) given below shows that f passes the horizontal line test, and is therefore 1 1 1 one-to-one, on its entire domain {x : x = −1}. Solving y = for x gives x = − 1. Thus, f −1 (x) = − 1. x +1 y x SOLUTION

y

y

4

4 y = f −1(x) 2

y = f(x) x

−4

2

−4

4

x

−2

−4

1 13. f (s) = 2 f (x)s=

1 7x − 3 SOLUTION To make f (s) = s −2 one-to-one, we must restrict the domain to either {s : s > 0} or {s : s < 0}. If we 1 1 1 choose the domain {s : s > 0}, then solving y = 2 for s yields s = √ . Hence, f −1 (s) = √ . Had we chosen the y s s 1 −1 domain {s : s < 0}, the inverse would have been f (s) = − √ . s y

y 8 6

4

4 y = f(s)

2 −2

−1

y = f −1(s)

2 s

1

2

s 1

2

3

4

15. f (z) = z 3 1 f (x) = 3 3 SOLUTION The x function 2 + 1 f (z) = z is one-to-one over its entire domain (see the graph below). Solving y = z for z 1/3 −1 1/3 yields y = z. Thus, f (z) = z .

346

CHAPTER 7

THE EXPONENTIAL FUNCTION y y = f(z) 3 2 y = f −1(z)

1

z

−3 −2 −1 −1

1

2

3

−2 −3

17. For each function shown in Figure 15, sketch the graph of the inverse (restrict the function’s domain if necessary). f (x) = x 3 + 9 y

y

y

x

x

x (A)

(B)

(C)

y

y

y

x x

x (E)

(D)

(F)

FIGURE 15

Here, we apply the rule that the graph of f −1 is obtained by reﬂecting the graph of f across the line y = x. For (C) and (D), we must restrict the domain of f to make f one-to-one. y (b) y (c) y (a) SOLUTION

x

x (A)

(d)

x

(B)

(e)

y

(C)

(f)

y

y

x x x (D)

(E)

(F)

19. Let n be a non-zero integer. Find a domain on which f (x) = (1 − x n )1/n coincides with its inverse. Hint: The Which ofon thewhether graphs ninisFigure 16odd. is the graph of a function satisfying f −1 = f ? answer depends even or SOLUTION

First note

1/n n 1/n = 1 − (1 − x n ) = (x n )1/n = x, f ( f (x)) = 1 − (1 − x n )1/n

so f (x) coincides with its inverse. For the domain and range of f , let’s ﬁrst consider the case when n > 0. If n is even, then f (x) is deﬁned only when 1 − x n ≥ 0. Hence, the domain is −1 ≤ x ≤ 1. The range is 0 ≤ y ≤ 1. If n is odd, then f (x) is deﬁned for all real numbers, and the range is also all real numbers. Now, suppose n < 0. Then −n > 0, and 1/−n 1 −1/−n x −n f (x) = 1 − −n = . x x −n − 1 If n is even, then f (x) is deﬁned only when x −n − 1 > 0. Hence, the domain is |x| > 1. The range is y > 1. If n is odd, then f (x) is deﬁned for all real numbers except x = 1. The range is all real numbers except y = 1. 21. Show that the inverse of f (x) = e−x exists (without ﬁnding it explicitly). What is the domain of f −1 ? Let f (x) = x 7 + x + 1. SOLUTION (a) Show that f −1 exists (but do not attempt to ﬁnd it). Hint: Show that f is strictly increasing. (b) What is the domain of f −1 ? (c) Find f −1 (3).

S E C T I O N 7.2

Inverse Functions

347

y

6 4 2

−2

y = e−x x

−1

1

2

Notice that the graph of f (x) = e−x (shown above) passes the horizontal line test. That is, f (x) = e−x is one-to-one. By Theorem 3, f −1 (x) exists and the domain of f −1 (x) is the range of f (x), namely (0, ∞) 23. Let f (x) = x 2 − 2x. Determine a domain on which f −1 exists and ﬁnd a formula for f −1 on this domain. Show that f (x) = (x 2 + 1)−1 is one-to-one on (−∞, 0] and ﬁnd a formula for f −1 for this domain. SOLUTION From the graph of y = x 2 − 2x shown below, we see that if the domain of f is restricted to either x ≤ 1 or x ≥ 1, then f is one-to-one and f −1 exists. To ﬁnd a formula for f −1 , we solve y = x 2 − 2x for x as follows: y + 1 = x 2 − 2x + 1 = (x − 1)2 x −1=± y+1 x =1± y+1 −1 If the √ domain of f is restricted to x ≤ 1, then we choose the negative sign in front of the radical and f −1(x) = 1 − √ x + 1. If the domain of f is restricted to x ≥ 1, we choose the positive sign in front of the radical and f (x) = 1 + x + 1. y y = x 2 − 2x 6 4 2 x

−1

2

3

two ways: using Theorem 2 and 25. Find the inverse g(x) of f (x) −1 = x 2 + 9 with domain x ≥ 0 and calculate g (x) in −1 Show that f (x) = x + x is one-to-one on [1, ∞) and ﬁnd a formula for f on this domain. What is the by direct calculation. −1 domain of f ? SOLUTION To ﬁnd a formula for g(x) = f −1 (x), solve y = x 2 + 9 for x. This yields x = ± y 2 − 9. Because the domain of f was restricted to x ≥ 0, we must choose the positive sign in front of the radical. Thus g(x) = f −1 (x) = x 2 − 9. Because x 2 + 9 ≥ 9 for all x, it follows that f (x) ≥ 3 for all x. Thus, the domain of g(x) = f −1 (x) is x ≥ 3. The range of g is the restricted domain of f : y ≥ 0. By Theorem 2, g (x) =

1 . f (g(x))

With f (x) = it follows that

x x2 + 9

x2 − 9 = f (g(x)) = ( x 2 − 9)2 + 9

,

x2 − 9 = √ x2

since the domain of g is x ≥ 3. Thus, g (x) =

1 f (g(x))

=

x x2 − 9

.

This agrees with the answer we obtain by differentiating directly: x 2x = . g (x) = 2 2 2 x −9 x −9

x2 − 9 x

348

CHAPTER 7

THE EXPONENTIAL FUNCTION

In Exercises 27–32, use Theorem 2 to calculate3 g (x), where g(x) is the inverse of f (x). Let g(x) be the inverse of f (x) = x + 1. Find a formula for g(x) and calculate g (x) in two ways: using Theorem 27. f (x) = 7x2 and + 6 then by direct calculation. Let f (x) = 7x + 6 then f (x) = 7. Solving y = 7x + 6 for x and switching variables, we obtain the inverse g(x) = (x − 6)/7. Thus, SOLUTION

1 1 = . f (g(x)) 7

g (x) =

29. f (x) = x −5√ f (x) = 3 − x SOLUTION Let f (x) = x −5 , then f (x) = −5x −6 . Solving y = x −5 for x and switching variables, we obtain the inverse g(x) = x −1/5 . Thus, g (x) =

1 1 = − x −6/5 . 5 −5(x −1/5 )−6

x 31. f (x) = f (x) x=+4x13 − 1 SOLUTION

x , then Let f (x) = x+1

(x + 1) − x 1 = . (x + 1)2 (x + 1)2

f (x) =

x for x and switching variables, we obtain the inverse g(x) = x . Thus Solving y = x+1 1−x

g (x) = 1

1 1 = . (x/(1 − x) + 1)2 (1 − x)2

33. Let g(x) be the inverse of f (x) = x 3 + 2x + 4. Calculate g(7) [without ﬁnding a formula for g(x)] and then −1 f (x) = 2 + x calculate g (7). SOLUTION

Let g(x) be the inverse of f (x) = x 3 + 2x + 4. Because f (1) = 13 + 2(1) + 4 = 7,

it follows that g(7) = 1. Moreover, f (x) = 3x 2 + 2, and g (7) =

1 1 1 = = . f (g(7)) f (1) 5

In Exercises 35–40, calculate g(b) and g (b), where g is the inverse of f (in the given domain, if indicated). x3 Find g (− 12 ), where g(x) is the inverse of f (x) = 2 . x +1 35. f (x) = x + cos x, b = 1 f (0) = 1, so g(1) = 0. f (x) = 1 − sin x so f (g(1)) = f (0) = 1 − sin 0 = 1. Thus, g (1) = 1/1 = 1. 37. f (x) = x 2 +3 6x for x ≥ 0, b = 4 f (x) = 4x − 2x, b = −2 SOLUTION To determine g(4), we solve f (x) = x 2 + 6x = 4 for x. This yields: SOLUTION

x 2 + 6x = 16 x 2 + 6x − 16 = 0 (x + 8)(x − 2) = 0 or x = −8, 2. Because the domain of f has been restricted to x ≥ 0, we have g(4) = 2. With f (x) =

x +3 x 2 + 6x

,

it then follows that g (4) = 1 1 = for x ≤ −6, 39. f (x)f (x) = = x,2 +b 6x x +1 4

b=4

1

1

4

= = . f (g(4)) f (2) 5

S E C T I O N 7.3 SOLUTION

f (3) = 1/4, so g(1/4) = 3. f (x) =

g (1/4) = −16.

Logarithms and Their Derivatives

349

−1 so f (g(1/4)) = f (3) = −1 2 = −1/16. Thus, (x+1)2 (3+1)

41. Let f (x) = xxn and g(x) = x 1/n . Compute g (x) using Theorem 2 and check your answer using the Power Rule. f (x) = e , b = e SOLUTION Note that g(x) = f −1 (x) (see problem 55 from 6.1). Therefore, g (x) =

1 f (g(x))

=

x 1/n−1 1 1 1 1 = = = = (x 1/n−1 ) n−1 1/n n−1 1−1/n n n n(g(x)) n(x ) n(x )

which agrees with the Power Rule. graphical reasoning to determine if the following statements are true or false. If false, modify the statement Further Use Insights and Challenges

to make it correct. 43. Show that if f (x) is odd and f −1 (x) exists, then f −1 (x) is odd. Show, on the other hand, that an even function does (a) If (x) is strictly increasing, then f −1 (x) is strictly decreasing. not have anfinverse. (b) If f (x) is strictly decreasing, then−1f −1 (x) is strictly decreasing. SOLUTION Suppose f (x) is odd and f (x) exists. Because f (x) is odd, f (−x) = − f (x). Let y = f −1 (x), then (c) If f (x) is concave up, then f −1 (x) is concave up. f (y) = x. Since f (x) is odd, f (−y) = − f (y) = −x. Thus f −1 (−x) = −y = − f −1 (x). Hence, f −1 is odd. is concave (d)the If other f (x) hand, is concave down, thenthen f −1 (x) On if f (x) is even, f (−x) = f (x).up. Hence, f is not one-to-one and f −1 does not exist. (e) Linear functions f (x) = ax + b (a = 0) are always one-to-one. (x). thatone-to-one. g (x) x −1 . We will this[a, in the next 45. Let gQuadratic bethe theIntermediate inverse of a function f satisfying f (x) + show bx + c (a= are always (f) Use polynomials f (x) = ax 2xto Value Theorem that if = ff0) (x) isShow continuous and= one-to-one on anapply interval b], then section to show that the inverse of f (x) = e (the natural logarithm) is an antiderivative of x −1 . f(g) (x)sin is xanisincreasing or a decreasing function. not one-to-one. SOLUTION

g (x) =

1 1 1 1 = −1 = = . f (g(x)) x f ( f (x)) f ( f −1 (x))

7.3 Logarithms and Their Derivatives Preliminary Questions 1. Compute logb2 (b4 ). SOLUTION

Because b4 = (b2 )2 , logb2 (b4 ) = 2.

2. When is ln x negative? SOLUTION

ln x is negative for 0 < x < 1.

3. What is ln(−3)? Explain. SOLUTION

ln(−3) is not deﬁned.

4. Explain the phrase “the logarithm converts multiplication into addition.” SOLUTION

This phrase is a verbal description of the general property of logarithms that states log(ab) = log a + log b.

5. What are the domain and range of ln x? SOLUTION

The domain of ln x is x > 0 and the range is all real numbers.

6. Does x −1 have an antiderivative for x < 0? If so, describe one. SOLUTION

Yes, ln(−x) is an antiderivative of x −1 for x < 0.

Exercises In Exercises 1–12, calculate directly (without using a calculator). 1. log3 27 SOLUTION

log3 27 = log3 33 = 3 log3 3 = 3.

3. log2 (25/31) log5 25 SOLUTION

log2 25/3 =

5. log64 4 log2 (85/3 )

5 5 log2 2 = . 3 3

350

CHAPTER 7

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SOLUTION

log64 4 = log64 641/3 =

1 1 log64 64 = . 3 3

7. log8 2 + log4 2 log7 (492 ) 1/3 + log 41/2 = 1 + 1 = 5 . SOLUTION log8 2 + log4 2 = log8 8 4 3 2 6 9. log4 48 − log4 12 log25 30 + log25 65 48 SOLUTION log4 48 − log4 12 = log4 = log4 4 = 1. 12 11. ln(e3 )√ + ln(e4 ) ln( e · e7/5 ) SOLUTION ln(e3 ) + ln(e4 ) = 3 + 4 = 7. 13. Write as the natural log of a single expression: 4 (a) 2 ln 5log +23 3ln+ 4 log2 24

(b) 5 ln(x 1/2 ) + ln(9x)

SOLUTION

(a) 2 ln 5 + 3 ln 4 = ln 52 + ln 43 = ln 25 + ln 64 = ln(25 · 64) = ln 1600. (b) 5 ln x 1/2 + ln 9x = ln x 5/2 + ln 9x = ln(x 5/2 · 9x) = ln(9x 7/2 ). In Exercises 15–20, solve 2for the unknown. Solve for x: ln(x + 1) − 3 ln x = ln(2). 15. 7e5t = 100 SOLUTION

Divide the equation by 7 and then take the natural logarithm of both sides. This gives 1 100 100 or t = ln . 5t = ln 7 5 7

2 17. 2x −2x =8 6e−4t = 2 SOLUTION Since 8 = 23 , we have x 2 − 2x − 3 = 0 or (x − 3)(x + 1) = 0. Thus, x = −1 or x = 3.

19. ln(x 42t+1 ) − ln(x 2 1−t )=2 e = 9e SOLUTION

ln(x 4 ) − ln(x 2 ) = ln

x4 x2

= ln(x 2 ) = 2 ln x. Thus, 2 ln x = 2 or ln x = 1. Hence, x = e.

21. The population of a city millions) at time t (years) is P(t) = 2.4e0.06t , where t = 0 is the year 2000. When 2 ) =(in log y + 3 log (y 14 3 3 will the population double from its size at t = 0? SOLUTION

Population doubles when 4.8 = 2.4e.06t . Thus, 0.06t = ln 2 or t =

ln 2 ≈ 11.55 years. 0.06

In Exercises 23–40, ﬁnd theGutenberg–Richter derivative. According to the law, the number N of earthquakes worldwide of Richter magnitude M approximately satisﬁes a relation ln N = 16.17 − bM for some constant b. Find b, assuming that there are 800 23. y = x ln x earthquakes of magnitude M = 5 per year. How many earthquakes of magnitude M = 7 occur per year? d x SOLUTION x ln x = ln x + = ln x + 1. dx x 2 25. y =y (ln = tx)ln t − t 1 2 d SOLUTION (ln x)2 = (2 ln x) = ln x. dx x x

27. y = ln(x 3 +23x + 1) y = ln(x ) d 3x 2 + 3 . SOLUTION ln(x 3 + 3x + 1) = 3 dx x + 3x + 1 29. y = ln(sin t s+ 1) y = ln(2 ) cos t d ln(sin t + 1) = . SOLUTION dt sin t + 1 ln x 31. y =y = x 2 ln x x SOLUTION

1 (x) − ln x 1 − ln x d ln x = . = x dx x x2 x2

33. y = ln(ln x) 2 y = e(ln x)

S E C T I O N 7.3

SOLUTION

Logarithms and Their Derivatives

351

d 1 ln(ln x) = . dx x ln x

35. y = ln((ln x)3 ) 3 y = (ln(ln x)) 3(ln x)2 3 d ln((ln x)3 ) = . SOLUTION = dx x ln x x(ln x)3 Alternately, because ln((ln x)3 ) = 3 ln(ln x), d 1 d ln((ln x)3 ) = 3 ln(ln x) = 3 · . dx dx x ln x 37. y = ln(tan x) y = ln((x + 1)(2x + 9)) d 1 1 SOLUTION ln(tan x) = · sec2 x = . dx tan x sin x cos x 39. y = 5x x +1 y = lnd x3 + ln 1 5 · 5x . SOLUTION 5x = dx In Exercises 41–44, compute the derivative using Eq. (1). 2 y = 5x −x 41. f (x), f (x) = log2 x SOLUTION

f (x) = log2 x =

ln x 1 1 . Thus, f (x) = · . ln 2 x ln 2

43. f (3), f (x) = log5 x d 2 log10 (x 3 + ln xx ) 1 1 d SOLUTIONx f (x) = , so f (x) = . Thus, f (3) = . ln 5 x ln 5 3 ln 5 In Exercises d 45–56, ﬁnd an equation of the tangent line at the point indicated. log (sin t) dt= 4x3, x = 3 45. f (x) SOLUTION Let f (x) = 4 x . Then f (3) = 43 = 64. f (x) = ln 4 · 4 x , so f (3) = ln 4 · 64. Therefore, the equation of the tangent line is y = 64 ln 4(x − 3) + 64.

√ 47. s(t) = 37t , √t =x2 f (x) = ( 2) , x = 2 SOLUTION Let s(t) = 37t . Then s(2) = 314 . s (t) = 7 ln 3 · 37t , so s (2) = 7 ln 3 · 314 . Therefore the equation of the tangent line is y = 7 ln 3 · 314 (t − 2) + 314 . 49. f (x) = 5x −2x+9 , x =1 f (x) = π 3x+9 , x = 1 2 2 SOLUTION Let f (x) = 5 x −2x+9 . Then f (1) = 58 . f (x) = ln 5 · 5 x −2x+9 (2x − 2), so f (1) = ln 5(0) = 0. Therefore, the equation of the tangent line is y = 58 . 2

51. s(t) = ln(8 − 4t), t = 1 s(t) = ln t, t = 5 −4 SOLUTION Let s(t) = ln(8 − 4t). Then s(1) = ln(8 − 4) = ln 4. s (t) = 8−4t , so s (1) = −4/4 = −1. Therefore the equation of the tangent line is y = −1(t − 1) + ln 4. 53. f (x) = 4 ln(9x + 2), x = 2 f (x) = ln(x 2 ), x = 4 36 SOLUTION Let f (x) = 4 ln(9x + 2). Then f (2) = 4 ln 20. f (x) = 9x+2 , so f (2) = 9/5. Therefore the equation of the tangent line is y = (9/5)(x − 2) + 4 ln 20. 55. f (x) = log5 x, x = 2 π f (x) = ln(sin x), x = 2 1 1 4 SOLUTION Let f (x) = log5 x. Then f (2) = log5 2 = ln ln 5 . f (x) = x ln 5 , so f (2) = 2 ln 5 . Therefore the equation of the tangent line is y=

1 ln 2 . (x − 2) + 2 ln 5 ln 5

In Exercises 57–60, ﬁnd the−1 local extreme values in the domain {x : x > 0} and use the Second Derivative Test to f (x) = log2 (x + x ), x = 1 determine whether these values are local minima or maxima. 57. g(x) =

ln x x

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THE EXPONENTIAL FUNCTION SOLUTION

Let g(x) = lnxx . Then g (x) =

x(1/x) − ln x 1 − ln x = . 2 x x2

We know g (x) = 0 when 1 − ln x = 0, or when x = e. g (x) = so g (e) =

x 2 (−1/x) − (1 − ln x)(2x) −3 + 2 ln x = x4 x3

−1 < 0. Thus, g(e) is a local maximum. e3

59. g(x) = x − ln x ln x g(x) Let = g(x) 2 SOLUTION x 2 = x − ln x. Then g (x) = 1 − 1/x, and g (x) = 0 when x = 1. g (x) = 1/x , so g (1) = 1 > 0. Thus g(1) is a local minimum. In Exercises g(x)61–66, = x lnﬁnd x the derivative using the methods of Example 9. 61. f (x) = x 2x SOLUTION

Method 1: x 2x = e2x ln x , so d 2x x = e2x ln x (2 + 2 ln x) = x 2x (2 + 2 ln x). dx

Method 2: Let y = x 2x . Then, ln y = 2x ln x. By logarithmic differentiation 1 y = 2x · + 2 ln x, y x so y = y(2 + 2 ln x) = x 2x (2 + 2 ln x) . x

e 63. f (x)f (x) = x= x cos x x x SOLUTION Method 1: x e = ee ln x , so x d ex x = ee ln x dx

x x x e e + e x ln x = x e + e x ln x . x x

x Method 2: Let y = x e . Then ln y = e x ln x. By logarithmic differentiation

y 1 = e x · + e x ln x, y x so y = y

x x x e e + e x ln x = x e + e x ln x . x x

x

65. f (x) = x 2 x 2 f (x) = x x x SOLUTION Method 1: x 2 = e2 ln x , so x x x ln x x 2 2 d 2x 2 x 2 x x =e + (ln x)(ln 2)2 = x + (ln x)(ln 2)2 . dx x x x Method 2: Let y = x 2 . Then ln y = 2x ln x. By logarithmic differentiation

1 y = 2x + (ln x)(ln 2)2x , y x so x y = x 2

x 2 x + (ln x)(ln 2)2 . x

In Exercises 67–74,x xevaluate the derivative using logarithmic differentiation as in Example 8. f (x) = e

Logarithms and Their Derivatives

S E C T I O N 7.3

353

67. y = (x + 2)(x + 4) SOLUTION Let y = (x + 2)(x + 4). Then ln y = ln((x + 2)(x + 4)) = ln(x + 2) + ln(x + 4). By logarithmic differentiation

y 1 1 = + y x +2 x +4 or y = (x + 2)(x + 4)

1 1 + x +2 x +4

= (x + 4) + (x + 2) = 2x + 6.

x(x + 1)3 + 2)(x + 4) 69. y =y = (x + 1)(x (3x − 1)2 SOLUTION

3 Let y = x(x+1)2 . Then ln y = ln x + 3 ln(x + 1) − 2 ln(3x − 1). By logarithmic differentiation

(3x−1)

y 1 3 6 = + − , y x x + 1 3x − 1 so y =

(x + 1)3 3x(x + 1)2 6x(x + 1)3 + − . (3x − 1)2 (3x − 1)2 (3x − 1)3

√ 2) x − 9 71. y = (2x + 1)(4x 2 x(x + 1) y = Let √ y = (2x + 1)(4x 2 )√x − 9. Then SOLUTION x +1 ln y = ln(2x + 1) + ln 4x 2 + ln(x − 9)1/2 = ln(2x + 1) + ln 4 + 2 ln x +

1 ln(x − 9). 2

By logarithmic differentiation 2 2 1 y = + + , y 2x + 1 x 2(x − 9) so

√ y = (2x + 1)(4x 2 ) x − 9

2 2 1 + + . 2x + 1 x 2(x − 9)

73. y = (x 2 + 1)(x 2 + 2)(x 2 + 3)2 x(x + 2) y = Let y = (x 2 + 1)(x 2 + 2)(x 2 + 3)2 . Then ln y = ln(x 2 + 1) + ln(x 2 + 2) + 2 ln(x 2 + 3). By logarithmic SOLUTION (2x + 1)(2x + 2) differentiation y 2x 2x 4x = 2 + 2 + 2 , y x +1 x +2 x +3 so y = (x 2 + 1)(x 2 + 2)(x 2 + 3)2

2x 2x 4x + + . x2 + 1 x2 + 2 x2 + 3

In Exercises 75–78, x cosﬁnd x the local extrema and points of inﬂection, and sketch the graph of y = f (x) over the given interval. y = (x + 1) sin x 75. y = x 2 − ln x, SOLUTION

[ 12 , 2]

Let y = x 2 − ln x. Then y = 2x −

1 2x 2 − 1 = x x

√

√

√

and y = 2 + 12 . Thus, the function is decreasing on ( 12 , 22 ), is increasing on ( 22 , 2), has a local minimum at x = 22 , x and is concave up over the entire interval ( 12 , 2). A graph of y = x 2 − ln x is shown below.

354

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THE EXPONENTIAL FUNCTION y 3 2 1 x 0

0.5

1

1.5

2

2 + 4), [0, ∞) 77. y = ln(xln t y = 3 , [1, ∞)2 SOLUTION Let t y = ln(x + 4). Then

2x y = 2 x +4

y =

and

(x 2 + 4)(2) − 2x(2x) 8 − 2x 2 = 2 . 2 2 (x + 4) (x + 4)2

Thus, the function is increasing on (0, ∞), is concave up on (0, 2), is concave down on (2, ∞) and has a point of inﬂection at x = 2. A graph of y = ln(x 2 + 4) is shown below. y 4 3 2 1 x 0

2

4

6

8

In Exercises 79–96, evaluate the indeﬁnite integral, using substitution if necessary. y = 2−1/t , (0, 4) " dx 79. 2x + 4 SOLUTION

81.

Let u = 2x + 4. Then du = 2 d x, and " " 1 1 1 dx = du = ln |2x + 4| + C. 2x + 4 2 u 2

" "2 x dtxdt x3 + t 2 2+ 4

SOLUTION

Let u = x 3 + 2. Then du = 3x 2 d x, and "

x2 dx 1 = 3 3 x +2

"

du 1 = ln |x 3 + 2| + C. u 3

"

" tan(4x (3x+−1)1)d dx x 9 − 2x + 3x 2 ! ! sin(4x+1) SOLUTION First we rewrite tan(4x + 1) d x as cos(4x+1) d x. Let u = cos(4x + 1). Then du = −4 sin(4x + 1) d x, and " " sin(4x + 1) 1 du 1 dx = − = − ln | cos(4x + 1)| + C. cos(4x + 1) 4 u 4 83.

" 85.

" cos x dx 2 sin cotx x+d3x

SOLUTION

Let u = 2 sin x + 3. Then du = 2 cos x d x, and " " cos x 1 du 1 dx = = ln(2 sin x + 3) + C, 2 sin x + 3 2 u 2

where we have used the fact that 2 sin x + 3 ≥ 1 to drop the absolute value. " 4" ln x + 5 87. ln x d x x dx x

S E C T I O N 7.3 SOLUTION

Logarithms and Their Derivatives

355

Let u = 4 ln x + 5. Then du = (4/x)d x, and " " 1 1 1 4 ln x + 5 u du = u 2 + C = (4 ln x + 5)2 + C. dx = x 4 8 8

"

"d x 2 x ln(ln x x) d x x SOLUTION Let u = ln x. Then du = (1/x)d x, and " " dx 1 = du = ln |u| + C = ln | ln x| + C. x ln x u 89.

"

ln(ln x) " dx dx x ln x (4x − 1) ln(8x − 2) 1 1 · d x and SOLUTION Let u = ln(ln x). Then du = ln x x " " ln(ln x) u2 (ln(ln x))2 d x = u du = +C = + C. x ln x 2 2 91.

"

" 3x d x cot x ln(sin x) d x " 3x 3x d x = SOLUTION + C. ln 3 " " x dx 95. cos x 3xsin 2 x3 d x 93.

SOLUTION

Let u = sin x. Then du = cos x d x, and "

" cos x 3sin x d x =

3u du =

3u 3sin x +C = + C. ln 3 ln 3

In Exercises evaluate the deﬁnite integral. " 97–102, 1 3x+2 dx " 2 1 2 97. dx 1 x 2 " 2 1 SOLUTION d x = ln |x| = ln 2 − ln 1 = ln 2. x 1 1 " e 1 " 12 99. d x1 1 x dx e 4 "x e 1 SOLUTION d x = ln |x| = ln e − ln 1 = 1. x 1

" −e " 41 dtdt 101. −e2 t 4 2 3t "+ −e SOLUTION

1

−e e 1 = ln | − e| − ln | − e2 | = ln 2 = ln(1/e) = −1. dt = ln |t| 2 e −e2 t −e

103. " e2 Find a good numerical approximation to the coordinates of the point on the graph of y = ln x − x closest 1 10). to the origin (Figure dt e t ln t y 0.5 x 1

2

3

4

5

FIGURE 10 Graph of y = ln x − x.

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The distance from the origin to the point (x, ln x − x) on the graph of y = ln x − x is d = As usual, we will minimize d 2 . Let d 2 = f (x) = x 2 + (ln x − x)2 . Then 1 f (x) = 2x + 2(ln x − x) −1 . x SOLUTION

x 2 + (ln x − x)2 .

To determine x, we need to solve 4x +

2 ln x − 2 ln x − 2 = 0. x

This yields x ≈ .632784. Thus, the point on the graph of y = ln x − x that is closest to the origin is approximately (0.632784, −1.090410). x 105. the formula (ln of f (x)) forf x(x) > to 0. show that ln x and ln(2x) have the same derivative. Is there a Find Use the minimum value f (x)== fx (x)/ simpler explanation of this result? SOLUTION

Observe (ln x) =

1 x

and (ln 2x) =

2 1 = . 2x x

As an alternative explanation, note that ln(2x) = ln 2 + ln x. Hence, ln x and ln(2x) differ by a constant, which implies the two functions have the same derivative. 107. What is b if the slope of the tangent line to the curve y = b x at x = 0 is 4? Why is the area under the hyperbola y = 1/x between 1 and 3 equal to the area under the same hyperbola = ln b · b x so y (0) = ln b. Setting this equal to 4 and solving for b yields b = e4 . SOLUTION between y5 and 15? For which value of b is it also equal to the area between 10 and b? 109. The energy E (in joules) radiated as seismic waves from an earthquake of Richter magnitude M is given by the Let f E (x)==4.8 ln + x. 1.5M. Use the Linear Approximation to estimate f = ln(e2 + 0.1) − ln(e2 ). formula log 10 (a) Express E as a function of M. (b) Show that when M increases by 1, the energy increases by a factor of approximately 31. (c) Calculate d E/d M. SOLUTION

(a) Solving log10 E = 4.8 + 1.5M for E yields E = 104.8+1.5M . (b) Using the formula from part (a), we ﬁnd 106.3+1.5M 104.8+1.5(M+1) E(M + 1) = = 101.5 ≈ 31.6228. = E(M) 104.8+1.5M 104.8+1.5M (c) Again using the formula from part (a), dE = 1.5(ln 10)104.8+1.5M . dM

Further Insights and Challenges 111. Show that loga b logb a = 1. (a) Show that if f and g are differentiable, then ln b ln a ln b ln a and logb a = . Thus loga b · logb a = · = 1. SOLUTION loga b = (x)b f (x)ln a g ln ln a ln bd ln( f (x)g(x)) = + f (x) g(x) 113. Use Exercise 112 to verify the formula d x loga x Verify the formula logb x = for a, b > 0. d 1 ( f (x)g(x)) log b logb x =that the left-hand side of Eq. (5) is equal to . (b) Give a new proof of the Producta Rule by observing dx (ln b)x f (x)g(x) SOLUTION

d ln x 1 d logb x = = . dx d x ln b (ln b)x

115. Show that if f (x) is strictly increasing and satisﬁes f (x y) = f (x) + f (y), then its inverse g(x) satisﬁes g(x + Deﬁning ln x as an Integral Deﬁne a function ϕ (x) in the domain x > 0: y) = g(x)g(y). " x SOLUTION Let x = f (w) and y = f (z). Then 1 ϕ (x) = dt t g(x + y) = g( f (w) + f (z)) = g( f 1(wz)) = wz = g(x) · g(y). This exercise proceeds as if we didn’t know that ϕ (x) = ln x and shows directly that ϕ (x) has all the basic properties Exercises provide a the mathematically elegant approach to the exponential and logarithm functions, which avoids of the114–116 logarithm. Prove following statements: x for irrational " bofisdeﬁning " eab This a continuation of the previous exercises. g(x) be the inverse of ϕ (x). Show that the problem x and two of proving thatLet e x is differentiable. 1 1 (a) g(x)g(y) dt = dt for all a, b > 0. Hint: Use the substitution u = t/a. = g(x + y). t 1 t a r number. (b) g(r ) ==e ϕfor (ab) (a)any + ϕrational (b). Hint: Break up the integral from 1 to ab into two integrals and use (a). (b) ϕ

S E C T I O N 7.4

Exponential Growth and Decay

357

7.4 Exponential Growth and Decay Preliminary Questions 1. Two quantities increase exponentially with growth constants k = 1.2 and k = 3.4, respectively. Which quantity doubles more rapidly? SOLUTION Doubling time is inversely proportional to the growth constant. Consequently, the quantity with k = 3.4 doubles more rapidly.

2. If you are given both the doubling time and the growth constant of a quantity that increases exponentially, can you determine the initial amount? SOLUTION

No. To determine the initial amount, we need to know the amount at one instant in time.

3. A cell population grows exponentially beginning with one cell. Does it take less time for the population to increase from one to two cells than from 10 million to 20 million cells? SOLUTION Because growth from one cell to two cells and growth from 10 million to 20 million cells both involve a doubling of the population, both increases take exactly the same amount of time.

4. Referring to his popular book A Brief History of Time, the renowned physicist Stephen Hawking said, “Someone told me that each equation I included in the book would halve its sales.” If this is so, write a differential equation satisﬁed by the sales function S(n), where n is the number of equations in the book. SOLUTION

Let S(0) denote the sales with no equations in the book. Translating Hawking’s observation into an equation

yields S(n) =

S(0) . 2n

Differentiating with respect to n then yields dS d = S(0) 2−n = − ln 2S(0)2−n = − ln 2S(n). dn dn 5. Carbon dating is based on the assumption that the ratio R of C14 to C12 in the atmosphere has been constant over the past 50,000 years. If R were actually smaller in the past than it is today, would the age estimates produced by carbon dating be too ancient or too recent? SOLUTION If R were actually smaller in the past than it is today, then we would be overestimating the amount of decay and therefore overestimating the age. Our estimates would be too ancient.

Exercises 1. A certain bacteria population P obeys the exponential growth law P(t) = 2,000e1.3t (t in hours). (a) How many bacteria are present initially? (b) At what time will there be 10,000 bacteria? SOLUTION

(a) P(0) = 2000e0 = 2000 bacteria initially. (b) We solve 2000e1.3t = 10, 000 for t. Thus, e1.3t = 5 or t=

1 ln 5 ≈ 1.24 hours. 1.3

3. A certain RNA molecule has a doubling time of 3 minutes. Find the growth constant k and the differential equation A quantity P of obeys the exponential P(t) = e5tStarting (t in years). for the number N (t) molecules present atgrowth time t law (in minutes). with one molecule, how many will be present At what time t is P = 10? after(a) 10 min? ln 2 (b) At what time t is P = 20? ln 2 SOLUTION The doubling time is so k = . Thus, the differential equation is N (t) = k N (t) = doubling time (c) What is the doubling time forkP? ln 2 N (t). With one molecule initially, 3 N (t) = e(ln 2/3)t = 2t/3 . Thus, after ten minutes, there are N (10) = 210/3 ≈ 10.079, or 10 molecules present. A quantity P obeys the exponential growth law P(t) = Cekt (t in years). Find the formula for P(t), assuming that the doubling time is 7 years and P(0) = 100.

358

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THE EXPONENTIAL FUNCTION

5. The decay constant of Cobalt-60 is 0.13 years−1 . What is its half-life? SOLUTION

Half-life =

ln 2 ≈ 5.33 years. 0.13

7. Find all solutions to the differential equation y = −5y. Which solution satisﬁes the initial condition y(0) = 3.4? Find the decay constant of Radium-226, given that its half-life is 1,622 years. SOLUTION y = −5y, so y(t) = Ce−5t for some constant C. The initial condition y(0) = 3.4 determines C = 3.4. Therefore, y(t) = 3.4e−5t . y(2) = 4. 9. Find the solution to y = 3y satisfying √ Find the solution to y = 2y satisfying y(0) = 20. 4 SOLUTION y = 3y, so y(t) = Ce3t for some constant C. The initial condition y(2) = 4 determines C = . e6 4 3t Therefore, y(t) = 6 e = 4e3(t−2) . e 11. (a) (b) (c)

The population of a city is P(t) = 2 · e0.06t (in millions), where t is measured in years. Find the function y = f (t) that satisﬁes the differential equation y = −0.7y and initial condition y(0) = 10. Calculate the doubling time of the population. How long does it take for the population to triple in size? How long does it take for the population to quadruple in size?

SOLUTION

(a) Since k = 0.06, the doubling time is ln 2 ≈ 11.55 years. k (b) The tripling time is calculated in the same way as the doubling time. Solve for in the equation P(t + ) = 3P(t) 2 · e0.06(t+) = 3(2e0.06t ) 2 · e0.06t e0.06 = 3(2e0.06t ) e0.06 = 3 0.06 = ln 3, or = ln 3/0.06 ≈ 18.31 years. (c) Since the population doubles every 11.55 years, it quadruples after 2 × 11.55 = 23.10 years. 13. Assuming that population growth is approximately exponential, which of the two sets of data is most likely to The population of Washington state increased from 4.86 million in 1990 to 5.89 million in 2000. Assuming represent the population (in millions) of a city over a 5-year period? exponential growth, (a) What will the population be in 2010? Year 2000 2001 2002 2003 2004 (b) What is the doubling time? Data I 3.14 3.36 3.60 3.85 4.11 Data II 3.14 3.24 3.54 4.04 4.74 SOLUTION If the population growth is approximately exponential, then the ratio between successive years’ data needs to be approximately the same.

Year

2000

2001

2002

2003

2004

Data I Ratios

3.14 3.36 3.60 3.85 4.11 1.07006 1.07143 1.06944 1.06753

Data II Ratios

3.14 3.24 3.54 4.04 4.74 1.03185 1.09259 1.14124 1.17327

As you can see, the ratio of successive years in the data from “Data I” is very close to 1.07. Therefore, we would expect exponential growth of about P(t) ≈ (3.14)(1.07t ). 15. The Beer–Lambert Law is used in spectroscopy to determine the molar absorptivity α or the concentration c of Lightdissolved intensityin The intensity of light passing through an 10). absorbing medium exponentially withasthe a compound a solution at low concentrations (Figure The law states decreases that the intensity I of light it distance traveled. Suppose the decay constant for a certain plastic block is k = 2 when the distance is measured in passes through the solution satisﬁes ln(I /I0 ) = α cx, where I0 is the initial intensity and x is the distance traveled by the How must the block be toequation reduce the byI afor factor one-third? light.feet. Show thatthick I satisﬁes a differential d I intensity /d x = −k someofconstant k.

S E C T I O N 7.4

Exponential Growth and Decay

359

Intensity I

Distance

x

Solution

I0 0

x

FIGURE 10 Light of intensity passing through a solution.

SOLUTION

ln

I I0

= α cx so

I = eα cx or I = I0 eα cx . Therefore, I0 dI = I0 eα cx (α c) = I (α c) = −k I, dx

where k = −α c is a constant. 17. A 10-kg quantity of a radioactive isotope decays to 3 kg after 17 years. Find the decay constant of the isotope. An insect population triples in size after 5 months. Assuming exponential growth, when will it quadruple in size? ln(3/10) SOLUTION P(t) = 10e−kt . Thus P(17) = 3 = 10e−17k , so k = ≈ 0.071 years−1 . −17 19. Chauvet Caves In 1994, rock climbers in southern France stumbled on a cave containing prehistoric cave paint14 to C12 ratio showedout that samplearcheologist of sheepskinHelene parchment discovered bythat archaeologists had byaFrench Valladas showed the paintings area C between 29,700 ings. A Measurements C14 -analysis carried equal to 40% of that found in the atmosphere. Approximately how old is the parchment? and 32,400 years old, much older than any previously known human art. Given that the C14 to C12 ratio of the atmosphere is R = 10−12 , what range of C14 to C12 ratios did Valladas ﬁnd in the charcoal specimens? SOLUTION

The C14 -C12 ratio found in the specimens ranged from 10−12 e−0.000121(32400) ≈ 1.98 × 10−14

to 10−12 e−0.000121(29700) ≈ 2.75 × 10−14 . 21. Atmospheric Pressure The atmospheric pressure P(h) (in pounds per square inch) at a height h (in miles) above A paleontologist has discovered the remains of animals that appear to have died at the onset of the Holocene ice sea level on earth satisﬁes a differential equation P = −k P for some positive constant k. age. She applies carbon dating to test her theory that the Holocene age started between 10,000 and 12,000 years ago. (a) Measurements with a barometer show that P(0) =to14.7 P(10) = 2.13. What is the decay constant k? 14 to C12 ratio would she expect ﬁnd and in the animal remains? What range of C (b) Determine the atmospheric pressure 15 miles above sea level. SOLUTION

(a) Because P = −k P for some positive constant k, P(h) = Ce−kh where C = P(0) = 14.7. Therefore, P(h) = 14.7e−kh . We know that P(10) = 14.7e−10k = 2.13. Solving for k yields 2.13 1 k = − ln ≈ 0.193 miles−1 . 10 14.7 (b) P(15) = 14.7e−0.193(15) ≈ 0.813 pounds per square inch. 23. A quantity P increases exponentially with doubling time 6 hours. After how many hours has P increased by 50%? Inversion of Sugar When cane sugar is dissolved in water, it converts to invert sugar over a period of several ln 2 −1 . P willexponentially. −0.2 f. hours. TheThe percentage (t) ofisunconverted at time t decreases Suppose thatwhen f =1.5P SOLUTION doublingf time have increased by 50% = 6 socane k ≈sugar 0.1155 hours 0 = k What percentage of cane sugar remains after 5 hours? After 10 hours? ln 1.5 P0 e0.1155t , or when t = ≈ 3.5 hours. 0.1155 25. Moore’s Law In 1965, Gordon Moore predicted that the number N of transistors on a microchip would increase Two bacteria colonies are cultivated in a laboratory. The ﬁrst colony has a doubling time of 2 hours and the second exponentially. a doubling time of 3 hours. Initially, the ﬁrst colony contains 1,000 bacteria and the second colony 3,000 bacteria. (a) Does thetime tablet of data below conﬁrm Moore’s prediction for the period from 1971 to 2000? If so, estimate the growth At what will sizes of the colonies be equal? constant k. Plot the data in the table. (b) (c) Let N (t) be the number of transistors t years after 1971. Find an approximate formula N (t) ≈ Cekt , where t is the number of years after 1971. (d) Estimate the doubling time in Moore’s Law for the period from 1971 to 2000. (e) If Moore’s Law continues to hold until the end of the decade, how many transistors will a chip contain in 2010?

360

CHAPTER 7

THE EXPONENTIAL FUNCTION

(f) Can Moore have expected his prediction to hold indeﬁnitely? Transistors

Year

No. Transistors

4004 8008 8080 8086 286 386 processor 486 DX processor Pentium processor Pentium II processor Pentium III processor Pentium 4 processor

1971 1972 1974 1978 1982 1985 1989 1993 1997 1999 2000

2,250 2,500 5,000 29,000 120,000 275,000 1,180,000 3,100,000 7,500,000 24,000,000 42,000,000

SOLUTION

(a) Yes, the graph looks like an exponential graph especially towards the latter years. We estimate the growth constant by setting 1971 as our starting point, so P0 = 2250. Therefore, P(t) = 2250ekt . In 2000, t = 29. Therefore, P(29) = 2250e29k = 42000000, so k = ln 18666.67 ≈ 0.339. Note: A better estimate can be found by calculating k for each time 29 period and then averaging the k values. (b) y 4×10 7 3×10 7 2×10 7 1×10 7 x 1980 1985 1990 1995 2000

(c) (d) (e) (f)

N (t) = 2250e0.339t The doubling time is ln 2/0.339 ≈ 2.04 years. In 2010, t = 39 years. Therefore, N (39) = 2250e0.339(39) ≈ 1,241,623,327. No, you can’t make a microchip smaller than an atom.

In Exercises 27–28, the Gompertz differential equation: Assume that we in aconsider certain country, the rate at which jobs are created is proportional to the number of people who million jobs 3 months later, how many jobs will

y15.1 al

W. H. Freeman and Company, 41 Madison Avenue, New York, NY 10010 Houndmills, Basingstoke RG21 6XS, England www.whfreeman.com

Student’s Solutions Manual to accompany Jon Rogawski’s

Single Variable

CALCULUS BRIAN BRADIE Christopher Newport University With Chapter 12 contributed by

GREGORY P. DRESDEN Washington and Lee University and additional contributions by

Art Belmonte Cindy Chang-Fricke Benjamin G. Jones Kerry Marsack Katherine Socha Jill Zarestky Kenneth Zimmerman

W. H. Freeman and Company New York

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CONTENTS Chapter 1 Precalculus Review 1.1 1.2 1.3 1.4 1.5

2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8

3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9

4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8

5.1 5.2 5.3 5.4 5.5

1

Real Numbers, Functions, and Graphs Linear and Quadratic Functions The Basic Classes of Functions Trigonometric Functions Technology: Calculators and Computers Chapter Review Exercises

1 8 13 16 23 26

Chapter 2 Limits

31

Limits, Rates of Change, and Tangent Lines Limits: A Numerical and Graphical Approach Basic Limit Laws Limits and Continuity Evaluating Limits Algebraically Trigonometric Limits Intermediate Value Theorem The Formal Deﬁnition of a Limit Chapter Review Exercises

31 36 45 48 55 59 65 68 72

Chapter 3 Differentiation

79

Deﬁnition of the Derivative The Derivative as a Function Product and Quotient Rules Rates of Change Higher Derivatives Trigonometric Functions The Chain Rule Implicit Differentiation Related Rates Chapter Review Exercises

79 90 99 105 112 118 123 132 141 148

Chapter 4 Applications of the Derivative

159

Linear Approximation and Applications Extreme Values The Mean Value Theorem and Monotonicity The Shape of a Graph Graph Sketching and Asymptotes Applied Optimization Newton’s Method Antiderivatives Chapter Review Exercises

159 165 173 180 188 202 216 221 228

Chapter 5 The Integral

237

Approximating and Computing Area The Deﬁnite Integral The Fundamental Theorem of Calculus, Part I The Fundamental Theorem of Calculus, Part II Net or Total Change as the Integral of a Rate

237 250 259 266 273

5.6

6.1 6.2 6.3 6.4 6.5

7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9

8.1 8.2 8.3 8.4 8.5 8.6

Substitution Method Chapter Review Exercises

276 283

Chapter 6 Applications of the Integral

292

Area Between Two Curves 292 Setting Up Integrals: Volume, Density, Average Value 300 Volumes of Revolution 308 The Method of Cylindrical Shells 316 Work and Energy 324 Chapter Review Exercises 331

Chapter 7 The Exponential Function

338

Derivative of b x and the Number e Inverse Functions Logarithms and Their Derivatives Exponential Growth and Decay Compound Interest and Present Value Models Involving y = k ( y − b) ˆ L’Hopital’s Rule Inverse Trigonometric Functions Hyperbolic Functions Chapter Review Exercises

338 344 349 357 362 365 369 377 386 392

Chapter 8 Techniques of Integration

404

Numerical Integration Integration by Parts Trigonometric Integrals Trigonometric Substitution The Method of Partial Fractions Improper Integrals Chapter Review Exercises

404 415 426 435 449 469 487

Chapter 9 Further Applications of the Integral and Taylor Polynomials 503 9.1 9.2 9.3 9.4

10.1 10.2 10.3 10.4

Arc Length and Surface Area Fluid Pressure and Force Center of Mass Taylor Polynomials Chapter Review Exercises

503 513 518 526 538

Chapter 10 Introduction to Differential Equations

545

Solving Differential Equations Graphical and Numerical Methods The Logistic Equation First-Order Linear Equations Chapter Review Exercises

545 558 563 568 578

iv

11.1 11.2 11.3 11.4 11.5 11.6 11.7

C A L C U L U S CONTENTS

Chapter 11 Inﬁnite Series

587

Sequences Summing an Inﬁnite Series Convergence of Series with Positive Terms Absolute and Conditional Convergence The Ratio and Root Tests Power Series Taylor Series Chapter Review Exercises

587 598 608 622 628 636 646 660

12.1 12.2 12.3 12.4 12.5

Chapter 12 Parametric Equations, Polar Coordinates, and Conic Sections

674

Parametric Equations Arc Length and Speed Polar Coordinates Area and Arc Length in Polar Coordinates Conic Sections Chapter Review Exercises

674 690 698 711 721 732

P R E CALCULUS 1 REVIEW 1.1 Real Numbers, Functions, and Graphs Preliminary Questions 1. Give an example of numbers a and b such that a < b and |a| > |b|. SOLUTION

Take a = −3 and b = 1. Then a < b but |a| = 3 > 1 = |b|.

2. Which numbers satisfy |a| = a? Which satisfy |a| = −a? What about | − a| = a? SOLUTION

The numbers a ≥ 0 satisfy |a| = a and | − a| = a. The numbers a ≤ 0 satisfy |a| = −a.

3. Give an example of numbers a and b such that |a + b| < |a| + |b|. SOLUTION

Take a = −3 and b = 1. Then |a + b| = | − 3 + 1| = | − 2| = 2,

but

|a| + |b| = | − 3| + |1| = 3 + 1 = 4.

Thus, |a + b| < |a| + |b|. 4. What are the coordinates of the point lying at the intersection of the lines x = 9 and y = −4? SOLUTION

The point (9, −4) lies at the intersection of the lines x = 9 and y = −4.

5. In which quadrant do the following points lie? (a) (1, 4) (c) (4, −3)

(b) (−3, 2) (d) (−4, −1)

SOLUTION

(a) Because both the x- and y-coordinates of the point (1, 4) are positive, the point (1, 4) lies in the ﬁrst quadrant. (b) Because the x-coordinate of the point (−3, 2) is negative but the y-coordinate is positive, the point (−3, 2) lies in the second quadrant. (c) Because the x-coordinate of the point (4, −3) is positive but the y-coordinate is negative, the point (4, −3) lies in the fourth quadrant. (d) Because both the x- and y-coordinates of the point (−4, −1) are negative, the point (−4, −1) lies in the third quadrant. 6. What is the radius of the circle with equation (x − 9)2 + (y − 9)2 = 9? SOLUTION

The circle with equation (x − 9)2 + (y − 9)2 = 9 has radius 3.

7. The equation f (x) = 5 has a solution if (choose one): (a) 5 belongs to the domain of f . (b) 5 belongs to the range of f . SOLUTION

The correct response is (b): the equation f (x) = 5 has a solution if 5 belongs to the range of f .

8. What kind of symmetry does the graph have if f (−x) = − f (x)? SOLUTION

If f (−x) = − f (x), then the graph of f is symmetric with respect to the origin.

Exercises 1. Use a calculator to ﬁnd a rational number r such that |r − π 2 | < 10−4 . r must satisfy π 2 − 10−4 < r < π 2 + 10−4 , or 9.869504 < r < 9.869705. r = 9.8696 = 12337 1250 would be one such number. SOLUTION

In Exercises express interval of an inequality involving absolute value. Let a3–8, = −3 and b the = 2. Whichinofterms the following inequalities are true? (a) a2]< b 3. [−2, (d) 3a < |x| 3b ≤ SOLUTION 5. (0, 4) (−4, 4)

(b) |a| < |b| 2

(e) −4a < −4b

(c) ab > 0 1 1 (f) < a b

2

CHAPTER 1

PRECALCULUS REVIEW SOLUTION The midpoint of the interval is c = (0 + 4)/2 = 2, and the radius is r = (4 − 0)/2 = 2; therefore, (0, 4) can be expressed as |x − 2| < 2.

7. [1, 5] [−4, 0] SOLUTION The midpoint of the interval is c = (1 + 5)/2 = 3, and the radius is r = (5 − 1)/2 = 2; therefore, the interval [1, 5] can be expressed as |x − 3| ≤ 2. In Exercises write the inequality in the form a < x < b for some numbers a, b. (−2, 9–12, 8) 9. |x| < 8 SOLUTION

−8 < x < 8

11. |2x + 1| < 5 |x − 12| < 8 SOLUTION −5 < 2x + 1 < 5 so −6 < 2x < 4 and −3 < x < 2 In Exercises |3x −13–18, 4| < 2express the set of numbers x satisfying the given condition as an interval. 13. |x| < 4 SOLUTION

(−4, 4)

15. |x − 4| < 2 |x| ≤ 9 SOLUTION The expression |x − 4| < 2 is equivalent to −2 < x − 4 < 2. Therefore, 2 < x < 6, which represents the interval (2, 6). 17. |4x − 1| ≤ 8 |x + 7| < 2 7 9 SOLUTION The expression |4x − 1| ≤ 8 is equivalent to −8 ≤ 4x − 1 ≤ 8 or −7 ≤ 4x ≤ 9. Therefore, − 4 ≤ x ≤ 4 , 7 9 which represents the interval [− 4 , 4 ]. In Exercises |3x +19–22, 5| < 1describe the set as a union of ﬁnite or inﬁnite intervals. 19. {x : |x − 4| > 2} SOLUTION

x − 4 > 2 or x − 4 < −2 ⇒ x > 6 or x < 2 ⇒ (−∞, 2) ∪ (6, ∞)

21. {x : |x 2 − 1| > 2}

{x : |2x + 4| > 3} √ √ x 2 − 1 > 2 or x 2 − 1 < −2 ⇒ x 2 > 3 or x 2 < −1 (this will never happen) ⇒ x > 3 or x < − 3 ⇒ √ √ (−∞, − 3) ∪ ( 3, ∞). SOLUTION

23. Match the inequalities (a)–(f) with the corresponding statements (i)–(vi). {x : |x 2 + 2x| > 2} 1 (a) a > 3 (b) |a − 5| < 3 1 (c) a − < 5 (d) |a| > 5 3 (e) |a − 4| < 3 (f) 1 < a < 5 (i) (ii) (iii) (iv) (v) (vi)

a lies to the right of 3. a lies between 1 and 7. The distance from a to 5 is less than 13 . The distance from a to 3 is at most 2. a is less than 5 units from 13 . a lies either to the left of −5 or to the right of 5.

SOLUTION

(a) On the number line, numbers greater than 3 appear to the right; hence, a > 3 is equivalent to the numbers to the right of 3: (i). (b) |a − 5| measures the distance from a to 5; hence, |a − 5| < 13 is satisﬁed by those numbers less than 13 of a unit from 5: (iii). (c) |a − 13 | measures the distance from a to 13 ; hence, |a − 13 | < 5 is satisﬁed by those numbers less than 5 units from 1 : (v). 3 (d) The inequality |a| > 5 is equivalent to a > 5 or a < −5; that is, either a lies to the right of 5 or to the left of −5: (vi). (e) The interval described by the inequality |a − 4| < 3 has a center at 4 and a radius of 3; that is, the interval consists of those numbers between 1 and 7: (ii). (f) The interval described by the inequality 1 < x < 5 has a center at 3 and a radius of 2; that is, the interval consists of those numbers less than 2 units from 3: (iv).

S E C T I O N 1.1

Real Numbers, Functions, and Graphs

3

25. Show that if a > b, then b−1 > a −1, provided that a and b have the same sign. What happens if a > 0 and b < 0? x Describe the set x : < 0 as an interval. b 1 1 SOLUTION Case 1a: If a and b are x+ 1 both positive, then a > b ⇒ 1 > a ⇒ b > a . b Case 1b: If a and b are both negative, then a > b ⇒ 1 < a (since a is negative) ⇒ b1 > a1 (again, since b is negative). Case 2: If a > 0 and b < 0, then a1 > 0 and b1 < 0 so b1 < a1 . (See Exercise 2f for an example of this). 27. Show that if |a − 5||x<−12 3| and − 8||x<−125| , then Which x satisfy < |b 2 and < 1?|(a + b) − 13| < 1. Hint: Use the triangle inequality. SOLUTION

|a + b − 13| = |(a − 5) + (b − 8)| ≤ |a − 5| + |b − 8| < 29. (a) (b)

(by the triangle inequality)

1 1 + = 1. 2 2

Suppose that |x − 4| ≤ 1. Suppose that |a| ≤ 2 and |b| ≤ 3. What is the maximum possible value of |x + 4|? (a) What is the largest possible value of |a + b|? Show that |x 2 − 16| ≤ 9. (b) What is the largest possible value of |a + b| if a and b have opposite signs?

SOLUTION

(a) |x − 4| ≤ 1 guarantees 3 ≤ x ≤ 5. Thus, 7 ≤ x + 4 ≤ 9, so |x + 4| ≤ 9. (b) |x 2 − 16| = |x − 4| · |x + 4| ≤ 1 · 9 = 9. 31. Express r1 = 0.27 as a fraction. Hint: 100r1 − r1 is an integer. Then express r2 = 0.2666 . . . as a fraction. Prove that |x| − |y| ≤ |x − y|. Hint: Apply the triangle inequality to y and x − y. SOLUTION Let r1 = .27. We observe that 100r1 = 27.27. Therefore, 100r1 − r1 = 27.27 − .27 = 27 and r1 =

27 3 = . 99 11

Now, let r2 = .2666. Then 10r2 = 2.666 and 100r2 = 26.666. Therefore, 100r2 − 10r2 = 26.666 − 2.666 = 24 and r2 =

24 4 = . 90 15

33. The text states the following: If the decimal expansions of two real numbers a and b agree to k places, then the Represent 1/7 and 4/27 as repeating decimals. distance |a − b| ≤ 10−k . Show that the converse is not true, that is, for any k we can ﬁnd real numbers a and b whose decimal expansions do not agree at all but |a − b| ≤ 10−k . SOLUTION Let a = 1 and b = .9 (see the discussion before Example 1). The decimal expansions of a and b do not agree, but |1 − .9| < 10−k for all k.

35. Determine the equation of the circle with center (2, 4) and radius 3. Plot each pair of points and compute the distance between them: 2 2 = 32 = 9. SOLUTION (y −(2, 4)1) (a) (1, 4)The andequation (3, 2) of the indicated circle is (x − 2) + (b) and (2, 4) 37. Find all0)points with3)integer coordinates located at a distance from−3) theand origin. (c) (0, and (−2, (d) 5(−3, (−2,Then 3) ﬁnd all points with integer Determine the equation of the circle with center (2, 4) passing through (1, −1). coordinates located at a distance 5 from (2, 3). SOLUTION

• To be located a distance 5 from the origin, the points must lie on the circle x 2 + y 2 = 25. This leads to 12 points

with integer coordinates: (5, 0) (3, 4) (4, 3)

(−5, 0) (−3, 4) (−4, 3)

(0, 5) (3, −4) (4, −3)

(0, −5) (−3, −4) (−4, −3)

• To be located a distance 5 from the point (2, 3), the points must lie on the circle (x − 2)2 + (y − 3)2 = 25, which

implies that we must shift the points listed above two units to the right and three units up. This gives the 12 points: (7, 3) (5, 7) (6, 6)

(−3, 3) (−1, 7) (−2, 6)

(2, 8) (5, −1) (6, 0)

(2, −2) (−1, −1) (−2, 0)

39. Give an example of a function whose domain D has three elements and range R has two elements. Does a function Determine the domain and range of the function exist whose domain D has two elements and range has three elements? f : {r, s, t, u} → { A, B, C, D, E} deﬁned by f (r ) = A, f (s) = B, f (t) = B, f (u) = E.

4

CHAPTER 1

PRECALCULUS REVIEW

Deﬁne f by f : {a, b, c} → {1, 2} where f (a) = 1, f (b) = 1, f (c) = 2. There is no function whose domain has two elements and range has three elements. If that happened, one of the domain elements would get assigned to more than one element of the range, which would contradict the deﬁnition of a function. SOLUTION

In Exercises 40–48, ﬁnd the domain and range of the function. 4 41. g(t)f = (x)t = −x SOLUTION D : all reals; R : {y : y ≥ 0} √ 43. g(t) = 2 −3t f (x) = x SOLUTION D : {t : t ≤ 2}; R : {y : y ≥ 0}

1 45. h(s)f = (x) = |x| s SOLUTION D : {s : s = 0}; R : {y : y = 0} 47. g(t) = 2 +1t 2 √ f (x) = SOLUTION D x: 2all reals; R : {y : y ≥ 2} In Exercises 49–52,1ﬁnd the interval on which the function is increasing. g(t) = cos 49. f (x) = |x + 1|t SOLUTION A graph of the function y = |x + 1| is shown below. From the graph, we see that the function is increasing on the interval (−1, ∞). y 2

1

−3

−2

x

−1

1

51. f (x) = x 4 3 f (x) = x SOLUTION A graph of the function y = x 4 is shown below. From the graph, we see that the function is increasing on the interval (0, ∞). y 12 8 4 −2

x

−1

1

2

In Exercises 53–58, 1 ﬁnd the zeros of the function and sketch its graph by plotting points. Use symmetry and inf (x) = information crease/decrease where appropriate. 2 x +1 53. f (x) = x 2 − 4 Zeros: ±2 Increasing: x > 0 Decreasing: x < 0 Symmetry: f (−x) = f (x) (even function). So, y-axis symmetry.

SOLUTION

y 4 2 −2 −1

x −2

1

2

−4

55. f (x) = x 3 − 4x f (x) = 2x 2 − 4 SOLUTION Zeros: 0, ±2; Symmetry: f (−x) = − f (x) (odd function). So origin symmetry.

Real Numbers, Functions, and Graphs

S E C T I O N 1.1

5

y 10 5 x

−2 −1 −5

1

2

−10

57. f (x) = 2 − x33 f (x) = x √ 3 SOLUTION This is an x-axis reﬂection of x 3 translated up 2 units. There is one zero at x = 2. y 20 10 x

−2 −1−10 −20

1

2

59. Which of the curves in Figure 26 is the graph of a function? 1 f (x) = y y (x − 1)2 + 1 x x

(A)

(B) y

y

x

x

(C)

(D)

FIGURE 26 SOLUTION

(B) is the graph of a function. (A), (C), and (D) all fail the vertical line test.

In Exercises be theisfunction whose graph is shown in Figure 27. State 61–66, whetherletthef (x) function increasing, decreasing, or neither. (a) (b) (c) (d)

Surface area of a sphere as a function of its radius y Temperature at a point on the equator as a function of time 4 Price of an airline ticket as a function of the price of oil 3 Pressure of the gas in a piston as a function of volume 2 1 x

0 1

2

3

4

FIGURE 27

61. What are the domain and range of f (x)? SOLUTION

D : [0, 4]; R : [0, 4]

1 , andf (x) 2 f (x). 63. Sketch the the graphs of fof(2x), f 2) 2 x and Sketch graphs f (x + + 2. SOLUTION The graph of y = f (2x) is obtained by compressing the graph of y = f (x) horizontally by a factor of 2 (see the graph below on the left). The graph of y = f ( 12 x) is obtained by stretching the graph of y = f (x) horizontally by a factor of 2 (see the graph below in the middle). The graph of y = 2 f (x) is obtained by stretching the graph of y = f (x) vertically by a factor of 2 (see the graph below on the right). y

y

y

4

4

8

3

3

6

2

2

4

1

1

2

x 1

2 f(2x)

3

4

x

x 2

4 f(x/2)

6

8

1

2 2 f(x)

3

4

6

CHAPTER 1

PRECALCULUS REVIEW

65. Extend the graph of f (x) to [−4, 4] so that it is an even function. Sketch the graphs of f (−x) and − f (−x). SOLUTION To continue the graph of f (x) to the interval [−4, 4] as an even function, reﬂect the graph of f (x) across the y-axis (see the graph below). y 4 3 2 1 x

−4 −2

2

4

67. Suppose that f (x) has domain [4, 8] and range [2, 6]. What are the domain and range of: Extend the graph of f (x) to [−4, 4] so that it is an odd function. (a) f (x) + 3 (b) f (x + 3) (c) f (3x) (d) 3 f (x) SOLUTION

(a) f (x) + 3 is obtained by shifting f (x) upward three units. Therefore, the domain remains [4, 8], while the range becomes [5, 9]. (b) f (x + 3) is obtained by shifting f (x) left three units. Therefore, the domain becomes [1, 5], while the range remains [2, 6]. (c) f (3x) is obtained by compressing f (x) horizontally by a factor of three. Therefore, the domain becomes [ 43 , 83 ], while the range remains [2, 6]. (d) 3 f (x) is obtained by stretching f (x) vertically by a factor of three. Therefore, the domain remains [4, 8], while the range becomes [6, 18]. 69. Suppose that the graph of f (x) = sin x is compressed horizontally by a factor of 2 and then shifted 5 units to the 2 right. Let f (x) = x . Sketch the graphs of the following functions over [−2, 2]: (a) f is (xthe + 1) (b) f (x) + 1 (a) What equation for the new graph? (c) f is (5x) (d)by52? f (x) (b) What the equation if you ﬁrst shift by 5 and then compress Verify your answers by plotting your equations.

(c) SOLUTION

(a) Let f (x) = sin x. After compressing the graph of f horizontally by a factor of 2, we obtain the function g(x) = f (2x) = sin 2x. Shifting the graph 5 units to the right then yields h(x) = g(x − 5) = sin 2(x − 5) = sin(2x − 10). (b) Let f (x) = sin x. After shifting the graph 5 units to the right, we obtain the function g(x) = f (x − 5) = sin(x − 5). Compressing the graph horizontally by a factor of 2 then yields h(x) = g(2x) = sin(2x − 5). (c) The ﬁgure below at the top left shows the graphs of y = sin x (the dashed curve), the sine graph compressed horizontally by a factor of 2 (the dash, double dot curve) and then shifted right 5 units (the solid curve). Compare this last graph with the graph of y = sin(2x − 10) shown at the bottom left. The ﬁgure below at the top right shows the graphs of y = sin x (the dashed curve), the sine graph shifted to the right 5 units (the dash, double dot curve) and then compressed horizontally by a factor of 2 (the solid curve). Compare this last graph with the graph of y = sin(2x − 5) shown at the bottom right. y

y

1

1

x

−6 −4 −2

2

4

6

x

−6 −4 −2

−1

6

2

4

6

y

y 1

x 2 −1

4

−1

1

−6 −4 −2

2

4

6

x

−6 −4 −2 −1

Figure 28 shows the graph of f (x) = |x| + 1. Match the functions (a)–(e) with their graphs (i)–(v). (a) f (x − 1) (b) − f (x) (c) − f (x) + 2

Real Numbers, Functions, and Graphs

S E C T I O N 1.1

7

71. Sketch the graph of f (2x) and f 12 x , where f (x) = |x| + 1 (Figure 28). SOLUTION The graph of y = f (2x) is obtained by compressing the graph of y = f (x) horizontally by a factor of 2 (see the graph below on the left). The graph of y = f ( 12 x) is obtained by stretching the graph of y = f (x) horizontally by a factor of 2 (see the graph below on the right). y

y 6

6

4

4

2

2 x

−3 −2 −1

1

2

x

−3 −2 −1

3

f(2x)

1

2

3

f (x/2)

73. Deﬁne f (x) to be the larger of x and 2 − x. Sketch the graph of f (x). What are its domain and range? Express f (x) theabsolute functionvalue f (x)function. whose graph is obtained by shifting the parabola y = x 2 three units to the right and four in termsFind of the units down as in Figure 29. SOLUTION y

2 1 x

−1

1

2

3

The graph of y = f (x) is shown above. Clearly, the domain of f is the set of all real numbers while the range is {y | y ≥ 1}. Notice the graph has the standard V-shape associated with the absolute value function, but the base of the V has been translated to the point (1, 1). Thus, f (x) = |x − 1| + 1. 75. Show that the sum of two even functions is even and the sum of two odd functions is odd. For each curve in Figure 30, state whether it is symmetrical with respect to the y-axis, the origin, both, or neither. even SOLUTION Even: ( f + g)(−x) = f (−x) + g(−x) = f (x) + g(x) = ( f + g)(x) odd

Odd: ( f + g)(−x) = f (−x) + g(−x) = − f (x) + −g(x) = −( f + g)(x) 77. Give an example of a curve that is symmetrical with respect to both the y-axis and the origin. Can the graph of a Suppose that f (x) and g(x) are both odd. Which of the following functions are even? Which are odd? function have both symmetries? Hint: Prove algebraically that f (x) = 0 is the only such function. (a) f (x)g(x) (b) f (x)3 SOLUTION A circle of radius 1 with its center at the origin is symmetrical both with respect to the y-axis and the f (x) (c) f (x) − g(x) (d) origin. g(x) The only function having both symmetries is f (x) = 0. For if f is symmetric with respect to the y-axis, then f (−x) = f (x). If f is also symmetric with respect to the origin, then f (−x) = − f (x). Thus f (x) = − f (x) or 2 f (x) = 0. Finally, f (x) = 0.

Further Insights and Challenges 79. Show that if r = a/b is a fraction in lowest terms, then r has a ﬁnite decimal expansion if and only if b = 2n 5m for Prove the triangle inequality by adding the two inequalities some n, m ≥ 0. Hint: Observe that r has a ﬁnite decimal expansion when 10 N r is an integer for some N ≥ 0 (and hence b divides 10 N ). −|a| ≤ a ≤ |a|, −|b| ≤ b ≤ |b| Suppose r has a ﬁnite decimal expansion. Then there exists an integer N ≥ 0 such that 10 N r is an integer, call it k. Thus, r = k/10 N . Because the only prime factors of 10 are 2 and 5, it follows that when r is written in lowest terms, its denominator must be of the form 2n 5m for some integers n, m ≥ 0. a a Conversely, suppose r = a/b in lowest with b = 2n 5m for some integers n, m ≥ 0. Then r = = n m or b 2 5 a2m−n 2n 5m r = a. If m ≥ n, then 2m 5m r = a2m−n or r = and thus r has a ﬁnite decimal expansion (less than or 10m n−m a5 equal to m terms, to be precise). On the other hand, if n > m, then 2n 5n r = a5n−m or r = and once again r has 10n a ﬁnite decimal expansion. SOLUTION

81. is symmetrical with respect vertical line x = a if f (a − x) = f (a + x). be an integer with digits p1 , . . .to, pthe that Let pA=function p1 . . . psf (x) s . Show (a) Draw the graph of a function that is symmetrical with respect to x = 2. p p . . . p = 0. s 1 (b) Show that if f (x) is symmetrical with respect to = 1a, then g(x) = f (x + a) is even. 10xs − SOLUTION

2 Use this ﬁnd the decimal expansion of r may = be . Note that (a) There aretomany possibilities, two of which 11 r=

18 2 = 2 11 10 − 1

8

CHAPTER 1

PRECALCULUS REVIEW y

2 1

−1

x 1

2

3

4

5

y = | x − 2|

(b) Let g(x) = f (x + a). Then g(−x) = f (−x + a) = f (a − x) = f (a + x)

symmetry with respect to x = a

= g(x) Thus, g(x) is even. Formulate a condition for f (x) to be symmetrical with respect to the point (a, 0) on the x-axis.

1.2 Linear and Quadratic Functions Preliminary Questions 1. What is the slope of the line y = −4x − 9? SOLUTION

The slope of the line y = −4x − 9 is −4, given by the coefﬁcient of x.

2. Are the lines y = 2x + 1 and y = −2x − 4 perpendicular? SOLUTION The slopes of perpendicular lines are negative reciprocals of one another. Because the slope of y = 2x + 1 is 2 and the slope of y = −2x − 4 is −2, these two lines are not perpendicular.

3. When is the line ax + by = c parallel to the y-axis? To the x-axis? SOLUTION

The line ax + by = c will be parallel to the y-axis when b = 0 and parallel to the x-axis when a = 0.

4. Suppose y = 3x + 2. What is y if x increases by 3? SOLUTION

Because y = 3x + 2 is a linear function with slope 3, increasing x by 3 will lead to y = 3(3) = 9.

5. What is the minimum of f (x) = (x + 3)2 − 4? SOLUTION

−4.

Because (x + 3)2 ≥ 0, it follows that (x + 3)2 − 4 ≥ −4. Thus, the minimum value of (x + 3)2 − 4 is

6. What is the result of completing the square for f (x) = x 2 + 1? SOLUTION

Because there is no x term in x 2 + 1, completing the square on this expression leads to (x − 0)2 + 1.

Exercises In Exercises 1–4, ﬁnd the slope, the y-intercept, and the x-intercept of the line with the given equation. 1. y = 3x + 12 Because the equation of the line is given in slope-intercept form, the slope is the coefﬁcient of x and the y-intercept is the constant term: that is, m = 3 and the y-intercept is 12. To determine the x-intercept, substitute y = 0 and then solve for x: 0 = 3x + 12 or x = −4. SOLUTION

3. 4x + 9y = 3 y =4−x SOLUTION To determine the slope and y-intercept, we ﬁrst solve the equation for y to obtain the slope-intercept form. This yields y = − 49 x + 13 . From here, we see that the slope is m = − 49 and the y-intercept is 13 . To determine the x-intercept, substitute y = 0 and solve for x: 4x = 3 or x = 34 .

In Exercises 5–8,1ﬁnd the slope of the line. y − 3 = 2 (x − 6) 5. y = 3x + 2 SOLUTION

m=3

7. 3x + 4y = 12 y = 3(x − 9) + 2 3 SOLUTION First solve the equation for y to obtain the slope-intercept form. This yields y = − 4 x + 3. The slope of 3 the line is therefore m = − 4 . 3x + 4y = −8

S E C T I O N 1.2

Linear and Quadratic Functions

9

In Exercises 9–20, ﬁnd the equation of the line with the given description. 9. Slope 3, y-intercept 8 SOLUTION

Using the slope-intercept form for the equation of a line, we have y = 3x + 8.

11. Slope 3, passes through (7, 9) Slope −2, y-intercept 3 SOLUTION Using the point-slope form for the equation of a line, we have y − 9 = 3(x − 7) or y = 3x − 12. 13. Horizontal, passes through (0, −2) Slope −5, passes through (0, 0) SOLUTION A horizontal line has a slope of 0. Using the point-slope form for the equation of a line, we have y − (−2) = 0(x − 0) or y = −2. 15. Parallel to y = 3x − 4, passes through (1, 1) Passes through (−1, 4) and (2, 7) SOLUTION Because the equation y = 3x − 4 is in slope-intercept form, we can readily identify that it has a slope of 3. Parallel lines have the same slope, so the slope of the requested line is also 3. Using the point-slope form for the equation of a line, we have y − 1 = 3(x − 1) or y = 3x − 2. 17. Perpendicular to 3x + 5y = 9, passes through (2, 3) Passes through (1, 4) and (12, −3) SOLUTION We start by solving the equation 3x + 5y = 9 for y to obtain the slope-intercept form for the equation of a line. This yields 3 9 y=− x+ , 5 5 from which we identify the slope as − 35 . Perpendicular lines have slopes that are negative reciprocals of one another, so the slope of the desired line is m ⊥ = 53 . Using the point-slope form for the equation of a line, we have y − 3 = 53 (x − 2) or y = 53 x − 13 . 19. Horizontal, passes through (8, 4) Vertical, passes through (−4, 9) SOLUTION A horizontal line has slope 0. Using the point slope form for the equation of a line, we have y − 4 = 0(x − 8) or y = 4. 21. Find the equation of the perpendicular bisector of joining (1, 2) and (5, 4) (Figure 11). Hint: The the segment Slope 3, x-intercept 6 a+c b+d midpoint Q of the segment joining (a, b) and (c, d) is , . 2 2 y

Perpendicular bisector (5, 4) Q (1, 2) x

FIGURE 11 SOLUTION

The slope of the segment joining (1, 2) and (5, 4) is m=

4−2 1 = 5−1 2

and the midpoint of the segment (Figure 11) is midpoint =

1+5 2+4 , 2 2

= (3, 3)

The perpendicular bisector has slope −1/m = −2 and passes through (3, 3), so its equation is: y − 3 = −2(x − 3) or y = −2x + 9. 23. Find the equation of the line with x-intercept x = 4 and y-intercept y = 3. Intercept-intercept Form Show that if a, b = 0, then the line with x-intercept x = a and y-intercept y = b y x SOLUTION From Exercise has equation (Figure 12) 22, 4 + 3 = 1 or 3x + 4y = 12. y x + cy = 1 25. Determine whether there exists a constant c such thatx the line 1 (3, y) lies on the line. A line of slope m = 2 passes through (1, 4). Find y+such=that a b Passes through (3, 1) (a) Has slope 4 (b) (c) Is horizontal (d) Is vertical SOLUTION

10

CHAPTER 1

PRECALCULUS REVIEW

(a) Rewriting the equation of the line in slope-intercept form gives y = − xc + 1c . To have slope 4 requires − 1c = 4 or c = − 14 . (b) Substituting x = 3 and y = 1 into the equation of the line gives 3 + c = 1 or c = −2. (c) From (a), we know the slope of the line is − 1c . There is no value for c that will make this slope equal to 0. (d) With c = 0, the equation becomes x = 1. This is the equation of a vertical line. 27. Materials expand when heated. Consider a metal rod of length L 0 at temperature T0 . If the temperature is changed Assume that the number N of concert tickets which can be sold at a price of P dollars per ticket is a linear by an amount T , then the rod’s length changes by L = α L 0 T , where α is the thermal expansion coefﬁcient. For function N (P) for 10 ≤ P ≤ 40. Determine N (P) (called the demand function) if N (10) = 500 and N (40) = 0. steel, α = 1.24 × 10−5 ◦ C−1 . What is the decrease N in the number of tickets sold if the price is increased by P = 5 dollars? (a) A steel rod has length L 0 = 40 cm at T0 = 40◦ C. What is its length at T = 90◦ C? (b) Find its length at T = 50◦ C if its length at T0 = 100◦ C is 65 in. (c) Express length L as a function of T if L 0 = 65 in. at T0 = 100◦ C. SOLUTION

(a) With T = 90◦ C and T0 = 40◦ C, T = 50◦ C. Therefore, L = α L 0 T = (1.24 × 10−5 )(40)(50) = .0248

and

L = L 0 + L = 40.0248 cm.

(b) With T = 50◦ C and T0 = 100◦ C, T = −50◦ C. Therefore, L = α L 0 T = (1.24 × 10−5 )(65)(−50) = −.0403

and

L = L 0 + L = 64.9597 in.

(c) L = L 0 + L = L 0 + α L 0 T = L 0 (1 + α T ) = 65(1 + α (T − 100)) 29. Find b such that (2, −1), (3, 2), and (b, 5) lie on a line. Do the points (0.5, 1), (1, 1.2), (2, 2) lie on a line? SOLUTION The slope of the line determined by the points (2, −1) and (3, 2) is 2 − (−1) = 3. 3−2 To lie on the same line, the slope between (3, 2) and (b, 5) must also be 3. Thus, we require 3 5−2 = = 3, b−3 b−3 or b = 4. 31. The period T of a pendulum is measured for pendulums of several different lengths L. Based on the following data, Find an expression for the velocity v as a linear function of t that matches the following data. does T appear to be a linear function of L? t (s) 0 2 4 6 L (ft) 2 3 4 5 v (m/s) 39.2 58.6 78 97.4 T (s) 1.57 1.92 2.22 2.48 SOLUTION

Examine the slope between consecutive data points. The ﬁrst pair of data points yields a slope of 1.92 − 1.57 = .35, 3−2

while the second pair of data points yields a slope of 2.22 − 1.92 = .3, 4−3 and the last pair of data points yields a slope of 2.48 − 2.22 = .26 5−4 Because the three slopes are not equal, T does not appear to be a linear function of L. 33. Find the roots of the quadratic polynomials: Show that f (x) is linear of slope m if and only if (a) 4x 2 − 3x − 1 (b) x 2 − 2x − 1 f (x + h) − f (x) = mh (for all x and h) SOLUTION √ √ 3 ± 9 − 4(4)(−1) 3 ± 25 1 (a) x = = = 1 or − 2(4) 8 4 √ √ √ 2 ± 4 − (4)(1)(−1) 2± 8 (b) x = = =1± 2 2 2

Linear and Quadratic Functions

S E C T I O N 1.2

11

In Exercises 34–41, complete the square and ﬁnd the minimum or maximum value of the quadratic function. 35. y = x 2 −26x + 9 y = x + 2x + 5 SOLUTION y = (x − 3)2 ; therefore, the minimum value of the quadratic polynomial is 0, and this occurs at x = 3. 37. y = x 2 + 6x2+ 2 y = −9x + x SOLUTION y = x 2 + 6x + 9 − 9 + 2 = (x + 3)2 − 7; therefore, the minimum value of the quadratic polynomial is −7, and this occurs at x = −3. 39. y = −4x 2 2+ 3x + 8 y = 2x − 4x − 7 3 9 9 3 137 SOLUTION y = −4x 2 + 3x + 8 = −4(x 2 − 4 x + 64 ) + 8 + 16 = −4(x − 8 )2 + 16 ; therefore, the maximum 3 value of the quadratic polynomial is 137 16 , and this occurs at x = 8 . 2 41. y = 4x − 12x y = 3x 2 + 12x − 5 1 ) + 1 = −12(x − 1 )2 + 1 ; therefore, the maximum value of the SOLUTION y = −12(x 2 − x3 ) = −12(x 2 − x3 + 36 3 6 3 quadratic polynomial is 13 , and this occurs at x = 16 .

43. Sketch the graph of y = x 2 +24x + 6 by plotting the minimum point, the y-intercept, and one other point. Sketch the graph of y = x − 6x + 8 by plotting the roots and the minimum point. SOLUTION y = x 2 + 4x + 4 − 4 + 6 = (x + 2)2 + 2 so the minimum occurs at (−2, 2). If x = 0, then y = 6 and if x = −4, y = 6. This is the graph of x 2 moved left 2 units and up 2 units. 10 8 6 4 2 –4

–3

–2

–1

45. ForIfwhich valuesAofand c does (x) cystic = x2 + cx + 1gene haveoccur a double No real roots? the alleles B off the ﬁbrosis in a root? population with frequencies p and 1 − p (where p is a fraction between 0 and 1), then carriers (carriers SOLUTION A double root occurs whenthe c2frequency − 4(1)(1)of =heterozygous 0 or c2 = 4. Thus, c= ±2. with both alleles) is 2 p(1 − p). Which of proots giveswhen the largest frequency There arevalue no real c2 − 4(1)(1) < 0oforheterozygous c2 < 4. Thus,carriers? −2 < c < 2. 1 2 for all x > 0. Hint: Consider (x 1/2 − x −1/2 )2 . 47. ProveLet that fx(x) + be ≥ x a quadratic function and c a constant. Which is correct? Explain graphically. (a) There is a unique value of c such that y = f (x) − c has a double root. SOLUTION Let x > 0. Then (b) There is a unique value of c such that y = f (x − c) has a double root. 2

1 x 1/2 − x −1/2 = x − 2 + . x Because (x 1/2 − x −1/2 )2 ≥ 0, it follows that x −2+

1 ≥0 x

x+

or

1 ≥ 2. x

49. If objects of weights x and w1 are suspended from the balance in Figure 13(A), the cross-beam is horizontal if √ a+b bx = aw If the a and are geometric known, wemean may useabthis determine an unknown selecting Hint: Use a is equation not largertothan the arithmetic meanweight x. by Let b >lengths 0. Show thatb the 1 . a, 2 First balance x w1 so that the cross-beam is horizontal. If a and b are not known precisely, we might proceed as follows. variation of the hint given in Exercise 47. by w1 on the left as in (A). Then switch places and balance x by w2 on the right as in (B). The average x¯ = 12 (w1 + w2 ) gives an estimate for x. Show that x¯ is greater than or equal to the true weight x. a

a

b

w1

x (A)

b

x

w2 (B)

FIGURE 13

12

CHAPTER 1

PRECALCULUS REVIEW SOLUTION

First note bx = aw1 and ax = bw2 . Thus, x¯ = = = ≥ =

1 (w1 + w2 ) 2 1 bx ax + 2 a b x b a + 2 a b x (2) by Exercise 47 2 x

51. Find a pair of numbers whose sum and product are both equal to 8. Find numbers x and y with sum 10 and product 24. Hint: Find a quadratic polynomial satisﬁed by x. SOLUTION Let x and y be numbers whose sum and product are both equal to 8. Then x + y = 8 and x y = 8. From the second equation, y = 8x . Substituting this expression for y in the ﬁrst equation gives x + 8x = 8 or x 2 − 8x + 8 = 0. By the quadratic formula, √ √ 8 ± 64 − 32 = 4 ± 2 2. x= 2 √ If x = 4 + 2 2, then √ √ 8 8 4−2 2 y= √ = √ · √ = 4 − 2 2. 4+2 2 4+2 2 4−2 2 √ On the other hand, if x = 4 − 2 2, then √ √ 8 8 4+2 2 y= √ = √ · √ = 4 + 2 2. 4−2 2 4−2 2 4+2 2 √ √ Thus, the two numbers are 4 + 2 2 and 4 − 2 2. 2 consists of all points P such that d = d , where d is the distance 1 2 1 from P to (0, 4 ) and d2 is the distance from P to the horizontal line y = − 14 (Figure 14). 53. Show that if f (x) and g(x) are linear, then so is f (x) + g(x). Is the same true of f (x)g(x)?

Show that the graph of the parabola y = x Further Insights and Challenges 1 SOLUTION

If f (x) = mx + b and g(x) = nx + d, then f (x) + g(x) = mx + b + nx + d = (m + n)x + (b + d),

which is linear. f (x)g(x) is not generally linear. Take, for example, f (x) = g(x) = x. Then f (x)g(x) = x 2 . overfthe ] is=not a constant, but=depends 55. Show thatthat the ifratio y/x forare thelinear function f (x) = x 2 that 1 , xf 2(1) Show f (x) and g(x) functions such (0) interval = g(0) [x and g(1), then f (x) g(x). on the interval. Determine the exact dependence of y/x on x 1 and x 2 . SOLUTION

For x 2 ,

x 2 − x 12 y = 2 = x2 + x1 . x x2 − x1

57. Let a, c = 0. Show that the roots of ax 2 + bx + c = 0 and cx 2 +2 bx + a = 0 are reciprocals of each other. Use Eq. (2) to derive the quadratic formula for the roots of ax + bx + c = 0. SOLUTION Let r1 and r2 be the roots of ax 2 + bx + c and r3 and r4 be the roots of cx 2 + bx + a. Without loss of generality, let −b + b2 − 4ac 2a 1 −b − b2 − 4ac r1 = = ⇒ · 2a r1 −b + b2 − 4ac −b − b2 − 4ac −b − b2 − 4ac 2a(−b − b2 − 4ac) = = r4 . = 2c b2 − b2 + 4ac Similarly, you can show

1 = r3 . r2

59. Prove Vi`ete’s Formulas, which state that the quadratic polynomial with given numbers α and β as roots is x 2 + square to show that. the parabolas y = ax 2 + bx + c and y = ax 2 have the same shape (show that bx + c, Complete where b =the −α − β and c = αβ the ﬁrst parabola is congruent to the second by a vertical and horizontal translation). SOLUTION If a quadratic polynomial has roots α and β , then the polynomial is (x − α )(x − β ) = x 2 − α x − β x + αβ = x 2 + (−α − β )x + αβ . Thus, b = −α − β and c = αβ .

S E C T I O N 1.3

The Basic Classes of Functions

13

1.3 The Basic Classes of Functions Preliminary Questions 1. Give an example of a rational function. SOLUTION

One example is

3x 2 − 2 . 7x 3 + x − 1

2. Is |x| a polynomial function? What about |x 2 + 1|? SOLUTION

|x| is not a polynomial; however, because x 2 + 1 > 0 for all x, it follows that |x 2 + 1| = x 2 + 1, which is

a polynomial. 3. What is unusual about the domain of f ◦ g for f (x) = x 1/2 and g(x) = −1 − |x|? SOLUTION Recall that ( f ◦ g)(x) = f (g(x)). Now, for any real number x, g(x) = −1 − |x| ≤ −1 < 0. Because we cannot take the square root of a negative number, it follows that f (g(x)) is not deﬁned for any real number. In other words, the domain of f (g(x)) is the empty set. x 4. Is f (x) = 12 increasing or decreasing? SOLUTION

function.

The function f (x) = ( 12 )x is an exponential function with base b = 12 < 1. Therefore, f is a decreasing

5. Give an example of a transcendental function. SOLUTION

One possibility is f (x) = e x − sin x.

Exercises In Exercises 1–12, determine the domain of the function. 1. f (x) = x 1/4 SOLUTION

x≥0

3. f (x) = x 3 + 3x − 4 g(t) = t 2/3 SOLUTION All reals 1 5. g(t)h(z) = = z 3 + z −3 t +2 SOLUTION

t = −2

1 7. G(u) = 2 1 f (x) u= −2 4 x +4 SOLUTION u = ±2 √(x − 1)−3 9. f (x) = x −4 + x f (x) = 20, 1 SOLUTION x = x −9 √ −1 11. g(y) = 10 y+y s F(s) = sin SOLUTION y > 0 s + 1 In Exercises 13–24, identify each of the following functions as polynomial, rational, algebraic, or transcendental. x + x −1 f (x) =3 2 − 8+ 4) +− 9x3)(x 13. f (x) = 4x (x Polynomial √ 15. f (x) = x f (x) = x −4 SOLUTION Algebraic SOLUTION

x2 17. f (x)f (x) = = 1 − x2 x + sin x SOLUTION

Transcendental

2x 3 x+ 3x 19. f (x)f (x) = =2 2 9 − 7x SOLUTION

Rational

21. f (x) = sin(x 2 ) 3x − 9x −1/2 f (x) = 9 − 7x 2

14

CHAPTER 1

PRECALCULUS REVIEW SOLUTION

Transcendental

23. f (x) = x 2 + 3xx −1 f (x) = √ SOLUTION Rational x +1 2

x a transcendental 25. Is f f(x) function? x) (x)==2sin(3 SOLUTION Yes.

In Exercises 27–34, calculate2 the composite functions f ◦ g and g ◦ f , and determine their domains. Show that f (x) = x + 3x −1 and g(x) = 3x 3 − 9x + x −2 are rational functions (show that each is a quotient √ polynomials). 27. of f (x) = x, g(x) = x + 1 √ √ SOLUTION f (g(x)) = x + 1; D: x ≥ −1, g( f (x)) = x + 1; D: x ≥ 0 29. f (x) = 2x , 1 g(x) = x 2 −4 f (x) = , g(x) = 2x SOLUTION f (g(x)) x = 2x ; D: R,

g( f (x)) = (2x )2 = 22x ; D: R

3 2 31. f (θf)(x) = cos θ , g(x) = |x|, g(θ )==xsin+θ x 3 SOLUTION f (g(x)) = cos(x + x 2 ); D: R,

g( f (θ )) = cos3 θ + cos2 θ ; D: R

1 33. f (t) = √ , g(t) 1 = −t 2 , g(x) = x −2 f (x) =t 2 x +1 1 SOLUTION f (g(t)) = ; D: Not valid for any t, −t 2

2 g( f (t)) = − √1 = − 1t ; D: t > 0 t

kt √ (in millions) of 35. The population 3 a country as a function of time t (years) is P(t) = 30 · 2 , with k = 0.1. Show f (t) = t, g(t) = 1 − t that the population doubles every 10 years. Show more generally that for any nonzero constants a and k, the function g(t) = a2kt doubles after 1/k years. SOLUTION

Let P(t) = 30 · 20.1t . Then P(t + 10) = 30 · 20.1(t+10) = 30 · 20.1t+1 = 2(30 · 20.1t ) = 2P(t).

Hence, the population doubles in size every 10 years. In the more general case, let g(t) = a2kt . Then 1 = a2k(t+1/k) = a2kt+1 = 2a2kt = 2g(t). g t+ k Hence, the function g doubles after 1/k years. x +1 is R. f (x) = 2 x + 2cx + 4 In Exercises 37–43, we deﬁne the ﬁrst difference δ f of a function f (x) by δ f (x) = f (x + 1) − f (x).

Further Insights Challenges Find all valuesand of c such that the domain of

37. Show that if f (x) = x 2 , then δ f (x) = 2x + 1. Calculate δ f for f (x) = x and f (x) = x 3 . f (x) = x 2 : δ f (x) = f (x + 1) − f (x) = (x + 1)2 − x 2 = 2x + 1 f (x) = x: δ f (x) = x + 1 − x = 1 f (x) = x 3 : δ f (x) = (x + 1)3 − x 3 = 3x 2 + 3x + 1

SOLUTION

39. Show that for any two functions f and g, δ ( f + g) = δ f + δ g and δ (c f ) = cδ ( f ), where c is any constant. Show that δ (10x ) = 9 · 10x , and more generally, δ (b x ) = c · b x for some constant c. SOLUTION δ ( f + g) = ( f (x + 1) + g(x + 1)) − ( f (x) − g(x)) = ( f (x + 1) − f (x)) + (g(x + 1) − g(x)) = δ f (x) + δ g(x)

δ (c f ) = c f (x + 1) − c f (x) = c( f (x + 1) − f (x)) = cδ f (x). x(x + 1) 41. First show that P(x)can = be used tosatisﬁes δ P = (x for + 1). 40 toSuppose conclude First differences derive formulas theThen sum apply of theExercise kth powers. wethat can ﬁnd a function 2 P(x) such that δ P = (x + 1)k and P(0) = 0. Prove that P(1) = 1k , P(2) = 1k + 2k , and more generally, for every n(n + 1) whole number n, 1 + 2 + 3 + ··· + n = 2 P(n) = 1k + 2k + · · · + n k SOLUTION Let P(x) = x(x + 1)/2. Then

δ P(x) = P(x + 1) − P(x) =

(x + 1)(x + 2) x(x + 1) (x + 1)(x + 2 − x) − = = x + 1. 2 2 2

Also, note that P(0) = 0. Thus, by Exercise 40, with k = 1, it follows that P(n) =

n(n + 1) = 1 + 2 + 3 + · · · + n. 2

Calculate δ (x 3 ), δ (x 2 ), and δ (x). Then ﬁnd a polynomial P(x) of degree 3 such that δ P = (x + 1)2 and P(0) = 0. Conclude that

S E C T I O N 1.3

The Basic Classes of Functions

15

43. This exercise combined with Exercise 40 shows that for all k, there exists a polynomial P(x) satisfying Eq. (1). The solution requires proof by induction and the Binomial Theorem (see Appendix C). (a) Show that

δ (x k+1 ) = (k + 1) x k + · · · where the dots indicate terms involving smaller powers of x. (b) Show by induction that for all whole numbers k, there exists a polynomial of degree k + 1 with leading coefﬁcient 1/(k + 1): P(x) =

1 x k+1 + · · · k+1

such that δ P = (x + 1)k and P(0) = 0. SOLUTION

(a) By the Binomial Theorem: n+1 n+1 δ (x n+1 ) = (x + 1)n+1 − x n+1 = x n+1 + xn + x n−1 + · · · + 1 − x n+1 1 2 n+1 n+1 xn + x n−1 + · · · + 1 = 1 2 Thus,

δ (x n+1 ) = (n + 1) x n + · · · where the dots indicate terms involving smaller powers of x. (b) For k = 0, note that P(x) = x satisﬁes δ P = (x + 1)0 = 1 and P(0) = 0. Now suppose the polynomial P(x) =

1 k x + pk−1 x k−1 + · · · + p1 x k

which clearly satisﬁes P(0) = 0 also satisﬁes δ P = (x + 1)k−1 . We try to prove the existence of Q(x) =

1 x k+1 + qk x k + · · · + q1 x k+1

such that δ Q = (x + 1)k . Observe that Q(0) = 0. If δ Q = (x + 1)k and δ P = (x + 1)k−1 , then

δ Q = (x + 1)k = (x + 1)δ P = x δ P(x) + δ P By the linearity of δ (Exercise 39), we ﬁnd δ Q − δ P = x δ P or δ (Q − P) = x δ P. By deﬁnition, 1 1 Q−P= x k+1 + qk − x k + · · · + (q1 − p1 )x, k+1 k so, by the linearity of δ ,

δ (Q − P) =

1 1 δ (x k+1 ) + qk − δ (x k ) + · · · + (q1 − p1 ) = x(x + 1)k−1 k+1 k

By part (a),

δ (x k+1 ) = (k + 1)x k + L k−1,k−1 x k−1 + . . . + L k−1,1 x + 1 δ (x k ) = kx k−1 + L k−2,k−2 x k−2 + . . . + L k−2,1 x + 1 .. .

δ (x 2 ) = 2x + 1 where the L i, j are real numbers for each i, j. To construct Q, we have to group like powers of x on both sides of (1). This yields the system of equations 1

(k + 1)x k = x k k+1

(1)

16

CHAPTER 1

PRECALCULUS REVIEW

1 1 L k−1,k−1 x k−1 + qk − kx k−1 = (k − 1)x k−1 k+1 k .. .

1 1 + qk − + (qk−1 − pk−1 ) + · · · + (q1 − p1 ) = 0. k+1 k The ﬁrst equation is identically true, and the second equation can be solved immediately for qk . Substituting the value of qk into the third equation of the system, we can then solve for qk−1 . We continue this process until we substitute the values of qk , qk−1 , . . . q2 into the last equation, and then solve for q1 .

1.4 Trigonometric Functions Preliminary Questions 1. How is it possible for two different rotations to deﬁne the same angle? SOLUTION Working from the same initial radius, two rotations that differ by a whole number of full revolutions will have the same ending radius; consequently, the two rotations will deﬁne the same angle even though the measures of the rotations will be different.

2. Give two different positive rotations that deﬁne the angle π4 . π SOLUTION The angle π /4 is deﬁned by any rotation of the form 4 + 2π k where k is an integer. Thus, two different positive rotations that deﬁne the angle π /4 are

9π π + 2π (1) = 4 4

and

41π π + 2π (5) = . 4 4

3. Give a negative rotation that deﬁnes the angle π3 . SOLUTION The angle π /3 is deﬁned by any rotation of the form π 3 + 2π k where k is an integer. Thus, a negative rotation that deﬁnes the angle π /3 is

5π π + 2π (−1) = − . 3 3 4. The deﬁnition of cos θ using right triangles applies when (choose the correct answer): π (a) 0 < θ < (c) 0 < θ < 2π (b) 0 < θ < π 2 The correct response is (a): 0 < θ < π2 . 5. What is the unit circle deﬁnition of sin θ ?

SOLUTION

SOLUTION Let O denote the center of the unit circle, and let P be a point on the unit circle such that the radius O P makes an angle θ with the positive x-axis. Then, sin θ is the y-coordinate of the point P.

6. How does the periodicity of sin θ and cos θ follow from the unit circle deﬁnition? SOLUTION Let O denote the center of the unit circle, and let P be a point on the unit circle such that the radius O P makes an angle θ with the positive x-axis. Then, cos θ and sin θ are the x- and y-coordinates, respectively, of the point P. The angle θ + 2π is obtained from the angle θ by making one full revolution around the circle. The angle θ + 2π will therefore have the radius O P as its terminal side. Thus

cos(θ + 2π ) = cos θ

and

sin(θ + 2π ) = sin θ .

In other words, sin θ and cos θ are periodic functions.

Exercises 1. Find the angle between 0 and 2π that is equivalent to 13π /4. SOLUTION Because 13π /4 > 2π , we repeatedly subtract 2π until we arrive at a radian measure that is between 0 and 2π . After one subtraction, we have 13π /4 − 2π = 5π /4. Because 0 < 5π /4 < 2π , 5π /4 is the angle measure between 0 and 2π that is equivalent to 13π /4.

3. Convert from radians to degrees: Describe the angle θ = π6 by 5 π an angle of negative radian measure. (c) (a) 1 (b) 3 12

(d) −

3π 4

Trigonometric Functions

S E C T I O N 1.4 SOLUTION

180◦ 180◦ ≈ 57.1◦ = π π 5 180◦ 75◦ (c) ≈ 23.87◦ = 12 π π

180◦ = 60◦ π 3π 180◦ (d) − = −135◦ 4 π

(a) 1

(b)

π 3

5. Find the lengths of the arcs subtended by the angles θ and φ radians in Figure 20. Convert from degrees to radians: (b) 30◦ (c) 25◦ (a) 1◦

(d) 120◦

4 q = 0.9 f =2

FIGURE 20 Circle of radius 4. SOLUTION

s = r θ = 4(.9) = 3.6; s = r φ = 4(2) = 8

7. Fill in the remaining values of (cos θ , sin θ ) for the points in Figure 22. Calculate the values of the six standard trigonometric functions for the angle θ in Figure 21. 3p 4

2p 3

p 2

p 3

5p 6

( 12 , 23 ) p 2 2 , 4 ( 2 2 ) p 3 1 , ( 6 2 2)

p

0 (0, 0)

7p 6

11p 6 5p 4 4p 3

7p 5p 4 3

3p 2

FIGURE 22 SOLUTION

θ

π 2

(cos θ , sin θ )

(0, 1)

2π 3 √

−1 , 3 2 2

θ

5π 4

4π 3

(cos θ , sin θ )

√

√ − 2, − 2 2 2

√

3π 4

√ − 2, 2 2 2 3π 2

√ −1 , − 3 2 2

5π 6

√ − 3, 1 2 2 5π 3 √

1, − 3 2 2

(0, −1)

(−1, 0)

7π 6

√ − 3 , −1 2 2

7π 4

11π 6

π

√

√ 2 − 2 2 , 2

π θsatisfying the given condition. In Exercises use Figure 22 to ﬁnd alltrigonometric angles between 0 and 2at Find 9–14, the values of the six standard functions = 11π /6. 9. cos θ = 12

θ = π3 , 53π 11. tan θ = −1 tan θ = 1 3π 7π SOLUTION θ = 4 , 4 √ 3 13. sin csc x =θ = 2 2 SOLUTION

x = π3 , 23π 15. Fill in the following table of values: sec t = 2 SOLUTION

θ tan θ sec θ SOLUTION

π 6

π 4

π 3

π 2

2π 3

3π 4

5π 6

√

3 −1 2 , 2

17

18

CHAPTER 1

PRECALCULUS REVIEW

π 6

π 4

tan θ

1 √ 3

1

sec θ

2 √ 3

θ

√

2

π 3 √ 3 2

π 2

3π 4

5π 6

und

2π 3 √ − 3

−1

1 −√ 3

und

−2

√ − 2

2 −√ 3

17. Show that if tan = c and 0table ≤ θof<signs π /2, for then θ = 1/ 1 +functions: c2 . Hint: Draw a right triangle whose opposite and Complete theθfollowing thecos trigonometric adjacent sides have lengths c and 1. SOLUTION Because 0 ≤ θ < π /2, deﬁnition the trigonometric in terms of right triangles. θ we can use the sin cos of tan cot sec functions csc tan θ is the ratio of the length of the side opposite the angle θ to the length of the adjacent side. With c = 1c , we label π the length of the opposite side as c 0and length of the θ< + adjacent + side as 1 (see the diagram below). By the Pythagorean < the theorem, the length of the hypotenuse is 12+ c2 . Finally, we use the fact that cos θ is the ratio of the length of the adjacent side to the length of the hypotenuse to obtain π <θ<π 2 1 . cos θ = 3π 1 + c2 π<θ< 2

3π < θ < 2π 2

1 + c2

c

q 1

In Exercises 19–24, assume that 0 ≤ θ < π /2. Suppose that cos θ = 13 . √ √ 5 . and tan = 13 19. Find sin θ that <θ π /2, then sin θ = 2 2/3 and tan θ = 2 2. (a) Show if 0θ≤ifθcos θ and tan if 3π /2below. ≤ θ

12 13

tan θ =

and

13

12 . 5

12

θ 5

21. Find sin θ , sec θ , and cot θ if tan θ 3= 27 . Find cos θ and tan θ if sin θ = 5 . 7 SOLUTION If tan θ = 2 7 , then cot θ = 2 . For the remaining trigonometric functions, consider the triangle below. The lengths of the sides opposite and adjacent to the angle θ have been labeled so that tan θ = 27 . The length of the √ hypotenuse has been calculated using the Pythagorean theorem: 22 + 72 = 53. From the triangle, we see that √ √ 2 53 2 53 and sec θ = . sin θ = √ = 53 7 53 53

2

q 7

1. 23. FindFind cossin 2θ θif, sin 5 sec θ if cot θ = 4. cosθθ ,=and

Using the double angle formula cos 2θ = cos2 θ − sin2 θ and the fundamental identity sin2 θ + cos2 θ = 1, we ﬁnd that cos 2θ = 1 − 2 sin2 θ . Thus, cos 2θ = 1 − 2(1/25) = 23/25. SOLUTION

Find sin 2θ and cos 2θ if tan θ =

√

2.

S E C T I O N 1.4

Trigonometric Functions

19

25. Find cos θ and tan θ if sin θ = 0.4 and π /2 ≤ θ < π . SOLUTION We can determine the “magnitude” of cos θ and tan θ using the triangle shown below. The lengths of the side opposite the angle θ and the hypotenuse have been labeled so that sin θ = 0.4 = 25 . The length of the side adjacent √ to the angle θ was calculated using the Pythagorean theorem: 52 − 22 = 21. From the triangle, we see that √ √ 21 2 21 2 and |tan θ | = √ = . |cos θ | = 5 21 21

Because π /2 ≤ θ < π , both cos θ and tan θ are negative; consequently, √ √ 2 21 21 and tan θ = − . cos θ = − 5 21 5

2

q 21

27. Find the values of sin θ , cos θ , and tan θ at the eight points in Figure 23. Find cos θ and sin θ if tan θ = 4 and π ≤ θ < 3π /2. (0.3965, 0.918)

(A)

(0.3965, 0.918)

(B)

FIGURE 23 SOLUTION

Let’s start with the four points in Figure 23(A).

• The point in the ﬁrst quadrant has coordinates (0.3965, 0.918). Therefore,

0.918 = 2.3153. 0.3965

sin θ = 0.918, cos θ = 0.3965, and tan θ =

• The coordinates of the point in the second quadrant are (−0.918, 0.3965). Therefore,

sin θ = 0.3965, cos θ = −0.918, and tan θ =

0.3965 = −0.4319. −0.918

• Because the point in the third quadrant is symmetric to the point in the ﬁrst quadrant with respect to the origin, its

coordinates are (−0.3965, −0.918). Therefore, sin θ = −0.918, cos θ = −0.3965, and tan θ =

−0.918 = 2.3153. −0.3965

• Because the point in the fourth quadrant is symmetric to the point in the second quadrant with respect to the origin,

its coordinates are (0.918, −0.3965). Therefore, sin θ = −0.3965, cos θ = 0.918, and tan θ =

−0.3965 = −0.4319. 0.918

Now consider the four points in Figure 23(B). • The point in the ﬁrst quadrant has coordinates (0.3965, 0.918). Therefore,

sin θ = 0.918, cos θ = 0.3965, and tan θ =

0.918 = 2.3153. 0.3965

• The point in the second quadrant is a reﬂection through the y-axis of the point in the ﬁrst quadrant. Its coordinates

are therefore (−0.3965, 0.918) and sin θ = 0.918, cos θ = −0.3965, and tan θ =

0.918 = −2.3153. 0.3965

20

CHAPTER 1

PRECALCULUS REVIEW • Because the point in the third quadrant is symmetric to the point in the ﬁrst quadrant with respect to the origin, its

coordinates are (−0.3965, −0.918). Therefore, sin θ = −0.918, cos θ = −0.3965, and tan θ =

−0.918 = 2.3153. −0.3965

• Because the point in the fourth quadrant is symmetric to the point in the second quadrant with respect to the origin,

its coordinates are (0.3965, −0.918). Therefore, sin θ = −0.918, cos θ = 0.3965, and tan θ =

−0.918 = −2.3153. 0.3965

29. Refer to Figure 24(B). Compute cos ψ , sin ψ , cot ψ , and csc ψ . Refer to Figure 24(A). Express the functions sin θ , tan θ , and csc θ in terms of c.

1

1

c

0.3

0.3

(A)

(B)

FIGURE 24

By the Pythagorean theorem, the length of the side opposite the angle ψ in Figure 24 (B) is 1 − 0.32 = 0.91. Consequently, √ 0.3 0.3 1 0.91 √ cos ψ = and csc ψ = √ . = 0.3, sin ψ = = 0.91, cot ψ = √ 1 1 0.91 0.91

SOLUTION

√

graph over

[0,π 2π ]. In Exercises 31–34, sketchπthe and sin θ + in terms of cos θ and sin θ . Hint: Find the relation between the coordiExpress cos θ + 2 2 θ b) and (c, d) in Figure 25. 31. 2nates sin 4(a, SOLUTION

y 2 1 x 1

−1

2

3

4

5

6

−2

π 33. cos 2θ −

π cos 2 2θ − 2 SOLUTION

y 1 0.5 x −0.5

1

2

3

4

5

6

−1

35. How many

points π lie onthe intersection of the horizontal line y = c and the graph of y = sin x for 0 ≤ x < 2π ? +2 sinanswer 2 θ− + πon c. Hint: The depends 2 SOLUTION Recall that for any x, −1 ≤ sin x ≤ 1. Thus, if |c| > 1, the horizontal line y = c and the graph of y = sin x never intersect. If c = +1, then y = c and y = sin x intersect at the peak of the sine curve; that is, they intersect at x = π2 . On the other hand, if c = −1, then y = c and y = sin x intersect at the bottom of the sine curve; that is, they intersect at x = 32π . Finally, if |c| < 1, the graphs of y = c and y = sin x intersect twice. In Exercises solve lie foron 0≤ < 2π (see Example 6). How 37–40, many points theθ intersection of the horizontal line y = c and the graph of y = tan x for 0 ≤ x < 2π ? 37. sin 2θ + sin 3θ = 0

S E C T I O N 1.4

Trigonometric Functions

21

SOLUTION sin α = − sin β when α = −β + 2π k or α = π + β + 2π k. Substituting α = 2θ and β = 3θ , we have either 2θ = −3θ + 2π k or 2θ = π + 3θ + 2π k. Solving each of these equations for θ yields θ = 25 π k or θ = −π − 2π k. The solutions on the interval 0 ≤ θ < 2π are then

θ = 0,

2π 4π 6π 8π , , π, , . 5 5 5 5

39. cos 4θ + cos 2θ = 0 sin θ = sin 2θ SOLUTION cos α = − cos β when α + β = π + 2π k or α = β + π + 2π k. Substituting α = 4θ and β = 2θ , we have either 6θ = π + 2π k or 4θ = 2θ + π + 2π k. Solving each of these equations for θ yields θ = π6 + π3 k or θ = π2 + π k. The solutions on the interval 0 ≤ θ < 2π are then

θ=

π π 5π 7π 3π 11π , , , , , . 6 2 6 6 2 6

In Exercises the identities using the identities listed in this section. = cos 2derive θ sin θ 41–50, 41. cos 2θ = 2 cos2 θ − 1 Starting from the double angle formula for cosine, cos2 θ = 12 (1 + cos 2θ ), we solve for cos 2θ . This gives 2 cos2 θ = 1 + cos 2θ and then cos 2θ = 2 cos2 θ − 1. θ θ 1 −1 cos θ θ + cos 43. sin = 2 cos 2 2 = 2 2 θ 1 SOLUTION Substitute x = θ /2 into the double angle formula for sine, sin2 x = 2 (1 − cos 2x) to obtain sin2 = 2 θ 1 − cos θ 1 − cos θ . Taking the square root of both sides yields sin = . 2 2 2 SOLUTION

45. cos(θ + π ) = − cos θ sin(θ + π ) = − sin θ SOLUTION From the addition formula for the cosine function, we have cos(θ + π ) = cos θ cos π − sin θ sin π = cos θ (−1) = − cos θ 47. tan(π − θ ) = −

tan π θ tan x = cot −x SOLUTION Using Exercises 44 and 45, 2 tan(π − θ ) =

sin(π − θ ) sin(π + (−θ )) − sin(−θ ) sin θ = − tan θ . = = = cos(π − θ ) cos(π + (−θ )) − cos(−θ ) − cos θ

The second to last equality occurs because sin x is an odd function and cos x is an even function. sin 2x 49. tan x = 2 tan x + cos 2x tan 2x1 = 1 − tan2 x SOLUTION Using the addition formula for the sine function, we ﬁnd sin 2x = sin(x + x) = sin x cos x + cos x sin x = 2 sin x cos x. By Exercise 41, we know that cos 2x = 2 cos2 x − 1. Therefore, 2 sin x cos x sin x 2 sin x cos x sin 2x = = = = tan x. 2 2 1 + cos 2x cos x 1 + 2 cos x − 1 2 cos x 51. Use Exercises 44 and 45 to show that tan θ and cot θ are periodic with period π . 1 − cos 4x sin2 x By cos2Exercises x= SOLUTION 44 8 and 45, tan(θ + π ) =

sin(θ + π ) − sin θ = tan θ , = cos(θ + π ) − cos θ

cot(θ + π ) =

cos(θ + π ) − cos θ = cot θ . = sin(θ + π ) − sin θ

and

Thus, both tan θ and cot θ are periodic with period π . Use the addition formula to compute cos

π π π π , noting that = − . 12 12 3 4

22

CHAPTER 1

PRECALCULUS REVIEW

53. Use the Law of Cosines to ﬁnd the distance from P to Q in Figure 26. Q

10

7π/9 8

P

FIGURE 26 SOLUTION

By the Law of Cosines, the distance from P to Q is 102 + 82 − 2(10)(8) cos

7π = 16.928. 9

Further Insights and Challenges 55. Use the addition formulas for sine and cosine to prove Use the addition formula to prove tan a + tan b 3 θ − 3 cos θ tan(a cos+ 3θb)==4 cos 1 − tan a tan b cot(a − b) =

cot a cot b + 1 cot b − cot a

SOLUTION

tan(a + b) =

sin a cos b cos a sin b sin(a + b) tan a + tan b sin a cos b + cos a sin b cos a cos b + cos a cos b = = = cos a cos b sin a sin b cos(a + b) cos a cos b − sin a sin b 1 − tan a tan b − cos a cos b cos a cos b

cot(a − b) =

cos a cos b + sin a sin b cos(a − b) cot a cot b + 1 cos a cos b + sin a sin b sin a sin b sin a sin b = = = sin a cos b cos a sin b sin(a − b) sin a cos b − cos a sin b cot b − cot a sin a sin b − sin a sin b

57. Let L 1 and L 2 be the lines of slope m 1 and m 2 [Figure 27(B)]. Show that the angle θ between L 1 and L 2 satisﬁes Let m 2θm 1be+the 1 angle between the line y = mx + b and the x-axis [Figure 27(A)]. Prove that m = tan θ . . cot θ = m2 − m1 SOLUTION Measured from the positive x-axis, let α and β satisfy tan α = m 1 and tan β = m 2 . Without loss of generality, let β ≥ α . Then the angle between the two lines will be θ = β − α . Then from Exercise 55,

cot θ = cot(β − α ) =

( m11 )( m12 ) + 1 cot β cot α + 1 1 + m1m2 = = 1 − 1 cot α − cot β m2 − m1 m1 m2

59. Apply the double-angle formula to prove: Perpendicular √Lines Use Exercise 57 to prove that two lines with nonzero slopes m 1 and m 2 are perpendicular π 1 (a) cos = if 2m+ = 2−1/m 1 . if and only 2 8 2 √ 1 π 2+ 2+ 2 = (b) cos 16 2 π π and of cos n for all n. Guess the values of cos 32 2 SOLUTION

√

√ 1 + cos π4 1 + 22 1 π π /4 = = = (a) cos = cos 2 + 2. 8 2 2 2 2 √

√ 1 + cos π8 1 + 12 2 + 2 π 1 (b) cos 2 + 2 + 2. = = = 16 2 2 2 π involves three (c) Observe that 8 = 23 and cos π8 involves two nested square roots of 2; further, 16 = 24 and cos 16 5 nested square roots of 2. Since 32 = 2 , it seems plausible that

√ 1 π 2 + 2 + 2 + 2, = cos 32 2

and that cos 2πn involves n − 1 nested square roots of 2. Note that the general case can be proven by induction.

S E C T I O N 1.5

Technology: Calculators and Computers

23

1.5 Technology: Calculators and Computers Preliminary Questions 1. Is there a deﬁnite way of choosing the optimal viewing rectangle, or is it best to experiment until you ﬁnd a viewing rectangle appropriate to the problem at hand? SOLUTION

It is best to experiment with the window size until one is found that is appropriate for the problem at hand.

2. Describe the calculator screen produced when the function y = 3 + x 2 is plotted with viewing rectangle: (a) [−1, 1] × [0, 2] (b) [0, 1] × [0, 4] SOLUTION

(a) Using the viewing rectangle [−1, 1] by [0, 2], the screen will display nothing as the minimum value of y = 3 + x 2 is y = 3. (b) Using the viewing rectangle [0, 1] by [0, 4], the screen will display the portion of the parabola between the points (0, 3) and (1, 4). 3. According to the evidence in Example 4, it appears that f (n) = (1 + 1/n)n never takes on a value greater than 3 for n ≥ 0. Does this evidence prove that f (n) ≤ 3 for n ≥ 0? SOLUTION

No, this evidence does not constitute a proof that f (n) ≤ 3 for n ≥ 0.

4. How can a graphing calculator be used to ﬁnd the minimum value of a function? Experiment with the viewing window to zoom in on the lowest point on the graph of the function. The y-coordinate of the lowest point on the graph is the minimum value of the function.

SOLUTION

Exercises The exercises in this section should be done using a graphing calculator or computer algebra system. 1. Plot f (x) = 2x 4 + 3x 3 − 14x 2 − 9x + 18 in the appropriate viewing rectangles and determine its roots. SOLUTION

Using a viewing rectangle of [−4, 3] by [−20, 20], we obtain the plot below. y 20 10 x

−4 −3 −2 −1 −10

1

2

3

−20

Now, the roots of f (x) are the x-intercepts of the graph of y = f (x). From the plot, we can identify the x-intercepts as −3, −1.5, 1, and 2. The roots of f (x) are therefore x = −3, x = −1.5, x = 1, and x = 2. 3. How many positive solutions does x 3 − 12x + 8 = 0 have? How many solutions does x 3 − 4x + 8 = 0 have? SOLUTION The graph of y = x 3 − 12x + 8 shown below has two x-intercepts to the right of the origin; therefore the equation x 3 − 12x + 8 = 0 has two positive solutions. y 60 40 20 −4

−2

x 2

−20 −40 −60

4

√ 5. Find all the solutions of sin x = x for x > 0. Does cos x + x = 0 have a solution? Does √ it have a positive solution? SOLUTION Solutions to the equation sin x = x correspond to points of intersection between the graphs of y = sin x √ and y = √ x. The two graphs are shown below; the only point of intersection is at x = 0. Therefore, there are no solutions of sin x = x for x > 0. y 2 1 x 1 −1

2

3

4

5

24

CHAPTER 1

PRECALCULUS REVIEW

7. Let f (x) = (x − 100)2 + 1,000. What 2will the display show if you graph f (x) in the viewing rectangle [−10, 10] How many solutions x = x have? by [−10, −10]? What woulddoes be ancos appropriate viewing rectangle? SOLUTION Because (x − 100)2 ≥ 0 for all x, it follows that f (x) = (x − 100)2 + 1000 ≥ 1000 for all x. Thus, using a viewing rectangle of [−10, 10] by [−10, 10] will display nothing. The minimum value of the function occurs when x = 100, so an appropriate viewing rectangle would be [50, 150] by [1000, 2000]. x in + a viewing rectangle that clearly displays the vertical and horizontal asymptotes. 9. Plot the graph of f (x) = 8x 1 Plot the graph of f (x)4 − =x in an appropriate viewing rectangle. What are the vertical and horizontal 8x − x4 asymptotes (seethe Example 3)?y = SOLUTION From graph of shown below, we see that the vertical asymptote is x = 4 and the horizontal 4−x asymptote is y = −1. y 2 −8 −4

x 4

8 12 16

−2

11. Plot f (x) = cos(x 2 ) sin x for 0 ≤ x ≤ 2π2 . Then illustrate local linearity at x = 3.8 by choosing appropriate viewing Illustrate local linearity for f (x) = x by zooming in on the graph at x = 0.5 (see Example 6). rectangles. SOLUTION The following three graphs display f (x) = cos(x 2 ) sin x over the intervals [0, 2π ], [3.5, 4.1] and [3.75, 3.85]. The ﬁnal graph looks like a straight line. y

y

y

1

0.4

1

0.2 x 1

2

3

4

5

x

6

3.5 3.6 3.7 3.8 3.9

−1

4

3.76 3.78 3.8 3.82 3.84 x −0.2

−1

In Exercises 13–18, the behavior of the function or xdeposit grows Plarge by making a table of function values A bank pays investigate r = 5% interest compounded monthly.asIf nyou 0 dollars at time t = 0, then the value of behavior and plotting a graph (see Example 4). Describe the in words. N 0.05 . Find to the nearest integer N the number of months after which your account after N months is P0 1 + 12 13. f (n) = n 1/n the account value doubles. SOLUTION The table and graphs below suggest that as n gets large, n 1/n approaches 1. n

n 1/n

10 102 103 104 105 106

1.258925412 1.047128548 1.006931669 1.000921458 1.000115136 1.000013816

y

y

1

1

x 0

2

4

6

8

10

x 0

200 400 600 800 1000

n 2 4n 1+ 1 15. f (n)f (n) = =1 + 6n n− 5 SOLUTION

The table and graphs below suggest that as n gets large, f (n) tends toward ∞.

Technology: Calculators and Computers

S E C T I O N 1.5

n 10 102 103 104 105 106

2 1 n 1+ n

13780.61234 1.635828711 × 1043 1.195306603 × 10434 5.341783312 × 104342 1.702333054 × 1043429 1.839738749 × 10434294

y

y 1 × 10 43

10,000

x 0

25

2

4

6

8

x 0

10

20

40

60

80 100

1 x 17. f (x) = x tanx + 6 x x f (x) = x −4 SOLUTION The table and graphs below suggest that as x gets large, f (x) approaches 1. 1 x x

x

x tan

10 102 103 104 105 106

1.033975759 1.003338973 1.000333389 1.000033334 1.000003333 1.000000333

y

y

1.5 1.4 1.3 1.2 1.1 1

1.5 1.4 1.3 1.2 1.1 1 x 5

10

15

x

20

20

40

60

80 100

19. The graph of f (θ ) = A2 cos θ + B sin θ is a sinusoidal wave for any constants A and B. Conﬁrm this for ( A, B) = (1, 1), (1, 2), and (3, 4)1by xplotting f (θ ). f (x) = x tan x of f (θ ) = cos θ + sin θ , f (θ ) = cos θ + 2 sin θ and f (θ ) = 3 cos θ + 4 sin θ are shown SOLUTION The graphs below. y

y

y

2

4

1 1 x

−2

2

4

6

8

−2

−1

2 x 2

−1

4

6

8

−2 (A, B) = (1, 1)

−2

x 2

−2

4

6

8

−4 (A, B) = (1, 2)

(A, B) = (3, 4)

21. Find the intervals on which f (x) = x(x + 2)(x − 3) is positive by plotting a graph. Find the maximum value of f (θ ) for the graphs produced in Exercise 19. Can you guess the formula for the SOLUTION The function f (x) = x(x + 2)(x − 3) is positive when the graph of y = x(x + 2)(x − 3) lies above the maximum value in terms of A and B? x-axis. The graph of y = x(x + 2)(x − 3) is shown below. Clearly, the graph lies above the x-axis and the function is positive for x ∈ (−2, 0) ∪ (3, ∞). y 20 −4

−2

x −20

2

4

−40

Find the set of solutions to the inequality (x 2 − 4)(x 2 − 1) < 0 by plotting a graph.

26

CHAPTER 1

PRECALCULUS REVIEW

Further Insights and Challenges 23. Let f 1 (x) = x and deﬁne a sequence of functions by f n+1 (x) = 12 ( f n (x) + x/ f n (x)). For example, √ 1 f 2 (x) = 2 (x + 1). Use a computer algebra system to compute f n (x) for n = 3, 4, 5 and plot f n (x) together with x for x ≥ 0. What do you notice? SOLUTION

With f 1 (x) = x and f 2 (x) = 12 (x + 1), we calculate 1 1 x 2 + 6x + 1 x = (x + 1) + 1 f 3 (x) = 2 2 4(x + 1) 2 (x + 1) ⎛ ⎞ 4 3 2 1 x 2 + 6x + 1 x ⎠ = x + 28x + 70x + 28x + 1 + 2 f 4 (x) = ⎝ x +6x+1 2 4(x + 1) 8(1 + x)(1 + 6x + x 2 ) 4(x+1)

and 1 + 120x + 1820x 2 + 8008x 3 + 12870x 4 + 8008x 5 + 1820x 6 + 120x 7 + x 8 . 16(1 + x)(1 + 6x + x 2 )(1 + 28x + 70x 2 + 28x 3 + x 4 ) √ √ A plot of f 1 (x), f 2 (x), f 3 (x), f 4 (x), f 5 (x) and x is shown below, with the graph of x shown as a dashed curve. It √ seems as if the f n are asymptotic to x. f 5 (x) =

y 12 8 4 x 20 40 60 80 100

Set P0 (x) = 1 and P1 (x) = x. The Chebyshev polynomials (useful in approximation theory) are deﬁned successively by the formula Pn+1 (x) = 2x Pn (x) − Pn−1 (x). CHAPTER REVIEW EXERCISES (a) Show that P2 (x) = 2x 2 − 1. for{x3 :≤|x n− ≤ computer algebra Compute 1. (b) Express (4, 10)Pnas(x) a set a| 6< using c} forasuitable a and c. system or by hand, and plot each Pn (x) over the interval [−1, 1]. 4+10 = 7 and the radius is 10−4 = 3. Therefore, the interval (4, 10) SOLUTION The center of the interval (4, 10) is 2 2 (c) Check that your plots conﬁrm two interesting properties of Chebyshev polynomials: (a) Pn (x) has n real roots is equivalent to the set {x : |x − 7| < 3}. in [−1, 1] and (b) for x ∈ [−1, 1], Pn (x) lies between −1 and 1. 3. Express {x : 2 ≤ |x − 1| ≤ 6} as a union of two intervals. Express as an interval: SOLUTION set<{x4}: 2 ≤ |x − 1| ≤ 6} consists of those numbers at ≤ least (a) {x : |xThe − 5| (b) {x that : |5xare + 3| 2} 2 but at most 6 units from 1. The numbers larger than 1 that satisfy these conditions are 3 ≤ x ≤ 7, while the numbers smaller than 1 that satisfy these conditions are −5 ≤ x ≤ −1. Therefore {x : 2 ≤ |x − 1| ≤ 6} = [−5, −1] ∪ [3, 7]. 5. Describe the pairs of numbers x, y such that |x + y| = x − y. Give an example of numbers x, y such that |x| + |y| = x − y. SOLUTION First consider the case when x + y ≥ 0. Then |x + y| = x + y and we obtain the equation x + y = x − y. The solution of this equation is y = 0. Thus, the pairs (x, 0) with x ≥ 0 satisfy |x + y| = x − y. Next, consider the case when x + y < 0. Then |x + y| = −(x + y) = −x − y and we obtain the equation −x − y = x − y. The solution of this equation is x = 0. Thus, the pairs (0, y) with y < 0 also satisfy |x + y| = x − y. In Exercises 7–10, let f (x) be the function whose graph is shown in Figure 1. Sketch the graph of y = f (x + 2) − 1, where f (x) = x 2 for −2 ≤ x ≤ 2. y 3 2 1 x

0 1

2

3

4

FIGURE 1

7. Sketch the graphs of y = f (x) + 2 and y = f (x + 2). SOLUTION The graph of y = f (x) + 2 is obtained by shifting the graph of y = f (x) up 2 units (see the graph below at the left). The graph of y = f (x + 2) is obtained by shifting the graph of y = f (x) to the left 2 units (see the graph below at the right).

Chapter Review Exercises y

y

5

5

4

4

3

3

2

2

1 −2 −1

27

1 x 1

2

3

x

−2 −1

4

1

f(x) + 2

2

3

4

f(x + 2)

9. Continue the graph of f (x) to the interval [−4, 4] as an even function. Sketch the graphs of y = 12 f (x) and y = f ( 12 x). SOLUTION To continue the graph of f (x) to the interval [−4, 4] as an even function, reﬂect the graph of f (x) across the y-axis (see the graph below). y 3 2 1 x

−4 −3 −2 −1

1

2

3

4

In Exercises 11–14, theofdomain and of the function. Continue theﬁnd graph f (x) to therange interval [−4, 4] as an odd function. √ 11. f (x) = x + 1 √ SOLUTION The domain of the function f (x) = x + 1 is {x : x ≥ −1} and the range is {y : y ≥ 0}. 2 13. f (x) = 4 f (x) 3=− x4 x +1 SOLUTION The domain of the function f (x) =

2 is {x : x = 3} and the range is {y : y = 0}. 3−x

15. Determine whether the function is increasing, decreasing, or neither: 2 −x +5 f (x) = x 1 (b) f (x) = 2 (a) f (x) = 3−x x +1 (c) g(t) = t 2 + t (d) g(t) = t 3 + t SOLUTION

(a) The function f (x) = 3−x can be rewritten as f (x) = ( 13 )x . This is an exponential function with a base less than 1; therefore, this is a decreasing function. (b) From the graph of y = 1/(x 2 + 1) shown below, we see that this function is neither increasing nor decreasing for all x (though it is increasing for x < 0 and decreasing for x > 0). y 1 0.8 0.6 0.4 0.2 x

−3 −2 −1

1

2

3

(c) The graph of y = t 2 + t is an upward opening parabola; therefore, this function is neither increasing nor decreasing for all t. By completing the square we ﬁnd y = (t + 12 )2 − 14 . The vertex of this parabola is then at t = − 12 , so the function is decreasing for t < − 12 and increasing for t > − 12 . (d) From the graph of y = t 3 + t shown below, we see that this is an increasing function. y 20 −3 −2 −1 −20

x 1

In Exercises 17–22,whether ﬁnd thethe equation of is theeven, line.odd, or neither: Determine function (a) f (x) = x 4 − 3x 2 (b) g(x) = sin(x + 1)

2

3

28

CHAPTER 1

PRECALCULUS REVIEW

17. Line passing through (−1, 4) and (2, 6) SOLUTION

The slope of the line passing through (−1, 4) and (2, 6) is m=

2 6−4 = . 2 − (−1) 3

The equation of the line passing through (−1, 4) and (2, 6) is therefore y − 4 = 23 (x + 1) or 2x − 3y = −14. 19. Line of slope 6 through (9, 1) Line passing through (−1, 4) and (−1, 6) SOLUTION Using the point-slope form for the equation of a line, the equation of the line of slope 6 and passing through (9, 1) is y − 1 = 6(x − 9) or 6x − y = 53. 21. Line through (2, 3) parallel to y = 4 − x 3 Line ofThe slope − through (4, −12) SOLUTION equation 2 y = 4 − x is in slope-intercept form; it follows that the slope of this line is −1. Any line parallel to y = 4 − x will have the same slope, so we are looking for the equation of the line of slope −1 and passing through (2, 3). The equation of this line is y − 3 = −(x − 2) or x + y = 5. 23. Does the following table of market data suggest a linear relationship between price and number of homes sold during Horizontal line through (−3, 5) a one-year period? Explain.

SOLUTION

Price (thousands of $)

180

195

220

240

No. of homes sold

127

118

103

91

Examine the slope between consecutive data points. The ﬁrst pair of data points yields a slope of 9 3 118 − 127 =− =− , 195 − 180 15 5

while the second pair of data points yields a slope of 15 3 103 − 118 =− =− 220 − 195 25 5 and the last pair of data points yields a slope of 91 − 103 12 3 =− =− . 240 − 220 20 5 Because all three slopes are equal, the data does suggest a linear relationship between price and the number of homes sold. 2 and sketch its graph. On which intervals is f (x) decreasing? 25. FindDoes the roots of f (x) = x4 − the following table of4x yearly revenue data for a computer manufacturer suggest a linear relation between revenue and SOLUTION Thetime? rootsExplain. of f (x) = x 4 − 4x 2 are obtained by solving the equation x 4 − 4x 2 = x 2 (x − 2)(x + 2) = 0, which yields x = −2, x = 0 and x = 2. The graph of y = f (x) is shown below. From this graph we see that f (x) is Year −1.4 and for x between 1995 0 and 1999 2001 2004 decreasing for x less than approximately approximately 1.4.

Revenue (billions of $)

y

13

18

15

11

20 10 −1

1

−3 −2

x 2

3

27. Let f (x) be the square of the distance from the point (2, 1) to a point (x, 3x + 2) on the line y = 3x + 2. Show that 2 + 12z + 3. Complete the square and ﬁnd the minimum value of h(z). h(z) = 2z f (x) is Let a quadratic function and ﬁnd its minimum value by completing the square. SOLUTION

3x + 2. Then

Let f (x) denote the square of the distance from the point (2, 1) to a point (x, 3x + 2) on the line y = f (x) = (x − 2)2 + (3x + 2 − 1)2 = x 2 − 4x + 4 + 9x 2 + 6x + 1 = 10x 2 + 2x + 5,

which is a quadratic function. Completing the square, we ﬁnd f (x) = 10

1 x2 + x + 5

1 100

1 1 2 49 + . +5− = 10 x + 10 10 10

1 )2 ≥ 0 for all x, it follows that f (x) ≥ 49 for all x. Hence, the minimum value of f (x) is 49 . Because (x + 10 10 10

Prove that x 2 + 3x + 3 ≥ 0 for all x.

Chapter Review Exercises

29

In Exercises 29–34, sketch the graph by hand. 29. y = t 4 y

SOLUTION 1 0.8 0.6 0.4 0.2 −1

x

−0.5

0.5

1

θ 31. y =y sin = t25 y

SOLUTION 1 0.5

x

−5

5

10

−0.5 −1

33. y = x 1/3 −x y = 10 y

SOLUTION 2 1 −4 −3 −2 −1 −1

x 1 2 3 4

−2

35. Show that1 the graph of y = f 13 x − b is obtained by shifting the graph of y = f 13 x to the right 3b units. Use 1 y = 2 to sketch the graph of y = x − 4. this observation 3 x SOLUTION

Let g(x) = f ( 13 x). Then

g(x − 3b) = f

1 1 (x − 3b) = f x −b . 3 3

Thus, the graph of y = f ( 13 x − b) is obtained by shifting the graph of y = f ( 13 x) to the right 3b units. The graph of y = | 13 x − 4| is the graph of y = | 13 x| shifted right 12 units (see the graph below). y 4 3 2 1 x 0

5

10

15

20

37. Find functions f and g such that the function Let h(x) = cos x and g(x) = x −1 . Compute the composite functions h(g(x)) and g(h(x)), and ﬁnd their dof (g(t)) = (12t + 9)4 mains. SOLUTION

One possible choice is f (t) = t 4 and g(t) = 12t + 9. Then f (g(t)) = f (12t + 9) = (12t + 9)4

as desired.

θ sin 2θ + sin to?the following three angles and ﬁnd the values of the six 39. What is thethe period of the function g(θ ) = Sketch points on the unit circle corresponding 2 standard trigonometric functions at each angle: SOLUTION of 4π . Because 4π is a 2π The function sin 2θ has a period 7ofπ π , and the function sin(θ /2) has a 7period π (a) of π , the period of the function g(θ(b) (c) multiple ) = sin 2 θ + sin θ /2 is 4 π . 3 4 6 Assume that sin θ =

4 π , where < θ < π . Find: 5 2

30

CHAPTER 1

PRECALCULUS REVIEW

41. Give an example of values a, b such that (a) cos(a + b) = cos a + cos b

(b) cos

a cos a = 2 2

SOLUTION

(a) Take a = b = π /2. Then cos(a + b) = cos π = −1 but cos a + cos b = cos

π π + cos = 0 + 0 = 0. 2 2

(b) Take a = π . Then cos

a 2

= cos

π 2

=0

but cos a cos π −1 1 = = =− . 2 2 2 2 43. Solve sin 2x + cos x = 0 for 0 ≤ x < 2π . π 1 π . equation as 2 sin x cos x + cos x = Let f (x) = cos x. Sketch the graph of y = 2 f x − forwe0 ≤ x ≤ 6the SOLUTION Using the double angle formula for the sine rewrite 3 function, 4 cos x(2 sin x + 1) = 0. Thus, either cos x = 0 or sin x = −1/2. From here we see that the solutions are x = π /2, x = 7π /6, x = 3π /2 and x = 11π /6. Use a graphing calculator to determine whether the equation cos x = 5x 2 − 8x 4 has any solutions. n2 How does h(n) = n behave for large whole 2 number 4 values of n? Does h(n) tend to inﬁnity? SOLUTION The graphs of 2 y = cos x and y = 5x − 8x are shown below. Because the graphs do not intersect, there are no solutions to the equation cos x = 5x 2 − 8x 4 . 45.

y 1

y = cos x x

−1

1 −1

y = 5x 2 − 8x 4

Using a graphing calculator, ﬁnd the number of real roots and estimate the largest root to two decimal places: (a) f (x) = 1.8x 4 − x 5 − x (b) g(x) = 1.7x 4 − x 5 − x

2 LIMITS 2.1 Limits, Rates of Change, and Tangent Lines Preliminary Questions 1. Average velocity is deﬁned as a ratio of which two quantities? SOLUTION

Average velocity is deﬁned as the ratio of distance traveled to time elapsed.

2. Average velocity is equal to the slope of a secant line through two points on a graph. Which graph? SOLUTION

Average velocity is the slope of a secant line through two points on the graph of position as a function of

time. 3. Can instantaneous velocity be deﬁned as a ratio? If not, how is instantaneous velocity computed? SOLUTION Instantaneous velocity cannot be deﬁned as a ratio. It is deﬁned as the limit of average velocity as time elapsed shrinks to zero.

4. What is the graphical interpretation of instantaneous velocity at a moment t = t0 ? SOLUTION Instantaneous velocity at time t = t0 is the slope of the line tangent to the graph of position as a function of time at t = t0 .

5. What is the graphical interpretation of the following statement: The average ROC approaches the instantaneous ROC as the interval [x 0 , x 1 ] shrinks to x0 ? SOLUTION

The slope of the secant line over the interval [x 0 , x 1 ] approaches the slope of the tangent line at x = x 0 .

6. The ROC of atmospheric temperature with respect to altitude is equal to the slope of the tangent line to a graph. Which graph? What are possible units for this rate? SOLUTION The rate of change of atmospheric temperature with respect to altitude is the slope of the line tangent to the graph of atmospheric temperature as a function of altitude. Possible units for this rate of change are ◦ F/ft or ◦ C/m.

Exercises 1. A ball is dropped from a state of rest at time t = 0. The distance traveled after t seconds is s(t) = 16t 2 ft. (a) How far does the ball travel during the time interval [2, 2.5]? (b) Compute the average velocity over [2, 2.5]. (c) Compute the average velocity over time intervals [2, 2.01], [2, 2.005], [2, 2.001], [2, 2.00001]. Use this to estimate the object’s instantaneous velocity at t = 2. SOLUTION

(a) Galileo’s formula is s(t) = 16t 2 . The ball thus travels s = s(2.5) − s(2) = 16(2.5)2 − 16(2)2 = 36 ft. (b) The average velocity over [2, 2.5] is s(2.5) − s(2) 36 s = = = 72 ft/s. t 2.5 − 2 0.5 (c) time interval

[2, 2.01]

[2, 2.005]

[2, 2.001]

[2, 2.00001]

average velocity

64.16

64.08

64.016

64.00016

The instantaneous velocity at t = 2 is 64 ft/s. √ 3. LetAv wrench = 20 Tisas in Example Estimate theatinstantaneous of vthe with respect to T when T =velocity 300 K. at t = 1, released from 2. a state of rest time t = 0. ROC Estimate wrench’s instantaneous 2 SOLUTION assuming that the distance traveled after t seconds is s(t) = 16t . T interval

[300, 300.01]

[300, 300.005]

average rate of change

0.577345

0.577348

T interval

[300, 300.001]

[300, 300.00001]

average ROC

0.57735

0.57735

32

CHAPTER 2

LIMITS

The instantaneous rate of change is approximately 0.57735 m/(s · K). In Exercises 5–6, y/x a stone is in the [2, air 5], from ground an initial of 15 m/s. time t to is Compute fortossed the interval where y =level 4x −with 9. What is thevelocity instantaneous ROCItsofheight y withatrespect h(t) x=at15t − 2? 4.9t 2 m. x= 5. Compute the stone’s average velocity over the time interval [0.5, 2.5] and indicate the corresponding secant line on a sketch of the graph of h(t). SOLUTION

The average velocity is equal to h(2.5) − h(0.5) = 0.3. 2

The secant line is plotted with h(t) below. h 10 8 6 4 2 t 0.5

1

1.5

2

2.5

3

7. With an initial deposit of $100, the balance in a bank account after t years is f (t) = 100(1.08)t dollars. Compute the stone’s average velocity over the time intervals [1, 1.01], [1, 1.001], [1, 1.0001] and [0.99, 1], (a) What are1],the units of1]. theUse ROC (t)? [0.999, [0.9999, thisoftofestimate the instantaneous velocity at t = 1. (b) Find the average ROC over [0, 0.5] and [0, 1]. (c) Estimate the instantaneous rate of change at t = 0.5 by computing the average ROC over intervals to the left and right of t = 0.5. SOLUTION

(a) The units of the rate of change of f (t) are dollars/year or $/yr. (b) The average rate of change of f (t) = 100(1.08)t over the time interval [t1 , t2 ] is given by f f (t2 ) − f (t1 ) . = t t2 − t 1 time interval

[0, .5]

[0, 1]

average rate of change

7.8461

8

(c) time interval

[.5, .51]

[.5, .501]

[.5, .5001]

average rate of change

8.0011

7.9983

7.9981

time interval

[.49, .5]

[.499, .5]

[.4999, .5]

average ROC

7.9949

7.9977

7.998

The rate of change at t = 0.5 is approximately $8/yr. In Exercises 9–12, estimate the instantaneous rate of change at3 the point indicated. The distance traveled by a particle at time t is s(t) = t + t. Compute the average velocity over the time interval 2 − 3; xthe 4] = and4xestimate velocity at t = 1. = instantaneous 2 9. [1, P(x) SOLUTION

x interval

[2, 2.01]

[2, 2.001]

[2, 2.0001]

[1.99, 2]

[1.999, 2]

[1.9999, 2]

average rate of change

16.04

16.004

16.0004

15.96

15.996

15.9996

The rate of change at x = 2 is approximately 16. 1 11. y(x)f (t) = = 3t − ; x5;=t 2= −9 x +2 SOLUTION

x interval

[2, 2.01]

[2, 2.001]

[2, 2.0001]

[1.99, 2]

[1.999, 2]

[1.9999, 2]

average ROC

−.0623

−.0625

−.0625

−.0627

−.0625

−.0625

S E C T I O N 2.1

Limits, Rates of Change, and Tangent Lines

33

The rate of change at x = 2 is approximately −0.06. 13. The atmospheric temperature T (in ◦ F) above a certain point on earth is T = 59 − 0.00356h, where h is the altitude √ 1; t = 1What are the average and instantaneous rates of change of T with respect to h? Why are y(t) =for h3t≤+37,000). in feet (valid they the same? Sketch the graph of T for h ≤ 37,000. The average and instantaneous rates of change of T with respect to h are both −0.00356◦ F/ft. The rates of change are the same because T is a linear function of h with slope −0.00356. SOLUTION

Temp (°F) 60 40 20

20,000 10,000

−20 −40 −60

15. (a) (b)

30,000

Altitude (ft)

The number P(t) of E. coli cells at time t (hours) in a petri dish is plotted in Figure 9. The height (in feet) at time t (in seconds) of a small weight oscillating at the end of a spring is h(t) = 0.5 cos(8t). Calculate the average ROC of P(t) over the time interval [1, 3] and draw the corresponding secant line. (a) Calculate the weight’s average velocity over the time intervals [0, 1] and [3, 5]. Estimate the slope m of the line in Figure 9. What does m represent? (b) Estimate its instantaneous velocity at t = 3. P(t) 10,000 8,000 6,000 4,000 2,000 1,000 1

2

3 t (hours)

FIGURE 9 Number of E. coli cells at time t. SOLUTION

(a) Looking at the graph, we can estimate P(1) = 2000 and P(3) = 8000. Assuming these values of P(t), the average rate of change is P(3) − P(1) 6000 = = 3000 cells/hour. 3−1 2 The secant line is here: P(t) 10,000 8,000 6,000 4,000 2,000 1,000 1

2

3 t (hours)

(b) The line in Figure 9 goes through two points with approximate coordinates (1, 2000) and (2.5, 4000). This line has approximate slope m=

4000 4000 − 2000 = cells/hour. 2.5 − 1 3

m is close to the slope of the line tangent to the graph of P(t) at t = 1, and so m represents the instantaneous rate of change of P(t) at t = 1 hour. √ T (in seconds) of a pendulum (the√time required for a complete back-and-forth cycle) 17. Assume√that the period √Calculate x + 1 − x for x = 1, 10, 100, 1,000. Does x√change more rapidly when x is large or small? 3 where L is the pendulum’s length (into meters). is T Interpret = 2 L,your answer in terms of tangent lines the graph of y = x. (a) What are the units for the ROC of T with respect to L? Explain what this rate measures. (b) Which quantities are represented by the slopes of lines A and B in Figure 10? (c) Estimate the instantaneous ROC of T with respect to L when L = 3 m.

CHAPTER 2

LIMITS BA Period (s)

34

2

1

3 Length (m)

FIGURE 10 The period T is the time required for a pendulum to swing back and forth. SOLUTION

(a) The units for the rate of change of T with respect to L are seconds per meter. This rate measures the sensitivity of the period of the pendulum to a change in the length of the pendulum. (b) The slope of the line B represents the average rate of change in T from L = 1 m to L = 3 m. The slope of the line A represents the instantaneous rate of change of T at L = 3 m. (c) time interval

[3, 3.01]

[3, 3.001]

[3, 3.0001]

[2.99, 3]

[2.999, 3]

[2.9999, 3]

average velocity

0.4327

0.4330

0.4330

0.4334

0.4330

0.4330

The instantaneous rate of change at L = 1 m is approximately 0.4330 s/m. 19. The graphs in Figure 12 represent the positions s of moving particles as functions of time t. Match each graph with The graphs in Figure 11 represent the positions of moving particles as functions of time. one of the following statements: (a) Do the (a) Speeding upinstantaneous velocities at times t1 , t2 , t3 in (A) form an increasing or decreasing sequence? (b) Is theup particle speeding updown or slowing down in (A)? (b) Speeding and then slowing (c) Is thedown particle speeding up or slowing down in (B)? (c) Slowing (d) Slowing down and then speeding up s

s

t

t (A)

(B)

s

s

t

t

(C)

(D)

FIGURE 12 SOLUTION When a particle is speeding up over a time interval, its graph is bent upward over that interval. When a particle is slowing down, its graph is bent downward over that interval. Accordingly,

• In graph (A), the particle is (c) slowing down. • In graph (B), the particle is (b) speeding up and then slowing down. • In graph (C), the particle is (d) slowing down and then speeding up. • In graph (D), the particle is (a) speeding up.

21. (a) (b) (c)

The fraction of a city’s population infected by a ﬂu virus is plotted as a function of time (in weeks) in Figure 14. An epidemiologist ﬁnds that the percentage N (t) of susceptible children who were infected on day t during the Which quantities are represented by the slopes of lines A and B? Estimate these slopes. ﬁrst three weeks of a measles outbreak is given, to a reasonable approximation, by the formula Is the ﬂu spreading more rapidly at t = 1, 2, or 3? Is the ﬂu spreading more rapidly at t = 4, 5, or 6? 100t 2 N (t) = 3 Fraction infected 2 t + 5t − 100t + 380 A graph of N (t) appears in Figure 13.

0.3 B 0.2 A

(a) Draw the secant line whose slope is the0.1 average rate of increase in infected children over the intervals between days 4 and 6 and between days 12 and 14. Then compute these average rates (in units of percent per day). Weeks (b) Estimate the ROC of N (t) on day 12. 1 2 3 4 5 6 FIGURE 14

Limits, Rates of Change, and Tangent Lines

S E C T I O N 2.1

35

SOLUTION

(a) The slope of line A is the average rate of change over the interval [4, 6], whereas the slope of the line B is the instantaneous rate of change at t = 6. Thus, the slope of the line A ≈ (0.28 − 0.19)/2 = 0.045/week, whereas the slope of the line B ≈ (0.28 − 0.15)/6 = 0.0217/week. (b) Among times t = 1, 2, 3, the ﬂu is spreading most rapidly at t = 3 since the slope is greatest at that instant; hence, the rate of change is greatest at that instant. (c) Among times t = 4, 5, 6, the ﬂu is spreading most rapidly at t = 4 since the slope is greatest at that instant; hence, the rate of change is greatest at that instant. √ 23. Let v =fusarium 20 T as in Example 2. Is athe ROC of v plants with respect T greater at low temperatures high The fungus exosporium infects ﬁeld of ﬂax throughtothe roots and causes the plants or to wilt. temperatures? in terms ofinfected. the graph. Eventually,Explain the entire ﬁeld is The percentage f (t) of infected plants as a function of time t (in days) since planting is shown in Figure 15. SOLUTION (a) What are the units of the rate of change of f (t) with respect to t? What does this rate measure? (b) Use the graph to rank (from smallest (m/s) to largest) the average infection rates over the intervals [0, 12], [20, 32], 350 and [40, 52]. 300 (c) Use the following table to compute 250 the average rates of infection over the intervals [30, 40], [40, 50], [30, 50]: Days Percent

200 150 100 50 infected

0 0

10 18

20 56

30 82

50 100 150 200 250 300

T (K)

40 91

50 96

60 98

(d) Draw the tangent line at t = 40 and estimate its slope. Choose any two points on the tangent line for the computation. As the graph progresses to the right, the graph bends progressively downward, meaning that the slope of the tangent lines becomes smaller. This means that the ROC of v with respect to T is lower at high temperatures. Sketch the graph in of af straight (x) = x(1 x) over 1]. Refer to the graph without anythen computations, 25. If an object moving line−(but with [0, changing velocity) coversand, s feet in tmaking seconds, its average ﬁnd:velocity is v = s/t ft/s. Show that it would cover the same distance if it traveled at constant velocity v over 0 0 (a) The average overof [0,t 1] seconds. This is a justiﬁcation for calling s/t the average velocity. the same timeROC interval (b) The (instantaneous) ROC at x = 12 (c) The values of x at which the ROC is positive SOLUTION y 0.25 0.2 0.15 0.1 0.05 x 0.2

0.4

0.6

0.8

1

(a) f (0) = f (1), so there is no change between x = 0 and x = 1. The average ROC is zero. (b) The tangent line to the graph of f (x) is horizontal at x = 12 ; the instantaneous ROC is zero at this point. (c) The ROC is positive at all points where the graph is rising, because the slope of the tangent line is positive at these points. This is so for all x between x = 0 and x = 0.5. graphand in Figure 16 has the following property: For all x, the average ROC over [0, x] is greater than the Further Which Insights Challenges

instantaneous ROC at x? Explain. 27. The height of a projectile ﬁred in the air vertically with initial velocity 64 ft/s is h(t) = 64t − 16t 2 ft. (a) Compute h(1). Show that h(t) − h(1) can be factored with (t − 1) as a factor. (b) Using part (a), show that the average velocity over the interval [1, t] is −16(t − 3). (c) Use this formula to ﬁnd the average velocity over several intervals [1, t] with t close to 1. Then estimate the instantaneous velocity at time t = 1. SOLUTION

(a) With h(t) = 64t − 16t 2 , we have h(1) = 48 ft, so h(t) − h(1) = −16t 2 + 64t − 48. Taking out the common factor of −16 and factoring the remaining quadratic, we get h(t) − h(1) = −16(t 2 − 4t + 3) = −16(t − 1)(t − 3).

36

CHAPTER 2

LIMITS

(b) The average velocity over the interval [1, t] is h(t) − h(1) −16(t − 1)(t − 3) = = −16(t − 3). t −1 t −1

t

1.1

1.05

1.01

1.001

average velocity over [1, t]

30.4

31.2

31.84

31.984

(c)

The instantaneous velocity is approximately 32 ft/s. Plugging t = 1 second into the formula in (b) yields −16(1 − 3) = 32 ft/s exactly. 29. Show that the average ROC of f (x) = x 3 over [1, x] is equal to x 2 + x + 1. Use this to estimate the instantaneous 2 Q(t) ROC ofLet f (x) at x==t 1.. As in the previous exercise, ﬁnd a formula for the average ROC of Q over the interval [1, t] and use it to estimate the instantaneous ROC at t = 1. Repeat for the interval [2, t] and estimate the ROC at t = 2. SOLUTION The average ROC is f (x) − f (1) x3 − 1 = . x −1 x −1 Factoring the numerator as the difference of cubes means the average rate of change is (x − 1)(x 2 + x + 1) = x2 + x + 1 x −1 (for all x = 1). The closer x gets to 1, the closer the average ROC gets to 12 + 1 + 1 = 3. The instantaneous ROC is 3. √ 31. T = 32 forLthe as in Exercise 17.ofThe numbers the[2, second of estimate Table 4 are and those over x] andcolumn use it to theincreasing instantaneous ROC in at Find Let a formula average ROC f (x) = x 3 in the last are decreasing. Explain why in terms of the graph of T as a function of L. Also, explain graphically why x =column 2. the instantaneous ROC at L = 3 lies between 0.4329 and 0.4331. TABLE 4

Average Rates of Change of T with Respect to L

Interval

Average ROC

Interval

Average ROC

[3, 3.2] [3, 3.1] [3, 3.001] [3, 3.0005]

0.42603 0.42946 0.43298 0.43299

[2.8, 3] [2.9, 3] [2.999, 3] [2.9995, 3]

0.44048 0.43668 0.43305 0.43303

SOLUTION Since the average ROC is increasing on the intervals [3, L] as L get close to 3, we know that the slopes of the secant lines between points on the graph over these intervals are increasing. The more rows we add with smaller intervals, the greater the average ROC. This means that the instantaneous ROC is probably greater than all of the numbers in this column. Likewise, since the average ROC is decreasing on the intervals [L , 3] as L gets closer to 3, we know that the slopes of the secant lines between points over these intervals are decreasing. This means that the instantaneous ROC is probably less than all the numbers in this column. The tangent slope is somewhere between the greatest value in the ﬁrst column and the least value in the second column. Hence, it is between .43299 and .43303. The ﬁrst column underestimates the instantaneous ROC by secant slopes; this estimate improves as L decreases toward L = 3. The second column overestimates the instantaneous ROC by secant slopes; this estimate improves as L increases toward L = 3.

2.2 Limits: A Numerical and Graphical Approach Preliminary Questions 1. What is the limit of f (x) = 1 as x → π ? SOLUTION

limx→π 1 = 1.

2. What is the limit of g(t) = t as t → π ? SOLUTION

limt→π t = π .

3. Can f (x) approach a limit as x → c if f (c) is undeﬁned? If so, give an example.

Limits: A Numerical and Graphical Approach

S E C T I O N 2.2

37

SOLUTION Yes. The limit of a function f as x → c does not depend on what happens at x = c, only on the behavior of f as x → c. As an example, consider the function

f (x) =

x2 − 1 . x −1

The function is clearly not deﬁned at x = 1 but x2 − 1 = lim (x + 1) = 2. x→1 x − 1 x→1

lim f (x) = lim

x→1

4. Is lim 20 equal to 10 or 20? x→10

SOLUTION

limx→10 20 = 20.

5. What does the following table suggest about lim f (x) and lim f (x)? x→1−

x

0.9

0.99

0.999

1.1

1.01

1.001

7

25

4317

3.0126

3.0047

3.00011

f (x) SOLUTION

x→1+

The values in the table suggest that limx→1− f (x) = ∞ and limx→1+ f (x) = 3.

6. Is it possible to tell if lim f (x) exists by only examining values f (x) for x close to but greater than 5? Explain. x→5

SOLUTION No. By examining values of f (x) for x close to but greater than 5, we can determine whether the one-sided limit limx→5+ f (x) exists. To determine whether limx→5 f (x) exists, we must examine value of f (x) on both sides of x = 5.

7. If you know in advance that lim f (x) exists, can you determine its value just knowing the values of f (x) for all x→5

x > 5? SOLUTION

Yes. If limx→5 f (x) exists, then both one-sided limits must exist and be equal.

8. Which of the following pieces of information is sufﬁcient to determine whether lim f (x) exists? Explain. x→5

(a) (b) (c) (d) (e)

The values of The values of The values of The values of f (5)

f (x) for all x f (x) for x in [4.5, 5.5] f (x) for all x in [4.5, 5.5] other than x = 5 f (x) for all x ≥ 5

SOLUTION To determine whether lim x→5 f (x) exists, we must know the values of f (x) for values of x near 5, both smaller than and larger than 5. Thus, the information in (a), (b) or (c) would be sufﬁcient. The information in (d) does not include values of f for x < 5, so this information is not sufﬁcient to determine whether the limit exists. The limit does not depend at all on the value f (5), so the information in (e) is also not sufﬁcient to determine whether the limit exists.

Exercises In Exercises 1–4, ﬁll in the tables and guess the value of the limit. x3 − 1 . 1. lim f (x), where f (x) = 2 x→1 x −1 f (x)

x

f (x)

x

1.002

0.998

1.001

0.999

1.0005

0.9995

1.00001

0.99999

SOLUTION

x

0.998

0.999

0.9995

0.99999

1.00001

1.0005

1.001

1.002

f (x)

1.498501

1.499250

1.499625

1.499993

1.500008

1.500375

1.500750

1.501500

38

CHAPTER 2

LIMITS

The limit as x → 1 is 32 . 2− y−2 ycos t −1 3. limlim f (y), where f (y) = h(t), where h(t) =y 2 + 2y − 6. .Note that h(t) is even, that is, h(t) = h(−t). y→2 t→0 t

t

y±0.002 f (y) y ±0.0001

f (y) ±0.00005

h(t) 2.002

1.998

2.001

1.999

2.0001

1.9999

±0.00001

SOLUTION

y

1.998

1.999

1.9999

2.0001

2.001

2.02

f (y)

0.59984

0.59992

0.599992

0.600008

0.60008

0.601594

The limit as y → 2 is 35 . 5. Determine lim f (x) for the function f (x) shown in Figure 8. sin θ − θ x→0.5 lim f (θ ), where f (θ ) = . θ →0 θ3 y

θ f (θ )

±0.002

±0.0001

±0.00005

±0.00001

1.5

x .5

FIGURE 8 SOLUTION

The graph suggests that f (x) → 1.5 as x → .5.

In Exercises 7–8, of evaluate limit. functions in Figure 9 appear to approach a limit as x → 0? Do either the twothe oscillating 7. lim x x→21

SOLUTION

As x → 21, f (x) = x → 21. You can see this, for example, on the graph of f (x) = x.

√ verify each limit using the limit deﬁnition. For example, in Exercise 9, show that |2x − 6| can be made In Exercises 9–18, lim 3 as smallx→4.2 as desired by taking x close to 3. 9. lim 2x = 6 x→3

SOLUTION

|x − 3| small.

|2x − 6| = 2|x − 3|. |2x − 6| can be made arbitrarily small by making x close enough to 3, thus making

11. lim (4x + 3) = 11 x→2lim 4 = 4 x→3

|(4x + 3) − 11| = |4x − 8| = 4|x − 2|. Therefore, if you make |x − 2| small enough, you can make |(4x + 3) − 11| as small as desired. SOLUTION

13. lim (−2x) = −18 x→9lim (5x − 7) = 8 x→3

We have |−2x − (−18)| = |(−2)(x − 9)| = 2 |x − 9|. If you make |x − 9| small enough, you can make 2 |x − 9| = |−2x − (−18)| as small as desired. SOLUTION

x 2 =(10 − 2x) = 11 15. lim lim x→0 x→−5

As x → 0, we have |x 2 − 0| = |x + 0||x − 0|. To simplify things, suppose that |x| < 1, so that |x + 0||x − 0| = |x||x| < |x|. By making |x| sufﬁciently small, so that |x + 0||x − 0| = x 2 is even smaller, you can make |x 2 − 0| as small as desired. SOLUTION

+ 3) = 3 17. lim (x 2 + 2x x→0lim (2x 2 + 4) = 4 x→0

As x → 0, we have |x 2 + 2x + 3 − 3| = |x 2 + 2x| = |x||x + 2|. If |x| < 1, |x + 2| can be no bigger than 3, so |x||x + 2| < 3|x|. Therefore, by making |x − 0| = |x| sufﬁciently small, you can make |x 2 + 2x + 3 − 3| = |x||x + 2| as small as desired. SOLUTION

lim (x 3 + 9) = 9

x→0

Limits: A Numerical and Graphical Approach

S E C T I O N 2.2

In Exercises 19–32, estimate the limit numerically or state that the limit does not exist. √ x −1 19. lim x→1 x − 1 SOLUTION

x

.9995

.99999

1.00001

1.0005

f (x)

.500063

.500001

.49999

.499938

x

1.999

1.99999

2.00001

2.001

f (x)

1.666889

1.666669

1.666664

1.666445

x

−0.01

−0.005

0.005

0.01

f (x)

1.999867

1.999967

1.999967

1.999867

The limit as x → 1 is 12 . x2 + x − 6 2x 2 − 18 x→2 lim x2 − x − 2 x→−3 x + 3

21. lim

SOLUTION

The limit as x → 2 is 53 . sin 2x 23. lim x 3 − 2x 2 − 9 x→0lim x x→3 x 2 − 2x − 3 SOLUTION

The limit as x → 0 is 2. sin x 25. lim sin 5x x→0limx 2 x→0 x SOLUTION

x

−.01

−.001

−.0001

.0001

.001

.01

f (x)

−99.9983

−999.9998

−10000.0

10000.0

999.9998

99.9983

The limit does not exist. As x → 0−, f (x) → −∞; similarly, as x → 0+, f (x) → ∞. 1 27. lim cos cos θ − 1 h→0lim h θ θ →0 SOLUTION

h

±0.1

±0.01

±0.001

±0.0001

f (h)

−0.839072

0.862319

0.562379

−0.952155

The limit does not exist since cos (1/ h) oscillates inﬁnitely often as h → 0. 2h − 1 29. limlim sin h cos 1 h h→0 h h→0 SOLUTION

h

−.05

−.001

.001

.05

f (h)

.681273

.692907

.693387

.705298

The limit as x → 0 is approximately 0.693. (The exact answer is ln 2.) 5h −x25 x 2 −3 31. lim lim h→2 h − 2 x x→0 SOLUTION

39

40

CHAPTER 2

LIMITS

x

1.95

1.999

2.001

2.05

f (x)

38.6596

40.2036

40.2683

41.8992

The limit as h → 2 is approximately 40.2. (The exact answer is 25 ln 5.) 33. Determine x lim f (x) and lim f (x) for the function shown in Figure 10. lim |x| x→2+ x→2− x→0

y

2

1

x 2

FIGURE 10 SOLUTION

The left-hand limit is lim f (x) = 2, whereas the right-hand limit is lim f (x) = 1. Accordingly, the x→2−

x→2+

two-sided limit does not exist. 35. The greatest integer function is deﬁned by [x] = n, where n is the unique integer such that n ≤ x < n + 1. See Determine the one-sided limits at c = 1, 2, 4, 5 of the function g(t) shown in Figure 11 and state whether the Figure 12. limit exists at these points. (a) For which values of c does lim [x] exist? What about lim [x]? x→c−

x→c+

(b) For which values of c does lim [x] exist? x→c

y 2 1 x

−1

1

2

3

FIGURE 12 Graph of y = [x]. SOLUTION

(a) The one-sided limits exist for all real values of c. (b) For each integer value of c, the one-sided limits differ. In particular, lim [x] = c − 1, whereas lim [x] = c. (For x→c−

x→c+

noninteger values of c, the one-sided limits both equal [c].) The limit lim [x] exists when x→c

lim [x] = lim [x] ,

x→c−

x→c+

namely for noninteger values of c: n < c < n + 1, where n is an integer. In Exercises 37–39, determine the xone-sided limits numerically. −1 and use it to determine the one-sided limits lim f (x) and lim f (x). Draw a graph of f (x) = |x − 1| x→1+ x→1− sin x 37. lim |x| x→0± SOLUTION

x

−.2

−.02

.02

.2

f (x)

−.993347

−.999933

.999933

.993347

The left-hand limit is lim f (x) = −1, whereas the right-hand limit is lim f (x) = 1. x→0−

x→0+

x − sin(|x|) 39. limlim |x|1/x 3 x→0± x→0± x SOLUTION

x

−.1

−.01

.01

.1

f (x)

199.853

19999.8

.166666

.166583

Limits: A Numerical and Graphical Approach

S E C T I O N 2.2

The left-hand limit is lim f (x) = ∞, whereas the right-hand limit is lim f (x) = x→0−

x→0+

1 . 6

41. Determine the one-sided limits of f (x) at c = 2 and c = 4, for the function shown in Figure 14. Determine the one- or two-sided inﬁnite limits in Figure 13. y 15 10 5 x 2

4

−5

FIGURE 14 SOLUTION

• For c = 2, we have lim f (x) = ∞ and lim f (x) = ∞. x→2− x→2+ • For c = 4, we have lim f (x) = −∞ and lim f (x) = 10. x→4− x→4+

In Exercises 43–46,the draw the graph of atwo-sided function with theingiven Determine inﬁnite one- and limits Figurelimits. 15. 43. lim f (x) = 2, lim f (x) = 0, lim f (x) = 4 x→1

x→3−

x→3+

SOLUTION y 6 4 2

x 1

45.

2

3

4

lim f (x) = f (2) = 3, lim f (x) = −1, lim f (x) = 2 = f (4)

lim f (x) = ∞, lim x→2− f (x) = 0, lim fx→4 (x) = −∞ x→2+ x→1 x→3− x→3+

SOLUTION y 3 2 1 x −1

1

2

3

4

5

Inlim Exercises 47–52, use = the−∞ graph to estimate the value of the limit. f (x) = ∞, graph lim fthe (x)function = 3, limandf (x) x→1+

sin 3θ 47. lim θ →0 sin 2θ

x→1−

x→4

SOLUTION y 1.500 y= 1.495 1.490 1.485

The limit as θ → 0 is 32 . 2x − 1 x→0 4x − 1 lim

sin 3 sin 2

41

42

CHAPTER 2

LIMITS

2x − cos x x x→0

49. lim

SOLUTION y 0.6940 0.6935 y=

2x

− cos x x

0.6930

0.6925 0.6920

The limit as x → 0 is approximately 0.693. (The exact answer is ln 2.) cos 3θ − cos 4θ 51. lim sinθ224θ θ →0lim θ →0 cos θ − 1 SOLUTION

y 3.500

y=

cos 3 − cos 4 2

3.475 3.450 3.425

The limit as θ → 0 is 3.5. cos 3θ − cos 5θ FurtherlimInsights and Challenges 2

θ →0 θ 53. Light waves of frequency λ passing through a slit of width a produce a Fraunhofer diffraction pattern of light and dark fringes (Figure 16). The intensity as a function of the angle θ is given by sin(R sin θ ) 2 I (θ ) = I m R sin θ

where R = π a/λ and Im is a constant. Show that the intensity function is not deﬁned at θ = 0. Then check numerically that I (θ ) approaches Im as θ → 0 for any two values of R (e.g., choose two integer values).

a Incident light waves Slit

Viewing screen

Intensity pattern

FIGURE 16 Fraunhofer diffraction pattern. SOLUTION

If you plug in θ = 0, you get a division by zero in the expression

sin R sin θ ; R sin θ

thus, I (0) is undeﬁned. If R = 2, a table of values as θ → 0 follows:

θ

−0.01

−0.005

0.005

0.01

I (θ )

0.998667 Im

0.9999667 Im

0.9999667 Im

0.9998667 Im

The limit as θ → 0 is 1 · Im = Im . If R = 3, the table becomes:

θ

−0.01

−0.005

0.005

0.01

I (θ )

0.999700 Im

0.999925 Im

0.999925 Im

0.999700 Im

S E C T I O N 2.2

Limits: A Numerical and Graphical Approach

43

Again, the limit as θ → 0 is 1Im = Im . bx − 1 sin nlim θ 55. Show numerically that for b = 3, 5 appears to equal ln 3, ln 5, where ln x is the natural logarithm. Then for several values of n and then guess the value in general. Investigate lim x x→0numerically θ θ →0 make a conjecture (guess) for the value in general and test your conjecture for two additional values of b (these results are explained in Chapter 7). SOLUTION

•

−.1

−.01

−.001

.001

.01

.1

1.486601

1.596556

1.608144

1.610734

1.622459

1.746189

−.1

−.01

−.001

.001

.01

.1

1.040415

1.092600

1.098009

1.099216

1.104669

1.161232

x 5x − 1 x We have ln 5 ≈ 1.6094. •

x 3x − 1 x We have ln 3 ≈ 1.0986. • We conjecture that lim x→0

bx − 1 = ln b for any positive number b. Here are two additional test cases. x −.1

−.01

−.001

.001

.01

.1

−.717735

−.695555

−.693387

−.692907

−.690750

−.669670

x 1 x 2

−1

x

We have ln 12 ≈ −0.69315. x

−.1

−.01

−.001

.001

.01

.1

7x − 1 x

1.768287

1.927100

1.944018

1.947805

1.964966

2.148140

We have ln 7 ≈ 1.9459. xn − 1 for (m, n) equal to + (2,nx) 1),1/x (1,. 2), (2, 3), and (3, 2). Then guess the value of the limit in 57. Investigate lim number Let n be x→1 any lim (1 x m − 1 and consider x→0 general and check your guess for at least three additional pairs. (a) Give numerical evidence that for n = 1, the limit is e [where e = exp(1) can be found on any scientiﬁc SOLUTION calculator]. •(b) Estimate the limit for n = 2, 3 and try to express the results in terms of e. Then form a conjecture for the value of the limit in general. Is it valid for noninteger n? x .99 .9999 1.0001 1.01 x −1 x2 − 1

.502513

.500025

.499975

.497512

x

.99

.9999

1.0001

1.01

x2 − 1 x −1

1.99

1.9999

2.0001

2.01

The limit as x → 1 is 12 .

The limit as x → 1 is 2. x

.99

.9999

1.0001

1.01

x2 − 1 x3 − 1

0.670011

0.666700

0.666633

0.663344

44

CHAPTER 2

LIMITS

The limit as x → 1 is 23 . x

.99

.9999

1.0001

1.01

x3 − 1 x2 − 1

1.492513

1.499925

1.500075

1.507512

The limit as x → 1 is 32 .

xn − 1 n • For general m and n, we have lim = . m x→1 x m − 1 • x

.99

.9999

1.0001

1.01

x −1 x3 − 1

.336689

.333367

.333300

.330022

x

.99

.9999

1.0001

1.01

x3 − 1 x −1

2.9701

2.9997

3.0003

3.0301

x

.99

.9999

1.0001

1.01

x3 − 1 x7 − 1

.437200

.428657

.428486

.420058

The limit as x → 1 is 13 .

The limit as x → 1 is 3.

The limit as x → 1 is 37 ≈ 0.428571. 2x − 8 2 Sketch a graph of f (x) = with a graphing calculator. Observe that f (3) is not deﬁned. x − 3 k such that lim sin(sin x) exists. Find by experimentation the positive integers k x→0 x (a) Zoom in on the graph to estimate L = lim f (x).

59.

x→3

(b) Observe that the graph of f (x) is increasing. Explain how this implies that f (2.99999) ≤ L ≤ f (3.00001) Use this to determine L to three decimal places. SOLUTION

(a)

y 5.565 5.555 5.545

y=

2x − 8 x−3

5.535 5.525 x=3

(b) It is clear that the graph of f rises as we move to the right. Mathematically, we may express this observation as: whenever u < v, f (u) < f (v). Because 2.99999 < 3 = lim f (x) < 3.00001, x→3

it follows that f (2.99999) < L = lim f (x) < f (3.00001). x→3

With f (2.99999) ≈ 5.54516 and f (3.00001) ≈ 5.545195, the above inequality becomes 5.54516 < L < 5.545195; hence, to three decimal places, L = 5.545. 21/x − 2−1/x The function f (x) = 1/x is deﬁned for x = 0. −1/x

S E C T I O N 2.3

Basic Limit Laws

45

sin x is equal to the slope of a secant line through the origin and the point (x, sin x) on x the graph of y = sin x (Figure 17). Use this to give a geometric interpretation of lim f (x). Show that f (x) =

61.

x→0

y (x, sin x)

1 sin x

x x

FIGURE 17 Graph of y = sin x.

sin x sin x − 0 = . Since x −0 x f (x) is the slope of the secant line, the limit lim f (x) is equal to the slope of the tangent line to y = sin x at x = 0.

SOLUTION

The slope of a secant line through the points (0, 0) and (x, sin x) is given by x→0

2.3 Basic Limit Laws Preliminary Questions 1. State the Sum Law and Quotient Law. SOLUTION

Suppose limx→c f (x) and limx→c g(x) both exist. The Sum Law states that lim ( f (x) + g(x)) = lim f (x) + lim g(x).

x→c

x→c

x→c

Provided limx→c g(x) = 0, the Quotient Law states that lim

f (x)

x→c g(x)

=

limx→c f (x) . limx→c g(x)

2. Which of the following is a verbal version of the Product Law? (a) The product of two functions has a limit. (b) The limit of the product is the product of the limits. (c) The product of a limit is a product of functions. (d) A limit produces a product of functions. SOLUTION

The verbal version of the Product Law is (b): The limit of the product is the product of the limits.

3. Which of the following statements are incorrect (k and c are constants)? (b) lim k = k (a) lim k = c x→c

x→c

(c) lim x = c2 x→c2

(d) lim x = x x→c

SOLUTION Statements (a) and (d) are incorrect. Because k is constant, statement (a) should read lim x→c k = k. Statement (d) should be limx→c x = c.

4. Which of the following statements are incorrect? (a) The Product Law does not hold if the limit of one of the functions is zero. (b) The Quotient Law does not hold if the limit of the denominator is zero. (c) The Quotient Law does not hold if the limit of the numerator is zero. SOLUTION Statements (a) and (c) are incorrect. The Product Law remains valid when the limit of one or both of the functions is zero, and the Quotient Law remains valid when the limit of the numerator is zero.

Exercises In Exercises 1–22, evaluate the limits using the Limit Laws and the following two facts, where c and k are constants: lim x = c,

x→c

1. lim x x→9

SOLUTION

lim x = 9.

x→9

lim x

x→−3

lim k = k

x→c

46

CHAPTER 2

LIMITS

3. lim 14 x→9

SOLUTION

5.

lim 14 = 14.

x→9

lim (3x + 4)

lim 14 x→−3 x→−3

SOLUTION

We apply the Laws for Sums, Products, and Constants: lim (3x + 4) = lim 3x + lim 4

x→−3

x→−3

x→−3

= 3 lim x + lim 4 = 3(−3) + 4 = −5. x→−3

7.

x→−3

lim (y + 14)

lim 14y y→−3 y→−3

SOLUTION

lim (y + 14) = lim y + lim 14 = −3 + 14 = 11.

y→−3

y→−3

y→−3

9. lim (3t − 14) t→4 lim y(y + 14) y→−3

SOLUTION

lim (3t − 14) = 3 lim t − lim 14 = 3 · 4 − 14 = −2.

t→4

t→4

t→4

11. lim (4x + 1)(2x − 1) x→ 21 lim (x 3 + 2x) x→−5

SOLUTION

lim (4x + 1)(2x − 1) = 4 lim x + lim 1 2 lim x − lim 1

x→1/2

x→1/2

x→1/2

x→1/2

x→1/2

1 1 = 4 +1 2 − 1 = 3 · 0 = 0. 2 2

13. lim x(x + 1)(x + 2) x→2 lim (3x 4 − 2x 3 + 4x) x→−1 SOLUTION We apply the Product Law and Sum Law: lim (x + 1) lim (x + 2) lim x(x + 1)(x + 2) = lim x x→2

x→2

=2

x→2

x→2

lim x + lim 1 lim x + lim 2

x→2

x→2

x→2

= 2(2 + 1)(2 + 2) = 24 t 15. lim lim (x + 1)(3x 2 − 9) t→9 t + 1 x→2

SOLUTION

lim t t 9 9 t→9 = = = . lim t + lim 1 9+1 10 t→9 t + 1 lim

t→9

t→9

1−x 17. lim 3t − 14 1+x x→3lim t→4 t + 1 lim 1 − lim x 1−3 −2 1 1−x x→3 x→3 SOLUTION lim = = = =− . lim 1 + lim x 1+3 4 2 x→3 1 + x x→3

x→3

19. lim t −1 x t→2 lim x→−1 x 3 + 4x SOLUTION

We apply the deﬁnition of t −1 , and then the Quotient Law. lim 1 1 1 t→2 lim t −1 = lim = = . lim t 2 t→2 t→2 t t→2

21. lim (x 2 +−29x −3 ) x→3lim x x→5

x→2

S E C T I O N 2.3 SOLUTION

Basic Limit Laws

47

We apply the Sum, Product, and Quotient Laws. The Product Law is applied to the exponentiations x 2 =

x · x and x 3 = x · x · x. lim (x 2 + 9x −3 ) = lim x 2 + lim 9x −3 =

x→3

x→3

⎛

= 9 + 9⎝

x→3

x→3

⎞

lim 1

x→3

( lim x)3

2 lim x

⎠=9+9

1 27

+9

=

1 x→3 x 3

lim

28 . 3

x→3

23. Use the Quotient Law to prove that if lim f (x) exists and is nonzero, then x→c z −1 + z lim z→1 z + 1 1 1 lim = x→c f (x) lim f (x) x→c

SOLUTION

Since lim f (x) is nonzero, we can apply the Quotient Law: x→c

lim

x→c

1 f (x)

lim 1

1 x→c = =

. lim f (x) lim f (x) x→c x→c

In Exercises 25–28, the=limit assuming Assume that evaluate lim f (x) 4 and compute:that lim f (x) = 3 and lim g(x) = 1. x→−4

x→6

25. (a) lim limf (x)g(x) f (x)2 x→−4

(b) lim

1

(c) lim x f (x)

x→6 f (x)

x→6

SOLUTION

x→−4

x→6

lim f (x)g(x) = lim f (x) lim g(x) = 3 · 1 = 3.

x→−4

x→−4

x→−4

g(x) 27. limlim 2(2 f (x) + 3g(x)) x→−4 x x→−4 SOLUTION

Since lim x 2 = 0, we may apply the Quotient Law, then applying the Product Law (from x 2 = x · x): x→−4

lim g(x) g(x) x→−4 = = x→−4 x 2 lim x 2

1

lim

x→−4

lim x

1 2 = 16 .

x→−4

sin x 29. Can the Quotient ? Explain. f (x) +Law 1 be applied to evaluate lim x→0 x lim x→−4 3g(x) − 9 sin x SOLUTION The limit Quotient Law cannot be applied to evaluate lim since lim x = 0. This violates a condition x→0 x x→0 of the Quotient Law. Accordingly, the rule cannot be employed. 31. Give an example where lim ( f (x) + g(x)) exists but neither lim f (x) nor lim g(x) exists. Show that the Productx→0 Law cannot be used to evaluate the x→0 lim (x − π /2) tan x. x→0 x→π /2

SOLUTION

Let f (x) = 1/x and g(x) = −1/x. Then lim ( f (x) + g(x)) = lim 0 = 0 However, lim f (x) = x→0

lim 1/x and lim g(x) = lim −1/x do not exist.

x→0

x→0

x→0

x→0

x→0

Use the Limit Laws and the result lim lim x n = cn for all whole numbers n. If you are a x −x→c 1 x = c to show that x→c exists and that lim a x = 1 for all a > 0. Prove that L ab = L a + L b Assume that the limit L a = lim familiar with induction, give a formalx→0 proof by x induction. x→0 for a, b > 0. Hint: (ab)x − 1 = a x (b x − 1) + (a x − 1). Verify numerically that L 12 = L 3 + L 4 . SOLUTION Correct answers can vary. An example is given: Let P[n] be the proposition : lim x n = cn , and proceed by induction.

33.

x→c

P[1] is true, as lim x = c. Suppose that P[n] is true, so that lim x n = cn . We must prove P[n + 1], that is, that x→c

x→c

lim x n+1 = cn+1 .

x→c

Applying the Product Law and P[n], we see: lim x n+1 = lim x n · x =

x→c

x→c

lim x n

x→c

lim x = cn c = cn+1 .

x→c

Therefore, P[n] true implies that P[n + 1] true. By induction, the limit is equal to cn for all n. Extend Exercise 33 to negative integers.

48

CHAPTER 2

LIMITS

Further Insights and Challenges 35. Show that if both lim f (x) g(x) and lim g(x) exist and lim g(x) is nonzero, then lim f (x) exists. Hint: Write x→c

x→c

x→c

f (x) = ( f (x) g(x))/g(x) and apply the Quotient Law. SOLUTION

x→c

Given that lim f (x)g(x) = L and lim g(x) = M = 0 both exist, observe that x→c

x→c

lim f (x)g(x) f (x)g(x) L x→c = = x→c g(x) lim g(x) M

lim f (x) = lim

x→c

x→c

also exists. h(t) = 5,=then lim h(t) = 15.exists and equals 4. 37. Prove thatthat if lim 12, then lim g(t) Show if lim tg(t) t→3t→3t t→3 t→3 SOLUTION

h(t) = 5, observe that lim t = 3. Now use the Product Law: t→3 t t→3 h(t) h(t) lim = lim t = 3 · 5 = 15. lim h(t) = lim t t t→3 t→3 t→3 t→3 t

Given that lim

f (x) 39. Prove that if lim f (x) = Lf (x) = 0 and lim g(x) = 0, then lim does not exist. x→cthat lim x→c g(x) Assuming = 1, x→c which of the following statements is necessarily true? Why? x→0 x f (x) (a) f (0) Suppose =0 (b) lim f (x) = 0 SOLUTION that lim exists. Then x→c g(x) x→0 L = lim f (x) = lim g(x) · x→c

x→c

f (x) f (x) f (x) = lim g(x) · lim = 0 · lim = 0. x→c x→c g(x) x→c g(x) g(x)

But, we were given that L = 0, so we have arrived at a contradiction. Thus, lim

f (x)

x→c g(x)

does not exist.

There is a Limit Law for composite functions but it is not stated in the text. Which of the following is the correct statement? Give an intuitive explanation. 2.4 (a)Limits (g(x))Continuity = lim f (x) lim f and x→c

x→c

(b) lim f (g(x)) = lim f (x), where L = lim g(x) Preliminary Questions x→c

x→c

x→L

f (g(x)) where lim f (x) (c) lim 1. Which property of = f (x)lim = g(x), x 3 allows us Lto= conclude that lim x 3 = 8? x→c

x→c

x→L

x→2

Use the correct version to evaluate SOLUTION We can conclude that lim x→2 x 3 = 8 because the function x 3 is continuous at x = 2. π lim sin(g(x)), where lim 1 ? g(x) = f (x) = 2. What can be said about f (3) if f is continuous and lim 6 x→2 x→2 2 x→3

SOLUTION

If f is continuous and limx→3 f (x) = 12 , then f (3) = 12 .

3. Suppose that f (x) < 0 if x is positive and f (x) > 1 if x is negative. Can f be continuous at x = 0? SOLUTION

Since f (x) < 0 when x is positive and f (x) > 1 when x is negative, it follows that lim f (x) ≤ 0

x→0+

and

lim f (x) ≥ 1.

x→0−

Thus, limx→0 f (x) does not exist, so f cannot be continuous at x = 0. 4. Is it possible to determine f (7) if f (x) = 3 for all x < 7 and f is right-continuous at x = 7? SOLUTION No. To determine f (7), we need to combine either knowledge of the values of f (x) for x < 7 with left-continuity or knowledge of the values of f (x) for x > 7 with right-continuity.

5. Are the following true or false? If false, state a correct version. (a) f (x) is continuous at x = a if the left- and right-hand limits of f (x) as x → a exist and are equal. (b) f (x) is continuous at x = a if the left- and right-hand limits of f (x) as x → a exist and equal f (a). (c) If the left- and right-hand limits of f (x) as x → a exist, then f has a removable discontinuity at x = a. (d) If f (x) and g(x) are continuous at x = a, then f (x) + g(x) is continuous at x = a. (e) If f (x) and g(x) are continuous at x = a, then f (x)/g(x) is continuous at x = a. SOLUTION

(a) False. The correct statement is “ f (x) is continuous at x = a if the left- and right-hand limits of f (x) as x → a exist and equal f (a).”

S E C T I O N 2.4

Limits and Continuity

49

(b) True. (c) False. The correct statement is “If the left- and right-hand limits of f (x) as x → a are equal but not equal to f (a), then f has a removable discontinuity at x = a.” (d) True. (e) False. The correct statement is “If f (x) and g(x) are continuous at x = a and g(a) = 0, then f (x)/g(x) is continuous at x = a.”

Exercises 1. Find the points of discontinuity of the function shown in Figure 14 and state whether it is left- or right-continuous (or neither) at these points. y 5 4 3 2 1 x 1

2

3

4

5

6

FIGURE 14 SOLUTION

• The function f is discontinuous at x = 1; it is left-continuous there. • The function f is discontinuous at x = 3; it is neither left-continuous nor right-continuous there. • The function f is discontinuous at x = 5; it is left-continuous there.

In Exercises 2–4, refer to the function f (x) in Figure 15. y 5 4 3 2 1 x 1

2

3

4

5

6

FIGURE 15

3. At which point c does f (x) have a removable discontinuity? What value should be assigned to f (c) to make f Find the points of discontinuity of f (x) and state whether f (x) is left- or right-continuous (or neither) at these continuous at x = c? points. SOLUTION Because limx→3 f (x) exists, the function f has a removable discontinuity at x = 3. Assigning f (3) = 4.5 makes f continuous at x = 3. 5. (a) For the function shown in Figure 16, determine the one-sided limits at the points of discontinuity. Find the point c1 at which f (x) has a jump discontinuity but is left-continuous. What value should be assigned (b) Which discontinuities is removable to f (c1of ) tothese make f right-continuous at x = and c1 ? how should f be redeﬁned to make it continuous at this point? y

6

x

−2

2

4

FIGURE 16 SOLUTION

(a) The function f is discontinuous at x = 0, at which lim f (x) = ∞ and lim f (x) = 2. The function f is also x→0−

discontinuous at x = 2, at which lim f (x) = 6 and lim f (x) = 6. x→2−

x→0+

x→2+

(b) The discontinuity at x = 2 is removable. Assigning f (2) = 6 makes f continuous at x = 2. In Exercises Laws of Continuity Let f7–14, (x) beuse thethe function (Figure 17): and Theorems 2–3 to show that the function is continuous. ⎧ 2 ⎪ ⎨x + 3

for x < 1

50

CHAPTER 2

LIMITS

7. f (x) = x + sin x Since x and sin x are continuous, so is x + sin x by Continuity Law (i).

SOLUTION

9. f (x) = 3x + 4 sin x f (x) = x sin x SOLUTION Since x and sin x are continuous, so are 3x and 4 sin x by Continuity Law (iii). Thus 3x + 4 sin x is continuous by Continuity Law (i). 1 11. f (x)f (x) = =2 3x 3 + 8x 2 − 20x x +1 SOLUTION

• Since x is continuous, so is x 2 by Continuity Law (ii). • Recall that constant functions, such as 1, are continuous. Thus x 2 + 1 is continuous. • Finally,

1 x2 + 1

is continuous by Continuity Law (iv) because x 2 + 1 is never 0.

3x 13. f (x) = x 2x− cos x f (x) 1=+ 4 3 + cos x SOLUTION The functions 3 x , 1 and 4 x are each continuous. Therefore, 1 + 4 x is continuous by Continuity Law (i). 3x Because 1 + 4x is never zero, it follows that is continuous by Continuity Law (iv). 1 + 4x In Exercises 15–32, determine the points at which the function is discontinuous and state the type of discontinuity: x f (x) = 10x cos removable, jump, inﬁnite, or none of these. 15. f (x) = SOLUTION

1 x The function 1/x is discontinuous at x = 0, at which there is an inﬁnite discontinuity.

x −2 17. f (x)f (x) = = |x| |x − 1| SOLUTION

The function

x −2 is discontinuous at x = 1, at which there is an inﬁnite discontinuity. |x − 1|

19. f (x) = [x] x −2 f (x) = SOLUTION This function has a jump discontinuity at x = n for every integer n. It is continuous at all other values of |x − 2| x. For every integer n, lim [x] = n

x→n+

since [x] = n for all x between n and n + 1. This shows that [x] is right-continuous at x = n. On the other hand, lim [x] = n − 1

x→n−

since [x] = n − 1 for all x between n − 1 and n. Thus [x] is not left-continuous. 1 21. g(t) = 2 1 f (x)t =− 1 x 2 1 1 = SOLUTION The function f (t) = is discontinuous at t = −1 and t = 1, at which there are 2 (t − 1)(t + 1) t −1 inﬁnite discontinuities. 23. f (x) = 3x 3/2 − 19x 3 x+ f (x) = SOLUTION The f (x) = 3x 3/2 − 9x 3 is continuous for x > 0. At x = 0 it is right-continuous. (It is not 4xfunction −2 deﬁned for x < 0.) 1 − 2z 25. h(z)g(t) = =2 3t −3/2 − 9t 3 z −z−6 1 − 2z 1 − 2z The function f (z) = 2 is discontinuous at z = −2 and z = 3, at which there = (z + 2)(z − 3) z −z−6 are inﬁnite discontinuities. SOLUTION

x 2 1−−3x2z+ 2 27. f (x) = h(z) = |x2 − 2| z +9

S E C T I O N 2.4

SOLUTION

Limits and Continuity

51

(x − 2)(x − 1) x 2 − 3x + 2 = . For x > 2, the function |x − 2| |x − 2| f (x) =

For x < 2, f (x) =

(x − 2)(x − 1) (x − 2)(x − 1) = = x − 1. |x − 2| (x − 2)

(x − 2)(x − 1) = −(x − 1). This function has a jump discontinuity at x = 2. 2−x

29. f (x) == csctan x 22t g(t)

1 is discontinuous whenever sin(x 2 ) = 0; i.e., whenever x 2 = n π 2) sin(x √ or x = ± n π , where n is a positive integer. At every such value of x there is an inﬁnite discontinuity. SOLUTION

The function f (x) = csc(x 2 ) =

31. f (x) = tan(sin x) 1 f (x) = cosfunction f (x) = tan(sin x) is continuous everywhere. Reason: sin x is continuous everywhere and SOLUTION The x tan u is continuous on − π2 , π2 —and in particular on −1 ≤ u = sin x ≤ 1. Continuity of tan(sin x) follows by the continuity of composite functions. In Exercises the domain of the function and prove that it is continuous on its domain using the Laws of f (x)33–46, = x − determine |x| Continuity and the facts quoted in this section. 33. f (x) = 9 − x 2 √ SOLUTION The domain of 9 − x 2 is all x such that 9 − x 2 ≥ 0, or |x| ≤ 3. Since x and the polynomial 9 − x 2 are both continuous on this domain, so is the composite function 9 − x 2 . √ sin x 35. f (x) = x √ √ f (x) = x 2 + 9 SOLUTION This function is deﬁned as long as x ≥ 0. Since x and sin x are continuous, so is x sin x by Continuity Law (ii). 37. f (x) = x 2/3 2x 2 x 2/3 x 2/3 f (x) = SOLUTION The domain x+ x 1/4 of x 2 is all real numbers as the denominator of the rational exponent is odd. Both x x 2/3 x and 2 are continuous on this domain, so x 2 is continuous by Continuity Law (ii). 39. f (x) = x −4/3 f (x) = x 1/3 + x 3/4 SOLUTION This function is deﬁned for all x = 0. Because the function x 4/3 is continuous and not equal to zero for x = 0, it follows that 1 x −4/3 = 4/3 x is continuous for x = 0 by Continuity Law (iv). 41. f (x) = tan2 x 3 f (x) = cos x SOLUTION The domain of tan2 x is all x = ±(2n − 1)π /2 where n is a positive integer. Because tan x is continuous on this domain, it follows from Continuity Law (ii) that tan2 x is also continuous on this domain. 43. f (x) = (x 4 + 1)3/2 f (x) = cos(2x ) SOLUTION The domain of (x 4 + 1)3/2 is all real numbers as x 4 + 1 > 0 for all x. Because x 3/2 and the polynomial x 4 + 1 are both continuous, so is the composite function (x 4 + 1)3/2 . cos(x 2 ) 45. f (x)f (x) = =2 3−x 2 x −1 The domain for this function is all x = ±1. Because the functions cos x and x 2 are continuous on this domain, so is the composite function cos(x 2 ). Finally, because the polynomial x 2 − 1 is continuous and not equal to zero cos(x 2 ) for x = ±1, the function 2 is continuous by Continuity Law (iv). x −1 SOLUTION

47. Suppose that tan f (x) = 2 for x > 0 and f (x) = −4 for x < 0. What is f (0) if f is left-continuous at x = 0? What is =9 x f (0) if ff (x) is right-continuous at x = 0? SOLUTION

Let f (x) = 2 for positive x and f (x) = −4 for negative x.

• If f is left-continuous at x = 0, then f (0) = limx→0− f (x) = −4. • If f is right-continuous at x = 0, then f (0) = limx→0+ f (x) = 2.

Sawtooth Function Draw the graph of f (x) = x − [x]. At which points is f discontinuous? Is it left- or right-continuous at those points?

52

CHAPTER 2

LIMITS

In Exercises 49–52, draw the graph of a function on [0, 5] with the given properties. 49. f (x) is not continuous at x = 1, but lim f (x) and lim f (x) exist and are equal. x→1+

x→1−

SOLUTION y 4 3 2 1 x 1

2

3

4

5

51. f (x) has a removable discontinuity at x = 1, a jump discontinuity at x = 2, and f (x) is left-continuous but not continuous at x = 2 and right-continuous but not continuous at x = 3. lim f (x) = 2 lim f (x) = −∞, x→3−

x→3+

SOLUTION y 4 3 2 1 x 1

2

3

4

5

53. Each of the following statements is false. For each statement, sketch the graph of a function that provides a counf (x) is right- but not left-continuous at x = 1, left- but not right-continuous at x = 2, and neither left- nor terexample. right-continuous at x = 3. (a) If lim f (x) exists, then f (x) is continuous at x = a. x→a

(b) If f (x) has a jump discontinuity at x = a, then f (a) is equal to either lim f (x) or lim f (x). x→a−

x→a+

(c) If f (x) has a discontinuity at x = a, then lim f (x) and lim f (x) exist but are not equal. x→a−

x→a+

(d) The one-sided limits lim f (x) and lim f (x) always exist, even if lim f (x) does not exist. x→a−

x→a

x→a+

Refer to the four ﬁgures shown below. (a) The ﬁgure at the top left shows a function for which lim f (x) exists, but the function is not continuous at x = a

SOLUTION

x→a

because the function is not deﬁned at x = a. (b) The ﬁgure at the top right shows a function that has a jump discontinuity at x = a but f (a) is not equal to either lim f (x) or lim f (x). x→a−

x→a−

(c) This statement can be false either when the two one-sided limits exist and are equal or when one or both of the one-sided limits does not exist. The ﬁgure at the top left shows a function that has a discontinuity at x = a with both one-sided limits being equal; the ﬁgure at the bottom left shows a function that has a discontinuity at x = a with a one-sided limit that does not exist. (d) The ﬁgure at the bottom left shows a function for which lim f (x) does not exist and one of the one-sided limits x→a

also does not exist; the ﬁgure at the bottom right shows a function for which lim f (x) does not exist and neither of the x→a

one-sided limits exists.

S E C T I O N 2.4

Limits and Continuity

53

y

y

a

x

y

a

x

y

a

x

a

x

In Exercises 55–70,toevaluate using the method. According the Lawstheoflimit Continuity, if substitution f (x) and g(x) are continuous at x = c, then f (x) + g(x) is continuous at x =2 c. Suppose that f (x) and g(x) are discontinuous at x = c. Is it true that f (x) + g(x) is discontinuous at 55. lim x xx→5 = c? If not, give a counterexample. lim x 2 = 52 = 25.

SOLUTION

57.

x→5

lim (2x 33 − 4) lim x x→−1 x→8

lim (2x 3 − 4) = 2(−1)3 − 4 = −6.

SOLUTION

x→−1

x +9 59. limlim (5x − 12x −2 ) x −9 x→0 x→2 0+9 x +9 = = −1. x − 9 0−9 x→0

x 61. lim sin x + − 2π x→πlim 2 x→3 x 2 + 2x x SOLUTION lim sin( − π ) = sin(− π ) = −1. lim

SOLUTION

x→π

2

2

63. lim tan(3x) x→ π4lim sin(x 2 ) x→π

lim tan(3x) = tan(3 · π4 ) = tan( 34π ) = −1

SOLUTION

x→ π4

65. lim x −5/21 x→4lim x→π cos x 1 . SOLUTION lim x −5/2 = 4−5/2 = 32 x→4 67.

− 8x 3 )3/2 lim (1 x 3 + 4x

x→−1 lim x→2

lim (1 − 8x 3 )3/2 = (1 − 8(−1)3 )3/2 = 27.

SOLUTION

x→−1

69. lim 10x −2x 7x + 2 2/3 x→3lim x→2 4 − x 2 2 SOLUTION lim 10x −2x = 103 −2(3) = 1000. 2

x→3

In Exercises 71–74, sketch the graph of the given function. At each point of discontinuity, state whether f is left- or lim 3sin x π right-continuous. x→ 2 for x ≤ 1 x2 71. f (x) = 2 − x for x > 1

54

CHAPTER 2

LIMITS SOLUTION y 1

x

−1

1

2

3

−1

The function f is continuous everywhere. |x⎧− 3| for x ≤ 3 73. f (x) = ⎨x + 1 for x < 1 for x > 3 f (x) =x − 13 ⎩ for x ≥ 1 SOLUTION x y 4

2

x 2

4

6

The function f is continuous everywhere. ⎧ ﬁnd the value of c that makes the function continuous. In Exercises 75–76, ⎪x 3 + 1 for −∞ < x ≤ 0 ⎨ f (x) =x 2 − −xc + 1for x < 5 for 0 < x < 2 ⎪ 75. f (x) = ⎩ 2 10xx − −x2c +for 4x + ≥15 5 for x ≥ 2 As x → 5−, we have x 2 − c → 25 − c = L. As x → 5+, we have 4x + 2c → 20 + 2c = R. Match the limits: L = R or 25 − c = 20 + 2c implies c = 53 . SOLUTION

77. Find all constants a, b such that the following function has no discontinuities: 2x + 9 for x ≤ 3 ⎧ π f (x) = ⎪ ⎪ −4x + c for x > 3 ⎨ax + cos x for x ≤ 4 f (x) = ⎪ π ⎪ ⎩bx + 2 for x > 4 As x → π4 −, we have ax + cos x → a4π + √1 = L. As x → π4 +, we have bx + 2 → b4π + 2 = R. 2 √ Match the limits: L = R or a4π + √1 = b4π + 2. This implies π4 (a − b) = 2 − √1 or a − b = 8−2π 2 . SOLUTION

2

2

In of 1993, the amount T (x) ofwould federalbeincome tax owed on an income of x dollars the 79. Which the following quantities represented by continuous functions of time was and determined which wouldbyhave formula one or more discontinuities? ⎧ (a) Velocity of an airplane during a ﬂight ⎪0.15x for 0 ≤ x < 21,450 ⎨ (b) Temperature in aTroom under ordinary conditions (x) = 3,217.50 + 0.28(x − 21,450) for 21,450 ≤ x < 51,900 ⎪ ⎩ interest paid yearly (c) Value of a bank account with 11,743.50 + 0.31(x − 51,900) for x ≥ 51,900 (d) The salary of a teacher Sketch graph of T (x)ofand if it has any discontinuities. Explain why, if T (x) had a jump discontinuity, it (e)the The population the determine world might be advantageous in some situations to earn less money. SOLUTION T (x), the amount of federal income tax owed on an income of x dollars in 1993, might be a discontinuous function depending upon how the tax tables are constructed (as determined by that year’s regulations). Here is a graph of T (x) for that particular year. y 15,000 10,000 5,000 x 20,000 40,000 60,000

S E C T I O N 2.5

Evaluating Limits Algebraically

55

If T (x) had a jump discontinuity (say at x = c), it might be advantageous to earn slightly less income than c (say c − ) and be taxed at a lower rate than to earn c or more and be taxed at a higher rate. Your net earnings may actually be more in the former case than in the latter one.

Further Insights and Challenges 81. Give an example of functions f (x) and g(x) such that f (g(x)) is continuous but g(x) has at least one discontinuity. If f (x) has a removable discontinuity at x = c, then it is possible to redeﬁne f (c) so that f (x) is continuous SOLUTION Answers vary. The simplest at x = c. Can f (c) may be redeﬁned in more thanexamples one way?are the functions f (g(x)) where f (x) = C is a constant function, and g(x) is deﬁned for all x. In these cases, f (g(x)) = C. For example, if f (x) = 3 and g(x) = [x], g is discontinuous at all integer values x = n, but f (g(x)) = 3 is continuous. 83. Let f (x) = 1 if x is rational and f (x) = −1 if x is irrational. Show that f (x) is discontinuous at all points, whereas Continuous at Only One Point Let f (x) = x for x rational, f (x) = −x for x irrational. Show that f (x)2 isFunction continuous at all points. f is continuous at x = 0 and discontinuous at all points x = 0. SOLUTION lim f (x) does not exist for any c. If c is irrational, then there is always a rational number r arbitrarily x→c

close to c so that | f (c) − f (r )| = 2. If, on the other hand, c is rational, there is always an irrational number z arbitrarily close to c so that | f (c) − f (z)| = 2. f (x)2 , on the other hand, is a constant function that always has value 1, which is obviously continuous.

2.5 Evaluating Limits Algebraically Preliminary Questions 1. Which of the following is indeterminate at x = 1? x2 + 1 , x −1

x2 − 1 , x +2

√

x2 − 1 x +3−2

,

√

x2 + 1 x +3−2

2 At x = 1, √x −1

is of the form 00 ; hence, this function is indeterminate. None of the remaining functions x+3−2 2 +1 2 and √x +1 are undeﬁned because the denominator is zero but the numerator is not, is indeterminate at x = 1: xx−1 x+3−2 2 −1 while xx+2 is equal to 0. SOLUTION

2. Give counterexamples to show that each of the following statements is false: (a) If f (c) is indeterminate, then the right- and left-hand limits as x → c are not equal. (b) If lim f (x) exists, then f (c) is not indeterminate. x→c

(c) If f (x) is undeﬁned at x = c, then f (x) has an indeterminate form at x = c. SOLUTION 2 −1 (a) Let f (x) = xx−1 . At x = 1, f is indeterminate of the form 00 but

x2 − 1 x2 − 1 = lim (x + 1) = 2 = lim (x + 1) = lim . x→1− x − 1 x→1− x→1+ x→1+ x − 1 lim

2 −1 (b) Again, let f (x) = xx−1 . Then

x2 − 1 = lim (x + 1) = 2 x→1 x − 1 x→1

lim f (x) = lim

x→1

but f (1) is indeterminate of the form 00 . (c) Let f (x) = 1x . Then f is undeﬁned at x = 0 but does not have an indeterminate form at x = 0. 3. Although the method for evaluating limits discussed in this section is sometimes called “simplify and plug in,” explain how it actually relies on the property of continuity. SOLUTION If f is continuous at x = c, then, by deﬁnition, lim x→c f (x) = f (c); in other words, the limit of a continuous function at x = c is the value of the function at x = c. The “simplify and plug-in” strategy is based on simplifying a function which is indeterminate to a continuous function. Once the simpliﬁcation has been made, the limit of the remaining continuous function is obtained by evaluation.

56

CHAPTER 2

LIMITS

Exercises In Exercises 1–4, show that the limit leads to an indeterminate form. Then carry out the two-step procedure: Transform the function algebraically and evaluate using continuity. x 2 − 25 x→5 x − 5

1. lim

2 −25 When we substitute x = 5 into xx−5 , we obtain the indeterminate form 00 . Upon factoring the numerator and simplifying, we ﬁnd

SOLUTION

x 2 − 25 (x − 5)(x + 5) = lim = lim (x + 5) = 10. x −5 x→5 x − 5 x→5 x→5 lim

3. lim

2t − 14

x +1

5t − 35 t→7 lim x→−1 x 2 + 2x + 1

0 When we substitute t = 7 into 2t−14 5t−35 , we obtain the indeterminate form 0 . Upon dividing out the common factor of t − 7 from both the numerator and denominator, we ﬁnd SOLUTION

lim

2t − 14

t→7 5t − 35

= lim

2(t − 7)

t→7 5(t − 7)

= lim

2

t→7 5

=

2 . 5

In Exercises 5–32, the limit or state that it does not exist. h +evaluate 3 lim 2 h→−3 x 2 −h64− 9 5. lim x→8 x − 8 SOLUTION

x 2 − 64 (x + 8)(x − 8) = lim = lim (x + 8) = 16. x −8 x→8 x − 8 x→8 x→8 lim

x 2 −23x + 2 7. lim x − 64 x→2lim x − 2 x→8 x − 9 x 2 − 3x + 2 (x − 1)(x − 2) = lim = lim (x − 1) = 1. SOLUTION lim x −2 x −2 x→2 x→2 x→2 x −2 9. lim 3 x 3 − 4x x→2lim x − 4x x→2 x − 2 1 x −2 x −2 1 SOLUTION lim 3 = lim = . = lim 8 x→2 x − 4x x→2 x(x − 2)(x + 2) x→2 x(x + 2) (1 +3h)3 − 1 11. lim x − 2x h h→0lim x→2 x − 2 SOLUTION

(1 + h)3 − 1 1 + 3h + 3h 2 + h 3 − 1 3h + 3h 2 + h 3 = lim = lim h h h h→0 h→0 h→0 lim

= lim 3 + 3h + h 2 = 3 + 3(0) + 02 = 3. h→0

3x 2 − 4x −2 4 13. lim (h + 2) − 9h x→2lim 2x 2 − 8 h−4 h→4 3x 2 − 4x − 4 (3x + 2)(x − 2) 3x + 2 8 = lim SOLUTION lim = lim = = 1. 8 x→2 x→2 2(x − 2)(x + 2) x→2 2(x + 2) 2x 2 − 8 (y − 2)3 15. lim lim3 2t + 4 y→2 y − 5y + 22 t→−2 12 − 3t (y − 2)3 (y − 2)3 (y − 2)2 = lim = lim = 0. SOLUTION lim 3 2 2 y→2 y − 5y + 2 y→2 (y − 2) y + 2y − 1 y→2 y + 2y − 1 1√ 1 x−− 4 3 + 3 lim 17. limx→16 hx − 16 h h→0

S E C T I O N 2.5

1

Evaluating Limits Algebraically

57

1

−3 1 3 − (3 + h) −1 1 lim 3+h = lim = lim =− . h 9 h→0 h→0 h 3(3 + h) h→0 3(3 + h)

SOLUTION

√

2+h−2 1 1 − h 2 4 (h + 2) √ lim hh+2−2 h→0 SOLUTION lim does not exist. h h→0 √ √ √ h + 2 − 2 ( h + 2 + 2) h+2−2 h−2 • As h → 0+, we have = √ = √ → −∞. h h( h + 2 + 2) h( h + 2 + 2) √ √ √ h + 2 − 2 ( h + 2 + 2) h+2−2 h−2 • As h → 0−, we have = √ → ∞. = √ h h( h + 2 + 2) h( h + 2 + 2)

19. lim

h→0

x −2 √ 21. lim √ x√− 6 − 2 x→2limx − 4 − x x − 10 x→10 SOLUTION

√ √ √ √ x −2 (x − 2)( x + 4 − x) (x − 2)( x + 4 − x) lim √ = lim √ = lim √ √ √ √ x − (4 − x) x→2 x − 4 − x x→2 ( x − 4 − x)( x + 4 − x) x→2 √ √ √ √ (x − 2)( x + 4 − x) (x − 2)( x + 4 − x) = lim = lim 2x − 4 2(x − 2) x→2 x→2 √ √ √ √ ( x + 4 − x) 2+ 2 √ = lim = = 2. 2 2 x→2

√ 2−1− √ x√ x +1 1 + x − 1−x 23. lim x −3 x→2lim x x→0 √ √ √ x2 − 1 − x + 1 3− 3 SOLUTION lim = = 0. x −3 −1 x→2 4 √1 25. lim √ 5 − x−− 1 x→4lim x − 2 √ x − 4 x→4 2 − x √ √ 1 x +2−4 x −2 4 1 √ = lim √ √ = . SOLUTION lim √ − = lim √ x −4 4 x −2 x→4 x→4 x→4 x −2 x +2 x −2 x +2 27. lim

cot x

1 x→0 csc x 1 lim √ − 2 x x→0+ cot x x + x cos x

SOLUTION

29. lim

lim

x→0 csc x

= lim

x→0 sin x

· sin x = cos 0 = 1.

sin x − cos x cot θ

x→ π4limπtan x − 1 θ → 2 csc θ

SOLUTION

√ sin x − cos x cos x π 2 (sin x − cos x) cos x · = lim = cos = . cos x sin x − cos x 4 2 x→ π4 tan x − 1 x→ π4 lim

2 x + 3 cos x− 2 2 cos 31. limlimπ sec θ − tan θ 2 cos x − 1 x→θπ3→ 2 SOLUTION

5 2 cos2 x + 3 cos x − 2 π (2 cos x − 1) (cos x + 2) = lim = lim cos x + 2 = cos + 2 = . π π π 2 cos x − 1 2 cos x − 1 3 2 x→ 3 x→ 3 x→ 3 lim

x −2 Use to estimate lim f (x) to two decimal places. Compare with the answer √ cosaθplot − 1of f (x) = √ lim x→2 x − 4−x θ →0 sin θ obtained algebraically in Exercise 21.

33.

√ Let f (x) = √ x−2 . From the plot of f (x) shown below, we estimate lim f (x) ≈ 1.41; to two decimal x− 4−x x→2 √ places, this matches the value of 2 obtained in Exercise 21. SOLUTION

58

CHAPTER 2

LIMITS y 1.414 1.410 1.406 x

1.402 1.6

1.8

2.0

2.2

2.4

In Exercises 35–40, use the identity 1a 3 − b3 =4 (a − b)(a 2 + ab + b2 ). to evaluate lim f (x) numerically. Compare with the answer obtained Use a plot of f (x) = √ − x −2 x −4 x→4 x3 − 1 in Exercise 25. 35. algebraically lim x→1 x − 1

2 +x +1

3 (x − 1) x x −1 SOLUTION lim = lim = lim x 2 + x + 1 = 3. x −1 x→1 x − 1 x→1 x→1 x 2 −33x + 2 37. lim x −8 x→1lim x 3 − 1 x→2 x 2 − 4 x 2 − 3x + 2 (x − 1)(x − 2) x −2 1 = lim SOLUTION lim = lim =− . 3 x→1 x→1 (x − 1) x 2 + x + 1 x→1 x 2 + x + 1 x3 − 1 x4 − 1 3 39. lim 3 x −8 x→1lim x −1 x→2 x 2 − 6x + 8 SOLUTION

4 x4 − 1 (x 2 − 1)(x 2 + 1) (x − 1)(x + 1)(x 2 + 1) (x + 1)(x 2 + 1) = lim = lim = lim = . 3 x→1 x 3 − 1 x→1 (x − 1)(x 2 + x + 1) x→1 (x − 1)(x 2 + x + 1) x→1 (x 2 + x + 1) lim

In Exercises 41–50, evaluate the limits in terms of the constants involved. x − 27 lim 1/3 41. limx→27 (2a +x x) − 3 x→0

lim (2a + x) = 2a.

SOLUTION

x→0

43. lim (4t − 2at + 3a) t→−1lim (4ah + 7a) h→−2

SOLUTION

lim (4t − 2at + 3a) = −4 + 5a.

t→−1

2(x + h)2 −22x 2 2 (3a + h) − 9a h x→0lim h h→0 2(x + h)2 − 2x 2 4hx + 2h 2 = lim = lim (4x + 2h) = 2h. SOLUTION lim h h x→0 x→0 x→0 √ √ x− a 2 47. lim (x + a) − 4x 2 x→alim x − a x→a √ √ x√ −a √ 1 x− a x− a 1 SOLUTION lim = lim √ √ = √ . √ √ √ = lim √ x→a x − a x→a x→a x + a 2 a x− a x+ a 45. lim

√a)3 − a 3 √ (x + 49. lim a + 2h − a x x→0lim h h→0 (x + a)3 − a 3 x 3 + 3x 2 a + 3xa 2 + a 3 − a 3 SOLUTION lim = lim = lim (x 2 + 3xa + 3a 2 ) = 3a 2 . x x x→0 x→0 x→0 1 1 Further Insights and Challenges −

h a lim 51–52, In Exercises ﬁnd all values of c such that the limit exists. h→a h − a x 2 − 5x − 6 x→c x −c

51. lim

x 2 − 5x − 6 will exist provided that x − c is a factor of the numerator. (Otherwise there will be an x→c x −c inﬁnite discontinuity at x = c.) Since x 2 − 5x − 6 = (x + 1)(x − 6), this occurs for c = −1 and c = 6. SOLUTION

lim

lim

x 2 + 3x + c

S E C T I O N 2.6

Trigonometric Limits

59

53. For which sign ± does the following limit exist?

1 1 lim ± x(x − 1) x→0 x

SOLUTION

(x − 1) + 1 1 1 1 + = lim = lim = −1. x x(x − 1) x(x − 1) x − 1 x→0 x→0 1 1 • The limit lim − does not exist. x(x − 1) x→0 x

• The limit lim x→0

1 1 (x − 1) − 1 x −2 − = = → ∞. x x(x − 1) x(x − 1) x(x − 1) 1 1 (x − 1) − 1 x −2 – As x → 0−, we have − = = → −∞. x x(x − 1) x(x − 1) x(x − 1)

– As x → 0+, we have

For which sign ± does the following limit exist?

2.6 Trigonometric Limits

1 1 ± |x| x→0+ x

lim

Preliminary Questions 1. Assume that −x 4 ≤ f (x) ≤ x 2 . What is lim f (x)? Is there enough information to evaluate lim f (x)? Explain. x→ 12

x→0

Since limx→0 −x 4 = limx→0 x 2 = 0, the squeeze theorem guarantees that limx→0 f (x) = 0. Since 1 = 1 = lim 4 limx→ 1 −x = − 16 x 2 , we do not have enough information to determine limx→ 1 f (x). 4 x→ 21 2 2 SOLUTION

2. State the Squeeze Theorem carefully. SOLUTION

Assume that for x = c (in some open interval containing c), l(x) ≤ f (x) ≤ u(x)

and that lim l(x) = lim u(x) = L. Then lim f (x) exists and x→c

x→c

x→c

lim f (x) = L .

x→c

3. Suppose that f (x) is squeezed at x = c on an open interval I by two constant functions u(x) and l(x). Is f (x) necessarily constant on I ? SOLUTION

Yes. Because f (x) is squeezed at x = c on an open interval I by the two functions u(x) and l(x), we know

that lim u(x) = lim l(x).

x→c

x→c

Combining this relation with the fact that u(x) and l(x) are constant functions, it follows that u(x) = l(x) for all x on I . Finally, to have l(x) ≤ f (x) ≤ u(x) = l(x), it must be the case that f (x) = l(x) for all x on I . In other words, f (x) is constant on I . sin 5h , it is a good idea to rewrite the limit in terms of the variable (choose one): h→0 3h 5h (b) θ = 3h (c) θ = 3

4. If you want to evaluate lim (a) θ = 5h SOLUTION

To match the given limit to the pattern of sin θ , θ →0 θ lim

it is best to substitute for the argument of the sine function; thus, rewrite the limit in terms of (a): θ = 5h.

60

CHAPTER 2

LIMITS

Exercises 1. In Figure 6, is f (x) squeezed by u(x) and l(x) at x = 3? At x = 2? y

u(x) f(x) l(x)

1.5

x 1

2

3

4

FIGURE 6 SOLUTION

Because there is an open interval containing x = 3 on which l(x) ≤ f (x) ≤ u(x) and lim l(x) = lim u(x), x→3

x→3

f (x) is squeezed by u(x) and l(x) at x = 3. Because there is an open interval containing x = 2 on which l(x) ≤ f (x) ≤ u(x) but lim l(x) = lim u(x), f (x) is trapped by u(x) and l(x) at x = 2 but not squeezed. x→2

x→2

3. What information about f (x) does the Squeeze Theorem provide if we assume that the graphs of f (x), u(x), and What is lim f (x) if f (x) satisﬁes cos x ≤ f (x) ≤ 1 for x in (−1, 1)? l(x) are related as in Figure 7 and that lim u(x) = lim l(x) = 6? Note that the inequality f (x) ≤ u(x) is not satisﬁed x→0 x→7

x→7

for all x. Does this affect the validity of your conclusion? y f(x)

6 u(x) l(x) x 7

FIGURE 7 SOLUTION The Squeeze Theorem does not require that the inequalities l(x) ≤ f (x) ≤ u(x) hold for all x, only that the inequalities hold on some open interval containing x = c. In Figure 7, it is clear that l(x) ≤ f (x) ≤ u(x) on some open interval containing x = 7. Because lim u(x) = lim l(x) = 6, the Squeeze Theorem guarantees that lim f (x) = 6.

x→7

x→7

x→7

In Exercises use the Theorem tosufﬁcient evaluate the limit. to determine lim State 5–8, whether the Squeeze inequality provides information

x→1 f (x) and, if so, ﬁnd the limit. (a) 4x − 51≤ f (x) ≤ x 2 5. lim x cos 2 (b) x→02x − 1x≤ f (x) ≤ x 2+2 (c) 2x + We 1 ≤prove f (x) the ≤ xlimit SOLUTION to the right ﬁrst, and then the limit to the left. Suppose x > 0. Since −1 ≤ cos 1x ≤ 1, multiplication by x (positive) yields −x ≤ x cos 1x ≤ x. The squeeze theorem implies that lim −x ≤ lim x cos 1x ≤ x→0+

x→0+

lim x, so 0 ≤ lim x cos 1x ≤ 0. This gives us lim x cos 1x = 0. On the other hand, suppose x < 0. −1 ≤ cos 1x ≤ 1

x→0+

x→0+

x→0+

multiplied by x gives us x ≤ x cos 1x ≤ −x, and we can apply the squeeze theorem again to get lim x cos 1x = 0. x→0−

Therefore, lim x cos 1x = 0. x→0

1 1 x 7. lim cos 2 sin x→0lim x xsin x x→0

1 ≤ 1, observe that − sin x ≤ cos 1x sin x ≤ sin x. Since lim sin x = 0 = SOLUTION Since −1 ≤ cos x x→0 lim − sin x, the Squeeze Theorem shows that x→0

lim cos

x→0

1 sin x = 0. x

In Exercises 9–16, evaluate π the limit using Theorem 2 as necessary. lim (x − 1) sin x −1 x→1 sin x cos x 9. lim x x→0 sin x cos x sin x SOLUTION lim = lim · lim cos x = 1 · 1 = 1. x x→0 x→0 x x→0 sin2√t 11. lim t + 2 sin t t→0lim t t t→0

S E C T I O N 2.6

SOLUTION

sin2 t sin t sin t = lim sin t = lim · lim sin t = 1 · 0 = 0. t→0 t t→0 t t→0 t t→0 lim

x2 13. limlim 2tan x x→0 sin xx x→0 SOLUTION

x2

lim

x→0 sin2 x

1 1 1 1 1 = lim sin x sin x = lim sin x · lim sin x = · = 1. 1 1 x→0 x→0 x→0 x x x x

sin t 15. lim 1 − cos t πlimt t→ 4 π t t→ 2 sin t π SOLUTION is continuous at t = . Hence, by substitution t 4 √ √ 2 2 2 sin t 2 . = π = lim π t→ π4 t 4

sin 10x 17. Let L =cos limt − cos2 t . x x→0 lim t t→0 sin θ (a) Show, by letting θ = 10x, that L = lim 10 . θ θ →0 (b) Compute L. SOLUTION

θ . θ → 0 as x → 0, so we get: Since θ = 10x, x = 10

sin θ sin θ sin θ sin 10x = lim · = lim 10 = 10 lim · = 10. x θ x→0 θ →0 (θ /10) θ →0 θ →0 θ lim

In Exercises 19–40, evaluate sin 2h 2 sin 2h 5h sin 2h the limit. . Hint: = . Evaluate lim sin 6h h→0 sin 5h sin 5h 5 2h sin 5h 19. lim h→0 h SOLUTION

lim

h→0

sin 6h sin 6h = lim 6 = 6. h 6h h→0

sin 6h 21. lim 6 sin h h→0lim6h h→0 6h sin 6h sin x SOLUTION Substitute x = 6h. As h → 0, x → 0, so: lim = lim = 1. h→0 6h x→0 x sin 7x 23. lim sin h x→0lim3x h→0 6h sin 7x 7 sin 7x 7 = lim = . SOLUTION lim 3 x→0 3x x→0 3 7x 25. lim

tan 4x

x

x→0lim9x x→ π4 sin 11x

SOLUTION

27. lim

lim

x→0

tan 4t

4 4 tan 4x 1 sin 4x = lim · · = . 9x 4x cos 4x 9 x→0 9

x

t sec t t→0lim x→0 csc 25x

SOLUTION

tan 4t 4 sin 4t 4 cos t sin 4t = lim = lim · = 4. 4t t→0 t sec t t→0 4t cos(4t) sec(t) t→0 cos 4t lim

sin(z/3) sin 2h sin 3h z→0limsin z h→0 h2 sin(z/3) z/3 1 z sin(z/3) 1 SOLUTION lim · = lim · · = . z/3 z→0 3 sin z z/3 3 z→0 sin z

29. lim

31. lim

tan 4x sin(−3θ )

tan 9x x→0lim θ →0 sin(4θ )

SOLUTION

lim

lim

tan 4x

x→0 tan 9x

csc 8t

t→0 csc 4t

= lim

cos 9x

x→0 cos 4x

·

sin 4x 4 9x 4 · · = . 4x 9 sin 9x 9

Trigonometric Limits

61

62

CHAPTER 2

LIMITS

33. lim

sin 5x sin 2x

x→0 sin 3x sin 5x

SOLUTION

lim

sin 5x sin 2x

x→0 sin 3x sin 5x

= lim

x→0

sin 2x 2 3x 2 · · = . 2x 3 sin 3x 3

1 − cos 2h 35. lim sin 3x sin 2x h→0lim h x→0 x sin 5x 1 − cos 2h 1 − cos 2h 1 − cos 2h SOLUTION lim = lim 2 = 2 lim = 2 · 0 = 0. h 2h 2h h→0 h→0 h→0 1 − cos t sin(2h)(1 − cos h) t→0limsin t h h→0 t SOLUTION A single multiplication by t turns this limit into the quotient of two familiar limits.

37. lim

lim

t→0

1 − cos t t (1 − cos t) t (1 − cos t) t (1 − cos t) = lim = lim = lim lim = 1 · 0 = 0. sin t t sin t t t t→0 t→0 sin t t→0 sin t t→0

1 − cos 3h 39. lim cos 2θ − cos θ h h→ π2lim θ θ →0 π SOLUTION The function is continuous at 2 , so we may use substitution: 1 − cos 3 π2 1−0 2 1 − cos 3h = π = . = π π h π h→ 2 2 2 lim

sin x sin x 41. Calculate1 (a) lim and (b) lim . − cos 4θ lim x→0+ |x| x→0− |x| θ →0 sin 3θ SOLUTION

sin x = 1. x sin x sin x (b) lim = −1. = lim x→0− |x| x→0− −x (a)

lim

sin x

x→0+ |x|

= lim

x→0+

1 ≤ |tan x|. Then evaluate lim tan x cos 1 using the Squeeze Theorem. 43. Show thatthe −|tan x| ≤ tan x cosstated x x Prove following result in Theorem 2: x→0

1 1 − cos θ1 SOLUTION Since | cos x | ≤ 1, it follows that | tan lim x cos x =| 0≤ | tan x|, which is equivalent to −| tan x| ≤

θ θ →0 tan x cos 1x ≤ | tan x|. Because limx→0 −| tan x| = limx→0 | tan x| = 0, the Squeeze Theorem gives 1 − cos θ 1 1 − cos2 θ Hint: = · . 1 θ 1 + cos θ θ = 0. lim tan x cos x x→0

45. Plot the graphs of u(x) = 1 + |x − π2 | and l(x) = sin x on the same set of axes. What can you say about 1 using the Squeeze π tan x cos by sinl(x) Evaluate lim f (x) if f (x)lim is squeezed x and u(x) at x = 2 ?Theorem. x→ π2

x→0

SOLUTION y u(x) = 1 + | x −

1

/2 |

l(x) = sin x x /2

lim u(x) = 1 and lim l(x) = 1, so any function f (x) satisfying l(x) ≤ f (x) ≤ u(x) for all x near π /2 will satisfy

x→π /2

lim

x→π /2

f (x) = 1.

x→π /2

Use the Squeeze Theorem to prove that if lim | f (x)| = 0, then lim f (x) = 0. x→c

x→c

S E C T I O N 2.6

Trigonometric Limits

63

Further Insights and Challenges 1 − cos h numerically (and graphically if you have a graphing utility). Then prove that the h2 limit is equal to 12 . Hint: see the hint for Exercise 42.

47.

Investigate lim

h→0

SOLUTION

•

h 1 − cos h h2

−.1

−.01

.01

.1

.499583

.499996

.499996

.499583

The limit is 12 . •

y 0.5 0.4 0.3 0.2 0.1 −2

•

−1

x 1

2

1 1 − cos h 1 − cos2 h 1 sin h 2 = . = lim = lim h 1 + cos h 2 h→0 h→0 h 2 (1 + cos h) h→0 h2 lim

cos 3h − 1 49. Evaluate lim cos 3h −. 1 cos 2h − 1 . Hint: Use the result of Exercise 47. Evaluate h→0lim h→0 h2 SOLUTION

cos 3h − 1 cos 3h − 1 cos 3h + 1 cos 2h + 1 cos2 3h − 1 cos 2h + 1 · = lim · · = lim h→0 cos 2h − 1 h→0 cos 2h − 1 cos 3h + 1 cos 2h + 1 h→0 cos2 2h − 1 cos 3h + 1 lim

= lim

− sin2 3h

h→0 − sin2 2h

=

·

9 2h 2h cos 2h + 1 sin 3h sin 3h cos 2h + 1 = lim · · · cos 3h + 1 4 h→0 3h 3h sin 2h sin 2h cos 3h + 1

2 9 9 ·1·1·1·1· = . 4 2 4

51. Using a diagram of the unit circle and the Pythagorean Theorem, show that Use the result of Exercise 47 to prove that for m = 0, sin2 θ ≤ (1 − cos θ )2 + sin2 θ ≤ θ2 m2 cos mx − 1 lim =− Conclude that sin2 θ ≤ 2(1 − cos θ ) ≤ θ 2 and use this to give 2 proof of Eq. (7) in Exercise 42. Then give an x→0 x 2 an alternate alternate proof of the result in Exercise 47. SOLUTION

• Consider the unit circle shown below. The triangle B D A is a right triangle. It has base 1 − cos θ , altitude sin θ , and

hypotenuse h. Observe that the hypotenuse h is less than the arc length AB = radius · angle = 1 · θ = θ . Apply the Pythagorean Theorem to obtain (1 − cos θ )2 + sin2 θ = h 2 ≤ θ 2 . The inequality sin2 θ ≤ (1 − cos θ )2 + sin2 θ follows from the fact that (1 − cos θ )2 ≥ 0.

B A O

D

• Note that

(1 − cos θ )2 + sin2 θ = 1 − 2 cos θ + cos2 θ + sin2 θ = 2 − 2 cos θ = 2(1 − cos θ ).

64

CHAPTER 2

LIMITS

Therefore, sin2 θ ≤ 2(1 − cos θ ) ≤ θ 2 . • Divide the previous inequality by 2θ to obtain

1 − cos θ sin2 θ θ ≤ ≤ . 2θ θ 2 Because 1 1 sin2 θ sin θ = lim · lim sin θ = (1)(0) = 0, 2 θ →0 θ 2 θ →0 2θ θ →0 lim

and lim

θ

h→0 2

= 0, it follows by the Squeeze Theorem that lim

θ →0

1 − cos θ = 0. θ

• Divide the inequality

sin2 θ ≤ 2(1 − cos θ ) ≤ θ 2 by 2θ 2 to obtain sin2 θ 1 − cos θ 1 ≤ ≤ . 2 2θ 2 θ2 Because sin θ 2 sin2 θ 1 1 1 = = (12 ) = , lim 2 2 θ →0 θ 2 2 θ →0 2θ lim

and lim

1

h→0 2

=

1 , it follows by the Squeeze Theorem that 2 lim

θ →0

1 1 − cos θ = . 2 θ2

Let A(n) be sin the xarea of a regular n-gon inscribed in a unit circle (Figure 8). − sin c numerically for the ﬁve values c = 0, π , π , π , π . (a) Investigate lim1 6 4 3 2 x −2cπ . (a) Prove that A(n) x→c = n sin n for general c? (We’ll see why in Chapter 3.) (b) Can you guess 2the answer (b) Intuitively, might we expect A(n) to converge the values area ofof thec.unit circle as n → ∞? (c) Check why if your answer to part (b) works for two to other (c) Use Theorem 2 to evaluate lim A(n). 53.

n→∞

y

x 1

FIGURE 8 Regular n-gon inscribed in a unit circle. SOLUTION

(a) A(n) = n · a(n), where a(n) is the area of one section of the n-gon as shown. The sections into which the n-gon is divided are identical. By looking at the section lying just atop the x-axis, we can see that each section has a base length of 1 and a height of sin θ , where θ is the interior angle of the triangle.

θ=

2π , n

so a(n) =

2π 1 sin . 2 n

S E C T I O N 2.7

This means

Intermediate Value Theorem

65

2π 1 2π 1 = n sin . A(n) = n sin 2 n 2 n

(b) As n increases, the difference between the n-gon and the unit circle shrinks toward zero; hence, as n increases, A(n) approaches the area of the unit circle. (c) Substitute x = n1 . n = 1x , so A(n) = 12 1x sin(2π x). x → 0 as n → ∞, so lim A(n) = lim

1 1 sin(2π x) = (2π ) = π . x 2

x→0 2

n→∞

2.7 Intermediate Value Theorem Preliminary Questions 1. Explain why f (x) = x 2 takes on the value 0.5 in the interval [0, 1]. Observe that f (x) = x 2 is continuous on [0, 1] with f (0) = 0 and f (1) = 1. Because f (0) < 0.5 < f (1), the Intermediate Value Theorem guarantees there is a c ∈ [0, 1] such that f (c) = 0.5. SOLUTION

2. The temperature in Vancouver was 46◦ at 6 AM and rose to 68◦ at noon. What must we assume about temperature to conclude that the temperature was 60◦ at some moment of time between 6 AM and noon? SOLUTION

We must assume that temperature is a continuous function of time.

3. What is the graphical interpretation of the IVT? SOLUTION If f is continuous on [a, b], then the horizontal line y = k for every k between f (a) and f (b) intersects the graph of y = f (x) at least once.

4. Show that the following statement is false by drawing a graph that provides a counterexample: If f (x) is continuous and has a root in [a, b], then f (a) and f (b) have opposite signs. SOLUTION

f (a) f (b) a

b

Exercises 1. Use the IVT to show that f (x) = x 3 + x takes on the value 9 for some x in [1, 2]. SOLUTION Observe that f (1) = 2 and f (2) = 10. Since f is a polynomial, it is continuous everywhere; in particular on [1, 2]. Therefore, by the IVT there is a c ∈ [1, 2] such that f (c) = 9. 3. Show that g(t) = t 2 tan tt takes on the value 12 for some t in 0, π4 . Show that g(t) = takes on the value 0.499 for some t in [0, 1]. t + 1π π2 π 1 π2 SOLUTION g(0) = 0 and g( 4 ) = 16 . g(t) is continuous for all t between 0 and 4 , and 0 < 2 < 16 ; therefore, by the IVT, there is a c ∈ [0, π4 ] such that g(c) = 12 .

5. Show that cos x = x has a solution in the interval [0, 1]. Hint: Show that f (x) = x − cos x has a zero in [0, 1]. x2 on the value Show that SOLUTION Let f (x) = = x7 − costakes x. Observe that 0.4. f is continuous with f (0) = −1 and f (1) = 1 − cos 1 ≈ .46. x +1 Therefore, by the IVT there is a c ∈ [0, 1] such that f (c) = c − cos c = 0. Thus c = cos c and hence the equation cos x = x has a solution c in [0, 1]. In Exercises 7–14, use the IVT to prove each of the following statements. 1 3 √ Use√the IVT to ﬁnd an interval of length 2 containing a root of f (x) = x + 2x + 1. 7. c + c + 1 = 2 for some number c.

66

CHAPTER 2

LIMITS

√ √ Let f (x) = x + x + 1 − 2. Note that f is continuous on 14 , 1 with f ( 14 ) = 14 + 54 − 2 ≈ −.38 √ √ √ and f (1) = 2 − 1 ≈ .41. Therefore, by the IVT there is a c ∈ 14 , 1 such that f (c) = c + c + 1 − 2 = 0. Thus √ √ √ √ c + c + 1 = 2 and hence the equation x + x + 1 = 2 has a solution c in 14 , 1 . √ 9. 2For exists. Hint: Consider = xx 2for . some x ∈ [0, π ]. all integers n, sin nxf (x) = cos 2 SOLUTION Let f (x) = x . Observe that f is continuous with f (1) = 1 and f (2) = 4. Therefore, by the IVT there is √ a c ∈ [1, 2] such that f (c) = c2 = 2. This proves the existence of 2, a number whose square is 2. SOLUTION

11. ForAallpositive positivenumber integersck,has there x such cos x =integers xk. an exists nth root for that all positive n. (This fact is usually taken for granted, but it k SOLUTION For each positive integer k, let f (x) = x − cos x. Observe that f is continuous on 0, π2 with f (0) = −1 requires proof.) k and f ( π2 ) = π2 > 0. Therefore, by the IVT there is a c ∈ 0, π2 such that f (c) = ck − cos(c) = 0. Thus cos c = ck and hence the equation cos x = x k has a solution c in the interval 0, π2 . 13. 2x =x b has a solution for all b > 0 (treat b ≥ 1 ﬁrst). 2 = bx has a solution if b > 2. SOLUTION Let b ≥ 1. Let f (x) = 2 x . f is continuous for all x. f (0) = 1, and f (x) > b for some x > 0 (since f (x) increases without bound). Hence, by the IVT, f (x) = b for some x > 0. On the other hand, suppose b < 1. Since limx→−∞ 2x = 0, there is some x < 0 such that f (x) = 2x < b. f (0) = 1 > b, so the IVT states that f (x) = b for some x < 0. 15. (a) (b) (c)

Carry of themany Bisection Method for f (x) = 2x − x 3 as follows: tanout x =three x hassteps inﬁnitely solutions. Show that f (x) has a zero in [1, 1.5]. Show that f (x) has a zero in [1.25, 1.5]. Determine whether [1.25, 1.375] or [1.375, 1.5] contains a zero.

SOLUTION

Note that f (x) is continuous for all x.

(a) f (1) = 1, f (1.5) = 21.5 − (1.5)3 < 3 − 3.375 < 0. Hence, f (x) = 0 for some x between 0 and 1.5. (b) f (1.25) ≈ 0.4253 > 0 and f (1.5) < 0. Hence, f (x) = 0 for some x between 1.25 and 1.5. (c) f (1.375) ≈ −0.0059. Hence, f (x) = 0 for some x between 1.25 and 1.375. 17. Find an interval of length 14 in [0,31] containing a root of x 5 − 5x + 1 = 0. Figure 4 shows that f (x) = x − 8x − 1 has a root in the interval [2.75, 3]. Apply the Bisection Method twice SOLUTION f (x)of=length x 5 −15xcontaining + 1. Observe that f is continuous with f (0) = 1 and f (1) = −3. Therefore, this root. to ﬁnd an Let interval 16 by the IVT there is a c ∈ [0, 1] such that f (c) = 0. f (.5) = −1.46875 < 0, so f (c) = 0 for some c ∈ [0, .5]. f (.25) = −.0.249023 < 0, and so f (c) = 0 for some c ∈ [0, .25]. This means that [0, .25] is an interval of length 0.25 containing a root of f (x). In Exercises 19–22, draw the graph of a function f (x) on [0, 4] with the given property. Show that tan3 θ − 8 tan2 θ + 17 tan θ − 8 = 0 has a root in [0.5, 0.6]. Apply the Bisection Method twice to ﬁnd interval of lengthat0.025 root. the conclusion of the IVT. 19. an Jump discontinuity x = 2containing and does this not satisfy SOLUTION The function graphed below has a jump discontinuity at x = 2. Note that while f (0) = 2 and f (4) = 4, there is no point c in the interval [0, 4] such that f (c) = 3. Accordingly, the conclusion of the IVT is not satisﬁed. y 4 3 2 1 x 1

2

3

4

21. Inﬁnite one-sided limits at x = 2 and does not satisfy the conclusion of the IVT. Jump discontinuity at x = 2, yet does satisfy the conclusion of the IVT on [0, 4]. SOLUTION The function graphed below has inﬁnite one-sided limits at x = 2. Note that while f (0) = 2 and f (4) = 4, there is no point c in the interval [0, 4] such that f (c) = 3. Accordingly, the conclusion of the IVT is not satisﬁed. y 6 5 4 3 2 1 x −1

1

2

3

4

S E C T I O N 2.7

Intermediate Value Theorem

67

Corollary 2 islimits not foolproof. Letdoes f (x) = x 2the−conclusion 1 and explain why 23. Inﬁnite one-sided at x = 2, yet satisfy of the IVTtheoncorollary [0, 4]. fails to detect the roots x = ±1 if [a, b] contains [−1, 1]. If [a, b] contains [−1, 1], f (a) > 0 and f (b) > 0. If we were using the corollary, we would guess that f (x) has no roots in [a, b]. However, we can easily see that f (1) = 0 and f (−1) = 0, so that any interval [a, b] containing [−1, 1] contains two roots. SOLUTION

Further Insights and Challenges f (x) is continuous and that ≤ f (x) ≤ 1 on forit0 anywhere. ≤ x ≤ 1 (see Figure Show that f (c) =c 25. Take Assume any mapthat (e.g., of the United States) and0draw a circle Prove that 5). at any moment in time for some in [0,a1]. therecexists pair of diametrically opposite points on that circle corresponding to locations where the temperatures at that moment are equal. Hint: Let θ be an angular coordinate along the circle and let f (θ ) be the difference in y temperatures at the locations corresponding to θ and θ + π . y = x 1

y = f (x)

x c

1

FIGURE 5 A function satisfying 0 ≤ f (x) ≤ 1 for 0 ≤ x ≤ 1. SOLUTION If f (0) = 0, the proof is done with c = 0. We may assume that f (0) > 0. Let g(x) = f (x) − x. g(0) = f (0) − 0 = f (0) > 0. Since f (x) is continuous, the Rule of Differences dictates that g(x) is continuous. We need to prove that g(c) = 0 for some c ∈ [0, 1]. Since f (1) ≤ 1, g(1) = f (1) − 1 ≤ 0. If g(1) = 0, the proof is done with c = 1, so let’s assume that g(1) < 0. We now have a continuous function g(x) on the interval [0, 1] such that g(0) > 0 and g(1) < 0. From the IVT, there must be some c ∈ [0, 1] so that g(c) = 0, so f (c) − c = 0 and so f (c) = c. This is a simple case of a very general, useful, and beautiful theorem called the Brouwer ﬁxed point theorem.

27. Figure 6(B) shows a slice of ham a piece of bread. Prove thatthat it is to θ slice thisθ open-faced Ham Sandwich Theorem Figure 6(A)onshows a slice of ham. Prove forpossible any angle (0 ≤ ≤ π ), it is θ ≤ π a line sandwich so that each part has equal amounts of ham and bread. Hint: By Exercise 26, for all 0 ≤ givenisby the possible to cut the slice of ham in half with a cut of incline θ . Hint: The lines of inclination θ are there L(θ )equations of incliney θ=(which we+assume is unique) into twoline equal pieces. = 0into or πtwo , letpieces B(θ ) (tan θ )x b, where b varies that fromdivides −∞ tothe ∞.ham Each such divides the For hamθslice denote theofamount of bread to theLet leftA(b) of L(beθ )the minus the of amount theleft right. Notice that L( π )amount = L(0) θ = (one which may be empty). amount ham totothe of the line minus the to since the right and0 and let θ =A πbegive the same line, but B( π ) = −B(0) since left and right get interchanged as the angle moves from 0 to the total area of the ham. Show that A(b) = −A if b is sufﬁciently large and A(b) = A if b is sufﬁciently π π . Assume that B( θ ) is continuous and apply the IVT. (By a further extension of this argument, one can prove the full negative. Then use the IVT. This works if θ = 0 or 2 . If θ = 0, deﬁne A(b) as the amount of ham above the “Ham Sandwich Theorem,” which states thatHow if you knifethe to cut at a slant, then itwhen is possible cut a sandwich line y = b minus the amount below. canallow you the modify argument to work θ = πto 2 (in which case consisting of a slice of ham and two slices of bread so that all three layers are divided in half.) tan θ = ∞)? y

( )

L 2

y

L( )

L (0) = L( )

x (A) Cutting a slice of ham at an angle .

x (B) A slice of ham on top of a slice of bread.

FIGURE 6 SOLUTION For each angle θ , 0 ≤ θ < π , let L(θ ) be the line at angle θ to the x-axis that slices the ham exactly in half, as shown in ﬁgure 6. Let L(0) = L(π ) be the horizontal line cutting the ham in half, also as shown. For θ and L(θ ) thus deﬁned, let B(θ ) = the amount of bread to the left of L(θ ) minus that to the right of L(θ ). To understand this argument, one must understand what we mean by “to the left” or “to the right”. Here, we mean to the left or right of the line as viewed in the direction θ . Imagine you are walking along the line in direction θ (directly right if θ = 0, directly left if θ = π , etc). We will further accept the fact that B is continuous as a function of θ , which seems intuitively obvious. We need to prove that B(c) = 0 for some angle c. Since L(0) and L(π ) are drawn in opposite direction, B(0) = −B(π ). If B(0) > 0, we apply the IVT on [0, π ] with B(0) > 0, B(π ) < 0, and B continuous on [0, π ]; by IVT, B(c) = 0 for some c ∈ [0, π ]. On the other hand, if B(0) < 0, then we apply the IVT with B(0) < 0 and B(π ) > 0. If B(0) = 0, we are also done; L(0) is the appropriate line.

68

CHAPTER 2

LIMITS

2.8 The Formal Definition of a Limit Preliminary Questions 1. Given that lim cos x = 1, which of the following statements is true? x→0

(a) (b) (c) (d)

If |cos x − 1| is very small, then x is close to 0. There is an > 0 such that |x| < 10−5 if 0 < |cos x − 1| < . There is a δ > 0 such that |cos x − 1| < 10−5 if 0 < |x| < δ . There is a δ > 0 such that |cos x| < 10−5 if 0 < |x − 1| < δ .

SOLUTION

The true statement is (c): There is a δ > 0 such that |cos x − 1| < 10−5 if 0 < |x| < δ .

2. Suppose that for a given and δ , it is known that | f (x) − 2| < if 0 < |x − 3| < δ . Which of the following statements must also be true? (a) | f (x) − 2| < if 0 < |x − 3| < 2δ (b) | f (x) − 2| < 2 if 0 < |x − 3| < δ δ (c) | f (x) − 2| < if 0 < |x − 3| < 2 2 δ (d) | f (x) − 2| < if 0 < |x − 3| < 2 SOLUTION Statements (b) and (d) are true.

Exercises 1. Consider lim f (x), where f (x) = 8x + 3. x→4

(a) Show that | f (x) − 35| = 8|x − 4|. (b) Show that for any > 0, | f (x) − 35| < if |x − 4| < δ , where δ = 8 . Explain how this proves rigorously that lim f (x) = 35. x→4

SOLUTION

(a) | f (x) − 35| = |8x + 3 − 35| = |8x − 32| = |8(x − 4)| = 8 |x − 4|. (Remember that the last step is justiﬁed because 8 > 0). (b) Let > 0. Let δ = /8 and suppose |x − 4| < δ . By part (a), | f (x) − 35| = 8|x − 4| < 8δ . Substituting δ = /8, we see | f (x) − 35| < 8/8 = . We see that, for any > 0, we found an appropriate δ so that |x − 4| < δ implies | f (x) − 35| < . Hence lim f (x) = 35. x→4

3. Consider lim x 2 = 4 (refer to Example 2). Consider x→2lim f (x), where f (x) = 4x − 1. x→2 (a) Show that |x 2 − 4| < 0.05 if 0 < |x − 2| < 0.01.

(a) Show that | f (x) − 7| < 4δ if |x − 2| < δ . (b) Show thata|xδ 2such − 4|that: < 0.0009 if 0 < |x − 2| < 0.0002. (b) Find (c) Find a value of δ such that |x 2 − 4| is less than 10−4 if 0 < |x − 2| < δ . | f (x) − 7| < 0.01 if |x − 2| < δ SOLUTION |x −rigorously (a) If 2| < δ = that .01, then and x 2 − 4 = |x − 2||x + 2| ≤ |x − 2| (|x| + 2) < 5|x − 2| < .05. (c)0 < Prove lim f|x| (x)<=3 7. x→2

(b) If 0 < |x − 2| < δ = .0002, then |x| < 2.0002 and 2 x − 4 = |x − 2||x + 2| ≤ |x − 2| (|x| + 2) < 4.0002|x − 2| < .00080004 < .0009. (c) Note that x 2 − 4 = |(x + 2)(x − 2)| ≤ |x + 2| |x − 2|. Since |x − 2| can get arbitrarily small, we can require |x − 2| < 1 so that 1 < x < 3. This ensures that |x + 2| is at most 5. Now we know that x 2 − 4 ≤ 5|x − 2|. Let δ = 10−5 . Then, if |x − 2| < δ , we get x 2 − 4 ≤ 5|x − 2| < 5 × 10−5 < 10−4 as desired. 5. Refer to Example 3 to ﬁnd a value of δ > 0 such that With regard to the limit lim x2 = 25, x→5 1 1 −4 if Write 0< −25| 3| < < 10 2− x < 6. Hint: |x |x = δ|x + 5| · |x − 5|. (a) Show that |x 2 − 25| < 11|x −x 5|−if34 < (b) Find a δ such that |x 2 − 25| < 10−3 if |x − 5| < δ . SOLUTION The Example shows that for any > 0 we have2 (c) Give a rigorous proof of the limit by showing that |x − 25| < if 0 < |x − 5| < δ , where δ is the smaller of 1 11 and 1. − 1 ≤ if |x − 3| < δ x 3 where δ is the smaller of the numbers 6 and 1. In our case, we may take δ = 6 × 10−4 .

S E C T I O N 2.8

The Formal Deﬁnition of a Limit

69

Plot f (x) =√tan x together with the horizontal lines y = 0.99 and y = 1.01. Use this plot to ﬁnd a value of 7. π |

x 0.775

0.78

0.785

0.79

0.795

1 9. Consider lim . 4 . Observe that the limit L = lim f (x) has the value L = f ( 12 ) = 16 Let x→2 f (x) x= 2 5 . Let = 0.5 and, x x→ 12 (a) Show that if |x − 2| <+1,1then that |f (x) − 16 | < for |x − 1 | < δ . Repeat for = 0.2 and 0.1. using a plot of f (x), ﬁnd a value of δ > 0 such 5 2 1 − 1 < 1 |x − 2| x 2 2 (b) Let δ be the smaller of 1 and 2. Prove the following statement: 1 − 1 < if 0 < |x − 2| < δ x 2 (c) Find a δ > 0 such that 1x − 12 < 0.01 if |x − 2| < δ . (d) Prove rigorously that lim

1

x→2 x

=

1 . 2

SOLUTION

(a) Since |x − 2| < 1, it follows that 1 < x < 3, in particular that x > 1. Because x > 1, then 1 − 1 = 2 − x = |x − 2| < 1 |x − 2|. x 2 2x 2x 2

1 < 1 and x

(b) Let δ = min{1, 2} and suppose that |x − 2| < δ . Then by part (a) we have 1 − 1 < 1 |x − 2| < 1 δ < 1 · 2 = . x 2 2 2 2 1 1 1 (c) Choose δ = .02. Then − < δ = .01 by part (b). x 2 2 (d) Let > 0 be given. Then whenever 0 < |x − 2| < δ = min {1, 2}, we have 1 − 1 < 1 δ ≤ . x 2 2 Since was arbitrary, we conclude that lim

1

x→2 x

=

1 . 2

11. Based on the information conveyed in Figure 5(A), ﬁnd values of L, , and δ > 0 such that the following statement √ +< 3. δ . lim holds: | Consider f (x) − L|x→1 < ifx|x| √ √ SOLUTION forces < Hint: 4.7. Rewritten, thisinequality means that 0|3< implies that x + 3<−x2|<<.112 |x − 1| 3.3 if |x<− f1|(x) < 4. Multiply the by |x | − x+ + .1 2| and observe (a) ShowWe thatsee | −.1 √ | f (x) − |4| x<+.7. Looking that 3+ 2| > 2.at the limit deﬁnition |x − c| < δ implies | f (x) − L| < , we can replace so that L = 4, √ = .7, c = 0, and δ = .1. (b) Find δ > 0 such that | x + 3 − 2| < 10−4 for |x − 1| < δ . Prove rigorously that the is equal to 2. for5(B), 13. (c) Based A on calculator gives thelimit following values f (x)ﬁnd = sin x: of c, L, , and δ > 0 such that the following the information conveyed in Figure values statement holds: | f (x) −

πL| < if |x − c| < δ . π ≈ 0.707, f − 0.1 ≈ 0.633, f 4 4

f

π 4

+ 0.1 ≈ 0.774

Use these values and the fact that f (x) is increasing on [0, π2 ] to justify the statement

π f (x) − f < 0.08 4

Then draw a ﬁgure like Figure 3 to illustrate this statement.

if

π x − < 0.1 4

70

CHAPTER 2

LIMITS SOLUTION Since f (x) is increasing on the interval, the three f (x) values tell us that .633 ≤ f (x) ≤ .774 for all x between π4 − .1 and π4 + .1. We may subtract f ( π4 ) from the inequality for f (x). This show that, for π4 − .1 < x < π4 + .1, .633 − f ( π4 ) ≤ f (x) − f ( π4 ) ≤ .774 − f ( π4 ). This means that, if |x − π4 | < .1, then .633 − .707 ≤ f (x) − f ( π4 ) ≤ .774 − .707, so −0.074 ≤ f (x) − f ( π4 ) ≤ 0.067. Then −0.08 < f (x) − f ( π4 ) < 0.08 follows from this, so |x − π4 | < 0.1 implies | f (x) − f ( π4 )| < .08. The ﬁgure below illustrates this. y 1 0.8 0.6 0.4 0.2 x 0.25 0.5 0.75

1

1.25 1.5

15. Adapt the the argument in Example 2 to1prove rigorously thatthat lim x 2 = c2 for all c. Adapt argument in Example to prove rigorously x→clim (ax + b) = ac + b, where a, b, c are arbitrary. x→c

SOLUTION

To relate the gap to |x − c|, we take 2 x − c2 = |(x + c)(x − c)| = |x + c| |x − c| .

We choose δ in two steps. First, since we are requiring |x − c| to be small, we require δ < |c|, so that x lies between 0 and 2c. This means that |x + c| < 3|c|, so |x − c||x + c| < 3|c|δ . Next, we require that δ < , so 3|c| |x − c||x + c| <

3|c| = , 3|c|

and we are done. Therefore, given > 0, we let δ = min |c|, . 3|c| Then, for |x − c| < δ , we have |x 2 − c2 | = |x − c| |x + c| < 3|c|δ < 3|c|

= . 3|c|

In Exercises 17–22, use the formal deﬁnition of the limit to prove the statement rigorously. Adapt the argument in Example 3 to prove rigorously that lim x −1 = 1c for all c. x→c √ 17. lim x = 2 x→4

√ √ x +2 Let > 0 be given. We bound | x − 2| by multiplying √ . x +2 √ √ 1 √ x + 2 x − 4 . = √ = |x − 4| √ | x − 2| = x − 2 √ x + 2 x + 2 x + 2 √ √ We can assume δ < 1, so that |x − 4| < 1, and hence x + 2 > 3 + 2 > 3. This gives us 1 √ < |x − 4| 1 . | x − 2| = |x − 4| √ 3 x + 2

SOLUTION

Let δ = min(1, 3). If |x − 4| < δ ,

1 √ < |x − 4| 1 < δ 1 < 3 1 = , | x − 2| = |x − 4| √ 3 3 3 x + 2

thus proving the limit rigorously. 19. lim x 3 = 12 x→1lim (3x + x) = 4 x→1

SOLUTION

Let > 0 be given. We bound x 3 − 1 by factoring the difference of cubes: 3 x − 1 = (x 2 + x + 1)(x − 1) = |x − 1| x 2 + x + 1 .

S E C T I O N 2.8

The Formal Deﬁnition of a Limit

71

Let δ = min(1, 7 ), and assume |x − 1| < δ . Since δ < 1, 0 < x < 2. Since x 2 + x + 1 increases as x increases for x > 0, x 2 + x + 1 < 7 for 0 < x < 2, and so 3 x − 1 = |x − 1| x 2 + x + 1 < 7|x − 1| < 7 = 7 and the limit is rigorously proven. 21. lim |x| = |c| x→c 1 lim x −2 = |x| + |c| 4 x→2 SOLUTION Let > 0 be given. First we bound |x| − |c| by multiplying by . |x| + |c| 2 2 2 2 |x| − |c| < |x| − |c| |x| + |c| = ||x| − |c| | = x − c = (x + c)(x − c) ≤ |x − c| |x + c| . |x| + |c| |x| + |c| |x| + |c| |x| + |c| |x| + |c| Let δ = . We apply the Triangle Inequality—for all a, b: |a + b| ≤ |a| + |b|. This yields: |x| − |c| ≤ |x − c| |x + c| ≤ |x − c| = , |x| + |c| and the limit is rigorously proven. x 23. Let f (x) = 1 . Prove rigorously that lim f (x) does not exist. Hint: Show that no number L qualiﬁes as the limit x→0 lim x sin|x| = 0 x exists some x such that |x| < δ but | f (x) − L| ≥ 1 , no matter how small δ is taken. becausex→0 there always 2 Let L be any real number. Let δ > 0 be any small positive number. Let x = δ2 , which satisﬁes |x| < δ , and f (x) = 1. We consider two cases: SOLUTION

• (| f (x) − L| ≥ 1 ) : we are done. 2 • (| f (x) − L| < 1 ): This means 1 < L < 3 . In this case, let x = − δ . f (x) = −1, and so 3 < L − f (x). 2 2 2 2 2

In either case, there exists an x such that |x| < δ2 , but | f (x) − L| ≥ 12 . 25. Use the identity Prove rigorously that lim sin 1x does not exist. x+y x−y x→0 sin x + sin y = 2 sin cos 2 2 to verify the relation sin(a + h) − sin a = h

sin(h/2) h cos a + h/2 2

6

Conclude that |sin(a + h) − sin a| < |h| for all a and prove rigorously that lim sin x = sin a. You may use the inequality x→a sin x ≤ 1 for x = 0. x SOLUTION

We ﬁrst write sin(a + h) − sin a = sin(a + h) + sin(−a).

Applying the identity with x = a + h, y = −a, yields:

a+h−a 2a + h cos 2 2 h h h h h sin(h/2) h = 2 sin cos a + =2 sin cos a + =h cos a + . 2 2 h 2 2 h/2 2

sin(a + h) − sin a = sin(a + h) + sin(−a) = 2 sin

Therefore,

sin(h/2) cos a + h . |sin(a + h) − sin a| = |h| h/2 2

sin θ < 1 and that |cos θ | ≤ 1, we have Making the substitution h = x − a, we see that this last Using the fact that θ relation is equivalent to |sin x − sin a| < |x − a|.

72

CHAPTER 2

LIMITS

Now, to prove the desired limit, let > 0, and take δ = . If |x − a| < δ , then |sin x − sin a| < |x − a| < δ = , Therefore, a δ was found for arbitrary , and the proof is complete. Use Eq. (6) to prove rigorously that Further Insights and Challenges sin(a + h) − sin 27. Uniqueness of the Limit Show that a function converges to aat most one limiting value. In other words, use the = cos a lim limit deﬁnition to show that if lim f (x) = L 1h→0 and lim f (x) h = L 2 , then L 1 = L 2 . x→c

SOLUTION

x→c

Let > 0 be given. Since lim f (x) = L 1 , there exists δ1 such that if |x − c| < δ1 then | f (x) − L 1 | < . x→c

Similarly, since lim f (x) = L 2 , there exists δ2 such that if |x − c| < δ2 then | f (x) − L 2 | < . Now let |x − c| < x→c min(δ1 , δ2 ) and observe that |L 1 − L 2 | = |L 1 − f (x) + f (x) − L 2 | ≤ |L 1 − f (x)| + | f (x) − L 2 | = | f (x) − L 1 | + | f (x) − L 2 | < 2. So, |L 1 − L 2 | < 2 for any > 0. We have |L 1 − L 2 | = lim |L 1 − L 2 | < lim 2 = 0. Therefore, |L 1 − L 2 | = 0 →0

and, hence, L 1 = L 2 .

→0

In Exercises 28–30, prove the statement using the formal limit deﬁnition. 29. The Squeeze Theorem. (Theorem 1 in Section 2.6, p. 89) The Constant Multiple Law [Theorem 1, part (ii) in Section 2.3, p. 64] SOLUTION Proof of the Squeeze Theorem. Suppose that (i) the inequalities h(x) ≤ f (x) ≤ g(x) hold for all x near (but not equal to) a and (ii) lim h(x) = lim g(x) = L. Let > 0 be given. x→a

x→a

• By (i), there exists a δ1 > 0 such that h(x) ≤ f (x) ≤ g(x) whenever 0 < |x − a| < δ1 . • By (ii), there exist δ2 > 0 and δ3 > 0 such that |h(x) − L| < whenever 0 < |x − a| < δ2 and |g(x) − L| < whenever 0 < |x − a| < δ3 . • Choose δ = min {δ1 , δ2 , δ3 }. Then whenever 0 < |x − a| < δ we have L − < h(x) ≤ f (x) ≤ g(x) < L + ;

i.e., | f (x) − L| < . Since was arbitrary, we conclude that lim f (x) = L. x→a

31. Let f (x) = 1 if x is rational and f (x) = 0 if x is irrational. Prove that lim f (x) does not exist for any c. The Product Law [Theorem 1, part (iii) in Section 2.3, p. 64]. Hint: x→c Use the identity SOLUTION Let c be any number, and let δ > 0 be an arbitrary small number. We will prove that there is an x such f (x)g(x) − L M = ( f (x) − L) g(x) + L(g(x) − M) that |x − c| < δ , but | f (x) − f (c)| > 12 . c must be either irrational or rational. If c is rational, then f (c) = 1. Since the irrational numbers are dense, there is at least one irrational number z such that |z − c| < δ . | f (z) − f (c)| = 1 > 12 , so the function is discontinuous at x = c. On the other hand, if c is irrational, then there is a rational number q such that |q − c| < δ . | f (q) − f (c)| = |1 − 0| = 1 > 12 , so the function is discontinuous at x = c.

Here is a function with strange continuity properties: ⎧ p 1 if x is the rational number in ⎪ ⎨ q q lowest terms f (x) = 1. A particle’s position at time t (s) is s(t) =⎪ ⎩ t 2 + 1 m. Compute its average velocity over [2, 5] and estimate its 0 if x is an irrational number instantaneous velocity at t = 2. (a) ShowLet thats(t) f (x) at c if velocity c is rational. exist irrational numbers arbitrarily close to c. SOLUTION = is discontinuous t 2 + 1. The average over Hint: [2, 5] There is (b) Show that f (x) is continuous at c if c is irrational. Let I be the interval {x : |x − c| < 1}. Show that for √ Hint: p√ q < Q. Conclude that there is a δ such that all any Q > 0, I contains at most ﬁnitely s(5) −many s(2) fractions 26 −q with 5 = ≈ 0.954 m/s. larger than Q. fractions in {x : |x − c| < δ } have a denominator 5−2 3

CHAPTER REVIEW EXERCISES

From the data in the table below, we estimate that the instantaneous velocity at t = 2 is approximately 0.894 m/s. interval average ROC

[1.9, 2]

[1.99, 2]

[1.999, 2]

[2, 2.001]

[2, 2.01]

[2, 2.1]

0.889769

0.893978

0.894382

0.894472

0.894873

0.898727

3. For a whole number n, let P(n) be the number of partitions of n, that is, the number of ways of writing n as a sum The price p ofnumbers. natural gas the United States dollars per 1,000 ft3 ) be on partitioned the ﬁrst dayinofﬁve each monthways: in 2004 of one or more whole Forinexample, P(4) = 5(in since the number 4 can different 4, 3 per month) over the is listed in the table below. Compute the average rate of change of p (in dollars per 1,000 ft 3 + 1, 2 + 2, 2 + 1 + 1, 1 + 1 + 1 + 1. Treating P(n) as a continuous function, use Figure 1 to estimate the rate of quarterly periods April–June, and July–September. change of P(n) at n =January–March, 12. J

F

M

A

M

J

Chapter Review Exercises

73

SOLUTION The tangent line drawn in the ﬁgure appears to pass through the points (15, 140) and (10.5, 40). We therefore estimate that the rate of change of P(n) at n = 12 is

100 200 140 − 40 = = . 15 − 10.5 4.5 9 In Exercises 5–8, estimate the limit numerically to two decimal places or state that the limit does not exist.√ The average velocity v (m/s) of an oxygen molecule in the air at temperature T (◦ C) is v = 25.7 273.15 + T . 3 (x) What1is−the speed at T = 25◦ (room temperature)? Estimate the rate of change of average velocity with cosaverage 5. respect lim to temperature at T = 25◦ . What are the units of this rate? x→0 x2 SOLUTION

3x Let f (x) = 1−cos . The data in the table below suggests that 2 FIGURE 1 Graph of P(n).

x

1 − cos3 x ≈ 1.50. x→0 x2 lim

In constructing the table, we take advantage of the fact that f is an even function. x

±0.001

±0.01

±0.1

f (x)

1.500000

1.499912

1.491275

(The exact value is 32 .) xx − 4 7. limlim2 x 1/(x−1) x→2 x −4 x→1 SOLUTION

x Let f (x) = x 2 −4 . The data in the table below suggests that

x −4

xx − 4 ≈ 1.69. x→2 x 2 − 4 lim

x

1.9

1.99

1.999

2.001

2.01

2.1

f (x)

1.575461

1.680633

1.691888

1.694408

1.705836

1.828386

(The exact value is 1 + ln 2.) In Exercises 9–42, the limit if possible or state that it does not exist. 3x − evaluate 9 lim x 1/2 (3 +5x − 25 ) 9. limx→2 x→4 √ SOLUTION lim (3 + x 1/2 ) = 3 + 4 = 5. x→4

4 11. lim 35 − x 2 x→−2 limx x→1 4x + 7 4 4 1 SOLUTION lim = =− . 2 x→−2 x 3 (−2)3 x3 − x 2 3x + 4x + 1 x −1 x→1 lim x +1 x→−1 x3 − x x(x − 1)(x + 1) SOLUTION lim = lim = lim x(x + 1) = 1(1 + 1) = 2. x −1 x→1 x − 1 x→1 x→1 √ t −33 15. lim x − 2x t −9 t→9lim √ √ x→1 x − 1 1 1 t −3 t −3 1 SOLUTION lim = . = lim √ = √ = lim √ √ 6 t→9 t − 9 t→9 ( t − 3)( t + 3) t→9 t + 3 9+3 √ x +1−2 t −6 17. lim x −3 x→3lim √ t→9 t − 3 13. lim

SOLUTION

√ lim

x→3

x +1−2 = lim x −3 x→3

√

= lim √ x→3

√ x +1−2 x +1+2 (x + 1) − 4 = lim ·√ √ x −3 x + 1 + 2 x→3 (x − 3)( x + 1 + 2) 1 x +1+2

= √

1 3+1+2

=

1 . 4

74

CHAPTER 2

LIMITS

2 − 2a 2 2(a + h) 19. lim 1 − s2 + 1 h h→0lim s→0 s2 SOLUTION

2(a + h)2 − 2a 2 2a 2 + 4ah + 2h 2 − 2a 2 h(4a + 2h) = lim = lim = lim (4a + 2h) = 4a + 2(0) = 4a. h h h h→0 h→0 h→0 h→0 lim

1 21. lim 2 1 t→3 tlim −9 t→−1+ x + 1 SOLUTION Because the one-sided limits 1

lim

= −∞

t→3− t 2 − 9

and

lim

1

t→3+ t 2 − 9

= ∞,

are not equal, the two-sided limit lim

1

does not exist.

t→3 t 2 − 9

a 2 −33ab + 2b2 x − b3 a−b a→blim x→b x − b a 2 − 3ab + 2b2 (a − b)(a − 2b) SOLUTION lim = lim = lim (a − 2b) = b − 2b = −b. a−b a−b a→b a→b a→b 1 1 25. lim x 3−−x(x ax 2++3)ax − 1 x→0lim3x x − 1 x→1 1 (x + 3) − 3 1 1 1 1 SOLUTION lim − = lim = lim = = . x(x + 3) 3(0 + 3) 9 x→0 3x x→0 3x(x + 3) x→0 3(x + 3)

23. lim

27.

lim

[x]

x→1.5 limx x→1+

SOLUTION

1

1

− x− x2 − 1 1 [1.5] 2 [x]1 lim = = = . 1.5 1.5 3 x→1.5 x √

[x] t→0−limx

29. lim

1

x→4.3 x − [x]

SOLUTION

For x sufﬁciently close to zero but negative, [x] = −1. Therefore, lim

[x]

x→0− x

= lim

−1

x→0− x

= ∞.

31. lim sec θ [x] θ → π4 lim t→0+ x SOLUTION

lim sec θ = sec

θ → π4

√ π = 2. 4

cos θ − 2 33. lim lim θ sec θ θ →0 θ → π2 θ SOLUTION

Because the one-sided limits cos θ − 2 =∞ θ θ →0− lim

and

cos θ − 2 = −∞ θ θ →0+ lim

are not equal, the two-sided limit cos θ − 2 θ θ →0 lim

sin 4x 35. lim sin 5θ sin 3x θ →0lim θ →0 θ

does not exist.

Chapter Review Exercises

75

SOLUTION

lim

sin 4θ

θ →0 sin 3θ

=

4 3θ 4 4 sin 4θ sin 4θ 3θ 4 · = lim · lim = (1)(1) = . lim 3 θ →0 4θ sin 3θ 3 θ →0 4θ 3 3 θ →0 sin 3θ

37. lim tan x x→ π2 sin2 t lim 3 t→0 Because t SOLUTION the one-sided limits lim tan x = ∞

lim tan x = −∞

and

x→ π2 −

x→ π2 +

are not equal, the two-sided limit lim tan x

does not exist.

x→ π2

√ 1 t cos1 39. lim lim cos t t→0+ t t→0 SOLUTION For t > 0, −1 ≤ cos so

1 ≤ 1, t

√ √ √ 1 ≤ t. − t ≤ t cos t

Because √ √ t = 0, lim − t = lim

t→0+

t→0+

it follows from the Squeeze Theorem that √ 1 t cos = 0. t t→0+ lim

41. lim

cos x − 1 sin 7x

x→0lim sin x x→0 sin 3x

SOLUTION

cos x − 1 cos x − 1 cos x + 1 − sin2 x sin x 0 = lim · = lim = − lim =− = 0. cos x + 1 x→0 sin x(cos x + 1) 1+1 x→0 sin x x→0 sin x x→0 cos x + 1 lim

43. Find the left- and right-hand limits of the function f (x) in Figure 2 at x = 0, 2, 4. State whether f (x) is left- or tan θ − sin θ right-continuous (or both) at these points. lim θ →0 sin3 θ y

2 1 x 1

2

3

4

5

FIGURE 2 SOLUTION

According to the graph of f (x), lim f (x) = lim f (x) = 1

x→0−

x→0+

lim f (x) = lim f (x) = ∞

x→2−

x→2+

lim f (x) = −∞

x→4−

lim f (x) = ∞.

x→4+

The function is both left- and right-continuous at x = 0 and neither left- nor right-continuous at x = 2 and x = 4.

76

CHAPTER 2

LIMITS

45. Sketch the graph of a function g(x) such that Sketch the graph of a function f (x) such that lim g(x) = −∞, lim g(x) = ∞ lim g(x) = ∞, = 1, lim f (x) = 3 lim f (x) x→−3− x→−3+ x→4 x→2−

x→2+

SOLUTION

and such that lim f (x) but does not equal f (4). x→4

y 10 5 −4

x

−2

2

4

6

−5 −10

47. Find a constant b such that h(x) is continuous at x = 2, where Find the points of discontinuity of g(x) and determine the type of discontinuity, where ⎧x + 1 for π x |x| < 2 h(x) = ⎪ ⎨cos 2 for |x| < 1 b − x 2 for |x| ≥ 2 g(x) = ⎪ ⎩ for |x| ≥ 1 With this choice of b, ﬁnd all points of discontinuity. |x − 1| SOLUTION

To make h(x) continuous at x = 2, we must have the two one-sided limits as x approaches 2 be equal.

With lim h(x) = lim (x + 1) = 2 + 1 = 3

x→2−

x→2−

and lim h(x) = lim (b − x 2 ) = b − 4,

x→2+

x→2+

it follows that we must choose b = 7. Because x + 1 is continuous for −2 < x < 2 and 7 − x 2 is continuous for x ≤ −2 and for x ≥ 2, the only possible point of discontinuity is x = −2. At x = −2, lim

x→−2+

h(x) =

lim (x + 1) = −2 + 1 = −1

x→−2+

and lim

x→−2−

h(x) =

lim (7 − x 2 ) = 7 − (−2)2 = 3,

x→−2−

so h(x) has a jump discontinuity at x = −2. 49. Let f (x) and g(x) be functions such that g(x) = 0 for x = a, and let Calculate the following limits, assuming that B= lim g(x), L= = 4lim A = lim f (x), = x→a 6, lim g(x) lim f (x) x→a x→a x→3

x→3

f (x) g(x)

Prove(a)thatlim if the limits A, B, and L exist and L = 1, then A = (b) B. Hint: cannot use the Quotient Law if B = 0, so 2 f (x) ( f (x) − 2g(x)) lim xYou apply thex→3 Product Law to L and B instead. x→3 f (x) SOLUTION (d)Then lim (2g(x)3 − g(x)2 ) (c) lim Suppose the limits A, B, and L all exist and L = 1. x→3 g(x) + x x→3 f (x) f (x) = lim g(x) = lim f (x) = A. B = B · 1 = B · L = lim g(x) · lim x→a x→a g(x) x→a x→a g(x) 51. Let f (x) be a function deﬁned for all real numbers. Which of the following statements must be true, might be true, In true. the notation of Exercise 49, give an example in which L exists but neither A nor B exists. or are never (a) lim f (x) = f (3). x→3

= 1, then f (0) = 0. (b) If lim f (x) x→0 x 1

1 . 8 (d) If lim f (x) = 4 and lim f (x) = 8, then lim f (x) = 6. (c) If lim f (x) = 8, then lim x→−7

x→5+

x→−7 f (x)

x→5−

(e) If lim f (x) = 1, then lim f (x) = 0 x→0 x x→0

=

x→5

Chapter Review Exercises

77

(f) If lim f (x) = 2, then lim f (x)3 = 8. x→5

x→5

SOLUTION

(a) This statement might be true. If f (x) is continuous at x = 3, then the statement will be true; if f (x) is not continuous at x = 3, then the statement will not be true. (b) This statement might be true. If f (x) is continuous at x = 0, then the statement will be true; if f (x) is not continuous at x = 0, then the statement will not be true. (c) This statement is always true. (d) This statement is never true. If the two one-sided limits are not equal, then the two-sided limit does not exist. (e) This statement is always true. (f) This statement is always true. 1 , where [x] is the greatest integer function. LetSqueeze f (x) = xTheorem 53. x Use the to prove that the function f (x) = x cos 1 for x = 0 and f (0) = 0 is continuous at x

x = 0. the graph of f (x) on the interval [ 1 , 2]. (a) Sketch 4 (b) Show that for x = 0,

1 1 1 −1< ≤ x x x Then use the Squeeze Theorem to prove that 1 =1 x x→0 lim x

SOLUTION

(a) The graph of f (x) = x 1x over [ 14 , 2] is shown below. y 1 0.8 0.6 0.4 0.2 x 0.5

1

1.5

2

(b) Let y be any real number. From the deﬁnition of the greatest integer function, it follows that y − 1 < [y] ≤ y, with equality holding if and only if y is an integer. If x = 0, then 1x is a real number, so 1 1 1 −1< ≤ . x x x Upon multiplying this inequality through by x, we ﬁnd 1−x < x

1 ≤ 1. x

Because lim (1 − x) = lim 1 = 1,

x→0

x→0

it follows from the Squeeze Theorem that 1 = 1. x x→0 lim x

1 . 55. Let f (x) = x+2 Let r1 and r2 be the roots of the quadratic polynomial f (x) = ax 2 − 2x + 20. Observe that f (x) “approaches” |x−2| 1 the linear as < a 1. →Hint: 0. Since r = that 10 is the+unique = 1.0, we might expect (a) Show that function |x 20 − 2| Observe |4(x 2)| > root 12 if of |x L(x) − 2| < f (x) − L(x) 4 < =12−2xif + of f (x) to approach 10 as a → 0 (Figure 3). Prove that the roots can be labeled so that at least one of the roots 1 (b) Find lim rδ1 > = 010such andthat lim rf 2(x) =− ∞.4 < 0.01 for |x − 2| < δ . a→0

a→0

(c) Prove rigorously that lim f (x) = 14 . x→2

SOLUTION

FIGURE 3 Graphs of f (x) = ax 2 − 2x + 20.

78

CHAPTER 2

LIMITS 1 . Then (a) Let f (x) = x+2

f (x) −

1 4 − (x + 2) |x − 2| 1 1 = − = = . 4 x + 2 4 4(x + 2) |4(x + 2)|

If |x − 2| < 1, then 1 < x < 3, so 3 < x + 2 < 5 and 12 < 4(x + 2) < 20. Hence, 1 1 f (x) − 1 < |x − 2| . < and |4(x + 2)| 12 4 12 (b) If |x − 2| < δ , then by part (a),

f (x) −

1 δ < . 4 12

Choosing δ = 0.12 will then guarantee that | f (x) − 14 | < 0.01. (c) Let > 0 and take δ = min{1, 12}. Then, whenever |x − 2| < δ , f (x) − 1 = 1 − 1 = |2 − x| ≤ |x − 2| < δ = . 4 x + 2 4 4|x + 2| 12 12 57. Prove rigorously that lim (4 + 8x) = −4. x→−1 Plot the function f (x) = x 1/3 . Use the zoom feature to ﬁnd a δ > 0 such that |x 1/3 − 2| < 0.05 for δ . |x − 8| < SOLUTION Let > 0 and take δ = /8. Then, whenever |x − (−1)| = |x + 1| < δ , | f (x) − (−4)| = |4 + 8x + 4| = 8|x + 1| < 8δ = . 59. Use the IVT to prove that the curves y = x 2 and y = cos x intersect. Prove rigorously that lim (x 2 − x) = 6. SOLUTION Let f (x) = x 2x→3 − cos x. Note that any root of f (x) corresponds to a point of intersection between the curves y = x 2 and y = cos x. Now, f (x) is continuous over the interval [0, π2 ], f (0) = −1 < 0 and f ( π2 ) = π4 > 0. Therefore, by the Intermediate Value Theorem, there exists a c ∈ (0, π2 ) such that f (c) = 0; consequently, the curves y = x 2 and y = cos x intersect. 2

61. Use the Bisection Method to locate a root of x22 − 7 = 0 to two decimal places. +2 has a root in the interval [0, 2]. Use the IVT to prove that f (x) = x 3 − x x+2 SOLUTION Let f (x) = x 2 − 7. By trial andcos error, we ﬁnd that f (2.6) = −0.24 < 0 and f (2.7) = 0.29 > 0. Because f (x) is continuous on [2.6, 2.7], it follows from the Intermediate Value Theorem that f (x) has a root on (2.6, 2.7). We approximate the root by the midpoint of the interval: x = 2.65. Now, f (2.65) = 0.0225 > 0. Because f (2.6) and f (2.65) are of opposite sign, the root must lie on (2.6, 2.65). The midpoint of this interval is x = 2.625 and f (2.625) < 0; hence, the root must be on the interval (2.625, 2.65). Continuing in this fashion, we construct the following sequence of intervals and midpoints. interval

midpoint

(2.625, 2.65) (2.6375, 2.65) (2.64375, 2.65) (2.64375, 2.646875) (2.6453125, 2.646875)

2.6375 2.64375 2.646875 2.6453125 2.64609375

At this point, we note that, to two decimal places, one root of x 2 − 7 = 0 is 2.65. Give an example of a (discontinuous) function that does not satisfy the conclusion of the IVT on [−1, 1]. Then show that the function ⎧ ⎨sin 1 for x = 0 x f (x) = ⎩ 0 for x = 0 satisﬁes the conclusion of the IVT on every interval [−a, a], even though f is discontinuous at x = 0.

3 DIFFERENTIATION 3.1 Definition of the Derivative Preliminary Questions 1. Which of the lines in Figure 10 are tangent to the curve? D B

C

A

FIGURE 10 SOLUTION

Lines A and D are tangent to the curve.

2. What are the two ways of writing the difference quotient? SOLUTION

The difference quotient may be written either as f (x) − f (a) x −a

or as f (a + h) − f (a) . h 3. For which value of x is f (x) − f (3) f (7) − f (3) = ? x −3 4 SOLUTION

With x = 7, f (7) − f (3) f (x) − f (3) = . x −3 4

4. What do the following quantities represent in terms of the graph of f (x) = sin x? sin 1.3 − sin 0.9 (a) sin 1.3 − sin 0.9 (b) 0.4 (c) f (1.3) Consider the graph of y = sin x. (a) The quantity sin 1.3 − sin .9 represents the difference in height between the points (.9, sin .9) and (1.3, sin 1.3). sin 1.3 − sin .9 (b) The quantity represents the slope of the secant line between the points (.9, sin .9) and (1.3, sin 1.3) .4 on the graph. (c) The quantity f (1.3) represents the slope of the tangent line to the graph at x = 1.3. SOLUTION

5. For which values of a and h is and (5, f (5)) on the graph of f (x)?

f (a + h) − f (a) equal to the slope of the secant line between the points (3, f (3)) h

f (a + h) − f (a) is equal to the slope of the secant line between the points h (3, f (3)) and (5, f (5)) on the graph of f (x). SOLUTION

With a = 3 and h = 2,

6. To which derivative is the quantity tan π4 + 0.00001 − 1 0.00001 a good approximation? SOLUTION

tan( π4 + 0.00001) − 1 0.00001

is a good approximation to the derivative of the function f (x) = tan x at x = π4 .

80

CHAPTER 3

D I F F E R E N T I AT I O N

Exercises 1. Let f (x) = 3x 2 . Show that f (2 + h) = 3h 2 + 12h + 12 Then show that f (2 + h) − f (2) = 3h + 12 h and compute f (2) by taking the limit as h → 0. With f (x) = 3x 2 , it follows that

SOLUTION

f (2 + h) = 3(2 + h)2 = 3(4 + 4h + h 2 ) = 12 + 12h + 3h 2 . Using this result, we ﬁnd f (2 + h) − f (2) 12 + 12h + 3h 2 − 3 · 4 12 + 12h + 3h 2 − 12 12h + 3h 2 = = = = 12 + 3h. h h h h As h → 0, 12 + 3h → 12, so f (2) = 12. In Exercises 3–6, compute f (a) in two ways, using Eq. (1) and Eq. (2). Let f (x) = 2x 2 − 3x − 5. Show that the slope of the secant line through (2, f (2)) and (2 + h, f (2 + h)) is + 5. formula to compute the slope of: 9x, this a= 0 3. 2h f (x) = Then x 2 + use (a) The secant line through (2, f (2)) and (3, f (3)) SOLUTION Let f (x) = x 2 + 9x. Then (b) The tangent line at x = 2 (by taking a limit) f (0 + h) − f (0) (0 + h)2 + 9(0 + h) − 0 9h + h 2 = lim = lim = lim (9 + h) = 9. h h h h→0 h→0 h→0 h→0

f (0) = lim Alternately,

f (x) − f (0) x 2 + 9x − 0 = lim = lim (x + 9) = 9. x −0 x x→0 x→0 x→0

f (0) = lim

5. f (x) = 3x 2 + 4x + 2, a = −1 f (x) = x 2 + 9x, a = 2 SOLUTION Let f (x) = 3x 2 + 4x + 2. Then f (−1 + h) − f (−1) 3(−1 + h)2 + 4(−1 + h) + 2 − 1 = lim h h h→0 h→0

f (−1) = lim

3h 2 − 2h = lim (3h − 2) = −2. h h→0 h→0

= lim Alternately,

f (x) − f (−1) 3x 2 + 4x + 2 − 1 = lim x − (−1) x +1 x→−1 x→−1

f (−1) = lim = lim

x→−1

(3x + 1)(x + 1) = lim (3x + 1) = −2. x +1 x→−1

In Exercises 7–10, refer2 to the function whose graph is shown in Figure 11. f (x) = 9 − 3x , a = 0 7. Calculate the slope of the secant line through the points on the graph where x = 0 and x = 2.5. SOLUTION

f (0) ≈ 6 and f (2.5) ≈ 2, so the slope of the secant line connecting (0, f (0)) and (2.5, f (2.5)) is 2−6 −4 f (2.5) − f (0) ≈ = = −1.6. 2.5 − 0 2.5 2.5

f (2 + h) − f (2) Estimate for h = 0.5 and h = −0.5. Are these numbers larger or smaller than f (2)? f (2.5) − f (1) h . What does this quantity represent? Compute 2.5 − 1 Explain. 9.

S E C T I O N 3.1 SOLUTION

Deﬁnition of the Derivative

81

f (2) ≈ 1. If h = .5, f (2 + h) = f (2.5) ≈ 2, so 2−1 f (2 + h) − f (2) ≈ = 2. h .5

This number is probably greater than f (2). Since the curve is bending upward slightly at (2, f (2)), the secant line lies inside and over the curve, showing it has higher slope than the tangent line. If h = −.5, f (2 + h) = f (1.5) ≈ .5, so .5 − 1 f (2 + h) − f (2) ≈ = 1. h −.5 This number is smaller than f (2), since the curve is bending upwards near (2, f (2)). In Exercises 11–14, let f (x) be the function whose graph is shown in Figure 12. Estimate f (2). 5 4 3 2 1

y

x

1 2 3 4 5 6 7 8 9

FIGURE 12 Graph of f (x).

11. Determine f (a) for a = 1, 2, 4, 7. SOLUTION Remember that the value of the derivative of f at x = a can be interpreted as the slope of the line tangent to the graph of y = f (x) at x = a. From Figure 12, we see that the graph of y = f (x) is a horizontal line (that is, a line with zero slope) on the interval 0 ≤ x ≤ 3. Accordingly, f (1) = f (2) = 0. On the interval 3 ≤ x ≤ 5, the graph of y = f (x) is a line of slope 12 ; thus, f (4) = 12 . Finally, the line tangent to the graph of y = f (x) at x = 7 is horizontal, so f (7) = 0.

13. Which is larger: f (5.5) or f (6.5)? Estimate f (6). SOLUTION The line tangent to the graph of y = f (x) at x = 5.5 has a larger slope than the line tangent to the graph of y = f (x) at x = 6.5. Therefore, f (5.5) is larger than f (6.5). In Exercises the limit deﬁnition to ﬁnd the derivative of the linear function. (Note: The derivative does not Show15–18, that f use (3) does not exist. depend on a.) 15. f (x) = 3x − 2 SOLUTION

f (a + h) − f (a) 3(a + h) − 2 − (3a − 2) = lim = lim 3 = 3. h h h→0 h→0 h→0 lim

17. g(t) = 9 − t f (x) = 2 SOLUTION

g(a + h) − g(a) 9 − (a + h) − (9 − a) −h = lim = lim = lim (−1) = −1. h h h→0 h→0 h→0 h h→0 lim

1 1 1 1 Does or + ? Compute the difference quotient for f (x) at a = −2 19. Letk(z) f (x)==16z.+ 9 f (−2 + h) equal x −2 + h −2 h with h = 0.5. SOLUTION

Let f (x) = 1x . Then f (−2 + h) =

1 . −2 + h

With a = −2 and h = .5, the difference quotient is 1 − 1 f (−1.5) − f (−2) 1 f (a + h) − f (a) = = −1.5 −2 = − . h .5 .5 3

Let f (x) = with h = 1.

√

x. Does f (5 + h) equal

√

5 + h or

√

5+

√

h? Compute the difference quotient for f (x) at a = 5

82

CHAPTER 3

D I F F E R E N T I AT I O N

21. Let f (x) =

√

x. Show that 1 f (9 + h) − f (9) = √ h 9+h+3

Then use this formula to compute f (9) (by taking the limit). √ SOLUTION We multiply numerator and denominator by 9 + h + 3 to make a difference of squares. Thus, f (9 + h) − f (9) = h =

√ √ √ 9+h−3 ( 9 + h − 3)( 9 + h + 3) = √ h h( 9 + h + 3) 9+h−9 h 1 √ = √ = √ . h( 9 + h + 3) h( 9 + h + 3) 9+h+3

Applying this formula we ﬁnd: f (9) = lim

h→0

1 f (9 + h) − f (9) 1 1 = . = lim √ = √ h 6 h→0 9 + h + 3 9+3

√ In Exercises derivative at x = the limit x at x of = the 4. tangent line. First 23–40, ﬁnd thecompute slope andthethen an equation of atheusing tangent line todeﬁnition the graphand of ﬁnd f (x)an=equation 23. f (x) = 3x 2 + 2x,

a=2

Let f (x) = 3x 2 + 2x. Then

SOLUTION

f (2 + h) − f (2) 3(2 + h)2 + 2(2 + h) − 16 = lim h h h→0 h→0

f (2) = lim

12 + 12h + 3h 2 + 4 + 2h − 16 = lim (14 + 3h) = 14. h h→0 h→0

= lim At a = 2, the tangent line is

y = f (2)(x − 2) + f (2) = 14(x − 2) + 16 = 14x − 12. 25. f (x) = x 3 , a2 = 2 f (x) = 3x + 2x, a = −1 SOLUTION Let f (x) = x 3 . Then f (2 + h) − f (2) (2 + h)3 − 23 8 + 12h + 6h 2 + h 3 − 8 = lim = lim = lim (12 + 6h + h 2 ) = 12 h h h h→0 h→0 h→0 h→0

f (2) = lim

At a = 2, the tangent line is y = f (2)(x − 2) + f (2) = 12(x − 2) + 8 = 12x − 16. 27. f (x) = x 3 +3x, a = 0 f (x) = x , a = 3 SOLUTION Let f (x) = x 3 + x. Then

f (0 + h) − f (0) (0 + h)3 + (0 + h) − 0 h3 + h = lim = lim = lim h 2 + 1 = 1. h h h h→0 h→0 h→0 h→0

f (0) = lim

The tangent line at a = 0 is y = f (0) + f (0)(x − 0) = 0 + 1(x − 0) or y = x. 29. f (x) = x −1 , 3 a = 3 f (t) = 3t + 2t, a = 4 SOLUTION Let f (x) = x −1 . Then

1 − 1 3−3−h 1 f (3 + h) − f (3) −h 3+h 3 3(3+h) = lim = lim = lim =− f (3) = lim h h h 9 h→0 h→0 h→0 h→0 (9 + 3h)h The tangent at a = 3 is 1 1 1 2 y = f (3)(x − 3) + f (3) = − (x − 3) + = − x + . 9 3 9 3 31. f (x) = 9x − 4, a = −7 f (x) = x −2 , a = 1

Deﬁnition of the Derivative

S E C T I O N 3.1 SOLUTION

83

Let f (x) = 9x − 4. Then f (−7) = lim

h→0

= lim

h→0

f (−7 + h) − f (−7) (9(−7 + h) − 4) − (−67) = lim h h h→0 (−63 + 9h − 4) + 67 9h = lim = lim 9 = 9. h h→0 h h→0

The tangent line at a = −7 is y = f (−7)(x + 7) + f (−7) = 9(x + 7) − 67 = 9x − 4. In general, we can take a shorter path. The tangent line to a line is the line itself. 1√ a =a−2 33. f (x)f (t) = = t ,+ 1, =0 x +3 SOLUTION

1 . Then Let f (x) = x+3 1 1 −1 −1 f (−2 + h) − f (−2) −h −1 = lim −2+h+3 = lim 1+h = lim = lim = −1. h h h h→0 h→0 h→0 h→0 h(1 + h) h→0 1 + h

f (−2) = lim

The tangent line at a = −2 is y = f (−2)(x + 2) + f (−2) = −1(x + 2) + 1 = −x − 1. 2 35. f (t) = x ,+ 1a = −1 , a=3 f (x)1=− t x −1 2 SOLUTION Let f (t) = 1−t . Then 2 f (−1 + h) − f (−1) 2 − (2 − h) 1 1 1−(−1+h) − 1 = lim = lim = lim = . h h 2 h→0 h→0 h→0 h(2 − h) h→0 2 − h

f (−1) = lim

At a = −1, the tangent line is 1 1 3 y = f (−1)(x + 1) + f (−1) = (x + 1) + 1 = x + . 2 2 2 37. f (t) = t −3 , 1a = 1 f (t) = , a=2 t +f (t) 9 = 1 . Then SOLUTION Let t3 f (1 + h) − f (h) = lim h h→0 h→0

f (1) = lim

1 −1 (1+h)3

h

= lim

h→0

−h 3+3h+h 2 3 (1+h)

h

−(3 + 3h + h 2 ) = −3. h→0 (1 + h)3

= lim

The tangent line at a = 1 is y = f (1)(t − 1) + f (1) = −3(t − 1) + 1 = −3t + 4. 1 39. f (x)f (t) == √ t 4, , aa==92 x SOLUTION

1 Let f (x) = √ . Then x f (9 + h) − f (9) = lim h h→0 h→0

f (9) = lim

√1 − 13 9+h

h

= lim

h→0

√ √ 3−√ 9+h 3+√9+h · 3 9+h 3+ 9+h

h

= lim

h→0

−1

1 = lim √ =− . 54 h→0 9 9 + h + 3(9 + h) At a = 9 the tangent line is 1 1 1 1 y = f (9)(x − 9) + f (9) = − (x − 9) + = − x + . 54 3 54 2 41. What is an equation of the tangent line at x = 3, assuming that f (3) = 5 and f (3) = 2? f (x) = (x 2 + 1)−1 , a = 0

√ 9−9−h 9 9+h+3(9+h)

h

84

CHAPTER 3

D I F F E R E N T I AT I O N SOLUTION By deﬁnition, the equation of the tangent line to the graph of f (x) at x = 3 is y = f (3) + f (3)(x − 3) = 5 + 2(x − 3) = 2x − 1.

43. Consider the “curve” y = 2x + 8. What is the tangent line at the point (1, 10)? Describe the tangent line at an Suppose that y = 5x + 2 is an equation of the tangent line to the graph of y = f (x) at a = 3. What is f (3)? arbitrary point. What is f (3)? SOLUTION Since y = 2x + 8 represents a straight line, the tangent line at any point is the line itself, y = 2x + 8. 1 1 Verify thatf (x) P = and L(x) = 12 + m(x − 1) for every on the f (x) = 2 + 5h. Suppose that is a 1, function thatgraphs f (2 + of h) both − f (2) = 3h 2 liessuch 1 + x2 slope(a) m. What Plot fis(x)f and (2)? L(x) on the same axes for several values of m. Experiment until you ﬁnd a value of m for which y = (b) L(x)What appears tangent to the What(2, is your f (1)? is the slope of the graph secantof linef (x). through f (2))estimate and (6, for f (6))?

45.

SOLUTION

Let f (x) =

1 and L(x) = 12 + m(x − 1). Because 1 + x2 f (1) =

1 1 = 2 1 + 12

L(1) =

and

1 1 + m(1 − 1) = , 2 2

it follows that P = (1, 12 ) lies on the graphs of both functions. A plot of f (x) and L(x) on the same axes for several values of m is shown below. The graph of L(x) with m = − 12 appears to be tangent to the graph of f (x) at x = 1. We therefore estimate f (1) = − 12 . y 1 m = −1

0.8 0.6

m = −1/4

0.4 0.2 0.5

1

m = −1/2 x 2

1.5

Plot f (x) = xx x and the line y = x + c on the same set of axes for several values of c. Experiment until you 47. Plot f (x) that = x theforline 0 ≤is xtangent ≤ 1.5.to the graph. Then: ﬁnd a value c0 such are there thatthat f (xf0 )(x=)0? (a) How many points x (a) Use your plot to estimate0 the value such x 0 such 0 = 1. plotting horizontal lines y = c together with the plot of f (x) until you ﬁnd a value of c (b) Estimatef (x x 00 by + h) − f (xseveral 0) (b) Verify thatthe line is tangent to the is close for which graph.to 1 for h = 0.01, 0.001, 0.0001. h SOLUTION The ﬁgure below shows the graphs of the function f (x) = x x together with the lines y = x − 0.5, y = x, and y = x + 0.5. y 2 1.5 1 0.5 x 0.2

0.4

0.6

0.8

1

1.2

1.4

−0.5

(a) The graph of y = x appears to be tangent to the graph of f (x) at x = 1. We therefore estimate that f (1) = 1. (b) With x 0 = 1, we generate the table h

0.01

0.001

0.0001

( f (x 0 + h) − f (x 0 ))/ h

1.010050

1.001001

1.000100

(1)net Thefollowing vapor pressure water isa deﬁned as the quotients atmospheric at whichf no takes 49. Use the table toof calculate few difference of fpressure (x). ThenP estimate by evaporation taking an average place. The following table and Figure 13 give P (in atmospheres) as a function of temperature T in kelvins. of difference quotients at h and −h. For example, take the average for h = 0.03 and −0.03, etc. Recall that the a +or b P (350)? Answer by referring to the graph. (a) Which is larger: P (300) average of a and b is . 2 303, 313, 323, 333, 343 using the table and the average of the difference quotients for (b) Estimate P (T ) for T = h = 10 and −10: x 1.03 1.02 1.01 1 P(T + 10) − P(T − 10) )≈ 4 f (x)P (T0.5148 0.5234 0.5319 0.5403 20 x 0.97 0.98 0.99

f (x)

0.5653

0.557

0.5486

Deﬁnition of the Derivative

S E C T I O N 3.1

T (K)

293

303

313

323

333

343

85

353

P (atm) 0.0278 0.0482 0.0808 0.1311 0.2067 0.3173 0.4754 P (atm) 1 0.75 0.5 0.25 293 313 333 353 373 T (K)

FIGURE 13 Vapor pressure of water as a function temperature T in kelvins. SOLUTION

(a) Consider the graph of vapor pressure as a function of temperature. If we draw the tangent line at T = 300 and another at T = 350, it is clear that the latter has a steeper slope. Therefore, P (350) is larger than P (300). (b) Using equation (4), P (303) ≈ P (313) ≈ P (323) ≈ P (333) ≈ P (343) ≈

P(313) − P(293) 20 P(323) − P(303) 20 P(333) − P(313) 20 P(343) − P(323) 20 P(353) − P(333) 20

= = = = =

.0808 − .0278 20 .1311 − .0482 20 .2067 − .0808 20 .3173 − .1311 20 .4754 − .2067 20

= .00265 atm/K; = .004145 atm/K; = .006295 atm/K; = .00931 atm/K; = .013435 atm/K

In Exercises 50–51, trafﬁc speed S along a certain road (in mph) varies as a function of trafﬁc density q (number of cars per mile on the road). Use the following data to answer the questions: q (density)

100

110

120

130

140

S (speed)

45

42

39.5

37

35

(q) when The Squantity V q==q120 S is cars called volume. why V is equal to the atnumber of cars 51. Estimate pertrafﬁc mile using the Explain average of difference quotients h and −h as inpassing Exercisea (q) when q = 120. particular point per hour. Use the data to compute values of V as a function of q and estimate V 48. SOLUTION The trafﬁc speed S has units of miles/hour, and the trafﬁc density has units of cars/mile. Therefore, the trafﬁc volume V = Sq has units of cars/hour. A table giving the values of V follows.

q

100

110

120

130

140

V

4500

4620

4740

4810

4900

h

−20

−10

10

20

V (120 + h) − V (120) h

12

12

7

8

To estimate d V /dq, we take the difference quotient.

The mean of the difference quotients is 9.75. Hence d V /dq ≈ 9.75 mph when q = 120. and a.the derivative is positive. In Exercises 53–58, of the 14, limits represents derivative f (a). For the grapheach in Figure determine theaintervals along the Find x-axisf (x) on which (5 + h)3 − 125 h h→0

53. lim

SOLUTION

The difference quotient

sin( π63 + h) − .5 x − 125 h h→0lim x→5 x − 5

55. lim

(5 + h)3 − 125 f (a + h) − f (a) has the form where f (x) = x 3 and a = 5. h h

86

CHAPTER 3

D I F F E R E N T I AT I O N

SOLUTION

The difference quotient

sin( π6 + h) − .5 h

has the form

f (a + h) − f (a) where f (x) = sin x and a = π6 . h

52+h −1 − 25 x −4 h→0lim h 1 x→ 41 x − 4

57. lim

5(2+h) − 25 f (a + h) − f (a) has the form where f (x) = 5x and a = 2. h h 59. Sketch the graph of f (x) = sin x on [0, π ] and guess the value of f π2 . Then calculate the slope of the secant h π π 5 −1 line between lim x = 2 and x = 2 + h for at least three small positive and negative values of h. Are these calculations h h→0 consistent with your guess? SOLUTION

The difference quotient

SOLUTION

Here is the graph of y = sin x on [0, π ]. y 1 0.8 0.6 0.4 0.2 x 0.5 1 1.5 2 2.5 3

At x = π2 , we’re at the peak of the sine graph. The tangent line appears to be horizontal, so the slope is 0; hence, f ( π2 ) appears to be 0. h

−.01

−.001

−.0001

.0001

.001

.01

sin( π2 + h) − 1

.005

.0005

.00005

−.00005

−.0005

−.005

h

These numerical calculations are consistent with our guess. 4 √ Let15(A) f (x) = .graph of f (x) = x. The close-up in (B) shows that the graph is nearly a straight line Figure shows the 1 + 2x near x = 16. Estimate the slope of this line and take it as an estimate for f (16). Then compute f (16) and compare (a) Plot f (x) over [−2, 2]. Then zoom in near x = 0 until the graph appears straight and estimate the slope f (0). with your estimate. (b) Use your estimate to ﬁnd an approximate equation to the tangent line at x = 0. Plot this line and the graph on the same set of axes.

61.

SOLUTION

4 over [−2, 2]. The ﬁgure below at the right is a close-up (a) The ﬁgure below at the left shows the graph of f (x) = 1+2 x near x = 0. From the close-up, we see that the graph is nearly straight and passes through the points (−0.22, 2.15) and (0.22, 1.85). We therefore estimate

f (0) ≈

1.85 − 2.15 −0.3 = = −0.68 0.22 − (−0.22) 0.44

y

y

3 2.5

2.4 2.2

2 1.5 1 0.5 −2

−1

2.0 1.8 x 1

−0.2

2

−0.1

x 0.1

0.2

(b) Using the estimate for f (0) obtained in part (a), the approximate equation of the tangent line is y = f (0)(x − 0) + f (0) = −0.68x + 2. The ﬁgure below shows the graph of f (x) and the approximate tangent line. y 3 2.5 2 1.5 1 0.5 −2

−1

x 1

2

Deﬁnition of the Derivative

S E C T I O N 3.1

87

Let f (x) = cot x. Estimate f ( π2 ) graphically by zooming in on a plot of f (x) near x = π2 . Repeat Exercise 61 with f (x) = (2 + x)x . π SOLUTION The ﬁgure below shows a close-up of the graph of f (x) = cot x near x = 2 ≈ 1.5708. From the closeup, we see that the graph is nearly straight and passes through the points (1.53, 0.04) and (1.61, −0.04). We therefore estimate

π −0.04 − 0.04 −0.08 f ≈ = = −1 2 1.61 − 1.53 0.08 63.

y 1 0.05 1.58 1.6

x

1.54 1.56

−0.05 −1

65. Apply the method of Example 6 to f (x) = sin x to determine f π4 accurately to four decimal places. For each graph in Figure 16, determine whether f (1) is larger or smaller than the slope of the secant line SOLUTION between xWe = know 1 and that x = 1 + h for h > 0. Explain. √ f (π /4 + h) − f (π /4) sin(π /4 + h) − 2/2 f (π /4) = lim = lim . h h h→0 h→0 Creating a table of values of h close to zero: h

√

sin( π4 + h) − ( 2/2) h

−.001

−.0001

−.00001

.00001

.0001

.001

.7074602

.7071421

.7071103

.7071033

.7070714

.7067531

Accurate up to four decimal places, f ( π4 ) ≈ .7071. 67. Sketch the graph of f (x) = x 5/2 on [0, 6]. Apply the method of Example 6 to f (x) = cos x to determine f ( π5 ) accurately to four decimal places. Use a (a) Use theofsketch to explain justify the h > in 0: this case. graph f (x) to howinequalities the methodfor works f (4 + h) − f (4) f (4) − f (4 − h) ≤ f (4) ≤ h h (b) Use part (a) to compute f (4) to four decimal places. (c) Use a graphing utility to plot f (x) and the tangent line at x = 4 using your estimate for f (4). SOLUTION

(a) The slope of the secant line between points (4, f (4)) and (4 + h, f (4 + h)) is f (4 + h) − f (4) . h x 5/2 is a smooth curve increasing at a faster rate as x → ∞. Therefore, if h > 0, then the slope of the secant line is greater than the slope of the tangent line at f (4), which happens to be f (4). Likewise, if h < 0, the slope of the secant line is less than the slope of the tangent line at f (4), which happens to be f (4). (b) We know that f (4 + h) − f (4) (4 + h)5/2 − 32 = lim . h h h→0 h→0

f (4) = lim

Creating a table with values of h close to zero: h

−.0001

−.00001

.00001

.0001

(4 + h)5/2 − 32 h

19.999625

19.99999

20.0000

20.0000375

Thus, f (4) ≈ 20.0000. (c) Using the estimate for f (4) obtained in part (b), the equation of the line tangent to f (x) = x 5/2 at x = 4 is y = f (4)(x − 4) + f (4) = 20(x − 4) + 32 = 20x − 48.

88

CHAPTER 3

D I F F E R E N T I AT I O N y 80 60 40 20 x 1

− 20 − 40 − 60

2

3

4

5

6

69. The graph in Figure 18 (basedx on data collected by the biologist Julian Huxley, 1887–1975) gives the average antler of fas(x) = 2 is shown 17.the slope of the tangent line to the graph at t = 4. For which weight W The of agraph red deer a function of ageint. Figure Estimate (a)ofReferring to the explain thetoinequality values t is the slope ofgraph, the tangent linewhy equal zero? For which values is it negative? f (h) − f (0) Antler 8 f (0) ≤ Weight 7 h (kg) 6 5 holds for h > 0 and the opposite inequality holds for h < 0. 4 (b) Use part (a) to show that 0.69314 ≤ f3 (0) ≤ 0.69315. 2 (c) Use the same procedure to determine 1 f (x) to four decimal places for x = 1, 2, 3, 4. 0 0 x 2= 1,4 2, 3, 6 4. Can 8 10 14 an (approximate) you 12 guess (d) Now compute the ratios f (x)/ f (0) for Age (years) general x?

formula for f (x) for

FIGURE 18 SOLUTION Let W (t) denote the antler weight as a function of age. The “tangent line” sketched in the ﬁgure below passes through the points (1, 1) and (6, 5.5). Therefore

W (4) ≈

5.5 − 1 = 0.9 kg/year. 6−1

If the slope of the tangent is zero, the tangent line is horizontal. This appears to happen at roughly t = 10 and at t = 11.6. The slope of the tangent line is negative when the height of the graph decreases as we move to the right. For the graph in Figure 18, this occurs for 10 < t < 11.6. y 8 7 6 5 4 3 2 1 0

0

2

4

6

8

10

12

x 14

In Exercises 70–72, i(t) is the current (in amperes) at time t (seconds) ﬂowing in the circuit shown in Figure 19. According to Kirchhoff’s law, i(t) = Cv (t) + R −1 v(t), where v(t) is the voltage (in volts) at time t, C the capacitance (in farads), and R the resistance (in ohms, ). i + v

R

−

C

FIGURE 19

71. Use the following table to estimate v (10). For a better estimate, take the average of the difference quotients for h Calculate the current at t = 3 if v(t) = 0.5t + 4 V, C = 0.01 F, and R = 100 . and −h as described in Exercise 48. Then estimate i(10), assuming C = 0.03 and R = 1,000. t v(t) SOLUTION

9.8

9.9

10

10.1

10.2

256.52

257.32

258.11

258.9

259.69

We generate a table of difference quotients at 10 using the following table: h

−0.2

−0.1

0.1

0.2

v(10 + h) − v(10) h

7.95

7.9

7.9

7.9

Deﬁnition of the Derivative

S E C T I O N 3.1

89

The mean of the difference quotients is 7.9125, so v (10) ≈ 7.9125 volts/s. Thus, i(10) = 0.03(7.9125) +

1 (258.11) = 0.495485 amperes. 1000

Assume that Rand = 200 but C is unknown. Use the following data to estimate v (4) as in Exercise 71 and deduce Further Insights Challenges an approximate value for the capacitance C. In Exercises 73–76, we deﬁne the symmetric difference quotient (SDQ) at x = a for h = 0 by 3.8f (a + 3.9 h) − f (a4− h) 4.1 2h 420 436.2 388.8 404.2

t v(t)

73. Explain how SDQ can be interpreted slope of a secant34.1 line. 34.98 i(t)as the 32.34 33.22 SOLUTION The symmetric difference quotient

4.2

5

452.8 35.86

f (a + h) − f (a − h) 2h is the slope of the secant line connecting the points (a − h, f (a − h)) and (a + h, f (a + h)) on the graph of f ; the difference in the function values is divided by the difference in the x-values. 75. Showusually that if gives f (x) is a quadratic polynomial, then the SDQthan at xdoes = athe (for any h = 0) is equal to f (a). The SDQ a better approximation to the derivative ordinary difference quotient. Let x Explain of this the result. = 0. Compute SDQ with h = 0.001 and the ordinary difference quotients with h = ±0.001. f (x)the=graphical 2 and a meaning Compare with the actual value of f (0) that, to eight decimal places, is 0.69314718. SOLUTION Let f (x) = px 2 + qx + r be a quadratic polynomial. We compute the SDQ at x = a. f (a + h) − f (a − h) p(a + h)2 + q(a + h) + r − ( p(a − h)2 + q(a − h) + r ) = 2h 2h pa 2 + 2 pah + ph 2 + qa + qh + r − pa 2 + 2 pah − ph 2 − qa + qh − r 2h 4 pah + 2qh 2h(2 pa + q) = = = 2 pa + q 2h 2h =

Since this doesn’t depend on h, the limit, which is equal to f (a), is also 2 pa + q. Graphically, this result tells us that the secant line to a parabola passing through points chosen symmetrically about x = a is always parallel to the tangent line at x = a. 77. Which of the two functions in Figure 20 satisﬁes the inequality Let f (x) = x −2 . Compute f (1) by taking the limit of the SDQs (with a = 1) as h → 0. f (a + h) − f (a) f (a + h) − f (a − h) ≤ 2h h for h > 0? Explain in terms of secant lines. y

y

x

x a

a (A)

(B)

FIGURE 20 SOLUTION

Figure (A) satisﬁes the inequality f (a + h) − f (a − h) f (a + h) − f (a) ≤ 2h h

since in this graph the symmetric difference quotient has a larger negative slope than the ordinary right difference quotient. [In ﬁgure (B), the symmetric difference quotient has a larger positive slope than the ordinary right difference quotient and therefore does not satisfy the stated inequality.]

90

CHAPTER 3

D I F F E R E N T I AT I O N

3.2 The Derivative as a Function Preliminary Questions 1. What is the slope of the tangent line through the point (2, f (2)) if f is a function such that f (x) = x 3 ? SOLUTION

The slope of the tangent line through the point (2, f (2)) is given by f (2). Since f (x) = x 3 , it follows

that f (2) = 23 = 8.

2. Evaluate ( f − g) (1) and (3 f + 2g) (1) assuming that f (1) = 3 and g (1) = 5. Can we evaluate ( f g) (1) using the information given and the rules presented in this section? SOLUTION ( f − g) (1) = f (1) − g (1) = 3 − 5 = −2 and (3 f + 2g) (1) = 3 f (1) + 2g (1) = 3(3) + 2(5) = 19. Using the rules of this section, we cannot evaluate ( f g) (1).

3. Which of the following functions can be differentiated using the rules covered in this section? Explain. (b) f (x) = 2x (a) f (x) = x 2 1 (c) f (x) = √ (d) f (x) = x −4/5 x (e) f (x) = sin x (f) f (x) = (x + 5)3 SOLUTION

(a) Yes. x 2 is a power function, so the Power Rule can be applied. (b) No. 2x is an exponential function (the base is constant while the exponent is a variable), so the Power Rule does not apply. 1 (c) Yes. √ can be rewritten as x −1/2 . This is a power function, so the Power Rule can be applied. x −4/5 (d) Yes. x is a power function, so the Power Rule can be applied. (e) No. The Power Rule does not apply to the trigonometric function sin x. (f) Yes. (x + 5)3 can be expanded to x 3 + 15x 2 + 75x + 125. The Power Rule, Rule for Sums, and Rule for Constant Multiples can be applied to this function. 4. Which algebraic identity is used to prove the Power Rule for positive integer exponents? Explain how it is used. SOLUTION

The algebraic identity x n − a n = (x − a)(x n−1 + x n−2 a + x n−3 a 2 + · · · + xa n−2 + a n−1 )

is used to prove the Power Rule for positive integer exponents. With this identity, the difference quotient x n − an x −a can be simpliﬁed to x n−1 + x n−2 a + x n−3 a 2 + · · · + xa n−2 + a n−1 . This expression is continuous at x = a, so the limit as x → a can be evaluated by substitution. √ 5. Does the Power Rule apply to f (x) = 5 x? Explain. √ SOLUTION Yes. 5 x = x 1/5 , which is a power function. 6. In which of the following two cases does the derivative not exist? (a) Horizontal tangent (b) Vertical tangent SOLUTION

The derivative does not exist when there is a vertical tangent. At a horizontal tangent, the derivative is zero.

Exercises In Exercises 1–8, compute f (x) using the limit deﬁnition. 1. f (x) = 4x − 3 SOLUTION

Let f (x) = 4x − 3. Then, f (x) = lim

h→0

2 3. f (x) = 1 − 2x f (x) = x 2 + x

f (x + h) − f (x) 4(x + h) − 3 − (4x − 3) 4h = lim = lim = 4. h h h→0 h→0 h

The Derivative as a Function

S E C T I O N 3.2 SOLUTION

91

Let f (x) = 1 − 2x 2 . Then, f (x + h) − f (x) 1 − 2(x + h)2 − (1 − 2x 2 ) = lim h h h→0 h→0

f (x) = lim

−4xh − 2h 2 = lim (−4x − 2h) = −4x. h h→0 h→0

= lim 5. f (x) = x −1 3 f (x) = x SOLUTION Let f (x) = x −1 . Then,

x−x−h 1 − 1 f (x + h) − f (x) −1 1 x(x+h) x = lim x+h = lim = lim = − 2. h h h h→0 h→0 h→0 h→0 x(x + h) x

f (x) = lim

√ 7. f (x) = x x √ f (x) = SOLUTION Letx − f (x) 1 = x. Then, f (x + h) − f (x) = lim h h→0 h→0

f (x) = lim = lim

√

x +h− h

√

x

√ = lim

h→0

x +h− h

√

x

√ ·

√

√ x √ x +h+ x x +h+

1 (x + h) − x 1 √ √ = lim √ √ = √ . 2 x x + h + x) h→0 x + h + x

h→0 h(

In Exercises 9–16,−1/2 use the Power Rule to compute the derivative. f (x) = x d 4 9. x d x x=−2 d 4 d 4 3 x = 4x so x SOLUTION = 4(−2)3 = −32. dx d x x=−2 d 2/3 11. t d −4 dt x d x t=8x=3 d 2/3 2 1 d 2/3 2 −1/3 t SOLUTION so = (8)−1/3 = . t = t dt 3 dt 3 3 t=8 d 0.35 xd −2/3 dx t

dt d t=1 x 0.35 = 0.35(x 0.35−1 ) = 0.35x −0.65 . SOLUTION dx d √17 15. t d 14/3 dt x dx d √17 √ √17−1 SOLUTION = 17t t dt 13.

In Exercises compute f (a) and ﬁnd an equation of the tangent line to the graph at x = a. 2 d −17–20, t π dt= x 5 , a = 1 17. f (x) Let f (x) = x 5 . Then, by the Power Rule, f (x) = 5x 4 . The equation to the tangent line to the graph of f (x) at x = 1 is

SOLUTION

y = f (1)(x − 1) + f (1) = 5(x − 1) + 1 = 5x − 4. √ 19. f (x) = 3 x + 8x, a = 9 f (x) = x −2 , a = 3 3 17 SOLUTION Let f (x) = 3x 1/2 + 8x. Then f (x) = 2 x −1/2 + 8. In particular, f (9) = 2 . The tangent line at x = 9 is 17 17 9 y = f (9)(x − 9) + f (9) = (x − 9) + 81 = x+ . 2 2 2 √ In Exercises a = 8 the derivative of the function. f (x)21–32, = 3 x,calculate 21. f (x) = x 3 + x 2 − 12

92

CHAPTER 3

D I F F E R E N T I AT I O N

SOLUTION

d 3 x + x 2 − 12 = 3x 2 + 2x. dx

23. f (x) = 2x 3 −310x −12 f (x) = 2x − 3x + 2x d 2x 3 − 10x −1 = 6x 2 + 10x −2 . SOLUTION dx 25. g(z) = 7z −3 5+ z 2 +2 5 f (x) = x − 7x + 10x + 9 d SOLUTION 7z −3 + z 2 + 5 = −21z −4 + 2z. dz √ √ 3 27. f (s) = 4 s + √ s 1 √ +√ h(t) =f6(s)t = 4√ SOLUTION st + 3 s = s 1/4 + s 1/3 . In this form, we can apply the Sum and Power Rules. 1 1 1 1 d 1/4 + s 1/3 = (s (1/4)−1 ) + (s (1/3)−1 ) = s −3/4 + s −2/3 . s ds 4 3 4 3 29. f (x) = (x + 1)43 (Hint: Expand) + 7y 2/3 W (y) = 6y d 3 d

SOLUTION (x + 1)3 = x + 3x 2 + 3x + 1 = 3x 2 + 6x + 3. dx dx 31. P(z) = (3z − 1)(2z + 1) R(s) = (5s + 1)2 d d 2 SOLUTION 6z + z − 1 = 12z + 1. ((3z − 1)(2z + 1)) = dz dz √ calculate the derivative indicated. In Exercises 33–38, q(t) = t(t + 1) 3 33. f (2), f (x) = 4 x SOLUTION

With f (x) = 3x −4 , we have f (x) = −12x −5 , so 3 f (2) = −12(2)−5 = − . 8

d T √ , T = 3Cx2/3 +1 dCy C=8 (16), y = x dT SOLUTION With T (C) = 3C 2/3 , we have = 2C −1/3 . Therefore, dC d T = 2(8)−1/3 = 1. dC

35.

C=8

ds d P , s = 4z − 16z72 dz z=2 , P= d V V =−2 V ds SOLUTION With s = 4z − 16z 2 , we have = 4 − 32z. Therefore, dz ds = 4 − 32(2) = −60. dz z=2

37.

39. Match the functions in graphs (A)–(D) with their derivatives (I)–(III) in Figure 11. Note that two of the functions R derivative. Explain have thedsame why. , R = Wπ d W W =1 y

y

y

y x

x

x x (A)

(B)

(C)

y

y

(D)

y x

x (I)

x (II)

(III)

FIGURE 11 SOLUTION

S E C T I O N 3.2

The Derivative as a Function

93

• Consider the graph in (A). On the left side of the graph, the slope of the tangent line is positive but on the right

side the slope of the tangent line is negative. Thus the derivative should transition from positive to negative with increasing x. This matches the graph in (III). • Consider the graph in (B). This is a linear function, so its slope is constant. Thus the derivative is constant, which matches the graph in (I). • Consider the graph in (C). Moving from left to right, the slope of the tangent line transitions from positive to negative then back to positive. The derivative should therefore be negative in the middle and positive to either side. This matches the graph in (II). • Consider the graph in (D). On the left side of the graph, the slope of the tangent line is positive but on the right side the slope of the tangent line is negative. Thus the derivative should transition from positive to negative with increasing x. This matches the graph in (III). Note that the functions whose graphs are shown in (A) and (D) have the same derivative. This happens because the graph in (D) is just a vertical translation of the graph in (A), which means the two functions differ by a constant. The derivative of a constant is zero, so the two functions end up with the same derivative. 41. Sketch the graph of f (x) for f (x) as in Figure 13, omitting points where f (x) is not differentiable. Assign the labels f (x), g(x), and h(x) to the graphs in Figure 12 in such a way that f (x) = g(x) and g (x) = h(x). y 5 4 3 2 1

x

1 2 3 4 5 6 7 8 9

FIGURE 13 Graph of f (x).

On the interval 0 ≤ x ≤ 3, the function is constant, so f (x) = 0. On the interval 3 ≤ x ≤ 5, the function is linear with slope 12 , so f (x) = 12 . From x = 5 to x = 7, the derivative should be positive but decreasing to zero at x = 7. Finally, for 7 < x ≤ 9, the derivative is negative and decreasing. A sketch of f (x) is given below. SOLUTION

y

2 1 x

−1 −2

1 2 3 4 5 6 7 8 9

43. Let R be a variable and r a constant. Compute the derivatives f (x). Does f (x) appear to increased or decrease as a function of x? d The table below lists values of a function d (a) Is (A) R or (B) in Figure 14 the graph of (b)f (x)?rExplain. (c) r 2 R3 dR dR dR SOLUTION

x 0 0.5 1 1.5 2 d (a) R = 1, since R is a linear of R with f (x) function 10 55 98 slope 139 1. 177 dR d r = 0, since r is a constant. (b) dR (c) We apply the Linearity and Power Rules:

2.5

3

3.5

4

210

237

257

268

d 3 d 2 3 r R = r2 R = r 2 3(R 2 ) = 3r 2 R 2 . dR dR 45. Find the points on the curve y = x 2 + 23x − 7 at which the slope of the tangent line is equal to 4. Sketch the graph of f (x) = x − 3x and ﬁnd the values of x for which the tangent line is horizontal. 1 SOLUTION Let y = x 2 + 3x − 7. Solving d y/d x = 2x + 3 = 4 yields x = 2 . 47. Find all values of x where the tangent lines to y = x 3 and y = x 4 are parallel. Sketch the graphs of f (x) = x 2 − 5x + 4 and g(x) = −2x + 3. Find the value of x at which the graphs have SOLUTION Let f (x) = x 3 and let g(x) = x 4 . The two graphs have parallel tangent lines at all x where f (x) = g (x). parallel tangent lines. f (x) = g (x) 3x 2 = 4x 3 3x 2 − 4x 3 = 0 x 2 (3 − 4x) = 0 hence, x = 0 or x = 34 . Show that there is a unique point on the graph of the function f (x) = ax 2 + bx + c where the tangent line is horizontal (assume a = 0). Explain graphically.

94

CHAPTER 3

D I F F E R E N T I AT I O N

49. Determine coefﬁcients a and b such that p(x) = x 2 + ax + b satisﬁes p(1) = 0 and p (1) = 4. SOLUTION Let p(x) = x 2 + ax + b satisfy p(1) = 0 and p (1) = 4. Now, p (x) = 2x + a. Therefore 0 = p(1) = 1 + a + b and 4 = p (1) = 2 + a; i.e., a = 2 and b = −3.

m > −3, there are precisely two points 51. Let f (x) = x 3 − 3x + 1. Show that f (x) ≥ −3 for all x, and that for every + 11x + 2 isfor steeper than of themtangent line values of x the such that theoftangent line toand thethe graph of y = 4x 2tangent (x) all where fFind = m. Indicate position these points corresponding lines one value in a sketch 3 to y = x . of the graph of f (x). SOLUTION

Let P = (a, b) be a point on the graph of f (x) = x 3 − 3x + 1.

• The derivative satisﬁes f (x) = 3x 2 − 3 ≥ −3 since 3x 2 is nonnegative. • Suppose the slope m of the tangent line is greater than −3. Then f (a) = 3a 2 − 3 = m, whence

m+3 a2 = >0 3

and thus

a=±

m+3 . 3

• The two parallel tangent lines with slope 2 are shown with the graph of f (x) here. y

4 2 −2

1 −1

x

2

−2

Hint: Show that 53. Compute the derivative of f (x) = x −2 using the limit1 deﬁnition. Show that if the tangent lines to the graph of y = 3 x 3 − x 2 at x = a and at x = b are parallel, then either a = b or a + b = 2. (x + h)2 − x 2 1 f (x + h) − f (x) =− 2 h h x (x + h)2 SOLUTION

1

1

x 2 −(x+h)2

− 2 f (x + h) − f (x) 1 (x+h)2 x x 2 (x+h)2 = = = h h h h

x 2 − (x + h)2 1

1 x 2 (x + h)2

1 (x + h)2 − x 2 =− 2 . h x (x + h)2 We now compute the limits as h → 0 of each factor. By continuity, we see that, if x = 0, lim

−1

h→0 x 2 (x + h)2

1 = − 4. x

The second factor can be recognized easily as the limit deﬁnition of the derivative of x 2 , which is 2x. Applying the Product Rule, we get that: 1 2 d −2 = − 4 (2x) = − 3 . x dx x x √ 55. The average speed (in meters per second) of a gas molecule is vavg = 8RT /(π M), where T is the temperature (in 3/2 limit deﬁnition. the derivative of f (x)and = xR =using kelvin),Compute M is the molar mass (kg/mol) 8.31. the Calculate dvavg /d THint: at T Show = 300that K for oxygen, which has a molar mass of 0.032 kg/mol. f (x + h) − f (x) (x + √ h)3 − x 3 1 1/2 = 1/2 , where M √ and R are constant, we use the SOLUTION Using the form vav = (8RT /(π M))= h h8R/(π M)T (x + h)3 + x 3 Power Rule to compute the derivative dvav /d T . d d 1/2 1 = 8R/(π M) (T (1/2)−1 ). 8R/(π M)T 1/2 = 8R/(π M) T dT dT 2 In particular, if T = 300◦ K, 1 d vav = 8(8.31)/(π (0.032)) (300)−1/2 = 0.74234 m/(s · K). dT 2 57. Some studies suggest that kidney mass K in mammals (in kilograms) is related to body mass m (in kilograms) by Biologists have observed that the pulse rate P (in beats per minute) in animals is related to body mass (in the approximate formula K = 0.007m 0.85 . Calculate d K /dm at m = 68. Then calculate the derivative with respect to kilograms) by the approximate formula P = 200m −1/4 . This is one of many allometric scaling laws prevalent in m of the relative kidney-to-mass ratio K /m at m = 68. biology. Is the absolute value |d P/dm| increasing or decreasing as m increases? Find an equation of the tangent line at the points on the graph in Figure 15 that represent goat (m = 33) and man (m = 68).

S E C T I O N 3.2

The Derivative as a Function

95

SOLUTION

dK = 0.007(0.85)m −0.15 = 0.00595m −0.15 ; dm hence, d K = 0.00595(68)−0.15 = 0.00315966. dm m=68 Because K m 0.85 = 0.007 = 0.007m −0.15 , m m we ﬁnd d dm

K m

and d dm

= 0.007

d −0.15 m = −0.00105m −1.15 , dm

K = −8.19981 × 10−6 kg−1 . m m=68

59. Let L be a tangent line to the hyperbola x y = 1 at x = a, where a > 0. Show that the area of the triangle bounded The relation between the vapor pressure P (in atmospheres) of water and the temperature T (in kelvin) is given by L and the coordinate axes does not depend on a. by the Clausius–Clapeyron law: 1 1 SOLUTION Let f (x) = x −1 . The tangent line to f at x = a is y = f (a)(x − a) + f (a) = − 2 (x − a) + a . The a P dP 2 = k 2y = 0) is 2a. Hence the area of the triangle bounded y-intercept of this line (where x = 0) is a . Its x-intercept (where

dT T 1 1 by the tangent line and the coordinate axes is A = 2 bh = 2 (2a) a2 = 2, which is independent of a. where k is a constant. Use the table below and the approximation y dP P(T + 10) − P(T − 10) ≈ dT 20 1 y=

x

to estimate d P/d T for T = 303, 313, 323, 333, 343. Do your estimates seem to conﬁrm the Clausius–Clapeyron 2 = (0, a )are the units of k? law? What is the approximate value of k?P What

( 1)

R = a, a

T

293

303

313

323

333

343

353

x

P 0.0278 0.0482 0.0808 0.1311 Q = (2a, 0)0.2067 0.3173 0.4754 61. Match the functions (A)–(C) with their derivatives (I)–(III) in Figure 16. In the notation of Exercise 59, show that the point of tangency is the midpoint of the segment of L lying in the ﬁrst quadrant. y y

x

x

(A)

(I) y

y

x x

(B)

(II)

y

y

x x (C)

(III)

FIGURE 16

96

CHAPTER 3

D I F F E R E N T I AT I O N SOLUTION Note that the graph in (A) has three locations with a horizontal tangent line. The derivative must therefore cross the x-axis in three locations, which matches (III). The graph in (B) has only one location with a horizontal tangent line, so its derivative should cross the x-axis only once. Thus, (I) is the graph corresponding to the derivative of (B). Finally, the graph in (B) has two locations with a horizontal tangent line, so its derivative should cross the x-axis twice. Thus, (II) is the graph corresponding to the derivative of (C).

63. Make a rough sketch of the graph of the derivative of the function shown in Figure 17(A). Plot the derivative f (x) of f (x) = 2x 3 − 10x −1 for x > 0 (set the bounds of the viewing box appropriately) SOLUTION Thethat graph has>a 0. tangent negative slope onthe thegraph interval 3.6), andPlot has af (x) tangent and observe f (x) What line doeswith the positivity of f approximately (x) tell us about of (1, f (x) itself? and line with a positive slope elsewhere. This implies that the derivative must be negative on the interval (1, 3.6) and positive conﬁrm. elsewhere. The graph may therefore look like this: y

x 1

65.

2

3

4

thederivative two functions and g inshown Figurein18, which is theomitting derivative of thewhere other? your is answer. GraphOfthe of the ffunction Figure 17(B), points theJustify derivative not deﬁned. y f(x)

2

g(x) x

−1

1

FIGURE 18 SOLUTION g(x) is the derivative of f (x). For f (x) the slope is negative for negative values of x until x = 0, where there is a horizontal tangent, and then the slope is positive for positive values of x. Notice that g(x) is negative for negative values of x, goes through the origin at x = 0, and then is positive for positive values of x.

In Exercises 67–72, ﬁnd is thethe points c (if in any) such19 that f (c) does not At which points function Figure discontinuous? Atexist. which points is it nondifferentiable? 67. f (x) = |x − 1| SOLUTION y 2 1.5 1 0.5 x

−1

1

2

3

Here is the graph of f (x) = |x − 1|. Its derivative does not exist at x = 1. At that value of x there is a sharp corner. 2/3 69. f (x)f (x) = x= [x]

SOLUTION Here is the graph of f (x) = x 2/3 . Its derivative does not exist at x = 0. At that value of x, there is a sharp corner or “cusp”. y 1.5 1

−2

−1

x 1

2

71. f (x) = |x 2 −3/2 1| f (x) = x SOLUTION Here is the graph of f (x) = x 2 − 1. Its derivative does not exist at x = −1 or at x = 1. At these values of x, the graph has sharp corners.

S E C T I O N 3.2

The Derivative as a Function

97

y 3 2 1

−2

x

−1

1

2

f (x) = |x −73–78, 1|2 zoom in on a plot of f (x) at the point (a, f (a)) and state whether or not f (x) appears to be In Exercises differentiable at x = a. If nondifferentiable, state whether the tangent line appears to be vertical or does not exist. 73. f (x) = (x − 1)|x|,

a=0

SOLUTION The graph of f (x) = (x − 1)|x| for x near 0 is shown below. Because the graph has a sharp corner at x = 0, it appears that f is not differentiable at x = 0. Moreover, the tangent line does not exist at this point. y x

− 0.2 −0.1

0.1

0.2

−0.1 −0.2 −0.3

75. f (x) = (x − 3)1/3 ,5/3a = 3 f (x) = (x − 3) , a = 3 SOLUTION The graph of f (x) = (x − 3)1/3 for x near 3 is shown below. From this graph, it appears that f is not differentiable at x = 3. Moreover, the tangent line appears to be vertical.

2.9

2.95

3

3.05

3.1

77. f (x) = | sin x|, a = 0 f (x) = sin(x 1/3 ), a = 0 SOLUTION The graph of f (x) = | sin x| for x near 0 is shown below. Because the graph has a sharp corner at x = 0, it appears that f is not differentiable at x = 0. Moreover, the tangent line does not exist at this point. y 0.1 0.08 0.04

−0.1 −0.05

79.

x 0.05

0.1

thatx|, a nondifferentiable function is not continuous? If not, give a counterexample. f (x)Is=it|xtrue − sin a=0

SOLUTION

This statement is false. Indeed, the function |x| is not differentiable at x = 0 but it is continuous.

Sketch the graph of y = x |x| and show that it is differentiable for all x Further Insights and Challenges x < 0, x = 0, and x > 0).

(check differentiability separately for

81. Prove the following theorem of Apollonius of Perga (the Greek mathematician born in 262 BCE who gave the parabola, ellipse, and hyperbola their names): The tangent to the parabola y = x 2 at x = a intersects the x-axis at the midpoint between the origin and (a, 0). Draw a diagram. SOLUTION

Let f (x) = x 2 . The tangent line to f at x = a is y = f (a)(x − a) + f (a) = 2a(x − a) + a 2 = 2ax − a 2 .

The x-intercept of this line (where y = 0) is a2 , which is halfway between the origin and the point (a, 0).

98

CHAPTER 3

D I F F E R E N T I AT I O N y

(a, a 2) y=

x2

(–,a2 0)

x

83. Plot the graph of f (x) = (4 − x 2/3 )3/2 (the “astroid”). Let L be a tangent line to a point on the graph in 3 Apollonius’s in Exercise canﬁrst be generalized. that the tangent the ﬁrst quadrant. ShowTheorem that the portion of L 81 in the quadrant hasShow a constant length 8. to y = x at x = a intersects the x-axis at the point x = 23 a. Then formulate the general statement for the graph of y = x n and prove it. SOLUTION

• Here is a graph of the astroid. y 10

x

−10

10

−10

• Let f (x) = (4 − x 2/3 )3/2 . Since we have not yet encountered the Chain Rule, we use Maple throughout this

exercise. The tangent line to f at x = a is

4 − a 2/3 2/3 3/2 . y=− (x − a) + 4 − a a 1/3

The y-intercept of this line is the point P = 0, 4 4 − a 2/3 , its x-intercept is the point Q = 4a 1/3 , 0 , and the distance between P and Q is 8.

85. A vase is formed by rotating y = x 2 around the y-axis. If we drop in a marble, it will either the bottom point 2 fortouch −1marble ≤ x ≤be1 to and the Twoorsmall arches have thethe shape of parabolas. Thetheﬁrst is given by21). f (x) = small 1 − xmust of the vase be suspended above bottom by touching sides (Figure How the touch 2 for 2 ≤ x ≤ 6. A board is placed on top of these arches so it rests on both (Figure second by g(x) = 4 − (x − 4) the bottom? 20). What is the slope of the board? Hint: Find the tangent line to y = f (x) that intersects y = g(x) in exactly one point. FIGURE 20

FIGURE 21

Suppose a circle is tangent to the parabola y = x 2 at the point (t, t 2 ). The slope of the parabola at this 1 (since it is perpendicular to the tangent line of the point is 2t, so the slope of the radius of the circle at this point is − 2t 1 (x − t) + t 2 crosses the y-axis. We can circle). Thus the center of the circle must be where the line given by y = − 2t 1 2 ﬁnd the y-coordinate by setting x = 0: we get y = 2 + t . Thus, the radius extends from (0, 12 + t 2 ) to (t, t 2 ) and SOLUTION

r=

2 1 1 2 2 2 +t = +t −t + t 2. 2 4

This radius is greater than 12 whenever t > 0; so, if a marble has radius > 1/2 it sits on the edge of the vase, but if it has radius ≤ 1/2 it rolls all the way to the bottom. 87. Negative Exponents Let n be a whole number. Use the Power Rule for x n to calculate the derivative of f (x) = Let f (x) thatbe a differentiable function and set g(x) = f (x + c), where c is a constant. Use the limit deﬁnition to x −n by showing show that g (x) = f (x + c). Explain this result graphically, recalling that the graph of g(x) is obtained by shifting (x + − 0)f (x) the graph of f (x) c units to the fleft (ifh) c> or right (if −1 c < 0). (x + h)n − x n = n h x (x + h)n h

Product and Quotient Rules

S E C T I O N 3.3 SOLUTION

99

Let f (x) = x −n where n is a positive integer.

• The difference quotient for f is 1

1

x n −(x+h)n

(x + h)−n − x −n f (x + h) − f (x) (x+h)n − x n x n (x+h)n = = = h h h h (x + h)n − x n −1 . = n x (x + h)n h • Therefore,

f (x + h) − f (x) −1 (x + h)n − x n = lim n n h h h→0 h→0 x (x + h)

f (x) = lim

= lim

−1

lim

h→0 x n (x + h)n h→0

(x + h)n − x n d n = −x −2n x . h dx

d n x = −x −2n · nx n−1 = −nx −n−1 . Since n is a positive integer, dx d −n d k d xk = x x = kx k−1 = −nx −n−1 = kx k−1 ; i.e. k = −n is a negative integer and we have dx dx dx for negative integers k.

• From above, we continue: f (x) = −x −2n

89. Inﬁnitely Rapid Oscillations Deﬁne Verify the Power Rule for the exponent 1/n, where n is a positive integer, using the following trick: Rewrite the difference quotient for y = x 1/n at x = b in terms⎧ of u =1 (b + h)1/n and a = b1/n . ⎪ ⎨x sin if x = 0 x f (x) = ⎪ ⎩0 if x = 0 Show that f (x) is continuous at x = 0 but f (0) does not exist (see Figure 10).

if x = 0 x sin 1x SOLUTION Let f (x) = . As x → 0, 0 if x = 0 1 1 | f (x) − f (0)| = x sin − 0 = |x| sin →0 x x since the values of the sine lie between −1 and 1. Hence, by the Squeeze Theorem, lim f (x) = f (0) and thus f is continuous at x = 0. As x → 0, the difference quotient at x = 0,

x→0

x sin 1x − 0 1 f (x) − f (0) = = sin x −0 x −0 x does not converge to a limit since it oscillates inﬁnitely through every value between −1 and 1. Accordingly, f (0) does not exist.

3.3 Product and Quotient Rules Preliminary Questions 1. (a) (b) (c)

Are the following statements true or false? If false, state the correct version. The notation f g denotes the function whose value at x is f (g(x)). The notation f /g denotes the function whose value at x is f (x)/g(x). The derivative of the product is the product of the derivatives.

SOLUTION

(a) False. The notation f g denotes the function whose value at x is f (x)g(x). (b) True. (c) False. The derivative of the product f g is f (x)g(x) + f (x)g (x). 2. Are the following equations true or false? If false, state the correct version. d (a) = f (4)g (4) − g(4) f (4) ( f g) dx x=4

100

CHAPTER 3

D I F F E R E N T I AT I O N

d f f (4)g (4) + g(4) f (4) = d x g x=4 (g(4))2 d (c) = f (0)g (0) + g(0) f (0) ( f g) dx x=0

(b)

SOLUTION

d = f (4)g (4) + g(4) f (4). ( f g) dx x=4 d = [g(4) f (4) − f (4)g (4)]/g(4)2 . (b) False. ( f /g) dx x=4 (c) True. (a) False.

3. What is the derivative of f /g at x = 1 if f (1) = f (1) = g(1) = 2, and g (1) = 4? d SOLUTION ( f /g)x=1 = [g(1) f (1) − f (1)g (1)]/g(1)2 = [2(2) − 2(4)]/22 = −1. dx 4. Suppose that f (1) = 0 and f (1) = 2. Find g(1), assuming that ( f g) (1) = 10. ( f g) (1) = f (1)g (1) + f (1)g(1), so 10 = 0 · g (1) + 2g(1) and g(1) = 5.

SOLUTION

Exercises In Exercises 1–4, use the Product Rule to calculate the derivative. 1. f (x) = x(x 2 + 1) Let f (x) = x(x 2 + 1). Then

SOLUTION

f (x) = x

3.

d 2 d (x + 1) + (x 2 + 1) x = x(2x) + (x 2 + 1) = 3x 2 + 1. dx dx

d y √ (t 2 + 1)(t (x), = y =x(1 − x 4 ) + 9) dt ft=3 Let y = (t 2 + 1)(t + 9). Then

SOLUTION

d d dy = (t 2 + 1) (t + 9) + (t + 9) (t 2 + 1) = (t 2 + 1) + (t + 9)(2t) = 3t 2 + 18t + 1. dt dt dt Therefore, d y = 3(3)2 + 18(3) + 1 = 82. dt t=3 In Exercises Rule to calculate the derivative. dh 5–8, use the Quotient −1/2 + 2x)(7 − x −1 ) , h(x) = (x dx x 5. f (x) =x=4 x −2 x . Then Let f (x) = x−2

SOLUTION

f (x) =

(x − 2) ddx x − x ddx (x − 2) (x − 2) − x −2 = = . (x − 2)2 (x − 2)2 (x − 2)2

dg t2 + 1 x+ , g(t) =4 2 (x) = dt ft=−2 x 2 + x +t 1− 1 t2 + 1 SOLUTION Let g(t) = . Then t2 − 1 7.

d (t 2 + 1) − (t 2 + 1) d (t 2 − 1) (t 2 − 1) dt (t 2 − 1)(2t) − (t 2 + 1)(2t) 4t dg dt = =− 2 . = dt (t 2 − 1)2 (t 2 − 1)2 (t − 1)2

Therefore, 4(−2) 8 dg =− = . dt t=−2 9 ((−2)2 − 1)2 dw

, w= √

z2

S E C T I O N 3.3

Product and Quotient Rules

101

In Exercises 9–12, calculate the derivative in two ways: First use the Product or Quotient Rule, then rewrite the function algebraically and apply the Power Rule directly. 9. f (t) = (2t + 1)(t 2 − 2) SOLUTION

Let f (t) = (2t + 1)(t 2 − 2). Then, using the Product Rule, f (t) = (2t + 1)(2t) + (t 2 − 2)(2) = 6t 2 + 2t − 4.

Multiplying out ﬁrst, we ﬁnd f (t) = 2t 3 + t 2 − 4t − 2. Therefore, f (t) = 6t 2 + 2t − 4. x 3 + 2x 2 + 3x −1 11. g(x)f (x) = = x 2 (3 + x −1 ) x SOLUTION

3 2 −1 Let g(x) = x +2x x+3x . Using the quotient rule and the sum and power rules, and simplifying

g (x) =

x(3x 2 + 4x − 3x −2 ) − (x 3 + 2x 2 + 3x −1 )1 1 3 2 − 6x −1 = 2x + 2 − 6x −3 . 2x = + 2x x2 x2

Simplifying ﬁrst yields g(x) = x 2 + 2x + 3x −2 , from which we calculate g (x) = 2x + 2 − 6x −3 . In Exercises 13–32, calculate the derivative using the appropriate rule or combination of rules. t2 − 1 h(t) = 4 1 2 + x + 1) 13. f (x) = (x t−−4)(x

SOLUTION Let f (x) = x 4 − 4 x 2 + x + 1 . Then f (x) = (x 4 − 4)(2x + 1) + (x 2 + x + 1)(4x 3 ) = 6x 5 + 5x 4 + 4x 3 − 8x − 4. 15.

d y 1 2 + 9)(x + x −1 ) (x), = y(x= d x fx=2 x +4

SOLUTION

1 . Using the quotient rule: Let y = x+4

1 (x + 4)(0) − 1(1) dy =− . = dx (x + 4)2 (x + 4)2 Therefore, d y 1 1 =− =− . d x x=2 36 (2 + 4)2 √ √ 17. f (x) = ( x + 1)( x − 1) x dz z == 2(√x + 1)(√x − 1). Multiplying through ﬁrst yields f (x) = x − 1 for x ≥ 0. Therefore, SOLUTION Let , f (x) d x x=−1 x +1 f (x) = 1 for x ≥ 0. If we carry out the product rule on f (x) = (x 1/2 + 1)(x 1/2 − 1), we get 1 −1/2 1 −1/2 1 1 1 1 f (x) = (x 1/2 + 1) ) + (x 1/2 − 1) (x x = + x −1/2 + − x −1/2 = 1. 2 2 2 2 2 2 √ x4 − 4 d y 2 , y3 =x − (x) = d x fx=2 x2 − 5 x x4 − 4 SOLUTION Let y = . Then x2 − 5

x 2 − 5 4x 3 − x 4 − 4 (2x) 2x 5 − 20x 3 + 8x dy = . = 2 2 dx x2 − 5 x2 − 5

19.

Therefore, 2(2)5 − 20(2)3 + 8(2) d y = = −80. d x x=2 (22 − 5)2 21.

dz 1 4 + 2x + 1 , zx = d x fx=1 x3 + 1 (x) = x +1

102

CHAPTER 3

D I F F E R E N T I AT I O N SOLUTION

Let z =

1 . Using the quotient rule: x 3 +1

3x 2 (x 3 + 1)(0) − 1(3x 2 ) dz = − . = dx (x 3 + 1)2 (x 3 + 1)2 Therefore, 3(1)2 3 dz = − =− . 3 2 d x x=1 4 (1 + 1) t 23. h(t) = 4 3x 32 − 7x 2 + 2 (t + t )(t f (x) = √ + 1) x t t = SOLUTION Let h(t) = . Then t4 + t2 t7 + 1 t 11 + t 9 + t 4 + t 2

t 11 + t 9 + t 4 + t 2 (1) − t 11t 10 + 9t 8 + 4t 3 + 2t 10t 11 + 8t 9 + 3t 4 + t 2 =− h (t) = 2 2 . t 11 + t 9 + t 4 + t 2 t 11 + t 9 + t 4 + t 2 4 25. f (t) = 31/2 ·3/2 51/2 − 3x 2x √ f (x) = x √ + x −1/2 SOLUTION Let f (t) = 3 5. Then f (t) = 0, since f (t) is a constant function! 27. f (x) = (x + 3)(x − 1)(x − 5) h(x) = π 2 (x − 1) SOLUTION Let f (x) = (x + 3)(x − 1)(x − 5). Using the Product Rule inside the Product Rule with a ﬁrst factor of (x + 3) and a second factor of (x − 1)(x − 5), we ﬁnd f (x) = (x + 3) ((x − 1)(1) + (x − 5)(1)) + (x − 1)(x − 5)(1) = 3x 2 − 6x − 13. Alternatively,

f (x) = (x + 3) x 2 − 6x + 5 = x 3 − 3x 2 − 13x + 15. Therefore, f (x) = 3x 2 − 6x − 13. z2 − 1 z2 − 4 (x) = x(x 2 + 1)(x + 4) 29. g(z)f = z−1 z+2 SOLUTION

Hint: Simplify ﬁrst.

Let g(z) =

z2 − 4 z−1

z2 − 1 z+2

=

(z + 2)(z − 2) (z + 1)(z − 1) z−1 z+2

= (z − 2)(z + 1)

for z = −2 and z = 1. Then, g (z) = (z + 1)(1) + (z − 2)(1) = 2z − 1. d xt − 4 d (x constant) 2 (ax (a, b constants) dt t 2 − x + b)(abx + 1) dx xt−4 SOLUTION Let f (t) = 2 . Using the quotient rule:

31.

t −x

f (t) =

(t 2 − x)(x) − (xt − 4)(2t) xt 2 − x 2 − 2xt 2 + 8t −xt 2 + 8t − x 2 = = . (t 2 − x)2 (t 2 − x)2 (t 2 − x)2

x the derivative over [−4, 4]. Use the graph to determine the intervals on which f (x) > of f (x) = 2 d Plot ax + b x +1 (a, b, c, d constants) x

f (x) =

(x 2 + 1)(1) − x(2x) 1 − x2 = . (x 2 + 1)2 (x 2 + 1)2

The derivative is shown in the ﬁgure below at the left. From this plot we see that f (x) > 0 for −1 < x < 1 and f (x) < 0 for |x| > 1. The original function is plotted in the ﬁgure below at the right. Observe that the graph of f (x) is increasing whenever f (x) > 0 and that f (x) is decreasing whenever f (x) < 0.

Product and Quotient Rules

S E C T I O N 3.3 y

y 1 0.8 0.6 0.4 0.2

0.4 0.2

1

2

3

x

−4 −3 −2 −1 − 0.2

x −4 −3 −2 −1

103

1

2

3

4

−0.4

4

V2R 5. Calculate dP/dr, assuming that rthe is plot variable and R is whether constant. f (x) is positive 35. Let P Plot = f (x) =2 as xin Example (in a suitably bounded viewing box). Use to determine (R + r ) x 2 − 1 or negative on its {x :constant. x = ±1}.Let Then compute f (x) and conﬁrm your conclusion algebraically. SOLUTION Note thatdomain V is also f (r ) =

V2R V2R = 2 . 2 (R + r ) R + 2Rr + r 2

Using the quotient rule: f (r ) =

(R 2 + 2Rr + r 2 )(0) − (V 2 R)(2R + 2r ) 2V 2 R(R + r ) 2V 2 R = − = − . (R + r )4 (R + r )4 (R + r )3

x at volts), x = 2. and resistance R (in ohms) in a circuit are 37. FindAccording an equation of the tangent line to Ithe y= to Ohm’s Law, current (ingraph amps), voltage V (in x + x −1 related by I = V /R. x . The equation of the tangent line to the graph of f (x) at x = 2 is y = f (2)(x − 2) + SOLUTION Let f (x) = d I x+x −1 , assuming that V has the constant value V = 24. Calculate f (2).(a) Using the quotient rule, we compute: d R R=6

dV 2 (x+ x −1 )(1) − (x)(1 − x −2 ) 1 −1 − x + x −1 = (b) Calculate , assuming that I has the=constant value Ix = 4. + x . f (x) = d R R=6 (x + x −1 )2 (x + x −1 )2 x(x + x −1 )2 2 = 4 . This makes the equation of the tangent line Therefore, f (2) = 25 25 2

y=

2 12 4 4 (x − 2) + 5 = x+ . 25 25 5 2

39. Let f (x) = g(x) = x. Show that ( f /g) = f /g . 1 = 0.ofOn The curve Agnesi (Figure 3) (after Maria Agnesi SOLUTION ( f /g)y = (x/x) =is 1,called so ( fthe /g)witch the other hand, f /g the ) =Italian (x /x mathematician ) = (1/1) = 1. We see that x2 + 1 0 = 1. (1718–1799) 3who wrote one of the ﬁrst books on calculus. This strange name is the result of a mistranslation of the = 3 f 2 f . 41. Show that ( fla)versiera, Italian meaning “that( which Useword the Product Rule to show that f 2 ) =turns.” 2 f f . Find equations of the tangent lines at x = ±1. SOLUTION Let g = f 3 = f f f . Then

g = f 3 = [ f ( f f )] = f f f + f f + f f ( f ) = 3 f 2 f . In Exercises 42–45, use the following function values: f (4)

f (4)

g(4)

g (4)

2

−3

5

−1

43. Calculate F (4), where F(x) = x f (x). Find the derivative of fg and f /g at x = 4. SOLUTION Let F(x) = x f (x). Then F (x) = x f (x) + f (x), and F (4) = 4 f (4) + f (4) = 4(−3) + 2 = −10. x 45. Calculate H (4), where H (x) = = xg(x) f .(x). where G(x) Calculate G (4), g(x) f (x) x SOLUTION Let H (x) = . Then g(x) f (x)

g(x) f (x) · 1 − x g(x) f (x) + f (x)g (x) . H (x) = (g(x) f (x))2 Therefore, H (4) =

(5)(2) − 4 · ((5)(−3) + (2)(−1)) ((5)(2))2

=

78 39 = . 100 50

104

CHAPTER 3

D I F F E R E N T I AT I O N

47. Proceed as in Exercise 46 to calculate F (0), where Calculate F (0), where 3x 5 + 5x 4 + 5x + 1 9+ x 5/3 F(x) = 1 + x + x 4/3x+ x 8 + 4x 5 9− 7x 4 8x − 7x + 1 F(x) = 4 x − 3x 2 + 2x + 1 SOLUTION Write F(x) = f (x)(g(x)/ h(x)), where Hint: Do not calculate F (x). Instead, write F(x) = f (x)/g(x) and express F (0) directly in terms of f (0), f (0), g(0), g (0). f (x) = (1 + x + x 4/3 + x 5/3 ) g(x) = 3x 5 + 5x 4 + 5x + 1 and h(x) = 8x 9 − 7x 4 + 1. 1 2 Now, f (x) = 1 + 43 x 3 + 53 x 3 , g (x) = 15x 4 + 20x 3 + 5, and h (x) = 72x 8 − 28x 3 . Moreover, f (0) = 1, f (0) = 1, g(0) = 1, g (0) = 5, h(0) = 1, and h (0) = 0. From the product and quotient rules,

F (0) = f (0)

h(0)g (0) − g(0)h (0) 1(5) − 1(0) + f (0)(g(0)/ h(0)) = 1 + 1(1/1) = 6. 2 1 h(0)

Further Insights and Challenges 49. Prove the Quotient Rule using the limit deﬁnition of the derivative. Let f , g, h be differentiable functions. Show that ( f gh) (x) is equal to f . Suppose that f and g are differentiable at x = a and that g(a) = 0. Then SOLUTION Let p = g f (x)g(x)h (x) + f (x)g (x)h(x) + f (x)g(x)h(x) f (a + h) f (a + h)g(a) − f (a)g(a + h) f (a) Hint: Write f gh as f (gh). − p(a + h) − p(a) g(a + h) g(a) g(a + h)g(a) p (a) = lim = lim = lim h h h h→0 h→0 h→0 f (a + h)g(a) − f (a)g(a) + f (a)g(a) − f (a)g(a + h) hg(a + h)g(a) 1 f (a + h) − f (a) g(a + h) − g(a) = lim g(a) − f (a) h h h→0 g(a + h)g(a) 1 f (a + h) − f (a) g(a + h) − g(a) = lim g(a) lim − f (a) lim h h h→0 g(a + h)g(a) h→0 h→0

= lim

h→0

=

1 (g(a))2

In other words, p =

g(a) f (a) − f (a)g (a) g(a) f (a) − f (a)g (a) = (g(a))2

f g f − f g = . g g2

51. Derive the Quotient Rule using Eq. (6) and the Product Rule. Derivative of the Reciprocal Use the limit deﬁnition to show that if f (x) is differentiable and f (x) = 0, then f (x) 1 SOLUTION h(x) = g(x) 1/ f (x) is Let differentiable and. We can write h(x) = f (x) g(x) . Applying Eq. (6), 1 f (x) 1 1 d g−(x) − f (x)g (x) + f (x)g(x) f (x) = h (x) = f (x) . = − f (x) = + f (x) + 2 f 2(x) g(x) g(x) g(x) d x f (x) (g(x)) (g(x))2 Hint: Show that the difference quotient for 1/ f (x) is equal to 53. Carry out the details of Agnesi’s proof of the Quotient Rule from her book on calculus, published in 1748: Assume Show that Eq. (6) is a special case of the Quotient Rule. − f (x of + h) that f , g, and h = f /g are differentiable. Compute thef (x) derivative hg = f using the Product Rule and solve for h . h f (x) f (x + h) SOLUTION Suppose that f , g, and h are differentiable functions with h = f /g. • Then hg = f and via the product rule hg + gh = f .

f − hg • Solving for h yields h = = g

f − g

f g g

=

g f − f g . g2

In Exercises 55–56, let f (x) be a polynomial. A basic fact of algebra states that c is a root of f (x) if and only if The Power Rule Revisited If you are familiar with proof by induction, use induction to2prove the Power Rule h(x), where h(x) is a f (x) = (x − c)g(x) for some polynomial g(x). We say that c is a multiple root if f (x)n = (x − c) for all whole numbers n. Show that the Power Rule holds for n = 1, then write x as x · x n−1 and use the Product polynomial. Rule. 55. Show that c is a multiple root of f (x) if and only if c is a root of both f (x) and f (x).

S E C T I O N 3.4

Rates of Change

105

SOLUTION Assume ﬁrst that f (c) = f (c) = 0 and let us show that c is a multiple root of f (x). We have f (x) = (x − c)g(x) for some polynomial g(x) and so f (x) = (x − c)g (x) + g(x). However, f (c) = 0 + g(c) = 0, so c is also a root of g(x) and hence g(x) = (x − c)h(x) for some polynomial h(x). We conclude that f (x) = (x − c)2 h(x), which shows that c is a multiple root of f (x). Conversely, assume that c is a multiple root. Then f (c) = 0 and f (x) = (x − c)2 g(x) for some polynomial g(x). Then f (x) = (x − c)2 g (x) + 2g(x)(x − c). Therefore, f (c) = (c − c)2 g (c) + 2g(c)(c − c) = 0.

57. Figure 4 55 is the graph of awhether polynomial at A, B, and of these is a multiple root? Explain Use Exercise to determine c = with −1 isroots a multiple root ofC. theWhich polynomials 5 4 3 2 your(a) reasoning using the result of Exercise 55. x + 2x − 4x − 8x − x + 2 (b) x 4 + x 3 − 5x 2 − 3x + 2 y

x A

B

C

FIGURE 4 SOLUTION A on the ﬁgure is a multiple root. It is a multiple root because f (x) = 0 at A and because the tangent line to the graph at A is horizontal, so that f (x) = 0 at A. For the same reasons, f also has a multiple root at C.

3.4 Rates of Change Preliminary Questions 1. What units might be used to measure the ROC of: (a) Pressure (in atmospheres) in a water tank with respect to depth? (b) The reaction rate of a chemical reaction (the ROC of concentration with respect to time), where concentration is measured in moles per liter? SOLUTION

(a) The rate of change of pressure with respect to depth might be measured in atmospheres/meter. (b) The reaction rate of a chemical reaction might be measured in moles/(liter·hour). 2. Suppose that f (2) = 4 and the average ROC of f between 2 and 5 is 3. What is f (5)? SOLUTION

The average rate of change of f between 2 and 5 is given by f (5) − 4 f (5) − f (2) = . 5−2 3

Setting this equal to 3 and solving for f (5) yields f (5) = 13. 3. Two trains travel from New Orleans to Memphis in 4 hours. The ﬁrst train travels at a constant velocity of 90 mph, but the velocity of the second train varies. What was the second train’s average velocity during the trip? SOLUTION

Since both trains travel the same distance in the same amount of time, they have the same average velocity:

90 mph. 4. Estimate f (26), assuming that f (25) = 43 and f (25) = 0.75. SOLUTION

f (x) ≈ f (25) + f (25)(x − 25), so f (26) ≈ 43 + 0.75(26 − 25) = 43.75.

5. The population P(t) of Freedonia in 1933 was P(1933) = 5 million. (a) What is the meaning of the derivative P (1933)? (b) Estimate P(1934) if P (1933) = 0.2. What if P (1933) = 0? SOLUTION

(a) Because P(t) measures the population of Freedonia as a function of time, the derivative P (1933) measures the rate of change of the population of Freedonia in the year 1933. (b) P(1934) ≈ P(1933) + P (1933). Thus, if P (1933) = 0.2, then P(1934) ≈ 5.2 million. On the other hand, if P (1933) = 0, then P(1934) ≈ 5 million.

CHAPTER 3

D I F F E R E N T I AT I O N

Exercises 1. Find the ROC of the area of a square with respect to the length of its side s when s = 3 and s = 5. Let the area be A = f (s) = s 2 . Then the rate of change of A with respect to s is d/ds(s 2 ) = 2s. When s = 3, the area changes at a rate of 6 square units per unit increase. When s = 5, the area changes at a rate of 10 square units per unit increase. (Draw a 5 × 5 square on graph paper and trace the area added by increasing each side length by 1, excluding the corner, to see what this means.) SOLUTION

3. FindFind the the ROC of yof=the x −1 with respect to with x forrespect x = 1, to 10.the length of its side s when s = 3 and s = 5. ROC volume of a cube 1 . −2 SOLUTION The ROC of change is d y/d x = −x . If x = 1, the ROC is −1. If x = 10, the ROC is − 100 √ In Exercises 5–8,rate calculate the ROC. At what is the cube root 3 x changing with respect to x when x = 1, 8, 27? dV 5. , where V is the volume of a cylinder whose height is equal to its radius (the volume of a cylinder of height h dr and radius r is π r 2 h) SOLUTION

The volume of the cylinder is V = π r 2 h = π r 3 . Thus d V /dr = 3π r 2 .

4 3 7. ROC of the volume V of (the volume ROC of the volume V aofsphere a cubewith withrespect respecttotoitsitsradius surface area A of a sphere is V = 3 π r ) SOLUTION

The volume of a sphere of radius r is V = 43 π r 3 . Thus d V /dr = 4π r 2 .

In Exercises d A 9–10, refer to Figure 10, which shows the graph of distance (in kilometers) versus time (in hours) for2 a car , where A is the surface area of a sphere of diameter D (the surface area of a sphere of radius r is 4π r ) trip. dD Distance 150 (km) 100 50 0.5

1

1.5

2

2.5

3

Time (hours)

FIGURE 10 Graph of distance versus time for a car trip.

9. (a) Estimate the average velocity over [0.5, 1]. (b) Is average velocity greater over [1, 2] or [2, 3]? (c) At what time is velocity at a maximum? SOLUTION

(a) The average velocity over the interval [.5, 1] is 50 − 25 = 50 km/h. 1 − .5 (b) The average velocity over the interval [1, 2], 75 − 50 = 25 km/h, 2−1 which is less than that over the interval [2, 3], 150 − 75 = 75 km/h. 3−2 (c) The car’s velocity is maximum when the slope of the distance versus time curve is most positive. This appears to happen when t = 0.5 h, t = 1.25 h, or t = 2.5 h. 11. Figure 11 displays the voltage across a capacitor as a function of time while the capacitor is being charged. Estimate Match the description with the interval (a)–(d). the ROC of voltage at t = 20 s. Indicate the values in your calculation and include proper units. Does voltage change (i) Velocity increasing more quickly or more slowly as time goes on? Explain in terms of tangent lines. (ii) Velocity decreasing 4 (iii) Velocity negative 3 (iv) Average velocity of 50 kph 2 (a) [0, 0.5] 1 (b) [0, 1] (c) [1.5, 2] 10 20 30 40 (d) [2.5, 3] Time (s) Voltage (V)

106

FIGURE 11

S E C T I O N 3.4

Rates of Change

107

SOLUTION The tangent line sketched in the ﬁgure below appears to pass through the points (10, 3) and (30, 4). Thus, the ROC of voltage at t = 20 seconds is approximately

4−3 = 0.05 V/s. 30 − 10 As we move to the right of the graph, the tangent lines to it grow shallower, indicating that the voltage changes more slowly as time goes on. y 4 3 2 1

x 10

13. (a) (b) (c)

20

30

40

A stone is tossed vertically upward with an initial velocity of 25 ft/s from the top of a 30-ft building. Use Figure 12 to estimate d T /dh at h = 30 and 70, where T is atmospheric temperature (in degrees Celsius) What height the stone after 0.25iss?d T /dh equal to zero? and h is is the altitude (inofkilometers). Where Find the velocity of the stone after 1 s. When does the stone hit the ground?

1 SOLUTION We employ Galileo’s formula, s(t) = s0 + v0 t − 2 gt 2 = 30 + 25t − 16t 2 , where the time t is in seconds (s) and the height s is in feet (ft). (a) The height of the stone after .25 seconds is s(.25) = 35.25 ft. (b) The velocity at time t is s (t) = 25 − 32t. When t = 1, this is −7 ft/s. √ −25 ± 2545 2 or (c) When the stone hits the ground, its height is zero. Solve 30 + 25t − 16t = 0 to obtain t = −32 t ≈ 2.36 s. (The other solution, t ≈ −0.79, we discard since it represents a time before the stone was thrown.)

30tft.+Find 340 15. TheThe temperature an object (in degrees Fahrenheit) as a after function of time minutes) T (t) = 34 t 2−−15t height (inof feet) of a skydiver at time t (in seconds) opening his (in parachute is is h(t) = 2,000 for 0the ≤ tskydiver’s ≤ 20. At velocity what rateafter doesthe theparachute object cool after 10 min (give correct units)? opens. SOLUTION

Let T (t) = 34 t 2 − 30t + 340, 0 ≤ t ≤ 20. Then T (t) = 32 t − 30, so T (10) = −15◦ F/min.

2.99 × 1016 velocity centimetersforce per second) ﬂowing through a capillary radius 0.008 cm is 17. TheThe earth exerts a(in gravitational of F(r ) of = a blood2 molecule (in Newtons) on an object with a of mass of 75 kg, where r r is the distance from the molecule to the center of the given by the formula v = 6.4 × 10−8 − 0.001r 2 , where r is the distance (inthe meters) the center the earth. Find thewhen ROC rof=force with capillary. Find ROCfrom of velocity as a of function of distance 0.004 cm.respect to distance at the surface of the earth, assuming the radius of the earth is 6.77 × 106 m. SOLUTION

The rate of change of force is F (r ) = −5.98 × 1016 /r 3 . Therefore, F (6.77 × 106 ) = −5.98 × 1016 /(6.77 × 106 )3 = −1.93 × 10−4 N/m.

2.25R 7 −1/2 19. TheThe power delivered byata abattery to ranmeters apparatus R (in ohms) is P= = W. Find rate of (2.82 m/s.the Calculate escape velocity distance fromof theresistance center of the earth is vesc 2 (R × + 10 0.5))r the of rate at which with respect surface of the earth. esc changes change power withvrespect to resistance for Rto=distance 3 and Rat=the 5 . SOLUTION

P (R) =

(R + .5)2 2.25 − 2.25R(2R + 1) . (R + .5)4

Therefore, P (3) = −0.1312 W/ and P (5) = −0.0609 W/. 21. By Faraday’s Law, if a conducting wire of length meters moves at velocity v m/s perpendicular to a magnetic ﬁeld 2 − t + 10 cm. TheBposition of a aparticle line duringina 5-s s(t) = t that of strength (in teslas), voltagemoving of size in V a=straight −Bv is induced the trip wire.isAssume B = 2 and = 0.5. (a) What is the averagedvelocity (a) Find the rate of change V /dv. for the entire trip? (b) Isthe there at which instantaneous velocity to this average velocity? If so, ﬁnd it. (b) Find rateaoftime change of V the with respect to time t if v is = equal 4t + 9. SOLUTION

(a) Assuming that B = 2 and l = 0.5, V = −2(.5)v = −v. Therefore, dV = −1. dv (b) If v = 4t + 9, then V = −2(.5)(4t + 9) = −(4t + 9). Therefore, ddtV = −4. The height (in feet) of a helicopter at time t (in minutes) is s(t) = −3t 3 + 400t for 0 ≤ t ≤ 10.

D I F F E R E N T I AT I O N

23. The population P(t) of a city (in millions) is given by the formula P(t) = 0.00005t 2 + 0.01t + 1, where t denotes the number of years since 1990. (a) How large is the population in 1996 and how fast is it growing? (b) When does the population grow at a rate of 12,000 people per year? SOLUTION

Let P(t) = (0.00005)t 2 + (0.01)t + 1 be the population of a city in millions. Here t is the number of years

past 1990. (a) In 1996 (t = 6 years after 1990), the population is P(6) = 1.0618 million. The rate of growth of population is P (t) = 0.0001t + 0.01. In 1996, this corresponds to a growth rate of P (6) = 0.0106 million per year or 10,600 people per year. (b) When the growth rate is 12,000 people per year (or 0.012 million per year), we have P (t) = 0.0001t + 0.01 = 0.012. Solving for t gives t = 20. This corresponds to the year 2010; i.e., 20 years past 1990. Ethan to ﬁnds that with hours of tutoring, he isI able answer correctly S(h) percent of thebyproblems on a 25. According Ohm’s Law,hthe voltage V , current , andtoresistance R in a circuit are related the equation (h)? Which would you expect to be larger: S (3) or S (30)? Explain. mathVexam. What is the the derivative = I R, where themeaning units areofvolts, amperes, Sand ohms. Assume that voltage is constant with V = 12 V. Calculate (specifying thederivative units): S (h) measures the rate at which the percent of problems Ethan answers correctly changes SOLUTION The The to average ROC of hours I withof respect to R the interval from R = 8 to R = 8.1 with (a) respect the number tutoring hefor receives. One of S(h) is shown in the Rﬁgure (b) possible The ROCgraph of I with respect to R when = 8 below on the left. This graph indicates that in the early hours of working withROC the tutor, makes to rapid progress (c) The of R Ethan with respect I when I = in 1.5learning the material but eventually approaches either the limit of his ability to learn the material or the maximum possible score on the exam. In this scenario, S (3) would be larger than S (30). An alternative graph of S(h) is shown below on the right. Here, in the early hours of working with the tutor little progress is made (perhaps the tutor is assessing how much Ethan already knows, his learning style, his personality, etc.). This is followed by a period of rapid improvement and ﬁnally a leveling off as Ethan reaches his maximum score. In this scenario, S (3) and S (30) might be roughly equal. Percentage correct

CHAPTER 3

Percentage correct

108

Hours of tutoring

Hours of tutoring

Table 2 gives the total month 1999 as determined Department of 27. Suppose θ (t) measures the U.S. anglepopulation between a during clock’seach minute andof hour hands. What is θ by (t)the at 3U.S. o’clock? Commerce. (a) Estimate P (t) for each of the months January–November. (b) Plot these data points for P (t) and connect the points by a smooth curve. (c) Write a newspaper headline describing the information contained in this plot. Total U.S. Population in 1999

TABLE 2

t

P(t) in Thousands

January February March April May June July August September October November December

271,841 271,987 272,142 272,317 272,508 272,718 272,945 273,197 273,439 273,672 273,891 274,076

The table in the text gives the growing population P(t) of the United States. (a) Here are estimates of P (t) in thousands/month for January–November. The estimates are computed using the estimate f (t + 1) − f (t). SOLUTION

t P (t)

Jan

Feb Mar Apr May Jun

146 155

175

191

210

Jul

227 252

Aug Sep Oct Nov 242

233 219

185

S E C T I O N 3.4

Rates of Change

109

(b) Here is a plot of these estimates (1 = Jan, 2 = Feb, etc.) P 250 200 150 100 50 t 2

4

6

8

10

(c) “U.S. Growth Rate Declines After Midsummer Peak” 29. According to a formula widely used by doctors√to determine drug dosages, a person’s body surface area (BSA) (in steeper as xh increases. At what rate do the and slopes tangent The tangent lines to of f BSA (x) ==x 2 grow hw/60, where is the height in centimeters w of thethe weight in meters squared) is given bythe thegraph formula lines increase? kilograms. Calculate the ROC of BSA with respect to weight for a person of constant height h = 180. What is this ROC for w = 70 and w = 80? Express your result in the correct units. Does BSA increase more rapidly with respect to weight at lower or higher body weights? √ √ 5 SOLUTION Assuming constant height h = 180 cm, let f (w) = hw/60 = 10 w be the formula for body surface area in terms of weight. The ROC of BSA with respect to weight is √ √ 5 1 −1/2 5 w = √ . f (w) = 10 2 20 w If w = 70 kg, this is f (70) =

√

√ 5 14 m2 ≈ 0.0133631 . √ = 280 kg 20 70

If w = 80 kg, √

5 1 1 m2 . √ = √ = 80 kg 20 80 20 16 √ Because the rate of change of BSA depends on 1/ w, it is clear that BSA increases more rapidly at lower body weights. f (80) =

31. What is the velocity of an object dropped from a height of 300 m when it hits the ground? A slingshot is used to shoot a pebble in the air vertically from ground level with an initial velocity 200 m/s. Find SOLUTION Wemaximum employ Galileo’s s(t) = s0 + v0 t − 12 gt 2 = 300 − 4.9t 2 , where the time t is in seconds (s) the pebble’s velocityformula, and height. and the height s is in meters (m). When the ball hits the ground its height is 0. Solve s(t) = 300 − 4.9t 2 = 0 to obtain t ≈ 7.8246 s. (We discard the negative time, which took place before the ball was dropped.) The velocity at impact is v(7.8246) = −9.8(7.8246) ≈ −76.68 m/s. This signiﬁes that the ball is falling at 76.68 m/s. 33. A ball is tossed up vertically from ground level and returns to earth 4 s later. What was the initial velocity of the It takes a stone 3 s to hit the ground when dropped from the top of a building. How high is the building and what stone and how high did it go? is the stone’s velocity upon impact? 1 SOLUTION Galileo’s formula gives s(t) = s0 + v0 t − 2 gt 2 = v0 t − 4.9t 2 , where the time t is in seconds (s) and the height s is in meters (m). When the ball hits the ground after 4 seconds its height is 0. Solve 0 = s(4) = 4v0 − 4.9(4)2 to obtain v0 = 19.6 m/s. The stone reaches its maximum height when s (t) = 0, that is, when 19.6 − 9.8t = 0, or t = 2 s. At this time, t = 2 s, 1 s(2) = 0 + 19.6(2) − (9.8)(4) = 19.6 m. 2 35. A man on the tenth ﬂoor of a building sees a bucket (dropped by a window washer) pass his window and notes that An object is tossed up vertically from ground level and hits the ground T s later. Show that its maximum height it hits the ground 1.5 s later. Assuming a ﬂoor is 16 ft high (and neglecting air friction), from which ﬂoor was the bucket was reached after T /2 s. dropped? A falling object moves 16t 2 feet in t seconds. Suppose H is the unknown height from which the bucket fell starting at time zero. The man saw it at some time t (also unknown to us) and it hit the ground, 160 feet down at time t + 1.5. Thus SOLUTION

(H − 16t 2 ) − (H − 16(t + 1.5)2 ) = 160. The H ’s cancel as do the t 2 terms and solving for t gives t = 31/12 s. Thus the bucket fell 16(31/12)2 ≈ 16 · 6.67 feet before the man saw it. Since there are 16 feet in a ﬂoor the bucket was dropped from 6.67 ﬂoors above the 10th: either the 16th or 17th ﬂoor. 37. Show that for an object rising and falling according to Galileo’s formula in Eq. (3), the average velocity over any Which the following statements is true for an object falling under the inﬂuence of gravity near the surface time interval [t1 , tof 2 ] is equal to the average of the instantaneous velocities at t1 and t2 . of the earth? Explain. (a) The object covers equal distance in equal time intervals.

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The simplest way to proceed is to compute both values and show that they are equal. The average velocity

over [t1 , t2 ] is (s0 + v0 t2 − 12 gt22 ) − (s0 + v0 t1 − 12 gt12 ) v0 (t2 − t1 ) + g2 (t2 2 − t1 2 ) s(t2 ) − s(t1 ) = = t2 − t 1 t2 − t1 t2 − t 1 v (t − t1 ) g g = 0 2 − (t2 + t1 ) = v0 − (t2 + t1 ) t2 − t 1 2 2 Whereas the average of the instantaneous velocities at the beginning and end of [t1 , t2 ] is 1 g g s (t1 ) + s (t2 ) 1

= (v0 − gt1 ) + (v0 − gt2 ) = (2v0 ) − (t2 + t1 ) = v0 − (t2 + t1 ). 2 2 2 2 2 The two quantities are the same. In Exercises 39–46, use Eq.up (2)and to estimate A weight oscillates down at the the unit end change. of a spring. Figure 13 shows the height y of the weight through one √ oscillation. √ √ √ sketch of the graph of the velocity as a function of time. cycle of the Make a rough 39. Estimate 2 − 1 and 101 − 100. Compare your estimates with the actual values. √ SOLUTION Let f (x) = x = x 1/2 . Then f (x) = 12 (x −1/2 ). We are using the derivative to estimate the average ROC. That is, √ √ x +h− x ≈ f (x), h so that √

√

x ≈ h f (x). √ √ √ Thus, 2 − 1 ≈ 1 f (1) = 12 (1) = 12 . The actual value, to six decimal places, is 0.414214. Also, 101 − 100 ≈

1 = .05. The actual value, to six decimal places, is 0.0498756. 1 f (100) = 12 10 √

x +h−

as in Example 3. Calculate F(65) and estimate the increase in 41. Let F(s) = 1.1s + 0.03s 2 be the stopping distance −x Suppose that f (x) is a function with f (x) = 2 . Estimate f (7) − f (6). Then estimate f (5), assuming that stopping distance if speed is increased from 65 to 66 mph. Compare your estimate with the actual increase. f (4) = 7. SOLUTION Let F(s) = 1.1s + .03s 2 be as in Example 3. F (s) = 1.1 + 0.06s. • Then F(65) = 198.25 ft and F (65) = 5.00 ft/mph. • F (65) ≈ F(66) − F(65) is approximately equal to the change in stopping distance per 1 mph increase in speed

when traveling at 65 mph. Increasing speed from 65 to 66 therefore increases stopping distance by approximately 5 ft. • The actual increase in stopping distance when speed increases from 65 mph to 66 mph is F(66) − F(65) = 203.28 − 198.25 = 5.03 feet, which differs by less than one percent from the estimate found using the derivative. 3 Determine the cost of producing 43. TheAccording dollar costtoofKleiber’s producing x bagels is C(x)rate = 300 0.25x − 0.5(x/1,000) Law, the metabolic P (in+kilocalories per day) and.body mass m (in kilograms) of an 2,000animal bagelsare and estimate cost of the 2001st your estimate the withincrease the actual cost of therate 2001st bagel. . Estimate in metabolic when body related by athe three-quarter power bagel. law P Compare = 73.3m 3/4 SOLUTION Expanding thetopower mass increases from 60 61 kg.of

3 yields C(x) = 300 + .25x − 5 × 10−10 x 3 .

This allows us to get the derivative C (x) = .25 − 1.5 × 10−9 x 2 . The cost of producing 2000 bagels is C(2000) = 300 + 0.25(2000) − 0.5(2000/1000)3 = 796 dollars. The cost of the 2001st bagel is, by deﬁnition, C(2001) − C(2000). By the derivative estimate, C(2001) − C(2000) ≈ C (2000)(1), so the cost of the 2001st bagel is approximately C (2000) = .25 − 1.5 × 10−9 (20002 ) = $.244. C(2001) = 796.244, so the exact cost of the 2001st bagel is indistinguishable from the estimated cost. The function is very nearly linear at this point. 45. The demand for a commodity generally decreases as the price is raised. Suppose that the demand for oil (per capita 2 + 10−8 x 3 . of producing x video cameras is C(x) = 500xFind − 0.003x per year)Suppose is D( p)the = dollar 900/ pcost barrels, where p is the price per barrel in dollars. the demand when p = $40. Estimate (a) Estimate the marginal at production level if x p=is5,000 and to compare the decrease in demand if p risescost to $41 and the increase decreased $39. it with the actual cost C(5,001) − C(5,000). SOLUTION D( p) = 900 p −1 , so D ( p) = −900 p −2 . When the price is $40 a barrel, the per capita demand is (b)=Compare the marginal cost at = 5,000inwith average cost camera, D(40) 22.5 barrels per year. With anxincrease pricethe from $40 to $41per a barrel, thedeﬁned changeasinC(x)/x. demand D(41) − D(40) −2 is approximately D (40) = −900(40 ) = −.5625 barrels a year. With a decrease in price from $40 to $39 a barrel, the change in demand D(39) − D(40) is approximately −D (40) = +.5625. An increase in oil prices of a dollar leads to a decrease in demand of .5625 barrels a year, and a decrease of a dollar leads to an increase in demand of .5625 barrels a year.

S E C T I O N 3.4

Rates of Change

111

of A(s + 1) − melanogaster, A(s) provided grown by Eq.in(2) has error exactly equal to 1. Explain 47. LetThe A =reproduction s 2 . Show that ratethe of estimate the fruit ﬂy Drosophila bottles in a laboratory, decreases as the this result using Figure 14. bottle becomes more crowded. A researcher has found that when a bottle contains p ﬂies, the number of offspring per female per day is f ( p) = (34 − 0.612 p) p −0.658 1 2

(a) Calculate f (15) and f (15). (b) Estimate the decrease in daily offspring per female when p is increased from 15 to 16. Is this estimate larger or s smaller than the actual value f (16) − f (15)? (c) Plot f ( p) for 5 ≤ p ≤ 25 and verify that f ( p) is a decreasing function of p. Do you expect f ( p) to be 14 positive or negative? Plot f ( p) and conﬁrm yourFIGURE expectation. SOLUTION

Let A = s 2 . Then A(s + 1) − A(s) = (s + 1)2 − s 2 = s 2 + 2s + 1 − s 2 = 2s + 1,

while A (s) = 2s. Therefore, regardless of the value of s, A(s + 1) − A(s) − A (s) = (2s + 1) − 2s = 1. To understand this result, consider Figure 14, which illustrates a square of side length s expanded to a square of side length s + 1 by increasing the side length by 1/2 unit in each direction. The difference A(s + 1) − A(s) is the total area between the two squares. On the other hand, A (s) = 2s is the total area of the four rectangles of dimension s by 1/2 bordering the sides of the original square. The error in approximating A(s + 1) − A(s) by A (s) is given by the four 1/2 by 1/2 squares in the corners of the ﬁgure. The total area of these four small squares is always exactly 1. 49. Let M(t) be the mass (in kilograms) of a plant as a function of time (in years). Recent studies by Niklas and Enquist According to aSteven’s Law wide in psychology, the perceived magnitude of to a stimulus (howthe strong a person feels have suggested that for remarkably range of plants (from algae and grass palm trees), growth rate during the span stimulus be) is proportional to a power ofpower the actual I of the Although not an C. exact law, some constant the life of thetoorganism satisﬁes a three-quarter law, intensity that is, d M/dt = stimulus. C2/3 M 3/4 for experiments show that the perceived brightness B of a light satisﬁes B = k I , where I is the light intensity, (a) If a tree has a growth rate of 6 kg/year when M = 100 kg, what is its growth rate when M = 125 kg? whereas the perceived heaviness H of a weight W satisﬁes H = kW 3/2 (k is a constant that is different in the (b) If M = 0.5 kg, how much more mass must the plant acquire to double its growth rate? two cases). Compute d B/d I and d H /d W and state whether they are increasing or decreasing functions. Use this to SOLUTION justify the statements: √ (a) Suppose a tree increase has a growth rate d M/dtisoffelt6 more kg/yr strongly when Mwhen = 100, = C(100 = 10C 10, so that (a)√ A one-unit in light intensity I is then small6than when 3/4 I is)large. 3 10 another pound to a load W is felt more strongly when W is large than when W is small. C =(b) . When M = 125, 50Adding √ dM 3 10 = C(1253/4 ) = 25(51/4 ) = 7.09306. dt 50 (b) The growth rate when M = .5 kg is d M/dt = C(.53/4 ). To double the rate, we must ﬁnd M so that d M/dt = C M 3/4 = 2C(.53/4 ). We solve for M. C M 3/4 = 2C(.53/4 ) M 3/4 = 2(.53/4 ) M = (2(.53/4 ))4/3 = 1.25992. The plant must acquire the difference 1.25992 − .5 = .75992 kg in order to double its growth rate. Note that a doubling of growth rate requires more than a doubling of mass.

Further Insights and Challenges dP 51. TheAs size a certainspreads animalthrough population P(t) at time (in months)p satisﬁes = 0.2(300at−time P). t (in days) satisﬁes anof epidemic a population, the tpercentage of infected individuals dt thePequation a differential (a) Is growing(called or shrinking when Pequation) = 250? when P = 350? (b) Sketch the graph of d P/dt as a functiondof p P for 0 ≤ P ≤2300. = 4 p − 0.06 p 0 ≤ p ≤ 100 (c) Which of the graphs in Figure 15 is the graph dt of P(t) if P(0) = 200? (a) (b) (c) (d)

How fast is the epidemic spreading when p = 10% and Pwhen p = 70%? P For which p is the epidemic 300 neither spreading nor diminishing? 300 Plot d p/dt as a function of p. 200 200 What is the maximum possible rate of increase and for which p does this occur? 100

100 t

t 4

8

4

(A)

8 (B)

FIGURE 15

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D I F F E R E N T I AT I O N SOLUTION

Let P (t) = d P/dt = 0.2(300 − P).

(a) Since P (250) = 10, the population is growing when P = 250. Since P (350) = −10, the population is shrinking when P = 350. (b) Here is a graph of d P/dt for 0 ≤ t ≤ 300. dP/dt 60 50 40 30 20 10 50 100 150 200 250 300

P

(c) If P(0) = 200, as in graph (A), then P (0) = 20 > 0 and the population is growing as depicted. Accordingly, graph (A) has the correct shape for P(t). If P(0) = 200, as in graph (B), then P (0) = 20 > 0 and the population is growing, contradicting what is depicted. Thus graph (B) cannot be the correct shape for P(t). In Exercises 53–54, the average cost per unit at production level x is deﬁned as Cavg (x) = C(x)/x, where C(x) is the Studies of internet usage show that website popularity is described quite well by Zipf’s Law, according to cost function. Average cost is a measure of the efﬁciency of the production process. which the nth most popular website receives roughly the fraction 1/n of all visits. Suppose that on a particular day, the nththat most popular hadtoapproximately (n) = 106 /nthe visitors ≤ 15,000). (x) is site equal the slope of theV line through origin(for andn the point (x, C(x)) on the graph of C(x). 53. Show C avg (a)this Verify that the top 50 websites received nearly of the visits. LetatTpoints (N ) denote sum V (n)16. for Using interpretation, determine whether average cost45% or marginal cost isHint: greater A, B, the C, D in of Figure 1 ≤ n ≤ N . Use a computer algebra system to compute T (45) and T (15,000). Cost (b) Verify, by numerical experimentation, that when Eq. (2) is used to estimate V (n + 1) − V (n), the error in the D estimate decreases as n grows larger. Find (again, by experimentation) an N such that the error is at most 10 for n ≥ N. C received at most 100 more visitors than the (n + 1)st web B (c) Using Eq. (2), show that for n ≥ 100, the nth A website site. Production level

FIGURE 16 Graph of C(x). SOLUTION

By deﬁnition, the slope of the line through the origin and (x, C(x)), that is, between (0, 0) and (x, C(x))

is C(x) − 0 C(x) = = Cav . x −0 x At point A, average cost is greater than marginal cost, as the line from the origin to A is steeper than the curve at this point (we see this because the line, tracing from the origin, crosses the curve from below). At point B, the average cost is still greater than the marginal cost. At the point C, the average cost and the marginal cost are nearly the same, since the tangent line and the line from the origin are nearly the same. The line from the origin to D crosses the cost curve from above, and so is less steep than the tangent line to the curve at D; the average cost at this point is less than the marginal cost. The cost in dollars of producing alarm clocks is

3.5 Higher Derivatives

C(x) = 50x 3 − 750x 2 + 3,740x + 3,750

where x is in units of 1,000. (a) Calculate the average cost at x = 4, 6, 8, and 10. 1. An economist who announces that “America’s economic growth is slowing” is making a statement about the gross (b) Use the graphical interpretation of average cost to ﬁnd the production level x0 at which average cost is lowest. national product (GNP) as a function of time. Is the second derivative of the GNP positive? What about the ﬁrst derivative? What is the relation between average cost and marginal cost at x 0 (see Figure 17)? SOLUTION If America’s economic growth is slowing, then the GNP is increasing, but the rate of increase is decreasing. Thus, the second derivative of GNP is negative, while the ﬁrst derivative is positive. FIGURE 17 Cost function C(x) = 50x 3 − 750x 2 + 3,740x + 3,750. 2. On September 4, 2003, the Wall Street Journal printed the headline “Stocks Go Higher, Though the Pace of Their Gains Slows.” Rephrase as a statement about the ﬁrst and second time derivatives of stock prices and sketch a possible graph.

Preliminary Questions

SOLUTION Because stocks are going higher, stock prices are increasing and the ﬁrst derivative of stock prices must therefore be positive. On the other hand, because the pace of gains is slowing, the second derivative of stock prices must be negative.

S E C T I O N 3.5

Higher Derivatives

113

Stock price

Time

3. Is the following statement true or false? The third derivative of position with respect to time is zero for an object falling to earth under the inﬂuence of gravity. Explain. SOLUTION This statement is true. The acceleration of an object falling to earth under the inﬂuence of gravity is constant; hence, the second derivative of position with respect to time is constant. Because the third derivative is just the derivative of the second derivative and the derivative of a constant is zero, it follows that the third derivative is zero.

4. Which type of polynomial satisﬁes f (x) = 0 for all x? SOLUTION The second derivative of all linear polynomials (polynomials of the form ax + b for some constants a and b) is equal to 0 for all x.

Exercises In Exercises 1–12, calculate the second and third derivatives. 1. y = 14x 2 SOLUTION

Let y = 14x 2 . Then y = 28x, y = 28, and y = 0.

2 25x 3. y =y x=4 − 7− 2x + 2x SOLUTION Let y = x 4 − 25x 2 + 2x. Then y = 4x 3 − 50x + 2, y = 12x 2 − 50, and y = 24x.

4 3 5. y =y =π4t r 3 − 9t 2 + 7 3 SOLUTION

Let y = 43 π r 3 . Then y = 4π r 2 , y = 8π r , and y = 8π .

4/5 − 6t 2/3 7. y = 20t √ y= x 16 4 SOLUTION Let y = 20t 4/5 − 6t 2/3 . Then y = 16t −1/5 − 4t −1/3 , y = − 5 t −6/5 + 3 t −4/3 , and y = 96 t −11/15 − 16 t −7/3 . 25 9

1 9. y =y z=−x −9/5 z SOLUTION

Let y = z − z −1 . Then y = 1 + z −2 , y = −2z −3 , and y = 6z −4 .

11. y = (x 2 + x)(x 3 + 1) y = t 2 (t 2 + t) SOLUTION Since we don’t want to apply the product rule to an ever growing list of products, we multiply through ﬁrst. Let y = (x 2 + x)(x 3 + 1) = x 5 + x 4 + x 2 + x. Then y = 5x 4 + 4x 3 + 2x + 1, y = 20x 3 + 12x 2 + 2, and y = 60x 2 + 24x. In Exercises 13–24, calculate the derivative indicated. 1 y= x = x4 13. f (4) (1), 1 +f (x) Let f (x) = x 4 . Then f (x) = 4x 3 , f (x) = 12x 2 , f (x) = 24x, and f (4) (x) = 24. Thus f (4) (1) = 24. d 2 y 15. = 4t=−3 + 3t 2 4t −3 g (1),, yg(t) dt 2 t=1 SOLUTION

SOLUTION

2 Let y = 4t −3 + 3t 2 . Then ddty = −12t −4 + 6t and d 2y = 48t −5 + 6. Hence

d 2 y dt 2

dt

= 48(1)−5 + 6 = 54. t=1

√ 17. h (9), h(x) = x 4 d f √ f (t)== 6tx9 − SOLUTION4 Let, h(x) = 2tx51/2 . Then h (x) = 12 x −1/2 , h (x) = − 14 x −3/2 , and h (x) = 38 x −5/2 . Thus dt t=1 1 h (9) = 648 . g (9),

g(x) = x −1/2

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D I F F E R E N T I AT I O N

d 4 x 19. , dt 4 t=16 SOLUTION

Thus

x = t −3/4

2 −11/4 , d 3 x = − 231 t −15/4 , and d 4 x = 3465 t −19/4 . Let x(t) = t −3/4 . Then ddtx = − 34 t −7/4 , d 2x = 21 16 t 64 256 dt dt 3 dt 4

d 4 x dt 4

=

t=16

3465 −19/4 3465 16 . = 256 134217728

x 21. g (1), g(x) = 2 f (4), f (t)x=+2t 1 −t x SOLUTION Let g(x) = . Then x +1 g (x) =

(x + 1)(1) − (x)(1) 1 1 = = 2 (x + 1)2 (x + 1)2 x + 2x + 1

and g (x) =

(x 2 + 2x + 1)(0) − 1(2x + 2) 2x + 2 2 =− =− . (x 2 + 2x + 1)2 (x + 1)3 (x + 1)4

1 Thus, g (1) = − . 4 1 23. h (1), h(x) = √ 1 1 f (1), f (t) =x + t3 + 1 1 SOLUTION Let h(x) = √ . Then x +1 − 1 x −1/2 − 12 x −1/2 h (x) = √2 , √ 2 = x +2 x +1 x +1 and √

3 x −1/2 + x −1 + 1 x −3/2 x + 1) 14 x −3/2 + 12 x −1/2 (1 + x −1/2 ) 4 4 = √ √ (x + 2 x + 1)2 ( x + 1)4

√ 3 x −1 + 1 x −3/2 ( x + 1) 34 x −1 + 14 x −3/2 4 = = . √ √ 4 ( x + 1)4 ( x + 1)3

(x + 2 h (x) =

= 18 . Accordingly, h (1) = (3/4)+(1/4) 23 25. Calculate y (k) (0) for 0 ≤2 k ≤ 5, where y = x 4 + ax 3 + bx 2 + cx + d (with a, b, c, d the constants). x F(x) =the power, constant multiple, and sum rules at each stage, we get (note y (0) is y by convention): F (2),Applying SOLUTION x −3 k

y (k)

0

x 4 + ax 3 + bx 2 + cx + d

1

4x 3 + 3ax 2 + 2bx + c

2

12x 2 + 6ax + 2b

3

24x + 6a

4

24

5

0

from which we get y (0) (0) = d, y (1) (0) = c, y (2) (0) = 2b, y (3) (0) = 6a, y (4) (0) = 24, and y (5) (0) = 0. d 6 −1 (k) 27. UseWhich the result in Example to ﬁnd satisfy x .f (x) = 0 for all k ≥ 6? of the following2functions dx6 (b) f (x) = x 3 − 2 (a) f (x) = 7x 4 + 4 + x −1 SOLUTION The√equation in Example 2 indicates that (d) f (x) = 1 − x 6 (c) f (x) = x (e) f (x) = x 9/5

d 6 −1 (f) f (x) = 2x 2 + 3x 5 x = (−1)6 6!x −6−1 . 6 dx

S E C T I O N 3.5

Higher Derivatives

115

(−1)6 = 1 and 6! = 6 × 5 × 4 × 3 × 2 × 1 = 720, so d 6 −1 x = 720x −7 . dx6 √ In Exercises 29–32, general formulaoffor f (n) Calculate theﬁnd ﬁrstaﬁve derivatives f (x) =(x).x. (n) −n+1/2 . 29. (a) f (x)Show = (x that + 1)f−1 (x) is a multiple of x (n) 1 as (−1)n−1 for n ≥ 1. −2 −3 (b) ShowLet thatf (x) f (x) SOLUTION = (xalternates + 1)−1 in = sign x+1 . By Exercise 12, f (x) = −1(x + 1) , f (x) = 2(x + 1) , f (x) = 2n − 3 −4 (4) −5 −6(x(c) + Find 1) ,a fformula (x) = . . 2. From we conclude that the nth can (x)1)for ,n. ≥ Hint:this Verify that the coefﬁcient ofderivative x −n+1/2 is ±1be· 3written · 5 · · · as fn(n) (x) . = for24(x f (n)+ 2 n −(n+1) . (−1) n!(x + 1) 31. f (x) = x −1/2 f (x) = x −2 −1 −3/2 SOLUTION f (x) = 2 x . We will avoid simplifying numerators and denominators to ﬁnd the pattern: f (x) =

−3 −1 −5/2 3×1 = (−1)2 2 x −5/2 x 2 2 2

f (x) = −

5 3 × 1 −7/2 5 × 3 × 1 −7/2 x = (−1)3 x 2 22 23

.. . f (n) (x) = (−1)n

(2n − 1) × (2n − 3) × . . . × 1 −(2n+1)/2 x . 2n

33. (a) Find the −3/2 acceleration at time t = 5 min of a helicopter whose height (in feet) is h(t) = −3t 3 + 400t. f (x) = x (b) Plot the acceleration h (t) for 0 ≤ t ≤ 6. How does this graph show that the helicopter is slowing down during this time interval? SOLUTION

(a) Let h(t) = −3t 3 + 400t, with t in minutes and h in feet. The velocity is v(t) = h (t) = −9t 2 + 400 and acceleration is a(t) = h (t) = −18t. Thus a(5) = −90 ft/min2 . (b) The acceleration of the helicopter for 0 ≤ t ≤ 6 is shown in the ﬁgure below. As the acceleration of the helicopter is negative, the velocity of the helicopter must be decreasing. Because the velocity is positive for 0 ≤ t ≤ 6, the helicopter is slowing down. y 1

2

3

4

5

6

x

−20 −40 −60 −80 −100

35. Figure 5 shows f , f , and f . Determine which is which. Find an equation of the tangent to the graph of y = f (x) at x = 3, where f (x) = x 4 . y

y

y

1

2

3

x

1

(A)

2

3

x

1

2

3

x

(C)

(B)

FIGURE 5

(a) f (b) f (c) f . The tangent line to (c) is horizontal at x = 1 and x = 3, where (b) has roots. The tangent line to (b) is horizontal at x = 2 and x = 0, where (a) has roots. 37. Figure 7 shows the graph of the position of an object as a function of time. Determine the intervals on which the The second derivative f is shown in Figure 6. Determine which graph, (A) or (B), is f and which is f . acceleration is positive. SOLUTION

Position

10

20

30

40 Time

FIGURE 7

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CHAPTER 3

D I F F E R E N T I AT I O N SOLUTION Roughly from time 10 to time 20 and from time 30 to time 40. The acceleration is positive over the same intervals over which the graph is bending upward.

+ frespect (x) = xto2 .the length of a side. 39. FindFind a polynomial (x) satisfying equation f (x)with the second fderivative of thethe volume of axcube SOLUTION Since x f (x) + f (x) = x 2 , and x 2 is a polynomial, it seems reasonable to assume that f (x) is a polynomial of some degree, call it n. The degree of f (x) is n − 2, so the degree of x f (x) is n − 1, and the degree of x f (x) + f (x) is n. Hence, n = 2, since the degree of x 2 is 2. Therefore, let f (x) = ax 2 + bx + c. Then f (x) = 2ax + b and f (x) = 2a. Substituting into the equation x f (x) + f (x) = x 2 yields ax 2 + (2a + b)x + c = x 2 , an identity in x. Equating coefﬁcients, we have a = 1, 2a + b = 0, c = 0. Therefore, b = −2 and f (x) = x 2 − 2x. 41. A servomotor controls the vertical movement of a drill bit that will drill a pattern of holes in sheet metal. The the following descriptions could and not apply Figurethe 8? Explain. maximumWhich verticalofspeed of the drill bit is 4 in./s, while to drilling hole, it must move no more than 2.6 in./s (a) Graph of acceleration when velocity to avoid warping the metal. During a cycle, is theconstant bit begins and ends at rest, quickly approaches the sheet metal, and quickly to its initialwhen position after the hole is drilled. Sketch possible graphs of the drill bit’s vertical velocity and (b) returns Graph of velocity acceleration is constant acceleration. Label the point where the bit enters the sheet metal. (c) Graph of position when acceleration is zero SOLUTION There will be multiple cycles, each of which will be more or less identical. Let v(t) be the downward vertical velocity of the drill bit, and let a(t) be the vertical acceleration. From the narrative, we see that v(t) can be no greater than 4 and no greater than 2.6 while drilling is taking place. During each cycle, v(t) = 0 initially, v(t) goes to 4 quickly. When the bit hits the sheet metal, v(t) goes down to 2.6 quickly, at which it stays until the sheet metal is drilled through. As the drill pulls out, it reaches maximum non-drilling upward speed (v(t) = −4) quickly, and maintains this speed until it returns to rest. A possible plot follows: y

4 Metal 2 x

0.5

1

1.5

2

−2 −4

A graph of the acceleration is extracted from this graph: y

40 20 0.5

1

x

1.5

2

−20 − 40 Metal

In Exercises 42–43, refer to the following. In their 1997 study, Boardman and Lave related the trafﬁc speed S on a twolane road to trafﬁc density Q (number of cars per mile of road) by the formula S = 2,882Q −1 − 0.052Q + 31.73 for 60 ≤ Q ≤ 400 (Figure 9). Speed S 70 (mph) 60 50 40 30 20 10 100

200

300

400

Density Q

FIGURE 9 Speed as a function of trafﬁc density.

43. (a) CalculateExplain d S/d Q intuitively and d 2 S/dwhy Q 2 . we should expect that d S/d Q < 0. (b) Show that d 2 S/d Q 2 > 0. Then use the fact that d S/d Q < 0 and d 2 S/d Q 2 > 0 to justify the following statement: A one-unit increase in trafﬁc density slows down trafﬁc more when Q is small than when Q is large. Plot d S/d Q. Which property of this graph shows that d 2 S/d Q 2 > 0? (c) SOLUTION

S E C T I O N 3.5

Higher Derivatives

117

(a) Trafﬁc speed must be reduced when the road gets more crowded so we expect d S/d Q to be negative. This is indeed the case since d S/d Q = −.052 − 2882/Q 2 < 0. (b) The decrease in speed due to a one-unit increase in density is approximately d S/d Q (a negative number). Since d 2 S/d Q 2 = 5764Q −3 > 0 is positive, this tells us that d S/d Q gets larger as Q increases—and a negative number which gets larger is getting closer to zero. So the decrease in speed is smaller when Q is larger, that is, a one-unit increase in trafﬁc density has a smaller effect when Q is large. (c) d S/d Q is plotted below. The fact that this graph is increasing shows that d 2 S/d Q 2 > 0. y 100

200

300

400

x

−0.2 −0.4 −0.6 −0.8 −1 −1.2

Use a computer algebra system to compute f (k) (x) for k = 1, 2, 3 for the functions: According to one model that attempts to account for air resistance, the distance s(t) (in feet) traveled by a 5/3 (a) falling f (x) =raindrop (1 + x 3 )satisﬁes 1 − x4 (b) f (x) = 0.0005 ds 2 d 2s 1 − 5x − 6x 2 = g − D dt dt 2 45.

SOLUTION

where D= is the and g = algebra 32 ft/s2 system, . Terminal velocity v 1/2 is deﬁned as the velocity at which the (a) Let f (x) (1 +raindrop x 3 )5/3 . diameter Using a computer drop has zero acceleration (one can show that velocity approaches v 1/2 as time proceeds). 2 3 2/3 √ = 5x 2000g D. (1 + x ) ; (a) Show that v 1/2 = f (x) (b) Find v 1/2 for drops (x)diameter = 10x(10.003 + x 3and )2/30.0003 + 10x 4ft.(1 + x 3 )−1/3 ; and f of (c) In this model, do fraindrops accelerate or lower (x) = 10(1 2/3 +rapidly 6 (1 + x 3 )−4/3 . + x 3 )more 60x 3 (1at+higher x 3 )−1/3 − 10xvelocities? (b) Let f (x) =

1 − x4 . Using a computer algebra system, 1 − 5x − 6x 2 f (x) =

12x 3 − 9x 2 + 2x + 5 ; (6x − 1)2

f (x) =

2(36x 3 − 18x 2 + 3x − 31) ; and (6x − 1)3

f (x) =

1110 . (6x − 1)4

x +2 Further Let Insights . Use a CAS to compute the f (x) = and Challenges x −1 47. Find the 100th derivative of f (k) (x)?

f (k) (x) for 1 ≤ k ≤ 4. Can you ﬁnd a general formula for

p(x) = (x + x 5 + x 7 )10 (1 + x 2 )11 (x 3 + x 5 + x 7 ) SOLUTION

This is a polynomial of degree 70 + 22 + 7 = 99, so its 100th derivative is zero.

49. Use the Product Rule twice to ﬁnd a formula for ( f g) in terms of the ﬁrst and second derivative of f and g. What is the p(99) (x) for p(x) as in Exercise 47? SOLUTION Let h = f g. Then h = f g + g f = f g + f g and h = f g + g f + f g + g f = f g + 2 f g + f g . 51. Compute Use the Product Rule to ﬁnd a formula for ( f g) and compare your result with the expansion of (a + b)3 . Then f (x + h) + f (x − h) − 2 f (x) try to guess the general formula for ( f g)(n) . f (x) = lim h→0 h2 for the following functions: (a) f (x) = x (b) f (x) = x 2 Based on these examples, can you formulate a conjecture about what f is?

(c) f (x) = x 3

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For f (x) = x, we have f (x + h) + f (x − h) − 2 f (x) = (x + h) + (x − h) − 2x = 0.

Hence, (x) = 0. For f (x) = x 2 , f (x + h) + f (x − h) − 2 f (x) = (x + h)2 + (x − h)2 − 2x 2 = x 2 + 2xh + h 2 + x 2 − 2xh + h 2 − 2x 2 = 2h 2 , so (x 2 ) = 2. Working in a similar fashion, we ﬁnd (x 3 ) = 6x. One can prove that for twice differentiable functions, f = f . It is an interesting fact of more advanced mathematics that there are functions f for which f exists at all points, but the function is not differentiable.

3.6 Trigonometric Functions Preliminary Questions 1. Determine the sign ± that yields the correct formula for the following: d (a) (sin x + cos x) = ± sin x ± cos x dx d (b) tan x = ± sec2 x dx d (c) sec x = ± sec x tan x dx d (d) cot x = ± csc2 x dx The correct formulas are d (sin x + cos x) = − sin x + cos x dx d tan x = sec2 x dx d sec x = sec x tan x dx d cot x = − csc2 x dx

SOLUTION

(a) (b) (c) (d)

2. Which of the following functions can be differentiated using the rules we have covered so far? (c) y = x 2 cos x (a) y = 3 cos x cot x (b) y = cos(x 2 ) SOLUTION

(a) 3 cos x cot x is a product of functions whose derivatives are known. This function can therefore be differentiated using the Product Rule. (b) cos(x 2 ) is a composition of the functions cos x and x 2 . We have not yet discussed how to differentiate composite functions. (c) x 2 cos x is a product of functions whose derivatives are known. This function can therefore be differentiated using the Product Rule. 3. Compute SOLUTION

d (sin2 x + cos2 x) without using the derivative formulas for sin x and cos x. dx Recall that sin2 x + cos2 x = 1 for all x. Thus, d d (sin2 x + cos2 x) = 1 = 0. dx dx

4. How is the addition formula used in deriving the formula (sin x) = cos x? SOLUTION The difference quotient for the function sin x involves the expression sin(x + h). The addition formula for the sine function is used to expand this expression as sin(x + h) = sin x cos h + sin h cos x.

S E C T I O N 3.6

Trigonometric Functions

Exercises In Exercises 1–4, ﬁnd an equation of the tangent line at the point indicated. 1. y = sin x, SOLUTION

x=

π 4

Let f (x) = sin x. Then f (x) = cos x and the equation of the tangent line is √ √ √

π √2

π

π π 2 2 2 π x− + f = x− + = x+ 1− . y = f 4 4 4 2 4 2 2 2 4

π 3. y = tan x, x = π y = cos x, x 4= 3 SOLUTION Let f (x) = tan x. Then f (x) = sec2 x and the equation of the tangent line is y = f

π

4

x−

π

π π π + f =2 x− + 1 = 2x + 1 − . 4 4 4 2

In Exercises 5–26, use theπProduct and Quotient Rules as necessary to ﬁnd the derivative of each function. y = sec x, x = 6 5. f (x) = sin x cos x SOLUTION

Let f (x) = sin x cos x. Then f (x) = sin x(− sin x) + cos x(cos x) = − sin2 x + cos2 x.

7. f (x) = sin2 x2 f (x) = x cos x SOLUTION Let f (x) = sin2 x = sin x sin x. Then f (x) = sin x(cos x) + sin x(cos x) = 2 sin x cos x. x x + 12 cot x 9. f (x)f (x) = x=3 sin 9 sec SOLUTION Let f (x) = x 3 sin x. Then f (x) = x 3 cos x + 3x 2 sin x. 11. f (θ ) = tan θ sec θ sin x f (x) = SOLUTION Let xf (θ ) = tan θ sec θ . Then

f (θ ) = tan θ sec θ tan θ + sec θ sec2 θ = sec θ tan2 θ + sec3 θ = tan2 θ + sec2 θ sec θ . 13. h(θ ) = cos2 θθ g(θ ) = SOLUTION Let h(θθ ) = cos2 θ = cos θ cos θ . Then cos h (θ ) = cos θ (− sin θ ) + cos θ (− sin θ ) = −2 cos θ sin θ . 15. f (x) = (x −2x 2 )2cot x k(θ ) = θ sin θ SOLUTION Let f (x) = (x − x 2 ) cot x. Then f (x) = (x − x 2 )(− csc2 x) + cot x(1 − 2x). sec x 17. f (x)f (z) = = 2z tan z x SOLUTION

Let f (x) =

2 sec x (x) = (x ) sec x tan x − 2x sec x = x sec x tan x − 2 sec x . . Then f x2 x4 x3

2 19. g(t) = sin t −x f (x) = cos t tan x SOLUTION

Let g(t) = sin t −

x 21. f (x) = cos y−1 sin x R(y) = + 2 sin y

2 = sin t − 2 sec t. Then g (t) = cos t − 2 sec t tan t. cos t

119

120

CHAPTER 3

D I F F E R E N T I AT I O N SOLUTION

Let f (x) =

x . Then 2 + sin x f (x) =

(2 + sin x) (1) − x cos x (2 + sin x)2

=

2 + sin x − x cos x (2 + sin x)2

.

1 + sin x 23. f (x)f (x) = = (x 2 − x − 4) csc x 1 − sin x 1 + sin x SOLUTION Let f (x) = . Then 1 − sin x f (x) =

(1 − sin x) (cos x) − (1 + sin x) (− cos x) (1 − sin x)2

=

2 cos x . (1 − sin x)2

sec x 25. g(x) = 1 + tan x f (x) =x 1 − tan x sec x SOLUTION Let g(x) = . Then x g (x) =

x sec x tan x − (sec x) (1) (x tan x − 1) sec x = . x2 x2

In Exercises 27–30, the second derivative. sincalculate x h(x) = 4 x++ cos x x 27. f (x) = 3 sin 4 cos SOLUTION

Let f (x) = 3 sin x + 4 cos x. Then f (x) = 3 cos x − 4 sin x and f (x) = −3 sin x − 4 cos x.

29. g(θ ) = θ sin θ f (x) = tan x SOLUTION Let g(θ ) = θ sin θ . Then, by the product rule, g (θ ) = θ cos θ + sin θ and g (θ ) = −θ sin θ + cos θ + cos θ = 2 cos θ − θ sin θ . In Exercises h(θ ) 31–36, = csc θﬁnd an equation of the tangent line at the point speciﬁed. 31. y = x 2 + sin x, SOLUTION

x =0

Let f (x) = x 2 + sin x. Then f (x) = 2x + cos x and f (0) = 1. The tangent line at x = 0 is y = f (0) (x − 0) + f (0) = 1 (x − 0) + 0 = x.

π 33. y = 2 sin x + 3 cos x, πx = 3 y = θ tan θ , θ = √ 4 π 3 3 SOLUTION Let f (x) = 2 sin x + 3 cos x. Then f (x) = 2 cos x − 3 sin x and f 3 = 1 − 2 . The tangent line at x = π3 is √

π

π 3 3 3

π π √ x− + f = 1− x− + 3+ y= f 3 3 3 2 3 2 √ √ √ 3 3 3 3 π π− . = 1− x + 3+ + 2 2 2 3 π 35. y =y csc sinx,θ , xθ== 40 = θx2−+cot SOLUTION Let f (x) = csc x − cot x. Then f (x) = csc2 x − csc x cot x and f

π 4

=2−

√

2·1=2−

√ 2.

Hence the tangent line is

π

π

√

π π √ x− + f = 2− 2 x − + y = f 2−1 4 4 4 4

√ √ π √ 2−2 . = 2− 2 x + 2−1+ 4

S E C T I O N 3.6

Trigonometric Functions

121

In Exercises 37–39, using (sin x) = cos x and (cos x) = − sin x. cos θ verify the formula π y= , θ= 1 + sin θ 3 d 37. cot x = − csc2 x dx cos x . Using the quotient rule and the derivative formulas, we compute: SOLUTION cot x = sin x d d cos x sin x(− sin x) − cos x(cos x) −(sin2 x + cos2 x) −1 cot x = = = = = − csc2 x. dx d x sin x sin2 x sin2 x sin2 x d csc d x = − csc x cot x dx sec x = sec x tan x dx 1 , we can apply the quotient rule and the two known derivatives to get: SOLUTION Since csc x = sin x

39.

d − cos x cos x 1 d 1 sin x(0) − 1(cos x) = =− csc x = = = − cot x csc x. dx d x sin x sin x sin x sin2 x sin2 x 41. Calculate thevalues ﬁrst ﬁve cos x. determine f (8)graph and of f (37) . sin x cos x is horizontal. where theThen tangent line to the y= Find the of xderivatives between 0of andf (x) 2π = SOLUTION Let f (x) = cos x. • Then f (x) = − sin x, f (x) = − cos x, f (x) = sin x, f (4) (x) = cos x, and f (5) (x) = − sin x. • Accordingly, the successive derivatives of f cycle among

{− sin x, − cos x, sin x, cos x} in that order. Since 8 is a multiple of 4, we have f (8) (x) = cos x. • Since 36 is a multiple of 4, we have f (36) (x) = cos x. Therefore, f (37) (x) = − sin x. 43. Calculate f (x) and f (x), where f (x) = tan x. Find y (157) , where y = sin x. SOLUTION Let f (x) = tan x. Then f (x) = sec2 x = sec x sec x f (x) = 2 sec x sec x tan x = 2 sec2 x tan x, and

f (x) = 2 sec2 x sec2 x + (tan x)(2 sec2 x tan x) = 2 sec4 x + 4 sec2 x tan2 x

sin x LetLet g(t)f (x) = t= − sinx t. for x = 0 and f (0) = 1. (a) Plot (a) Plot f (x)the ongraph [−3π ,of3πg].with a graphing utility for 0 ≤ t ≤ 4π . (b) Show that that f (c) 0 ifofc the = tangent tan c. Use numerical root and ﬁnder on this a computer (b) Show the=slope linethe is always positive verify on your algebra graph. system to ﬁnd a good approximation to the smallest valuerange c0 such that f (c0 )line = 0. (c) For which values of t positive in the given is the tangent horizontal? (c) Verify that the horizontal line y = f (c0 ) is tangent to the graph of y = f (x) at x = c0 by plotting them on the same set of axes. 45.

SOLUTION

(a) Here is the graph of f (x) over [−3π , 3π ]. y 1 0.8 0.4 −5

5

−10

(b) Let f (x) =

x 10

sin x . Then x f (x) =

x cos x − sin x . x2

To have f (c) = 0, it follows that c cos c − sin c = 0, or tan c = c. Using a computer algebra system, we ﬁnd that the smallest positive value c0 such that f (c0 ) = 0 is c0 = 4.493409.

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D I F F E R E N T I AT I O N

(c) The horizontal line y = f (c0 ) = −0.217234 and the function y = f (x) are both plotted below. The horizontal line is clearly tangent to the graph of f (x). y 1 0.8 0.4 −5

5

x

−10

10

47. The height at time t (s) of a weight, oscillating up and down at the end of a spring, is s(t) = 300 + 40 sin t cm. Find π the Show no tangent graph of f (x) = tan x has zero slope. What is the least slope of a tangent line? the velocity and that acceleration at tline = to 3 s. Justify your response by sketching the graph of (tan x) . SOLUTION Let s(t) = 300 + 40 sin t be the height. Then the velocity is v(t) = s (t) = 40 cos t and the acceleration is a(t) = v (t) = −40 sin t. √ At t = π3 , the velocity is v 3 = 20 cm/sec and the acceleration is a π3 = −20 3 cm/sec2 . π

If you stand 1 m Rfrom wall and mark off points on the wall increments δ of angular 49. θ and initial velocityelevation v0 m/s The horizontal range of aa projectile launched from ground levelatatequal an angle (Figure these sin points grow increasingly farθapart. howwill thisthe illustrates the fact or thatdecrease the derivative of tan is θ is R4),=then (v02 /9.8) θ cos θ . Calculate d R/d . If θ Explain = 7π /24, range increase if the angle is increasing. increased slightly? Base your answer on the sign of the derivative. h4

h3 h2 h1 1

FIGURE 4 SOLUTION Let D(θ ) = tan θ represent the distance from the horizontal base to the point on the wall at angular elevation θ . Based on the derivative estimate of the rate of change, for a small change δ in the angle of elevation, the change in the distance can be approximated by:

D(θ + δ ) − D(θ ) d ≈ D (θ ) = tan θ . δ dθ The change in distance resulting from a δ increase in angle is then D(θ + δ ) − D(θ ) ≈ δ

d tan θ . dθ

Knowing that D(θ + δ ) − D(θ ) increases as θ increases, it follows that D (θ ) = ddθ tan θ must increase as θ increases.

Further Insights and Challenges 51. Show that a nonzero polynomial function y = f (x) cannot satisfy the equation y = −y. Use this to prove that Usex the of the derivative and the addition law for the cosine to prove that (cos x) = − sin x. neither sin norlimit cos xdeﬁnition is a polynomial. SOLUTION

• Let p be a nonzero polynomial of degree n and assume that p satisﬁes the differential equation y + y = 0. Then p + p = 0 for all x. There are exactly three cases.

(a) If n = 0, then p is a constant polynomial and thus p = 0. Hence 0 = p + p = p or p ≡ 0 (i.e., p is equal to 0 for all x or p is identically 0). This is a contradiction, since p is a nonzero polynomial. (b) If n = 1, then p is a linear polynomial and thus p = 0. Once again, we have 0 = p + p = p or p ≡ 0, a contradiction since p is a nonzero polynomial. (c) If n ≥ 2, then p is at least a quadratic polynomial and thus p is a polynomial of degree n − 2 ≥ 0. Thus q = p + p is a polynomial of degree n ≥ 2. By assumption, however, p + p = 0. Thus q ≡ 0, a polynomial of degree 0. This is a contradiction, since the degree of q is n ≥ 2.

S E C T I O N 3.7

The Chain Rule

123

CONCLUSION: In all cases, we have reached a contradiction. Therefore the assumption that p satisﬁes the differential equation y + y = 0 is false. Accordingly, a nonzero polynomial cannot satisfy the stated differential equation. • Let y = sin x. Then y = cos x and y = − sin x. Therefore, y = −y. Now, let y = cos x. Then y = − sin x and y = − cos x. Therefore, y = −y. Because sin x and cos x are nonzero functions that satisfy y = −y, it follows that neither sin x nor cos x is a polynomial. 53. Let f (x) = x sin x and g(x) = x cos x. Verify the following identity and use it to give another proof of the formula sin x = cos x: (a) Show that f (x) = g(x) + sin x and g (x) = − f (x) + cos x.

x and g (x) =x−g(x) − 2xsin (b) Verify that f (x) = − f (x) + 2 cos sin(x + h) − sin = 2 cos +x.12 h sin 12 h (c) By further experimentation, try to ﬁnd formulas for all higher derivatives of f and g. Hint: The kth derivative depends on whether k =the 4n,addition 4n + 1,formula 4n + 2,toorprove 4n + that 3. sin(a + b) − sin(a − b) = 2 cos a sin b. Hint: Use Let f (x) = x sin x and g(x) = x cos x. (a) We examine ﬁrst derivatives: f (x) = x cos x + (sin x) · 1 = g(x) + sin x and g (x) = (x)(− sin x) + (cos x) · 1 = − f (x) + cos x; i.e., f (x) = g(x) + sin x and g (x) = − f (x) + cos x. (b) Now look at second derivatives: f (x) = g (x) + cos x = − f (x) + 2 cos x and g (x) = − f (x) − sin x = −g(x) − 2 sin x; i.e., f (x) = − f (x) + 2 cos x and g (x) = −g(x) − 2 sin x. • The third derivatives are f (x) = − f (x) − 2 sin x = −g(x) − 3 sin x and g (x) = −g (x) − 2 cos x = (c) f (x) − 3 cos x; i.e., f (x) = −g(x) − 3 sin x and g (x) = f (x) − 3 cos x. • The fourth derivatives are f (4) (x) = −g (x) − 3 cos x = f (x) − 4 cos x and g (4) (x) = f (x) + 3 sin x = g(x) + 4 sin x; i.e., f (4) = f (x) − 4 cos x and g (4) (x) = g(x) + 4 sin x. • We can now see the pattern for the derivatives, which are summarized in the following table. Here n = 0, 1, 2, . . . SOLUTION

k

4n

4n + 1

4n + 2

4n + 3

f (k) (x)

f (x) − k cos x

g(x) + k sin x

− f (x) + k cos x

−g(x) − k sin x

g (k) (x)

g(x) + k sin x

− f (x) + k cos x

−g(x) − k sin x

f (x) − k cos x

Show that if π /2 < θ < π , then the distance along the x-axis between θ and the point where the tangent line intersects the x-axis is equal to |tan θ | (Figure 5).

3.7 The Chain Rule

Preliminary Questions 1. Identify the outside and inside functions for each of these composite functions. 4x + 9x 2 (b) y = tan(x 2 + 1) (c) y = sec5 x

(a) y =

SOLUTION

√ (a) The outer function is x, and the inner function is 4x + 9x 2 . (b) The outer function is tan x, and the inner function is x 2 + 1. (c) The outer function is x 5 , and the inner function is sec x. 2. Which of the following can be differentiated easily without using the Chain Rule? y = tan(7x 2 + 2), y=

√

x · sec x,

x , x +1 √ y = x cos x y=

x can be differentiated using the Quotient Rule, and the function √ x · sec x can be differThe function x+1 √ entiated using the Product Rule. The functions tan(7x 2 + 2) and x cos x require the Chain Rule. SOLUTION

3. Which is the derivative of f (5x)? (a) 5 f (x) SOLUTION

(b) 5 f (5x)

(c) f (5x)

The correct answer is (b): 5 f (5x).

4. How many times must the Chain Rule be used to differentiate each function? (b) y = cos((x 2 + 1)4 ) (a) y = cos(x 2 + 1) (c) y = cos((x 2 + 1)4 ) SOLUTION

(a) To differentiate cos(x 2 + 1), the Chain Rule must be used once. (b) To differentiate cos((x 2 + 1)4 ), the Chain Rule must be used twice.

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D I F F E R E N T I AT I O N

(c) To differentiate

cos((x 2 + 1)4 ), the Chain Rule must be used three times.

5. Suppose that f (4) = g(4) = g (4) = 1. Do we have enough information to compute F (4), where F(x) = f (g(x))? If not, what is missing? SOLUTION If F(x) = f (g(x)), then F (x) = f (g(x))g (x) and F (4) = f (g(4))g (4). Thus, we do not have enough information to compute F (4). We are missing the value of f (1).

Exercises In Exercises 1–4, ﬁll in a table of the following type: f (g(x))

1. f (u) = u 3/2 ,

f (u)

f (g(x))

g (x)

( f ◦ g)

g(x) = x 4 + 1

SOLUTION

f (g(x))

f (u)

f (g(x))

g (x)

( f ◦ g)

(x 4 + 1)3/2

3 u 1/2 2

3 (x 4 + 1)1/2 2

4x 3

6x 3 (x 4 + 1)1/2

3. f (u) = tan u,3 g(x) = x 4 f (u) = u , g(x) = 3x + 5 SOLUTION

f (g(x))

f (u)

f (g(x))

g (x)

( f ◦ g)

tan(x 4 )

sec2 u

sec2 (x 4 )

4x 3

4x 3 sec2 (x 4 )

In Exercises 5–6, 4write the function as a composite f (g(x)) and compute the derivative using the Chain Rule. f (u) = u + u, g(x) = cos x 5. y = (x + sin x)4 SOLUTION

Let f (x) = x 4 , g(x) = x + sin x, and y = f (g(x)) = (x + sin x)4 . Then dy = f (g(x))g (x) = 4(x + sin x)3 (1 + cos x). dx

d 3cos u for the following choices of u(x): 7. Calculate y = cos(x dx ) 2 (b) u = x −1 (a) u = 9 − x

(c) u = tan x

SOLUTION

(a) cos(u(x)) = cos(9 − x 2 ). d cos(u(x)) = − sin(u(x))u (x) = − sin(9 − x 2 )(−2x) = 2x sin(9 − x 2 ). dx (b) cos(u(x)) = cos(x −1 ). d 1 sin(x −1 ) . cos(u(x)) = − sin(u(x))u (x) = − sin(x −1 ) − 2 = dx x x2 (c) cos(u(x)) = cos(tan x). d cos(u(x)) = − sin(u(x))u (x) = − sin(tan x)(sec2 x) = − sec2 x sin(tan x). dx In Exercises 9–14, duse the General Power Rule or the Shifting and Scaling Rule to ﬁnd the derivative. Calculate f (x 2 + 1) for the following choices of f (u): 9. y = (x 2 + 9)4d x (a) f (u) = sin u (b) f (u) = 3u 3/2 SOLUTION Let g(x) = x 2 + 9. We apply the general power rule. 2 (c) f (u) = u − u d 2 d d g(x)4 = (x + 9)4 = 4(x 2 + 9)3 (x 2 + 9) = 4(x 2 + 9)3 (2x) = 8x(x 2 + 9)3 . dx dx dx y = sin5 x

S E C T I O N 3.7

11. y =

√

The Chain Rule

125

11x + 4

SOLUTION

Let g(x) = 11x + 4. We apply the general power rule.

1 d d √ 1 11 d g(x)1/2 = 11x + 4 = (11x + 4)−1/2 (11x + 4) = (11x + 4)−1/2 (11) = √ . dx dx 2 dx 2 2 11x + 4 √ Alternately, let f (x) = x and apply the shifting and scaling rule. Then 1 d √ d 11 11x + 4 = (11) f (11x + 4) = (11x + 4)−1/2 = √ . dx dx 2 2 11x + 4 13. y = sin(1 − 4x) y = (7x − 9)5 SOLUTION Let f (x) = sin x. We apply the shifting and scaling rule. d d f (1 − 4x) = sin(1 − 4x) = −4 cos(1 − 4x). dx dx In Exercises 15–18, ﬁnd the derivative of f ◦ g. y = (x 4 − x + 2)−3/2 15. f (u) = sin u, g(x) = 2x + 1 SOLUTION

Let h(x) = f (g(x)) = sin(2x + 1). Then, applying the shifting and scaling rule, h (x) = 2 cos(2x + 1).

Alternately, d f (g(x)) = f (g(x))g (x) = cos(2x + 1) · 2 = 2 cos(2x + 1). dx 9 , g(x) = x + x −1 17. f (u)f (u) = u= 2u + 1, g(x) = sin x SOLUTION

Let h(x) = f (g(x)) = (x + x −1 )9 . Then, applying the general power rule, h (x) = 9(x + x −1 )8 (1 − 12 ). x

Alternately, d f (g(x)) = f (g(x))g (x) = 9(x + x −1 )8 (1 − x −2 ). dx In Exercises 19–20,u ﬁnd the derivatives of f (g(x)) and g( f (x)). f (u) = , g(x) = csc x u−1 19. f (u) = cos u, g(x) = x 2 + 1 SOLUTION

d f (g(x)) = f (g(x))g (x) = − sin(x 2 + 1)(2x) = −2x sin(x 2 + 1). dx d g( f (x)) = g ( f (x)) f (x) = 2(cos x)(− sin x) = −2 sin x cos x. dx In Exercises 21–32, use the Chain 1 Rule to ﬁnd the derivative. f (u) = u 3 , g(x) = x +1 21. y = sin(x 2 )

SOLUTION Let y = sin x 2 . Then y = cos x 2 · 2x = 2x cos x 2 . 23. y = cot(4t 22 + 9) y = sin x

SOLUTION Let y = cot 4t 2 + 9 . Then

y = − csc2 4t 2 + 9 · 8t = −8t csc2 4t 2 + 9 . 25. y = (t 2 + 3t + 1)−5/2 y = 1 − t2

−5/2 SOLUTION Let y = t 2 + 3t + 1 . Then y = −

y = (x 4 − x 3 − 1)2/3

−7/2 5 2 5 (2t + 3) t + 3t + 1 (2t + 3) = − 7/2 . 2 2 t 2 + 3t + 1

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27. y =

x +1 4 x −1

SOLUTION

Let y =

x +1 4 . Then x −1

y = 4

x + 1 3 (x − 1) · 1 − (x + 1) · 1 8 (x + 1)3 8(1 + x)3 · =− = . 2 5 x −1 (1 − x)5 (x − 1) (x − 1)

29. y = cos3 (θ 21)

3 y = sec SOLUTION Letx y = cos3 θ 2 = cos θ 2 . Here, we note that calculating the derivative of the inside function, cos(θ 2 ), requires the chain rule. After two applications of the chain rule, we have

2

y = 3 cos θ 2 − sin θ 2 · 2θ = −6θ sin θ 2 cos2 θ 2 . 1 31. y =y = tan(θ + cos θ ) cos(x 2 ) + 1

−1/2 SOLUTION Let y = 1 + cos x 2 . Here, we note that calculating the derivative of the inside function, 1 + cos(x 2 ), requires the chain rule. After two applications of the chain rule, we have

−3/2

x sin x 2 1 y = − 1 + cos x 2 · − sin x 2 · (2x) = 3/2 . 2 1 + cos x 2

√ In Exercises 33–62, ﬁnd the derivative using the appropriate rule or combination of rules. y = ( x + 1 − 1)3/2 33. y = tan 5x SOLUTION

Let y = tan 5x. By the scaling and shifting rule, dy = 5 sec2 (5x). dx

35. y = x cos(1 − 3x) y = sin(x 2 + 4x) SOLUTION Let y = x cos (1 − 3x). Applying the product rule and then the scaling and shifting rule, y = x (− sin (1 − 3x)) · (−3) + cos (1 − 3x) · 1 = 3x sin (1 − 3x) + cos (1 − 3x) . 37. y = (4t + 9)1/2 y = sin(x 2 ) cos(x 2 ) SOLUTION Let y = (4t + 9)1/2 . By the shifting and scaling rule, dy 1 =4 (4t + 9)−1/2 = 2(4t + 9)−1/2 . dt 2 3 + cos x)−4 39. y =y (x = sin(cos θ) SOLUTION Let y = (x 3 + cos x)−4 . By the general power rule,

y = −4(x 3 + cos x)−5 (3x 2 − sin x) = 4(sin x − 3x 2 )(x 3 + cos x)−5 . √ 41. y =y =sin x cos x x)) sin(cos(sin SOLUTION We start by using a trig identity to rewrite √ 1 1 y = sin x cos x = sin 2x = √ (sin 2x)1/2 . 2 2 Then, after two applications of the chain rule, 1 1 cos 2x . y = √ · (sin 2x)−1/2 · cos 2x · 2 = √ 2 sin 2x 2 2 y = x 2 tan 2x

S E C T I O N 3.7

The Chain Rule

127

43. y = (z + 1)4 (2z − 1)3 SOLUTION

Let y = (z + 1)4 (2z − 1)3 . Applying the product rule and the general power rule,

dy = (z + 1)4 (3(2z − 1)2 )(2) + (2z − 1)3 (4(z + 1)3 )(1) = (z + 1)3 (2z − 1)2 (6(z + 1) + 4(2z − 1)) dz = (z + 1)3 (2z − 1)2 (14z + 2). √ 45. y = (x + x −1 )√ x + 1 y = 3 + 2s s √ SOLUTION Let y = (x + x −1 ) x + 1. Applying the product rule and the shifting and scaling rule, and then factoring, we get: √ 1 1 (x + x −1 + 2(x + 1)(1 − x −2 )) y = (x + x −1 ) (x + 1)−1/2 + x + 1(1 − x −2 ) = √ 2 2 x +1 =

2x 2

1 1 √ √ (x 3 + x + 2x 3 + 2x 2 − 2x − 2) = (3x 3 + 2x 2 − x − 2). 2 x +1 2x x + 1

47. y = (cos 6x2 + sin x 2 )1/2 y = cos (8x) SOLUTION Let y = (cos 6x + sin(x 2 ))1/2 . Applying the general power rule followed by both the scaling and shifting rule and the chain rule, y =

−1/2 x cos(x 2 ) − 3 sin 6x 1 − sin 6x · 6 + cos(x 2 ) · 2x = . cos 6x + sin(x 2 ) 2 cos 6x + sin(x 2 )

3) 49. y = tan3 x + tan(x (x + 1)1/2 y = Let y = tan3 x + tan(x 3 ) = (tan x)3 + tan(x 3 ). Applying the general power rule to the ﬁrst term and the SOLUTION x +2 chain rule to the second term, y = 3(tan x)2 sec2 x + sec2 (x 3 ) · 3x 2 = 3 x 2 sec2 (x 3 ) + sec2 x tan2 x .

z√ +1 51. y =y = 4 − 3 cos x z−1 z + 1 1/2 SOLUTION Let y = . Applying the general power rule followed by the quotient rule, z−1 dy 1 = dz 2

−1 z + 1 −1/2 (z − 1) · 1 − (z + 1) · 1 · = √ . 2 z−1 z + 1 (z − 1)3/2 (z − 1)

cos(1 + x) 53. y =y = (cos3 x + 3 cos x + 7)9 1 + cos x SOLUTION

Let y=

cos(1 + x) . 1 + cos x

Then, applying the quotient rule and the shifting and scaling rule, dy −(1 + cos x) sin(1 + x) + cos(1 + x) sin x cos(1 + x) sin x − cos x sin(1 + x) − sin(1 + x) = = dx (1 + cos x)2 (1 + cos x)2 =

sin(−1) − sin(1 + x) . (1 + cos x)2

The last line follows from the identity sin( A − B) = sin A cos B − cos A sin B with A = x and B = 1 + x. ) 55. y = cot7 (x 5 y = sec( t 2 − 9)

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Let y = cot7 x 5 . Applying the general power rule followed by the chain rule,

dy = 7 cot6 x 5 · − csc2 x 5 · 5x 4 = −35x 4 cot6 x 5 csc2 x 5 . dx

57. y = (1 + (x 2 2+ 2)5 )3 cos(x ) y = Let y2 = (1 + (x 2 + 2)5 )3 . Then, applying the general power rule twice, we obtain: SOLUTION 1+x dy = 3(1 + (x 2 + 2)5 )2 (5(x 2 + 2)4 (2x)) = 30x(1 + (x 2 + 2)5 )2 (x 2 + 2)4 . dx

√

59. y =y =1 +1 +1cot +5 (xx4 + 1) 9 1/2 1/2

SOLUTION Let y = 1 + 1 + x 1/2 . Applying the general power rule twice,

1/2 −1/2 1

−1/2 1 dy 1 1 + x 1/2 · · x −1/2 = = 1 + 1 + x 1/2 dx 2 2 2

1

. √ √ √ 8 x 1+ x 1+ 1+ x

√ 61. y = kx + √ b; k and b any constants x +1+1 y= SOLUTION Let y = (kx + b)1/2 , where b and k are constants. By the scaling and shifting rule, y =

1 k . (kx + b)−1/2 · k = √ 2 2 kx + b

df df du 63. Compute 1if = 2 and = 6. du; k, b constants, dx not both zero y = d x kt 4 + b SOLUTION Assuming f is a function of u, which is in turn a function of x, df d f du = · = 2(6) = 12. dx du d x 65. With notation as in Example 5, calculate √ molecular velocity v of a gas in a certain container is given by v = 29 T m/s, where T is the d The average d (a) sin θ (b) θ + tan θ dv dθ d θ (in atmospheres) . temperature in ◦kelvins. The temperature is related to the pressure ◦ T = 200P. Find θ =60 θ =45by d P P=1.5 SOLUTION

(a)

π

π

π d d π 1 π sin θ = sin θ = cos (60) = = . θ =60◦ d θ θ =60◦ dθ 180 180 180 180 2 360

(b)

π

π d

π π d θ + tan θ = θ + tan θ =1+ sec2 =1+ . ◦ ◦ θ =45 θ =45 dθ dθ 180 180 4 90

67. Compute the derivative of h(sin x) at x = π6 , assuming that h (0.5) = 10. Assume that f (0) = 2 and f (0) = 3. Find the derivatives of ( f (x))3 and f (7x) at x = 0. SOLUTION Let u = sin x and suppose that h (0.5) = 10. Then d dh du dh = cos x. (h(u)) = dx du d x du √ When x = π6 , we have u = .5. Accordingly, the derivative of h(sin x) at x = π6 is 10 cos π6 = 5 3. In Exercises 69–72, the table of values to calculate function at the given and f (g(2)) from the Let F(x) = use f (g(x)), where the graphs of f andthe g derivative are shown of in the Figure 1. Estimate g (2) point. graph and compute F (2).

69. f (g(x)),

x =6

x

1

4

6

f (x) f (x) g(x) g (x)

4 5 4 5

0 7 1

6 4 6 3

1 2

S E C T I O N 3.7

SOLUTION

The Chain Rule

129

d = f (g(6))g (6) = f (6)g (6) = 4 × 3 = 12. f (g(x)) dx x=6

√ 71. g( x), x = 16 sin( f (x)), x =4 √ 1 1 d √ 1 1 1 g( x) . SOLUTION = g (4) (1/ 16) = = dx 2 2 2 4 16 x=16 In Exercises compute f (2x73–76, + g(x)), x = the 1 indicated higher derivatives. d2 sin(x 2 ) dx2

73.

SOLUTION

Let f (x) = sin x 2 . Then, by the chain rule, f (x) = 2x cos x 2 and, by the product rule and the chain

rule,

f (x) = 2x − sin x 2 · 2x + 2 cos x 2 = 2 cos x 2 − 4x 2 sin x 2 . d3 2 d(3x + 9)11 d x 3 2 (x 2 + 9)5 dx SOLUTION Let f (x) = (3x + 9)11 . Then, by repeated use of the scaling and shifting rule,

75.

f (x) = 11(3x + 9)10 · 3 = 33(3x + 9)10 f (x) = 330(3x + 9)9 · 3 = 990(3x + 9)9 , f (x) = 8910(3x + 9)8 · 3 = 26730 (3x + 9)8 . Use a computer algebra system to compute f (k) (x) for k = 1, 2, 3 for the following functions: d3 2 sin(2x) (a) f (x) (b) f (x) = x 3 + 1 d x=3 cot(x )

77.

SOLUTION

(a) Let f (x) = cot(x 2 ). Using a computer algebra system, f (x) = −2x csc2 (x 2 ); f (x) = 2 csc2 (x 2 )(4x 2 cot(x 2 ) − 1); and

f (x) = −8x csc2 (x 2 ) 6x 2 cot2 (x 2 ) − 3 cot(x 2 ) + 2x 2 . (b) Let f (x) =

x 3 + 1. Using a computer algebra system, 3x 2 ; f (x) = 2 x3 + 1 f (x) =

3x(x 3 + 4) ; and 4(x 3 + 1)3/2

f (x) = −

3(x 6 + 20x 3 − 8) . 8(x 3 + 1)5/2

79. Compute the second derivative of sin(g(x)) at x = 2, assuming that g(2) = π4 , g (2) = 5, and g (2) = 3. Use the Chain Rule to express the second derivative of f ◦ g in terms of the ﬁrst and second derivatives of f and g. SOLUTION Let f (x) = sin(g(x)). Then f (x) = cos(g(x))g (x) and f (x) = cos(g(x))g (x) + g (x)(− sin(g(x)))g (x) = cos(g(x))g (x) − (g (x))2 sin(g(x)). Therefore, √ √ 2 f (2) = g (2) cos (g(2)) − g (2) sin (g(2)) = 3 cos π4 − (5)2 sin π4 = −22 · 22 = −11 2

where is at resistance i the current. d P/dt at t volume. = 2 if RFind = 1,000 81. TheAn power P in a sphere circuit is = Ri 2r, = expanding hasPradius 0.4t Rcm time t (inand seconds). Let V Find be the sphere’s d V /dt and iwhen varies according to i = sin(4 π t) (time in seconds). (a) r = 3 and (b) t = 3. di d 2 Ri SOLUTION = 2Ri = 2(1000)4π sin(4π t) cos(4π t)|t=2 = 0. dt dt t=2 t=2 The price (in dollars) of a computer component is P = 2C − 18C −1 , where C is the manufacturer’s cost to produce it. Assume that cost at time t (in years) is C = 9 + 3t −1 and determine the ROC of price with respect to

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83. The force F (in Newtons) between two charged objects is F = 100/r 2 , where r is the distance (in meters) between them. Find d F/dt at t = 10 if the distance at time t (in seconds) is r = 1 + 0.4t 2 . Let F(r ) = 100/r 2 , and let r (t) = 1 + .4t 2 . We compute F (r ) = −200/r 3 , and r (t) = .8t. This gives us r (10) = 41 and r (10) = 8. SOLUTION

dF d F dr = , dt dr dt so

d F = −200(41)−3 (8) ≈ −0.0232 N/s. dt t=10

85. Conservation of Energy The position at time t (in seconds) of a weight of mass m oscillating at the end of a According to the U.S. standard atmospheric model, developed by the National Oceanic and Atmospheric Adspring is x(t) = L sin(2πω t). Here, L is the maximum length of the spring, and ω the frequency (number of oscillations ministration for use in aircraft and rocket design, atmospheric temperature T (in degrees Celsius), pressure P per second). Let v and a be the velocity and acceleration of the weight. (kPa = 1,000 Pascals), and altitude h (meters) are related by the formulas (valid in the troposphere h ≤ 11,000): (a) By Hooke’s Law, the spring exerts a force of magnitude where k is the spring constant. √ F = −kx onthe weight, Use Newton’s Second Law, F = ma, to show that 2πω = k/m. T + 273.1 5.256 T = 15.04 0.000649h, = 101.29 1 mv 2 and potential Penergy U =+12 kx 2 .288.08 Prove that the total energy E = K + U (b) The weight has kinetic energy K =− 2 is conserved, that is, d E/dt = 0. Calculate d P/dh. Then estimate the change in P (in Pascals, Pa) per additional meter of altitude when h = 3,000. SOLUTION

(a) Let x(t) = L sin(2πω t). Then v(t) = x (t) = 2πω L cos(2πω t) and a(t) = v (t) = −(2πω )2 L sin(2πω t). Equating Hooke’s Law, F = −kx, to Newton’s second law, F = ma, yields −k L sin(2πω t) = −m(2πω )2 L sin(2πω t) or k/m = (2πω )2 . The desired result follows upon taking square roots. (b) We have dx dU = kx = kxv dt dt and dK dv = mv = mva dt dt But F = ma = −kx by Hooke’s Law, so a = (−k/m)x and we get dK = mva = mv(−k/m)x = −kvx. dt This shows that the derivative of U + K is zero.

Further Insights and Challenges 87. if f (−x) = fthen (x) and odd if f (−x) = − f (x). ShowRecall that ifthat f , g,f (x) andish even are differentiable, (a) Sketch a graph of any even function and explain graphically why its derivative is an odd function. [ f (g(h(x)))] = f (g(h(x)))g (h(x))h (x) (b) Show that differentiation reverses parity: If f is even, then f is odd, and if f is odd, then f is even. Hint: Differentiate f (−x). (c) Suppose that f is even. Is f necessarily odd? Hint: Check if this is true for linear functions. A function is even if f (−x) = f (x) and odd if f (−x) = − f (x). (a) The graph of an even function is symmetric with respect to the y-axis. Accordingly, its image in the left half-plane is a mirror reﬂection of that in the right half-plane through the y-axis. If at x = a ≥ 0, the slope of f exists and is equal to m, then by reﬂection its slope at x = −a ≤ 0 is −m. That is, f (−a) = − f (a). Note: This means that if f (0) exists, then it equals 0. SOLUTION

y 4 3 2 1 −2

−1

x 1

2

S E C T I O N 3.7

The Chain Rule

131

(b) By the chain rule, ddx f (−x) = − f (−x). Now suppose that f is even. Then f (−x) = f (x) and d d f (−x) = f (x) = f (x). dx dx Hence, when f is even, − f (−x) = f (x) or f (−x) = − f (x) and f is odd. On the other hand, suppose f is odd. Then f (−x) = − f (x) and d d f (−x) = − f (x) = − f (x). dx dx Hence, when f is odd, − f (−x) = − f (x) or f (−x) = f (x) and f is even. (c) Suppose that f is even. Then f is not necessarily odd. Let f (x) = 4x + 7. Then f (x) = 4, an even function. But f is not odd. For example, f (2) = 15, f (−2) = −1, but f (−2) = − f (2). In Exercises 89–91, use the following fact (proved in Chapter 4):q If a differentiable function f satisﬁes f (x) = 0 for all Power Rule for Fractional Exponents Let f (u) = u and g(x) = x p/q . Show that f (g(x)) = x p (recall the x, then f is a constant function. laws of exponents). Apply the Chain Rule and the Power Rule for integer exponents to show that f (g(x)) g (x) = px p−1 . ThenEquation derive theofPower RuleCosine for x p/qSuppose . 89. Differential Sine and that f (x) satisﬁes the following equation (called a differential equation): f (x) = − f (x)

3

(a) Show that f (x)2 + f (x)2 = f (0)2 + f (0)2 . Hint: Show that the function on the left has zero derivative. (b) Verify that sin x and cos x satisfy Eq. (3) and deduce that sin2 x + cos2 x = 1. SOLUTION

(a) Let g(x) = f (x)2 + f (x)2 . Then g (x) = 2 f (x) f (x) + 2 f (x) f (x) = 2 f (x) f (x) + 2 f (x)(− f (x)) = 0, where we have used the fact that f (x) = − f (x). Because g (0) = 0 for all x, g(x) = f (x)2 + f (x)2 must be a constant function. In other words, f (x)2 + f (x)2 = C for some constant C. To determine the value of C, we can substitute any number for x. In particular, for this problem, we want to substitute x = 0 and ﬁnd C = f (0)2 + f (0)2 . Hence, f (x)2 + f (x)2 = f (0)2 + f (0)2 . (b) Let f (x) = sin x. Then f (x) = cos x and f (x) = − sin x, so f (x) = − f (x). Next, let f (x) = cos x. Then f (x) = − sin x, f (x) = − cos x, and we again have f (x) = − f (x). Finally, if we take f (x) = sin x, the result from part (a) guarantees that sin2 x + cos2 x = sin2 0 + cos2 0 = 0 + 1 = 1. 91. Use the result of Exercise 90 to show the following: f (x) = sin x is the unique solution of Eq. (3) such that f (0) = 0 = g (0). Suppose thatg(x) both=f cos andxgissatisfy Eq. (3) and have thethat same initial that=is,0.f This (0) =result g(0)provides and f (0) = 1; and the unique solution such g(0) = 1values, and g (0) a means of and f (0) Show that f (x) = g(x) for all x. Hint: Show that f − g satisﬁes Eq. (3) and apply Exercise 89(a). deﬁning and developing all the properties of the trigonometric functions without reference to triangles or the unit circle. SOLUTION In part (b) of Exercise 89, it was shown that f (x) = sin x satisﬁes Eq. (3), and we can directly calculate that f (0) = sin 0 = 0 and f (0) = cos 0 = 1. Suppose there is another function, call it F(x), that satisﬁes Eq. (3) with the same initial conditions: F(0) = 0 and F (0) = 1. By Exercise 90, it follows that F(x) = sin x for all x. Hence, f (x) = sin x is the unique solution of Eq. (3) satisfying f (0) = 0 and f (0) = 1. The proof that g(x) = cos x is the unique solution of Eq. (3) satisfying g(0) = 1 and g (0) = 0 is carried out in a similar manner.

93. Chain Rule This exercise proves the Chain Rule without the special assumption made in the text. For any number lim g (x), where b, deﬁneAaDiscontinuous new function Derivative Use the limit deﬁnition to show that g (0) exists but g (0) = x→0 f (u) ⎧ − f (b) 1 ⎪ F(u) = for all u = b ⎨ 2 u −xb sin x if x = 0 g(x) = ⎪ Observe that F(u) is equal to the slope of the secant line⎩through (b,iff x(b)) 0 = 0and (u, f (u)). (a) Show that if we deﬁne F(b) = f (b), then F(u) is continuous at u = b. (b) Take b = g(a). Show that if x = a, then for all u, u − g(a) f (u) − f (g(a)) = F(u) x −a x −a Note that both sides are zero if u = g(a).

4

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(c) Substitute u = g(x) in Eq. (4) to obtain g(x) − g(a) f (g(x)) − f (g(a)) = F(g(x)) x −a x −a Derive the Chain Rule by computing the limit of both sides as x → a. SOLUTION

For any differentiable function f and any number b, deﬁne F(u) =

f (u) − f (b) u−b

for all u = b. (a) Deﬁne F(b) = f (b). Then lim F(u) = lim

u→b

u→b

f (u) − f (b) = f (b) = F(b), u−b

i.e., lim F(u) = F(b). Therefore, F is continuous at u = b. u→b

(b) Let g be a differentiable function and take b = g(a). Let x be a number distinct from a. If we substitute u = g(a) into Eq. (4), both sides evaluate to 0, so equality is satisﬁed. On the other hand, if u = g(a), then f (u) − f (g(a)) f (u) − f (g(a)) u − g(a) f (u) − f (b) u − g(a) u − g(a) = = = F(u) . x −a u − g(a) x −a u−b x −a x −a (c) Hence for all u, we have u − g(a) f (u) − f (g(a)) = F(u) . x −a x −a (d) Substituting u = g(x) in Eq. (4), we have g(x) − g(a) f (g(x)) − f (g(a)) = F(g(x)) . x −a x −a Letting x → a gives f (g(x)) − f (g(a)) g(x) − g(a) = lim F(g(x)) = F(g(a))g (a) = F(b)g (a) = f (b)g (a) x→a x→a x −a x −a lim

= f (g(a))g (a) Therefore ( f ◦ g) (a) = f (g(a))g (a), which is the Chain Rule.

3.8 Implicit Differentiation Preliminary Questions 1. Which differentiation rule is used to show SOLUTION

d dy sin y = cos y ? dx dx

The chain rule is used to show that ddx sin y = cos y dd xy .

2. One of (a)–(c) is incorrect. Find the mistake and correct it. (a)

d sin(y 2 ) = 2y cos(y 2 ) dy

d sin(x 2 ) = 2x cos(x 2 ) dx d sin(y 2 ) = 2y cos(y 2 ) (c) dx

(b)

SOLUTION

(a) This is correct. Note that the differentiation is with respect to the variable y. (b) This is correct. Note that the differentiation is with respect to the variable x. (c) This is incorrect. Because the differentiation is with respect to the variable x, the chain rule is needed to obtain d dy sin(y 2 ) = 2y cos(y 2 ) . dx dx

S E C T I O N 3.8

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133

3. On an exam, Jason was asked to differentiate the equation x 2 + 2x y + y 3 = 7 What are the errors in Jason’s answer? 2x + 2x y + 3y 2 = 0 There are two mistakes in Jason’s answer. First, Jason should have applied the product rule to the second

SOLUTION

term to obtain d dy (2x y) = 2x + 2y. dx dx Second, he should have applied the general power rule to the third term to obtain dy d 3 y = 3y 2 . dx dx 4. Which of (a) or (b) is equal to (a) (x cos t) SOLUTION

d (x sin t)? dx

dt dt (b) (x cos t) + sin t dx dx Using the product rule and the chain rule we see that dt d (x sin t) = x cos t + sin t, dx dx

so the correct answer is (b).

Exercises 1. Show that if you differentiate both sides of x 2 + 2y 3 = 6, the result is 2x + 6y 2 y = 0. Then solve for y and calculate d y/d x at the point (2, 1). SOLUTION

d d 2 (x + 2y 3 ) = 6 dx dx 2x + 6y 2 y = 0 2x + 6y 2 y = 0 6y 2 y = −2x y =

−2x . 6y 2

2 At (2, 1), dd xy = −4 6 = −3.

In Exercises the expression withofrespect to x. x y + 3x + y = 1, the result is (x + 1)y + y + 3 = 0. Show3–8, thatdifferentiate if you differentiate both sides the equation Then solve for y and calculate d y/d x at the point (1, −1). 3. x 2 y 3 SOLUTION

Assuming that y depends on x, then d 2 3 x y = x 2 · 3y 2 y + y 3 · 2x = 3x 2 y 2 y + 2x y 3 . dx

5.

y3 3 x x y

SOLUTION

Assuming that y depends on x, then d −y 3 3y 2 y y3 x(3y 2 y ) − y 3 = 2 + = . 2 dx x x x x

7. z +tan(xt) z2 SOLUTION

Assuming that z depends on x, then

(x 2 + y 2 )3/2

d (z + z 2 ) = z + 2z(z ) dx

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In Exercises 9–24, calculate the derivative of y (or other variable) with respect to x. 9. 3y 3 + x 2 = 5 SOLUTION

2x Let 3y 3 + x 2 = 5. Then 9y 2 y + 2x = 0, and y = − 2 . 9y

11. x 2 y + 2x y 2 = x + y y4 − y = x 3 + x SOLUTION Let x 2 y + 2x y 2 = x + y. Then x 2 y + 2x y + 2x · 2yy + 2y 2 = 1 + y x 2 y + 4x yy − y = 1 − 2x y − 2y 2 y =

1 − 2x y − 2y 2 . x 2 + 4x y − 1

13. x 2 ysin(xt) + y 4 −=3x t =8 SOLUTION We apply the product rule and the chain rule together: d d 2 (x y + y 4 − 3x) = 8 dx dx x 2 y + 2x y + 4y 3 y − 3 = 0 x 2 y + 4y 3 y = 3 − 2x y y (x 2 + 4y 3 ) = 3 − 2x y 3 − 2x y . y = 2 x + 4y 3 15. x 4 +3z 45= 1 x R =1 SOLUTION Let x 4 + z 4 = 1. Then 4x 3 + 4z 3 z = 0, and z = −x 3 /z 3 . 3/2 = 1 17. y −3/2 y +xx + = 2y x yLet y −3/2 + x 3/2 = 1. Then − 32 y −5/2 y + 32 x 1/2 = 0, and y = x 1/2 y 5/2 = x y 5 . SOLUTION

19.

√

1 1 1/2 x x+ s= +=x+y + yx2/3 s

SOLUTION

Let (x + s)1/2 = x −1 + s −1 . Then 1 (x + s)−1/2 1 + s = −x −2 − s −2 s . 2

√ Multiplying by 2x 2 s 2 x + s and then solving for s gives √ √ x 2 s 2 1 + s = −2s 2 x + s − 2x 2 s x + s √ √ x 2 s 2 s + 2x 2 s x + s = −2s 2 x + s − x 2 s 2

√ √ x 2 s 2 + 2 x + s s = −s 2 x 2 + 2 x + s

√ s2 x 2 + 2 x + s . s = − 2 2 √ x s +2 x +s x 1 √ 21. y +√x = y + y + y = 2x SOLUTION

Let y + x y −1 = 1. Then y + x −y −2 y + y −1 · 1 = 0, and y =

23. sin(x + y) = x + cos y y2 x + =0 y x +1

y −1 y y2 = . · x y −2 − 1 y 2 x − y2

S E C T I O N 3.8 SOLUTION

Implicit Differentiation

135

Let sin(x + y) = x + cos y. Then (1 + y ) cos(x + y) = 1 − y sin y cos(x + y) + y cos(x + y) = 1 − y sin y (cos(x + y) + sin y) y = 1 − cos(x + y) y =

1 − cos(x + y) . cos(x + y) + sin y

In Exercises 25–26, ﬁnd d y/d x at the given point. tan(x 2 y) = x + y 25. (x + 2)2 − 6(2y + 3)2 = 3, SOLUTION

(1, −1)

By the scaling and shifting rule, 2(x + 2) − 24(2y + 3)y = 0.

If x = 1 and y = −1, then 2(3) − 24(1)y = 0. so that 24y = 6, or y = 14 . In Exercises 27–30, ﬁnd anequation 2 − π πof the tangent line at the given point. , sin2 (3y) = x + y; 4 4 27. x y − 2y = 1, (3, 1) SOLUTION

Taking the derivative of both sides of x y − 2y = 1 yields x y + y − 2y = 0. Substituting x = 3, y = 1

yields 3y + 1 − 2y = 0. Solving, we get:

3y + 1 − 2y = 0 y + 1 = 0 y = −1. Hence, the equation of the tangent line at (3, 1) is y − 1 = −(x − 3), or y = 4 − x. 29. x 2/3 2+ 3y 2/3 = 2, (1, 1) x y + 2y = 3x, (2, 1) SOLUTION Taking the derivative of both sides of x 2/3 + y 2/3 = 2 yields 2 −1/3 2 −1/3 + y y = 0. x 3 3 Substituting x = 1, y = 1 yields 23 + 23 y = 0, so that 1 + y = 0, or y = −1. Hence, the equation of the tangent line at (1, 1) is y − 1 = −(x − 1), or y = 2 − x. 31. Find 1/2 the points on the graph of y 2 = x 3 − 3x + 1 (Figure 5) where the tangent line is horizontal. + y −1/2 = 2x y, 2 (1, 1) x (a) First show that 2yy = 3x − 3, where y = d y/d x. (b) Do not solve for y . Rather, set y = 0 and solve for x. This gives two possible values of x where the slope may be zero. (c) Show that the positive value of x does not correspond to a point on the graph. (d) The negative value corresponds to the two points on the graph where the tangent line is horizontal. Find the coordinates of these two points. y 2

x

−2 −1

1

2

−2

FIGURE 5 Graph of y 2 = x 3 − 3x + 1. SOLUTION

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D I F F E R E N T I AT I O N

(a) Applying implicit differentiation to y 2 = x 3 − 3x + 1, we have 2y

dy = 3x 2 − 3. dx

(b) Setting y = 0 we have 0 = 3x 2 − 3, so x = 1 or x = −1. (c) If we return to the equation y 2 = x 3 − 3x + 1 and substitute x = 1, we obtain the equation y 2 = −1, which has no real solutions. (d) Substituting x = −1 into y 2 = x 3 − 3x + 1 yields y 2 = (−1)3 − 3(−1) + 1 = −1 + 3 + 1 = 3, √ √ √ √ so y = 3 or − 3. The tangent is horizontal at the points (−1, 3) and (−1, − 3). 33. Show that no point on the graph of x 22 − 3x y2 + y 2 = 1 has a horizontal tangent line. Find all points on the graph of 3x + 4y + 3x y = 24 where the tangent line is horizontal (Figure 6). SOLUTION Let the implicit curve x 2 − 3x y + y 2 = 1 be given. Then (a) By differentiating the equation of the curve implicitly and setting y = 0, show that if the tangent line is horizontal at (x, y), then y = −2x. 2x − 3x y − 3y + 2yy = 0, (b) Solve for x by substituting y = −2x in the equation of the curve. so y =

2x − 3y . 3x − 2y

Setting y = 0 leads to y = 23 x. Substituting y = 23 x into the equation of the implicit curve gives 2 2 2 = 1, x + x x 2 − 3x 3 3 or − 59 x 2 = 1, which has no real solutions. Accordingly, there are no points on the implicit curve where the tangent line has slope zero. 35. If the derivative d x/d y exists at a point and d x/d y = 0, then the tangent line is vertical. Calculate d x/d y for the 4 + x y = x 3 − x + 2. Find d y/d x at the two points on the graph with x-coordinate thex 2graph of ythe equationFigure y 4 + 11 shows = y2 + and ﬁnd points on the graph where the tangent line is vertical. 0 and ﬁnd an equation of 2the tangent line at (1, 1). 4 2 SOLUTION Let y + 1 = y + x . Differentiating this equation with respect to y yields 4y 3 = 2y + 2x

dx , dy

so 4y 3 − 2y y(2y 2 − 1) dx = = . dy 2x x √ 2 dx Thus, = 0 when y = 0 and when y = ± . Substituting y = 0 into the equation y 4 + 1 = y 2 + x 2 gives dy 2 √ √ 2 3 1 = x 2 , so x = ±1. Substituting y = ± , gives x 2 = 3/4, so x = ± . Thus, there are six points on the graph of 2 2 4 2 2 y + 1 = y + x where the tangent line is vertical: √ √ √ √ √ √ √ √ 3 2 3 2 3 2 3 2 (1, 0), (−1, 0), , , − , , ,− , − ,− . 2 2 2 2 2 2 2 2 37. Differentiate the equation x 3 + 3x y 2 = 1 with respect to the variable t and express d y/dtd yin termsy of d xd x/dt, as in =− . the equation x y = 1 with respect to the variable t and derive the relation ExerciseDifferentiate 36. dt x dt 3 2 SOLUTION Let x + 3x y = 1. Then 3x 2

dy dx dx + 6x y + 3y 2 = 0, dt dt dt

and x 2 + y2 d x dy =− . dt 2x y dt In Exercises 38–39, differentiate the equation with respect to t to calculate d y/dt in terms of d x/dt. x 2 − y2 = 1

S E C T I O N 3.8

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137

39. y 4 + 2x y + x 2 = 0 SOLUTION

Let y 4 + 2x y + x 2 = 0. Then 4y 3

dy dx dx dy + 2x + 2y + 2x = 0, dt dt dt dt

and x + y dx dy =− . dt x + 2y 3 dt discussed in 1638 by 41. The folium of Descartes is the curve with equation x 3 + y 3 = 3x y (Figure 7). It was ﬁrst The volume V and pressure P of gas in a piston (which vary in time t) satisfy P V 3/2 = C, where C is a the French philosopher-mathematician Ren´e Descartes. The name “folium” means leaf. Descartes’s scientiﬁc colleague constant. Prove that Gilles de Roberval called it the “jasmine ﬂower.” Both men believed incorrectly that the leaf shape in the ﬁrst quadrant was repeated in each giving the appearancedof petals of3a ﬂower. Find an equation of the tangent line to this P P/dt quadrant, =− curve at the point 23 , 43 . d V /dt 2 V The ratio of the derivatives is negative. Could you havey predicted this from the relation P V 3/2 = C? 2

x

−2

2

−2

FIGURE 7 Folium of Descartes: x 3 + y 3 = 3x y. SOLUTION

x2 − y 2 , 4 , we have Let x 3 + y 3 = 3x y. Then 3x 2 + 3y 2 y = 3x y + 3y, and y = . At the point 3 3 x − y2 4 − 4 − 89 4 3 y = 29 16 = 10 = . 5 −9 3 − 9

The tangent line at P is thus y − 43 = 45 x − 23 or y = 45 x + 45 .

43. Plot the equation x 3 + y 3 = 3x y + b for several values of b. Find all points on the folium x 3 + y 3 = 3x y at which the tangent line is horizontal. (a) Describe how the graph changes as b → 0. (b) Compute d y/d x (in terms of b) at the point on the graph where y = 0. How does this value change as b → ∞? Do your plots conﬁrm this conclusion? SOLUTION

(a) Consider the ﬁrst row of ﬁgures below. When b < 0, the graph of x 3 + y 3 = 3x y + b consists of two pieces. As b → 0−, the two pieces move closer to intersecting at the origin. From the second row of ﬁgures, we see that the graph of x 3 + y 3 = 3x y + b when b > 0 consists of a single piece that has a “loop” in the ﬁrst quadrant. As b → 0+, the loop comes closer to “pinching off” at the origin. b = −0.1

b = − 0.001

b = − 0.01

y

y

y

1.5

1.5

1.5

1

1

1

0.5

0.5

0.5

x

− 0.5

0.5 − 0.5

1

1.5

x

− 0.5

0.5 − 0.5

1

1.5

x

− 0.5

0.5 − 0.5

1

1.5

138

CHAPTER 3

D I F F E R E N T I AT I O N b = 0.1

b = 0.001

b = 0.01

y

y

y

1.5

1.5

1.5

1

1

1

0.5

0.5

0.5

x

− 0.5

0.5

1

1.5

x

−0.5

− 0.5

0.5

1

x

−0.5

1.5

0.5

1

1.5

−0.5

−0.5

(b) Differentiating the equation x 3 + y 3 = 3x y + b with respect to x yields 3x 2 + 3y 2 y = 3x y + 3y, so y − x2 y = 2 . y −x Substituting y = 0 into x 3 + y 3 = 3x y + b leads to x 3 = b, or x =

√ 3

b. Thus, at the point on the graph where y = 0,

√ 0 − x2 3 y = 2 = x = b. 0 −x Consequently, as b → ∞, y → ∞ at the point on the graph where y = 0. This conclusion is supported by the ﬁgures shown below, which correspond to b = 1, b = 10, and b = 100.

−4

b = 100 y

b = 01 y

b = 10 y

4

4

4

2

2

2

x

−2

2

4

−4

x

−2

2

−4

4

x

−2

2

−2

−2

−2

−4

−4

−4

4

√ a trident curve (Figure 9), named by Isaac Newton in his treatise on 45. The equation x y = x 3 − 5x 2 + 2x − 1 deﬁnes Show thatin the1710. tangent lines x = 1where ± 2the to the so-called 8) with equation (x − 1)2 (x 2 + y 2 ) = curves 2published Find theatpoints tangent to theconchoid trident is(Figure horizontal as follows. 2x are vertical. (a) Show that x y + y = 3x 2 − 10x + 2. (b) Set y = 0 in (a), replace y by x −1 (x 3 − 5x 2 + 2x − 1), and solve the resulting equation for x. y 20 4 −2

2

x 6

8

−20

FIGURE 9 Trident curve: x y = x 3 − 5x 2 + 2x − 1. SOLUTION

Consider the equation of a trident curve: x y = x 3 − 5x 2 + 2x − 1

or y = x −1 (x 3 − 5x 2 + 2x − 1). (a) Taking the derivative of x y = x 3 − 5x 2 + 2x − 1 yields x y + y = 3x 2 − 10x + 2.

S E C T I O N 3.8

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139

(b) Setting y = 0 in (a) gives y = 3x 2 − 10x + 2. Thus, we have x −1 (x 3 − 5x 2 + 2x − 1) = 3x 2 − 10x + 2. Collecting like terms and setting to zero, we have 0 = 2x 3 − 5x 2 + 1 = (2x − 1)(x 2 − 2x − 1). Hence, x = 12 , 1 ±

√

2.

47. Find equations of the tangent lines at the points where x = 1 on the so-called folium (Figure 11): Find an equation of the tangent line at the four points on the curve (x 2 + y 2 − 4x)2 = 2(x 2 + y 2 ), where x = 1. 25 2 after the father of the French philosopher Blaise This curve (Figure 10) is an example of a limac xy )2 = named (x¸2on+ofy 2Pascal, 4 Pascal, who ﬁrst described it in 1650. y 2

x 1

−2

FIGURE 11 SOLUTION

2 First, ﬁnd the points (1, y) on the curve. Setting x = 1 in the equation (x 2 + y 2 )2 = 25 4 x y yields

25 2 y 4 25 2 y 4 + 2y 2 + 1 = y 4 (1 + y 2 )2 =

4y 4 + 8y 2 + 4 = 25y 2 4y 4 − 17y 2 + 4 = 0 (4y 2 − 1)(y 2 − 4) = 0 y2 =

1 or y 2 = 4 4

Hence y = ± 12 or y = ±2. Taking ddx of both sides of the original equation yields 25 2 25 y + x yy 4 2 25 2 25 y + x yy 4(x 2 + y 2 )x + 4(x 2 + y 2 )yy = 4 2 25 25 2 (4(x 2 + y 2 ) − x)yy = y − 4(x 2 + y 2 )x 2 4 2(x 2 + y 2 )(2x + 2yy ) =

25 y 2 − 4(x 2 + y 2 )x y = 4 y(4(x 2 + y 2 ) − 25 2 x) • At (1, 2), x 2 + y 2 = 5, and 25

22 − 4(5)(1) 1 y = 4 = . 25 3 2(4(5) − 2 (1)) Hence, at (1, 2), the equation of the tangent line is y − 2 = 13 (x − 1) or y = 13 x + 53 . • At (1, −2), x 2 + y 2 = 5 as well, and 25

(−2)2 − 4(5)(1) 1 y = 4 =− . 25 3 −2(4(5) − 2 (1)) Hence, at (1, −2), the equation of the tangent line is y + 2 = − 13 (x − 1) or y = − 13 x − 53 .

140

CHAPTER 3

D I F F E R E N T I AT I O N • At (1, 1 ), x 2 + y 2 = 5 , and 2 4

− 4 54 (1) 11

= y = . 1 4 5 − 25 (1) 12 2 4 2 25 4

2 1 2

11 5 Hence, at (1, 12 ), the equation of the tangent line is y − 12 = 11 12 (x − 1) or y = 12 x − 12 . • At (1, − 1 ), x 2 + y 2 = 5 , and 2 4

− 4 54 (1) 11

=− . y = 1 5 25 12 − 2 4 4 − 2 (1) 25 − 1 4 2

2

11 5 Hence, at (1, − 12 ), the equation of the tangent line is y + 12 = − 11 12 (x − 1) or y = − 12 x + 12 . The folium and its tangent lines are plotted below: y 2 1 x 1

0.5

1.5

2

−1 −2

Use a computer algebra system to plot y 2 = x 3 − 4x for −4 ≤ x, y ≤ 4. Show that if d x/d y = 0, then 49. Use a computer algebra system to plot (x 2 + y 2 )2 = 12(x 2 − y 2 ) + 2 for −4 ≤ x, y ≤ 4. How many y = 0. Conclude that the tangent line is vertical at the points where the curve intersects the x-axis. Does your plot conﬁrm horizontal tangent lines does the curve appear to have? Find the points where these occur. this conclusion? SOLUTION

A plot of the curve y 2 = x 3 − 4x is shown below. y 2 1

−2

x

−1

1

2

3

−1 −2

Differentiating the equation y 2 = x 3 − 4x with respect to y yields 2y = 3x 2

dx dx −4 , dy dy

or dx 2y . = 2 dy 3x − 4 From here, it follows that dd xy = 0 when y = 0, so the tangent line to this curve is vertical at the points where the curve intersects the x-axis. This conclusion is conﬁrmed by the plot of the curve shown above. In Exercises 50–53, use implicit differentiation to calculate higher derivatives. 51. Use the method of the previous exercise to show that y = −y −3 if x 2 + y 2 = 1. Consider the equation y 3 − 32 x 2 = 1. x SOLUTION Let x 2 + y 2 =21. Then 2x + 2yy = 0, and y = − . Thus (a) Show that y = x/y and differentiate again to show that y

2 y −yx =− xyy − 2x yy y · 1 − x y y2 + x 2 1 4 y =− =− = = − 3 = −y −3 . y− 2 2 3 y y y y (b) Express y in terms of x and y using part (a). Calculate y at the point (1, 1) on the curve x y 2 + y − 2 = 0 by the following steps:

S E C T I O N 3.9

Related Rates

141

53. Use the method of the previous exercise to compute y at the point 23 , 43 on the folium of Descartes with the equation x 3 + y 3 = 3x y.

x2 − y 2 , 4 , we ﬁnd SOLUTION Let x 3 + y 3 = 3x y. Then 3x 2 + 3y 2 y = 3x y + 3y, and y = . At y) = (x, 3 3 x − y2 4 − 4 4 − 12 −8 4 3 y = 29 16 = = = . 6 − 16 −10 5 3 − 9

Similarly,

y =

x − y2

2x − y − x 2 − y 1 − 2yy 162 =− 2 125 2 x−y

when (x, y) = 23 , 43 and y = 45 .

Further Insights and Challenges 55. The lemniscate curve (x 2 + y 2 )2 = 4(x 2 − y 2 ) was discovered by Jacob Bernoulli in 1694, who noted that it is if P8, lies the or intersection two curves x 2coordinates − y 2 = c of andthe x yfour = dpoints (c, d atconstants), the “shapedShow like athat ﬁgure or aon knot, the bow ofofa the ribbon.” Find the which thethen tangent tangents to the curves at P are perpendicular. line is horizontal (Figure 12). y 1 x

−1

1 −1

FIGURE 12 Lemniscate curve: (x 2 + y 2 )2 = 4(x 2 − y 2 ).

2

Consider the equation of a lemniscate curve: x 2 + y 2 = 4 x 2 − y 2 . Taking the derivative of both sides of this equation, we have

2 x 2 + y 2 2x + 2yy = 4 2x − 2yy . SOLUTION

Therefore,

x 2 + y2 − 2 x 8x − 4x x 2 + y 2 . =− y = 8y + 4y x 2 + y 2 x 2 + y2 + 2 y If y = 0, then either x = 0 or x 2 + y 2 = 2.

• If x = 0 in the lemniscate curve, then y 4 = −4y 2 or y 2 y 2 + 4 = 0. If y is real, then y = 0. The formula for y

in (a) is not deﬁned at the origin (0/0). An alternative parametric analysis shows that the slopes of the tangent lines to the curve at the origin are ±1.

• If x 2 + y 2 = 2 or y 2 = 2 − x 2 , then plugging this into the lemniscate equation gives 4 = 4 2x 2 − 2 which

√ √ yields x = ± 32 = ± 26 . Thus y = ± 12 = ± 22 . Accordingly, the four points at which the tangent lines to the √ √ √ √ √

√ √

√ lemniscate curve are horizontal are − 26 , − 22 , − 26 , 22 , 26 , − 22 , and 26 , 22 . Divide the curve in Figure 13 into ﬁve branches, each of which is the graph of a function. Sketch the branches.

3.9 Related Rates Preliminary Questions 1. Assign variables and restate the following problem in terms of known and unknown derivatives (but do not solve it): How fast is the volume of a cube increasing if its side increases at a rate of 0.5 cm/s? SOLUTION

Let s and V denote the length of the side and the corresponding volume of a cube, respectively. Determine

d V if ds = 0.5 cm/s. dt dt

142

CHAPTER 3

D I F F E R E N T I AT I O N

2. What is the relation between d V /dt and dr/dt if

4 V = πr 3 3

SOLUTION

Applying the general power rule, we ﬁnd ddtV = 4π r 2 dr dt .

In Questions 3–4, suppose that water pours into a cylindrical glass of radius 4 cm. The variables V and h denote the volume and water level at time t, respectively. 3. Restate in terms of the derivatives d V /dt and dh/dt: How fast is the water level rising if water pours in at a rate of 2 cm3 /min? dV 3 Determine dh dt if dt = 2 cm /min. 4. Repeat the same for this problem: At what rate is water pouring in if the water level rises at a rate of 1 cm/min?

SOLUTION

SOLUTION

Determine ddtV if dh dt = 1 cm/min.

Exercises In Exercises 1–2, consider a rectangular bathtub whose base is 18 ft2 . 1. How fast is the water level rising if water is ﬁlling the tub at a rate of 0.7 ft3 /min? Let h be the height of the water in the tub and V be the volume of the water. Then V = 18h and dh dV = 18 . Thus dt dt

SOLUTION

1 dV 1 dh = = (.7) ≈ .039 ft/min. dt 18 dt 18 3. The radius of a circular oil slick expands at a rate of 2 m/min. At what rate is water pouring into the tub if the water level rises at a rate of 0.8 ft/min? (a) How fast is the area of the oil slick increasing when the radius is 25 m? (b) If the radius is 0 at time t = 0, how fast is the area increasing after 3 min? Let r be the radius of the oil slick and A its area. dA dr = 2π r . Substituting r = 25 and dr (a) Then A = π r 2 and dt = 2, we ﬁnd dt dt

SOLUTION

dA = 2π (25) (2) = 100π ≈ 314.16 m2 /min. dt (b) Since dr dt = 2 and r (0) = 0, it follows that r (t) = 2t. Thus, r (3) = 6 and dA = 2π (6) (2) = 24π ≈ 75.40 m2 /min. dt In Exercises 5–8,rate assume the radius r ofincreasing a sphere isif expanding at increasing a rate of 14 volume of a sphere is At what is the that diagonal of a cube its edges are at in./min. a rate of The 2 cm/s? V = 43 π r 3 and its surface area is 4π r 2 . 5. Determine the rate at which the volume is changing with respect to time when r = 8 in. d As the radius is expanding at 14 inches per minute, we know that dr dt = 14 in./min. Taking dt of the equation V = 43 π r 3 yields dr dr dV 4 = π 3r 2 = 4π r 2 . dt 3 dt dt SOLUTION

Substituting r = 8 and dr dt = 14 yields dV = 4π (8)2 (14) = 3584π in.3 /min. dt 7. Determine the rate at which the surface area is changing when the radius is r = 8 in. Determine the rate at which the volume is changing with respect to time at t = 2 min, assuming that r = 0 at dA dr dr 2 SOLUTION t = 0. Taking the derivative of both sides of A = 4π r with respect to t yields dt = 8π r dt . dt = 14, so dA = 8π (8)(14) = 896π in.2 /min. dt Determine the rate at which the surface area is changing with respect to time at t = 2 min, assuming that r = 3 at t = 0.

S E C T I O N 3.9

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143

9. A road perpendicular to a highway leads to a farmhouse located 1 mile away (Figure 9). An automobile travels past the farmhouse at a speed of 60 mph. How fast is the distance between the automobile and the farmhouse increasing when the automobile is 3 miles past the intersection of the highway and the road?

l 60 mph

Automobile

FIGURE 9 SOLUTION Let l denote the distance between the automobile and the farmhouse, and let s denote the distance past the intersection of the highway and the road. Then l 2 = 1 + s 2 . Taking the derivative of both sides of this equation yields dl = 2s ds , so 2l dt dt

s ds dl = . dt l dt When the auto is 3 miles past the intersection, we have √ 3 · 60 dl 180 = √ = 18 10 ≈ 56.92 mph. = 2 2 dt 10 1 +3 11. Follow the same set-up as Exercise 10, but assume that the water level is rising at a rate of 0.3 m/min when it is 2 m. 3 conical tankﬂowing has height At whatArate is water in? 3 m and radius 2 m at the top. Water ﬂows in at a rate of 2 m /min. How fast is the water level rising when it is 2 m? SOLUTION Consider the cone of water in the tank at a certain instant. Let r be the radius of its (inverted) base, h its 4 π h 3 . Accordingly, height, and V its volume. By similar triangles, hr = 23 or r = 23 h and thus V = 13 π r 2 h = 27 dV 4 dh = π h2 . dt 9 dt Substituting h = 2 and dh dt = 0.3 yields dV 4 = π (2)2 (.3) ≈ 1.68 m3 /min. dt 9 13. Answer (a) and (b) in Exercise 12 assuming that Sonya begins moving 1 minute after Isaac takes off. Sonya and Isaac are in motorboats located at the center of a lake. At time t = 0, Sonya begins traveling south at SOLUTION IsaacAtx the miles easttime, of the center of the and Sonya miles south its center, let h be the distance a speed ofWith 32 mph. same Isaac takes off, lake heading east at ya speed of 27 of mph. between them. (a) How far have Sonya and Isaac each traveled after 12 min? 12 = 1 hour, Isaac has traveled 1 × 27 = 27 miles. After 11 minutes or 11 hour, Sonya has (a) After minutes (b) At12what rate isorthe between them increasing 60 distance 5 5 at t = 125 min? 60 11 88 traveled 60 × 32 = 15 miles. dh dx dy = 2x + 2y . Thus, (b) We have h 2 = x 2 + y 2 and 2h dt dt dt x d x + y ddty x d x + y ddty dh . = dt = dt dt h x 2 + y2 dy dx 88 Substituting x = 27 5 , dt = 27, y = 15 , and dt = 32 yields

dh = dt

27 (27) + 5

27 5

2

88 15

+ 88 15

(32)

2

5003 = √ ≈ 41.83 mph. 14305

15. At a given moment, a plane passes directly above a radar station at an altitude of 6 miles. A 6-ft man walks away from a 15-ft lamppost at a speed of 3 ft/s (Figure 10). Find the rate at which his shadow (a) If the plane’sinspeed is 500 mph, how fast is the distance between the plane and the station changing half an hour is increasing length. later? (b) How fast is the distance between the plane and the station changing when the plane passes directly above the station? SOLUTION

Let x be the distance of the plane from the station along the ground and h the distance through the air.

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(a) By the Pythagorean Theorem, we have h 2 = x 2 + 62 = x 2 + 36. dh dx dh x dx = 2x , and = . After an half hour, x = 12 × 500 = 250 miles. With x = 250, h = 2502 + 36, dt dt dt h dt and ddtx = 500, Thus 2h

250 dh = × 500 ≈ 499.86 mph. dt 2502 + 36 (b) When the plane is directly above the station, x = 0, so the distance between the plane and the station is not changing, for at this instant we have 0 dh = × 500 = 0 mph. dt 6 17. A hot air balloon rising vertically is tracked by an observer located 2 miles from the lift-off point. At a certain In the setting of Exercise 15, suppose that the line through the radar station and the plane makes an angle θ with moment, the angle between the observer’s line-of-sight and the horizontal is π5 , and it is changing at a rate of 0.2 rad/min. the horizontal. How fast is θ changing 10 min after the plane passes over the radar station? How fast is the balloon rising at this moment? SOLUTION

Let y be the height of the balloon (in miles) and θ the angle between the line-of-sight and the horizontal. y . Therefore, 2

Via trigonometry, we have tan θ =

sec2 θ ·

1 dy dθ = , dt 2 dt

and dy dθ =2 sec2 θ . dt dt Using ddtθ = 0.2 and θ = π5 yields dy 1 ≈ .61 mi/min. = 2 (.2) 2 dt cos (π /5) In Exercises referaway to a 16-ft down wall, Figuresmoves 1 and twice 2. Theasvariable h isdoes. the height the As a 19–23, man walks from ladder a 12-ftsliding lamppost, thea tip of as hisinshadow fast as he What of is the ladder’s topheight? at time t, and x is the distance from the wall to the ladder’s bottom. man’s 19. Assume the bottom slides away from the wall at a rate of 3 ft/s. Find the velocity of the top of the ladder at t = 2 if the bottom is 5 ft from the wall at t = 0. SOLUTION Let x denote the distance from the base of the ladder to the wall, and h denote the height of the top of the ladder from the ﬂoor. The ladder is 16 ft long, so h 2 + x 2 = 162 . At any time t, x = 5 + 3t. Therefore, at time t = 2, the base is x = 5 + 3(2) = 11 ft from the wall. Furthermore, we have

2h Substituting x = 11, h =

dh dx + 2x = 0 so dt dt

162 − 112 and ddtx = 3, we obtain

dh x dx =− . dt h dt

11 dh 11 (3) = − √ ft/s ≈ −2.84 ft/s. = − dt 15 162 − 112 21. Suppose that h(0) = 12 and the top slides down the wall at a rate of 4 ft/s. Calculate x and d x/dt at t = 2 s. Suppose that the top is sliding down the wall at a rate of 4 ft/s. Calculate d x/dt when h = 12. SOLUTION Let h and x be the height of the ladder’s top and the distance from the wall of the ladder’s bottom, respectively. After 2 seconds, h = 12 + 2 (−4) = 4 ft. Since h 2 + x 2 = 162 , √ x = 162 − 42 = 4 15 ft. Furthermore, we have 2h ﬁnd

√ dh dx dx h dh + 2x = 0, so that =− . Substituting h = 4, x = 4 15, and dh dt = −4, we dt dt dt x dt dx 4 4 = − √ (−4) = √ ≈ 1.03 ft/s. dt 4 15 15

What is the relation between h and x at the moment when the top and bottom of the ladder move at the same speed?

S E C T I O N 3.9

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145

23. Show that the velocity dh/dt approaches inﬁnity as the ladder slides down to the ground (assuming d x/dt is constant). This suggests that our mathematical description is unrealistic, at least for small values of h. What would, in fact, happen as the top of the ladder approaches the ground? SOLUTION Let L be the (constant) length of the ladder, let x be the distance from the base of the ladder to the point of contact of the ladder with the ground, and let h be the height of the point of contact of the ladder with the wall. By Pythagoras’ theorem,

x 2 + h2 = L 2. Taking derivatives with respect to t yields: 2x

dh dx + 2h = 0. dt dt

Therefore, dh x dx =− . dt h dt As ddtx is constant and positive and x approaches the length of the ladder as it falls, − hx ddtx gets arbitrarily large as h → 0. In a real situation, the top of the ladder would slide down the wall only part of the way. At some point it would lose contact with the wall and fall down freely with acceleration g. 25. Suppose that both the radius r and height h of a circular cone change at a rate of 2 cm/s. How fast is the volume of The radius r of a right circular cone of ﬁxed height h = 20 cm is increasing at a rate of 2 cm/s. How fast is the the cone increasing when r = 10 and h = 20? volume increasing when r = 10? 1 SOLUTION Let r be the radius, h be the height, and V be the volume of a right circular cone. Then V = 3 π r 2 h, and dV dh 1 dr = π r2 + 2hr . dt 3 dt dt dh When r = 10, h = 20, and dr dt = dt = 2, we ﬁnd 1000π dV π 2 10 · 2 + 2 · 20 · 10 · 2 = = ≈ 1047.20 cm3 /s. dt 3 3

27. A searchlight rotates at a rate of 3 revolutions per minute. The beam hits a wall located 10 miles away and produces 2 + 16y 2 = 25 (Figure 11). particle the How ellipse 9xis a dot ofAlight that moves moves counterclockwise horizontally alongaround the wall. fast this dot moving when the angle θ between the beam π and the line through the searchlight perpendicular to the wall is 6 ? Note that d θ /dt = 3(2π ) = 6π . SOLUTION Let y be the distance between the dot of light and the point of intersection of the wall and the line through the searchlight perpendicular to the wall.is Let be the angle between the Explain beam ofyour lightanswer. and the line. Using trigonometry, (a) In which of the four quadrants the θderivative d x/dt positive? y we have tan θ = 10 . Therefore, (b) Find a relation between d x/dt and d y/dt. (c) At what rate is the x-coordinate changing when the the point (1, 1) if its y-coordinate is increasing d θparticle 1 passes dy = , sec2 θ · at a rate of 6 ft/s? dt 10 dt and (d) What is d y/dt when the particle is at the top and bottom of the ellipse?

dy dθ = 10 sec2 θ . dt dt With θ = π6 and ddtθ = 6π , we ﬁnd 1 dy = 80π ≈ 251.33 mi/min ≈ 15,079.64 mph. = 10 (6π ) dt cos2 (π /6) 29. A plane traveling at an altitude of 20,000 ft passes directly overhead at time t = 0. One minute later you observe A rocket travels vertically at a speed of 800 mph. The rocket is tracked through a telescope by an observer located that the angle between the vertical and your line of sight to the plane is 1.14 rad and that this angle is changing at a rate 10 miles from the launching pad. Find the rate at which the angle between the telescope and the ground is increasing of 0.38 rad/min. Calculate the velocity of the airplane. 3 min after lift-off. SOLUTION Let x be the distance of the plane from you along the ground and θ the angle between the vertical and your x line of sight to the plane. Then tan θ = 20000 and dθ dx = 20000 sec2 θ · . dt dt Substituting θ = 1.14 and ddtθ = 0.38, we ﬁnd dx 20000 = (.38) ≈ 43581.69 ft/min dt cos2 (1.14) or roughly 495.25 mph.

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31. A jogger runs around a circular track of radius 60 ft. Let (x, y) be her coordinates, where the origin is at the center 2 /s) at which area is swept out by the second hand of a circular clock as a function of Calculate (in cmcoordinates of the track. Whenthe therate jogger’s are (36, 48), her x-coordinate is changing at a rate of 14 ft/s. Find d y/dt. the clock’s radius. dx dy dy x dx SOLUTION We have x 2 + y 2 = 602 . Thus 2x + 2y = 0, and =− . With x = 36, y = 48, and dt dt dt y dt d x = 14, dt dy 36 21 = − (14) = − = −10.5 ft/s. dt 48 2 3 ) of an expanding gas are related In Exercises the pressure P (in volume cmthe A car33–34, travelsassume down a that highway at 55 mph. Ankilopascals) observer is and standing 500Vft(in from highway. b by P(a) V How = C, fast where b and C arebetween constants holds and in adiabatic expansion,atwithout heat gain or passes loss). in front of the is the distance the(this observer the car increasing the moment the car

observer? Can answer without relying 2 , and on 33. Find d P/dt if byou = justify 1.2, P your = 8 kPa, V = 100 cm d Vany /dt calculations? = 20 cm3 /min. (b) How fast is the distance between the observer and the car increasing 1 min later? b SOLUTION Let P V = C. Then PbV b−1

dV dP + Vb = 0, dt dt

and Pb d V dP =− . dt V dt Substituting b = 1.2, P = 8, V = 100, and ddtV = 20, we ﬁnd dP (8) (1.2) =− (20) = −1.92 kPa/min. dt 100 35. A point moves along the parabola y = x 2 + 1. Let (t) be the2 distance between the point and the origin. Calculate , and /dt = 20 cm3 /min. Find b ifthat P =the25x-coordinate kPa, d P/dt of = the 12 kPa/min, V = 100atcm (t), assuming point is increasing a rate ofd9Vft/s. A point moves along the parabola y = x 2 + 1. Let be the distance between the point and the origin. By the distance formula, we have = (x 2 + (x 2 + 1)2 )1/2 . Hence SOLUTION

−1/2 d x

(2x 2 + 3)x ddtx dx 1 2 d = x + (x 2 + 1)2 + 2 x 2 + 1 · 2x = 2x . dt 2 dt dt x 4 + 3x 2 + 1 With ddtx = 9, we have 9x(2x 2 + 3) (t) = ft/s. x 4 + 3x 2 + 1 37. A water tank in the shape of a right circular cone of radius 300 cm and height 500 cm leaks water from the vertex at The base x of the right triangle in Figure 12 increases at a rate of 5 cm/s, while the height remains constant at a rate of 10 cm3 /min. Find the rate at which the water level is decreasing when it is 200 cm. h = 20. How fast is the angle θ changing when x = 20? SOLUTION Consider the cone of water in the tank at a certain instant. Let r be the radius of its (inverted) base, h its 3 1 2 3 3 height, and V its volume. By similar triangles, hr = 300 500 or r = 5 h and thus V = 3 π r h = 25 π h . Therefore, dV 9 dh π h2 , = dt 25 dt and 25 d V dh = . dt 9π h 2 dt We are given ddtV = −10. When h = 200, it follows that dh 25 1 = ≈ −2.21 × 10−4 cm/min. (−10) = − 2 dt 1440 π 9π (200) Thus, the water level is decreasing at the rate of 2.21 × 10−4 cm/min. Two parallel paths 50 ft apart run through the woods. Shirley jogs east on one path at 6 mph, while Jamail walks west on the other at 4 mph. If they pass each other at time t = 0, how far apart are they 3 s later, and how fast is the distance between them changing at that moment?

S E C T I O N 3.9

Related Rates

147

Further Insights and Challenges 39. Henry is pulling on a rope that passes through a pulley on a 10-ft pole and is attached to a wagon (Figure 13). Assume that the rope is attached to a loop on the wagon 2 ft off the ground. Let x be the distance between the loop and the pole. (a) Find a formula for the speed of the wagon in terms of x and the rate at which Henry pulls the rope. (b) Find the speed of the wagon when it is 12 ft from the pole, assuming that Henry pulls the rope at a rate of 1.5 ft/sec.

2 x

FIGURE 13 SOLUTION Let h be the distance from the pulley to the loop on the wagon. (Note that the rate at which Henry pulls the rope is the rate at which h is decreasing.) Using the Pythagorean Theorem, we have h 2 = x 2 + (10 − 2)2 = x 2 + 82 . dh dx dx h dh (a) Thus 2h = 2x , and = . dt dt dt x dt (b) As Henry pulls the rope at the rate of 1.5 = 32 ft/s, the distance h is decreasing at that rate; i.e., dh/dt = − 32 . When the wagon is 12 feet from the pole, we thus have √ dx 122 + 82 3 = − = − 13/2 ft/s. dt 12 2 √ 13 Thus, the speed of the wagon is ft/s. 2

41. Using a telescope, you track a rocket that was launched 2 miles away, recording the angle θ between the telescope A roller coaster has the shape of the graph in Figure 14. Show that when the roller coaster passes the point and the ground at half-second intervals. Estimate the velocity of the rocket if θ (10) = 0.205 and θ (10.5) = 0.225. (x, f (x)), the vertical velocity of the roller coaster is equal to f (x) times its horizontal velocity. h SOLUTION Let h be the height of the vertically ascending rocket. Using trigonometry, tan θ = , so 2 dθ dh = 2 sec2 θ · . dt dt We are given θ (10) = 0.205, and we can estimate θ (10.5) − θ (10) d θ = 0.04. ≈ dt t=10 0.5 Thus, dh = 2 sec2 (0.205) · (0.04) ≈ 0.083 mi/s, dt or roughly 300 mph. 43. A baseball player runs from home plate toward ﬁrst base at 20 ft/s. How fast is the player’s distance from second Two trains leave a station at t = 0 and travel with constant velocity v along straight tracks that make an angle θ . base changing when the player is halfway to ﬁrst base? See Figure 15. √ (a) Show that the trains are separating from each other at a rate of v 2 − 2 cos θ . (b) What does this formula give for θ = π ?

20 ft/s

FIGURE 15 Baseball diamond. SOLUTION In baseball, the distance between bases is 90 feet. Let x be the distance of the player from home plate and h the player’s distance from second base. Using the Pythagorean theorem, we have h 2 = 902 + (90 − x)2 . Therefore, dh dx 2h = 2 (90 − x) − , dt dt

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and dh 90 − x d x =− . dt h dt We are given ddtx = 20. When the player is halfway to ﬁrst base, x = 45 and h =

902 + 452 , so

√ 45 dh = − (20) = −4 5 ≈ −8.94 ft/s. 2 2 dt 90 + 45 45. A spectator seated 300 m away from the center of a circular track of radius 100 m watches an athlete run laps at As the wheel of radius r cm in Figure 16 rotates, the rod of length L attached at the point P drives a piston back a speed of 5 m/s. How fast is the distance between the spectator and athlete changing when the runner is approaching and forth in a straight line. Let x be the distance from the origin to the point Q at the end of the rod as in the ﬁgure. the spectator and the distance between them is 250 m? Hint: The diagram for this problem is similar to Figure 16, with (a) Use Theorem to show that r = 100 and xthe=Pythagorean 300. SOLUTION

2 θ those of Q are (x, 0). From the diagram, the coordinates cosθθ)2, r+sin ) and L 2 of =P (xare − r(rcos r 2 θsin

• The distance formula gives (b) Differentiate Eq. (8) with respect to t to prove that

d x − r cosdθθ)2 + (−r2 sin θ )2 . d θ L 2(x − r cos θ ) = (x + r sin θ + 2r sin θ cos θ =0 dt dt dt

Thus, (c) Calculate the speed of the piston when θ = π2 , assuming that r = 10 cm, L = 30 cm, and the wheel rotates at 4 L 2 = (x − r cos θ )2 + r 2 sin2 θ . revolutions per minute. Note that x (the distance of the spectator from the center of the track) and r (the radius of the track) are constants. • Differentiating with respect to t gives

2L

dθ dθ dL = 2 (x − r cos θ ) r sin θ + 2r 2 sin θ cos θ . dt dt dt

Thus, dθ rx dL = sin θ . dt L dt

θ , namely s = r θ . Thus

• Recall the relation between arc length s and angle ds = −5, we have dt

dθ 1 ds = . Given r = 100 and dt r dt

dθ 1 1 = rad/s. (−5) = − dt 100 20 (Note: In this scenario, the runner traverses the track in a clockwise fashion and approaches the spectator from Quadrant 1.) • Next, the Law of Cosines gives L 2 = r 2 + x 2 − 2r x cos θ , so cos θ =

r 2 + x2 − L2 1002 + 3002 − 2502 5 = = . 2r x 2 (100) (300) 8

Accordingly, sin θ =

√ 2 5 39 1− = . 8 8

• Finally

dL (300) (100) = dt 250

√

39 8

−

1 20

=−

√ 3 39 ≈ −4.68 m/s. 4

CHAPTER REVIEW EXERCISES In Exercises 1–4, refer to the function f (x) whose graph is shown in Figure 1.

Chapter Review Exercises

7 6 5 4 3 2 1

149

y

0.5

1.0

1.5

x 2.0

FIGURE 1

1. Compute the average ROC of f (x) over [0, 2]. What is the graphical interpretation of this average ROC? SOLUTION

The average rate of change of f (x) over [0, 2] is f (2) − f (0) 7−1 = = 3. 2−0 2−0

Graphically, this average rate of change represents the slope of the secant line through the points (2, 7) and (0, 1) on the graph of f (x). f (0.7 + h) − f (0.7) for h+= this number larger or smaller than f (0.7)? 3. Estimate f (0.7 h)0.3. − fIs(0.7) equal to the slope of the secant line between the points where For which valuehof h is h SOLUTION Forxh==1.1. 0.3, x = 0.7 and f (0.7 + h) − f (0.7) f (1) − f (0.7) 2.8 − 2 8 = ≈ = . h 0.3 0.3 3 Because the curve is concave up, the slope of the secant line is larger than the slope of the tangent line, so the value of the difference quotient should be larger than the value of the derivative. using the limit deﬁnition and ﬁnd an equation of the tangent line to the graph of f (x) In Exercises 5–8, fcompute f (a) (0.7) and Estimate f (1.1). at x = a. 5. f (x) = x 2 − x, SOLUTION

a=1

Let f (x) = x 2 − x and a = 1. Then f (a + h) − f (a) (1 + h)2 − (1 + h) − (12 − 1) = lim h h h→0 h→0

f (a) = lim

1 + 2h + h 2 − 1 − h = lim (1 + h) = 1 h h→0 h→0

= lim

and the equation of the tangent line to the graph of f (x) at x = a is y = f (a)(x − a) + f (a) = 1(x − 1) + 0 = x − 1. −1 , a = 4 7. f (x)f (x) = x= 5 − 3x, a = 2 SOLUTION Let f (x) = x −1 and a = 4. Then 1 − 1 f (a + h) − f (a) 4 − (4 + h) 4 = lim 4+h = lim h h h→0 h→0 h→0 4h(4 + h)

f (a) = lim

−1 1 1 =− =− 4(4 + 0) 16 h→0 4(4 + h)

= lim

and the equation of the tangent line to the graph of f (x) at x = a is 1 1 1 1 y = f (a)(x − a) + f (a) = − (x − 4) + = − x + . 16 4 16 2 In Exercises 9–12,3 compute d y/d x using the limit deﬁnition. f (x) = x , a = −2 9. y = 4 − x 2 SOLUTION

Let y = 4 − x 2 . Then

4 − (x + h)2 − (4 − x 2 ) 4 − x 2 − 2xh − h 2 − 4 + x 2 dy = lim = lim = lim (−2x − h) = −2x − 0 = −2x. dx h h h→0 h→0 h→0 y=

√ 2x + 1

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11. y =

1 2−x

SOLUTION

Let y =

1 . Then 2−x 1

1

dy 1 (2 − x) − (2 − x − h) 1 2−(x+h) − 2−x . = lim = lim = lim = dx h h→0 h→0 h(2 − x − h)(2 − x) h→0 (2 − x − h)(2 − x) (2 − x)2 In Exercises 13–16, 1 express the limit as a derivative. y√ = 1)21 1(x +− h− 13. lim h h→0 √ SOLUTION Let f (x) = x. Then √ lim

h→0

15. lim

1+h−1 f (1 + h) − f (1) = lim = f (1). h h h→0

sin t cos t

3 t→π limt −xπ + 1 x→−1 x + 1

SOLUTION

Let f (t) = sin t cos t and note that f (π ) = sin π cos π = 0. Then lim

t→π

sin t cos t f (t) − f (π ) = lim = f (π ). t→π t −π t −π

17. Find f (4) and f (4) if the tangent line to the graph of f (x) at x = 4 has equation y = 3x − 14. cos θ − sin θ + 1 lim SOLUTION θ − π of the tangent line to the graph of f (x) at x = 4 is y = f (4)(x − 4) + f (4) = f (4)x + θ →π The equation ( f (4) − 4 f (4)). Matching this to y = 3x − 14, we see that f (4) = 3 and f (4) − 4(3) = −14, so f (4) = −2. 19. Which of (A), (B), or (C) is the graph of the derivative of the function f (x) shown in Figure 3? Each graph in Figure 2 shows the graph of a function f (x) and its derivative f (x). Determine which is the function and which is the derivative. y y = f(x) x

−2 −1 y

2

y x

−2 −1

1

1

2

x

−2 −1

(A)

y

1

2

x

−2 −1

(B)

1

2

(C)

FIGURE 3

The graph of f (x) has four horizontal tangent lines on [−2, 2], so the graph of its derivative must have four x-intercepts on [−2, 2]. This eliminates (B). Moreover, f (x) is increasing at both ends of the interval, so its derivative must be positive at both ends. This eliminates (A) and identiﬁes (C) as the graph of f (x). SOLUTION

21. A girl’s height h(t) (in centimeters) is measured at time t (years) for 0 ≤ t ≤ 14: Let N (t) be the percentage of a state population infected with a ﬂu virus on week t of an epidemic. What = 1.2? percentage is likely to be infected week 4 if 96.7, N (3) 104.5, = 8 and N (3) 52,in75.1, 87.5, 111.8, 118.7, 125.2, 131.5, 137.5, 143.3, 149.2, 155.3, 160.8, 164.7 (a) What is the girl’s average growth rate over the 14-year period? (b) Is the average growth rate larger over the ﬁrst half or the second half of this period? (c) Estimate h (t) (in centimeters per year) for t = 3, 8. SOLUTION

(a) The average growth rate over the 14-year period is 164.7 − 52 = 8.05 cm/year. 14

Chapter Review Exercises

151

(b) Over the ﬁrst half of the 14-year period, the average growth rate is 125.2 − 52 ≈ 10.46 cm/year, 7 which is larger than the average growth rate over the second half of the 14-year period: 164.7 − 125.2 ≈ 5.64 cm/year. 7 (c) For t = 3, h (3) ≈

h(4) − h(3) 104.5 − 96.7 = = 7.8 cm/year; 4−3 1

h (8) ≈

h(9) − h(8) 137.5 − 131.5 = = 6.0 cm/year. 9−8 1

for t = 8,

In Exercises 23–24, use the following table of values for the number A(t) of automobiles (in millions) manufactured3/2in A planet’s period P (time in years to complete one revolution around the sun) is approximately 0.0011 A , the United States in year t. where A is the average distance (in millions of miles) from the planet to the sun. (a) Calculate P and d P/d Mars using 142. 1974 1975 1976 t A for1970 1971the value 1972 A = 1973 (b) Estimate the increase in P if A is increased to 143. A(t) 6.55 8.58 8.83 9.67 7.32 6.72 8.50 23. What is the interpretation of A (t)? Estimate A (1971). Does A (1974) appear to be positive or negative? Because A(t) measures the number of automobiles manufactured in the United States in year t, A (t) measures the rate of change in automobile production in the United States. For t = 1971, SOLUTION

A (1971) ≈

A(1972) − A(1971) 8.83 − 8.58 = = 0.25 million automobiles/year. 1972 − 1971 1

Because A(t) decreases from 1973 to 1974 and from 1974 to 1975, it appears that A (1974) would be negative. In Exercises compute derivative. Given25–50, the data, whichthe of (A)–(C) in Figure 4 could be the graph of the derivative A (t)? Explain. 25. y = 3x 5 − 7x 2 + 4 SOLUTION

Let y = 3x 5 − 7x 2 + 4. Then dy = 15x 4 − 14x. dx

27. y = t −7.3 −3/2 y = 4x SOLUTION Let y = t −7.3 . Then dy = −7.3t −8.3 . dt x +1 29. y =y =2 4x 2 − x −2 x +1 SOLUTION

x +1 . Then Let y = 2 x +1 1 − 2x − x 2 (x 2 + 1)(1) − (x + 1)(2x) dy = . = 2 2 dx (x + 1) (x 2 + 1)2

6 31. y = (x 4 3t −− 9x) 2 y= SOLUTION 4tLet − 9y = (x 4 − 9x)6 . Then

d dy = 6(x 4 − 9x)5 (x 4 − 9x) = 6(4x 3 − 9)(x 4 − 9x)5 . dx dx 33. y = (2 + 9x2 2 )3/2 −3 6 y = (3t + 20t )

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D I F F E R E N T I AT I O N SOLUTION

Let y = (2 + 9x 2 )3/2 . Then 3 d dy = (2 + 9x 2 )1/2 (2 + 9x 2 ) = 27x(2 + 9x 2 )1/2 . dx 2 dx

z 35. y = √ 3 4 y =1(x−+ z 1) (x + 4) SOLUTION

Let y = √

z 1−z

. Then √

dy = dz

1 − z − (− 2z ) √ 1

1−z

1−z

=

1 − z + 2z (1 − z)3/2

=

2−z . 2(1 − z)3/2

√ x4 + x 3 37. y = 1 y = x 12 + x SOLUTION Let y=

√ x4 + x = x 2 + x −3/2 . x2

Then dy 3 = 2x − x −5/2 . dx 2 39. y = tan(t −3 ) 1 y= √ −3 SOLUTION (1Let y = 2tan(t − x) − x ). Then d dy = sec2 (t −3 ) t −3 = −3t −4 sec2 (t −3 ). dt dt 2x 41. y =y sin(2x) cos− = 4 cos(2 3x) SOLUTION Let y = sin(2x) cos2 x = 2 sin x cos3 x. Then

dy = −6 sin2 x cos2 x + 2 cos4 x. dx 43. y = tan3 θ y = sin( x 2 + 1) SOLUTION Let y = tan3 θ . Then d dy = 3 tan2 θ tan θ = 3 tan2 θ sec2 θ . dθ dθ t 45. y = sec t4 y 1=+sin θ SOLUTION Let y =

t . Then 1 + sec t 1 + sec t − t sec t tan t dy . = dt (1 + sec t)2

8 47. y =y = z csc(9z + 1) 1 + cot θ SOLUTION

Let y =

8 = 8(1 + cot θ )−1 . Then 1 + cot θ dy 8 csc2 θ d = −8(1 + cot θ )−2 (1 + cot θ ) = . dθ dθ (1 + cot θ )2

y x)√x + x+ 49. y = =xtan(cos

Chapter Review Exercises

SOLUTION

Let y =

x+

x+

√

153

x. Then

√ −1/2 d √ 1 dy = x+ x+ x x+ x+ x dx 2 dx −1/2

√ √ −1/2 d √ 1 1 = x+ x+ x x+ x x+ x 1+ 2 2 dx −1/2

√ √ −1/2 1 1 1 = x+ x+ x x+ x 1+ 1 + x −1/2 . 2 2 2 In Exercises 51–56, use the table of values to calculate the derivative of the given function at x = 2. y = cos(cos(cos( θ ))) x

f (x)

g(x)

f (x)

g (x)

2

5

4

−3

9

4

3

2

−2

3

51. S(x) = 3 f (x) − 2g(x) SOLUTION

Let S(x) = 3 f (x) − 2g(x). Then S (x) = 3 f (x) − 2g (x) and S (2) = 3 f (2) − 2g (2) = 3(−3) − 2(9) = −27.

f (x) 53. R(x) = = f (x)g(x) H (x) g(x) SOLUTION

Let R(x) = f (x)/g(x). Then R (x) =

g(x) f (x) − f (x)g (x) g(x)2

and R (2) =

g(2) f (2) − f (2)g (2) 4(−3) − 5(9) 57 = =− . 2 2 16 g(2) 4

55. F(x) = f (g(2x)) G(x) = f (g(x)) SOLUTION Let F(x) = f (g(2x)). Then F (x) = 2 f (g(2x))g (2x) and F (2) = 2 f (g(4))g (4) = 2 f (2)g (4) = 2(−3)(3) = −18. In Exercise 57–60, 2 ) f (x) = x 3 − 3x 2 + x + 4. K (x) = f (xlet 57. Find the points on the graph of f (x) where the tangent line has slope 10. Let f (x) = x 3 − 3x 2 + x + 4. Then f (x) = 3x 2 − 6x + 1. The tangent line to the graph of f (x) will have slope 10 when f (x) = 10. Solving the quadratic equation 3x 2 − 6x + 1 = 10 yields x = −1 and x = 3. Thus, the points on the graph of f (x) where the tangent line has slope 10 are (−1, −1) and (3, 7). SOLUTION

59. Find all values of b such that y = 25x + b is tangent to the graph of f (x). For which values of x are the tangent lines to the graph of f (x) horizontal? SOLUTION Let f (x) = x 3 − 3x 2 + x + 4. The equation y = 25x + b represents a line with slope 25; in order for this line to be tangent to the graph of f (x), we must therefore have f (x) = 3x 2 − 6x + 1 = 25. Solving for x yields x = 4 and x = −2. The equation of the line tangent to the graph of f (x) at x = 4 is y = f (4)(x − 4) + f (4) = 25(x − 4) + 24 = 25x − 76, while the equation of the tangent line at x = −2 is y = f (−2)(x + 2) + f (−2) = 25(x + 2) − 18 = 25x + 32. Thus, when b = −76 and when b = 32, the line y = 25x + b is tangent to the graph of f (x). 61. (a) Find Show there a unique of aonly suchone that f (x) line = x 3of−slope 2x 2 k. + x + 1 has the same slope at both a and allthat values of is k such that value f (x) has tangent a + 1. (b) Plot f (x) together with the tangent lines at x = a and x = a + 1 and conﬁrm your answer to part (a).

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D I F F E R E N T I AT I O N SOLUTION

(a) Let f (x) = x 3 − 2x 2 + x + 1. Then f (x) = 3x 2 − 4x + 1 and the slope of the tangent line at x = a is f (a) = 3a 2 − 4a + 1, while the slope of the tangent line at x = a + 1 is f (a + 1) = 3(a + 1)2 − 4(a + 1) + 1 = 3(a 2 + 2a + 1) − 4a − 4 + 1 = 3a 2 + 2a. In order for the tangent lines at x = a and x = a + 1 to have the same slope, we must have f (a) = f (a + 1), or 3a 2 − 4a + 1 = 3a 2 + 2a. The only solution to this equation is a = 16 . (b) The equation of the tangent line at x = 16 is y = f

1 1 5 1 241 5 113 1 x− + f = x− + = x+ , 6 6 6 12 6 216 12 108

and the equation of the tangent line at x = 76 is 7 7 5 7 223 5 59 7 x− + f = x− + = x+ . y = f 6 6 6 12 6 216 12 108 The graphs of f (x) and the two tangent lines appear below. y 3 2 −1

x −1 −2 −3

0.5

1

1.5

2

In Exercises 63–68, calculate y . The following table gives the percentage of voters supporting each of two candidates in the days before an − 5x 2 +the 3x average ROC of A’s percentage over the intervals from day 20 to 15, day 15 to 10, and day 10 63. election. y = 12x 3Compute to 5. If this trend continues over the last 5 days before the election, will A win? SOLUTION Let y = 12x 3 − 5x 2 + 3x. Then Days before2Election 20 15 10 5 y = 36x − 10x + 3 and y = 72x − 10. Candidate A 44.8% 46.8% 48.3% 49.3%

√ Candidate B 65. y = 2x −2/5 +3 y=x √ SOLUTION Let y = 2x + 3 = (2x + 3)1/2 . Then y =

1 d (2x + 3)−1/2 (2x + 3) = (2x + 3)−1/2 2 dx

55.2% 53.2% 51.7% 50.7%

and

1 d y = − (2x + 3)−3/2 (2x + 3) = −(2x + 3)−3/2 . 2 dx

2) 67. y = tan(x4x y= SOLUTION x Let + 1y = tan(x 2 ). Then

y = 2x sec2 (x 2 ) and d sec(x 2 ) + 2 sec2 (x 2 ) = 8x 2 sec2 (x 2 ) tan(x 2 ) + 2 sec2 (x 2 ). y = 2x 2 sec(x 2 ) dx 69. In Figure 5, label the graphs f , f , and f . y = sin2 (x + 9) y

y

x

FIGURE 5

x

Chapter Review Exercises

155

SOLUTION First consider the plot on the left. Observe that the green curve is nonnegative whereas the red curve is increasing, suggesting that the green curve is the derivative of the red curve. Moreover, the green curve is linear with negative slope for x < 0 and linear with positive slope for x > 0 while the blue curve is a negative constant for x < 0 and a positive constant for x > 0, suggesting the blue curve is the derivative of the green curve. Thus, the red, green and blue curves, respectively, are the graphs of f , f and f . Now consider the plot on the right. Because the red curve is decreasing when the blue curve is negative and increasing when the blue curve is positive and the green curve is decreasing when the red curve is negative and increasing when the red curve is positive, it follows that the green, red and blue curves, respectively, are the graphs of f , f and f .

q of frozen chocolate cakes a commercial can sell p, per week depends on the price 71. Let qThe be anumber differentiable function of p, say, q =that f ( p). If p changesbakery by an amount then the percentage change p. Ininthis the percentage ROC of q withpercentage respect to change p is called price elasticity of demand Assume p issetting, (100p)/ p, and the corresponding in q the is (100q)/q, where q = f E( ( p p). + p) − f (that p). q = The 50 p(10 − p) forrate 5

2(8) − 10 = −3. 8 − 10

Thus, with the price set at $8, a 1% increase in price results in a 3% decrease in demand. y The daily demand fordﬂights between Chicago and St. Louis at the price p is q = 350 − 0.7 p (in hundreds In Exercises 73–80, compute . x price elasticity of demand when p = $250 and estimate the number of additional of passengers). Calculatedthe passengers if the ticket price is lowered by 1%. 73. x 3 − y 3 = 4 SOLUTION

Consider the equation x 3 − y 3 = 4. Differentiating with respect to x yields 3x 2 − 3y 2

dy = 0. dx

Therefore, x2 dy = 2. dx y 75. y = x2y 2 + 2x2 2 4x − 9y = 36 SOLUTION Consider the equation y = x y 2 + 2x 2 . Differentiating with respect to x yields dy dy = 2x y + y 2 + 4x. dx dx Therefore, dy y 2 + 4x = . dx 1 − 2x y

77. x +yy = 3x 2 + 2y 2 =x+y x SOLUTION Squaring the equation x + y = 3x 2 + 2y 2 and combining like terms leaves 2x y = 2x 2 + y 2 . Differentiating with respect to x yields 2x

dy dy + 2y = 4x + 2y . dx dx

Therefore, y − 2x dy = . dx y−x y = sin(x + y)

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D I F F E R E N T I AT I O N

79. tan(x + y) = x y SOLUTION

Consider the equation tan(x + y) = x y. Differentiating with respect to x yields dy dy + y. sec2 (x + y) 1 + =x dx dx

Therefore, dy y − sec2 (x + y) = . dx sec2 (x + y) − x 81. Find the points on the2graph of x 3 − y 3 = 3x y − 3 where the tangent line is horizontal. (sin x)(cos y) = x − y SOLUTION Suppose x 3 − y 3 = 3x y − 3. Differentiating with respect to x leads to 3x 2 − 3y 2

dy dy = 3x + 3y, dx dx

or dy x2 − y . = dx x + y2 Tangents to the curve are therefore horizontal when y = x 2 . Substituting y = x 2 into the equation for the curve yields x 3 − x 6 = 3x 3 − 3 or x 6 + 2x 3 − 3 = (x 3 − 1)(x 3 + 3) = 0. √ √ Thus, x = 1 or x = − 3 3. The corresponding y-coordinates are y = 1 and y = 3 9. Thus, the points along the curve x 3 − y 3 = 3x y − 3 where the tangent line is horizontal are: √ √ 3 3 (1, 1) and (− 3, 9). 83. For which values of α is f (x) = |x|α differentiable at x = 0? Find the points on the graph of x 2/3 + y 2/3 = 1 where the tangent line has slope 1. SOLUTION Let f (x) = |x|α . If α < 0, then f (x) is not continuous at x = 0 and therefore cannot be differentiable at x = 0. If α = 0, then the function reduces to f (x) = 1, which is differentiable at x = 0. Now, suppose α > 0 and consider the limit f (x) − f (0) |x|α = lim . x −0 x→0 x→0 x lim

If 0 < α < 1, then |x|α = −∞ x→0− x lim

while

|x|α =∞ x→0+ x lim

and f (0) does not exist. If α = 1, then lim

|x|

x→0− x

= −1

while

lim

|x|

x→0+ x

=1

and f (0) again does not exist. Finally, if α > 1, then |x|α = 0, x→0 x lim

so f (0) does exist. In summary, f (x) = |x|α is differentiable at x = 0 when α = 0 and when α > 1. is the water level rising when the water level 85. Water pours into the tank−1 in Figure 7 at a rate of 20 m3 /min. How fast 2 sin(x ) for x = 0 and f (0) = 0. Show that f (x) exists for all x (including x = 0) but that Let f (x) = x is h = 4 m? f (x) is not continuous at x = 0 (Figure 6).

36 m 8m 10 m 24 m

FIGURE 7

Chapter Review Exercises

157

SOLUTION When the water level is at height h, the length of the upper surface of the water is 24 + 3 2 h and the volume of water in the trough is 3 15 1 V = h 24 + 24 + h (10) = 240h + h 2 . 2 2 2

Therefore, dh dV = (240 + 15h) = 20 m3 /min. dt dt When h = 4, we have 20 1 dh = = m/min. dt 240 + 15(4) 15 87. A light moving at 3 ft/s approaches a 6-ft man standing 12 ft from a wall (Figure 8). The light is 3 ft above the The minute hand of a clock is 4 in. long and the hour hand is 3 in. long. How fast is the distance between the tips ground. How fast is the tip P of the man’s shadow moving when the light is 24 ft from the wall? of the hands changing at 3 o’clock?

3 ft 12 ft

3 ft/s

FIGURE 8 SOLUTION

Let x denote the distance between the man and the light. Using similar triangles, we ﬁnd P −3 3 = x 12 + x

or

P=

36 + 6. x

Therefore, dP 36 d x =− 2 . dt x dt When the light is 24 feet from the wall, x = 12. With ddtx = −3, we have dP 36 = − 2 (−3) = 0.75 ft/s. dt 12 89. (a) Side x of the triangle in Figure 9 is increasing at 2 cm/s and side y is increasing at 3 cm/s. Assume that θ A bead slides down the curve x y = 10. Find the bead’s horizontal velocity if its height at time t seconds is decreases in such a2 way that the area of the triangle has the constant value 4 cm2 . How fast is θ decreasing when x = 4, y = 80 − 16t cm. y = 4? (b) How fast is the distance between P and Q changing when x = 4, y = 4? P x

y

Q

FIGURE 9 SOLUTION

(a) The area of the triangle is A=

1 x y sin θ = 4. 2

Differentiating with respect to t, we obtain dA 1 1 dθ dx dy = x y cos θ + y sin θ + x sin θ = 0. dt 2 dt 2 dt dt When x = y = 4, we have 12 (4)(4) sin θ = 4, so sin θ = 12 . Thus, θ = π6 and √ 1 1 1 1 1 3 dθ (4)(4) + (4) (2) + (4) (3) = 0. 2 2 dt 2 2 2 2

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D I F F E R E N T I AT I O N

Solving for d θ /dt, we ﬁnd dθ 5 = − √ ≈ −0.72 rad/s. dt 4 3 (b) By the Law of Cosines, the distance D between P and Q satisﬁes D 2 = x 2 + y 2 − 2x y cos θ , so 2D

dD dθ dy dx dx dy = 2x + 2y + 2x y sin θ − 2x cos θ − 2y cos θ . dt dt dt dt dt dt

With x = y = 4 and θ = π6 , D=

42 + 42 − 2(4)(4)

√

√ 3 = 4 2 − 3. 2

Therefore, √ √ 20 − 12 3 − 8 3 16 + 24 − √ dD 3 = ≈ −1.50 cm/s. √ dt 8 2− 3

APPLICATIONS OF 4 THE DERIVAT IVE 4.1 Linear Approximation and Applications Preliminary Questions 1. Estimate g(1.2) − g(1) if g (1) = 4. SOLUTION

Using the Linear Approximation, g(1.2) − g(1) ≈ g (1)(1.2 − 1) = 4(0.2) = 0.8.

2. Estimate f (2.1), assuming that f (2) = 1 and f (2) = 3. SOLUTION

Using the Linear Approximation, f (2.1) ≈ f (2) + f (2)(2.1 − 2) = 1 + 3(0.1) = 1.3

3. The velocity of a train at a given instant is 110 ft/s. How far does the train travel during the next half-second (use the Linear Approximation)? SOLUTION Using the Linear Approximation, we estimate that the train travels at the constant velocity of 110 ft/sec over the next half-second; hence, we estimate that the train will travel 55 ft over the next half-second.

4. Discuss how the Linear Approximation makes the following statement more precise: The sensitivity of the output to a small change in input depends on the derivative. SOLUTION The Linear Approximation tells us that the change in the output value of a function is approximately equal to the value of the derivative times the change in the input value.

5. Suppose that the linearization of f (x) at a = 2 is L(x) = 2x + 4. What are f (2) and f (2)? SOLUTION

The linearization of f (x) at a = 2 is L(x) = f (2) + f (2)(x − 2) = f (2) · x + ( f (2) − 2 f (2))

Matching this to the given expression, L(x) = 2x + 4, it follows that f (2) = 2. Moreover, f (2) − 2 f (2) = 4, so f (2) = 8.

Exercises In Exercises 1–4, use the Linear Approximation to estimate f = f (3.02) − f (3) for the given function. 1. f (x) = x 2 SOLUTION

Let f (x) = x 2 . Then f (x) = 2x and f ≈ f (3)x = 6(.02) = 0.12.

3. f (x) = x −1 4 f (x) = x 1 SOLUTION Let f (x) = x −1 . Then f (x) = −x −2 and f ≈ f (3)x = − (.02) = −.00222. 9 In Exercises 5–10, 1estimate f using the Linear Approximation and use a calculator to compute both the error and the f (x)error. = percentage x +1 5. f (x) = x − 2x 2 ,

a = 5,

x = −0.4

Let f (x) = x − 2x 2 , a = 5, and x = −0.4. Then f (x) = 1 − 4x, f (a) = f (5) = −19 and f ≈ f (a)x = (−19)(−0.4) = 7.6. The actual change is SOLUTION

f = f (a + x) − f (a) = f (4.6) − f (5) = −37.72 − (−45) = 7.28. The error in the Linear Approximation is therefore |7.28 − 7.6| = 0.32; in percentage terms, the error is 0.32 × 100% ≈ 4.40% 7.28 f (x) =

√

1 + x,

a = 8, x = 1

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A P P L I C AT I O N S O F T H E D E R I VATI V E

7. f (x) =

1 , 1 + x2

a = 3,

x = 0.5

Let f (x) = 1 2 , a = 3, and x = .5. Then f (x) = − 2x2 2 , f (a) = f (3) = −.06 and f ≈ 1+x (1+x ) f (a)x = −.06(.5) = −0.03. The actual change is

SOLUTION

f = f (a + x) − f (a) = f (3.5) − f (3) ≈ −.0245283. The error in the Linear Approximation is therefore | − .0245283 − (−.03)| = .0054717; in percentage terms, the error is .0054717 −.0245283 × 100% ≈ 22.31% 9. f (x) = tan x, a = π4 , x = 0.013 f (x) = sin x, a = 0, x = 0.02 π π SOLUTION Let f (x) = tan x, a = 4 , and x = .013. Then f (x) = sec2 x, f (a) = f ( 4 ) = 2 and f ≈ f (a)x = 2(.013) = .026. The actual change is

π

π ≈ 1.026344 − 1 = .026344. f = f (a + x) − f (a) = f + .013 − f 4 4 The error in the Linear Approximation is therefore |.026344 − .026| = .000344; in percentage terms, the error is .000344 × 100% ≈ 1.31% .026344 In Exercises estimate using the Linear Approximation and ﬁnd the error using a calculator. π , quantity x = 0.03 f (x) 11–16, = cos x, a = the 4 √ √ 11. 26 − 25 √ 1 1 SOLUTION Let f (x) = x, a = 25, and x = 1. Then f (x) = 2 x −1/2 and f (a) = f (25) = 10 . • The Linear Approximation is f ≈ f (a)x = 1 (1) = .1. 10 • The actual change is f = f (a + x) − f (a) = f (26) − f (25) ≈ .0990195. • The error in this estimate is |.0990195 − .1| = .000980486.

1 1 1/4 − 161/4 13. √ 16.5− 101 10 SOLUTION

−0.0005.

1 )= Let f (x) = √1 , a = 100, and x = 1. Then f (x) = ddx (x −1/2 ) = − 12 x −3/2 and f (a) = − 12 ( 1000 x

• The Linear Approximation is f ≈ f (a)x = −0.0005(1) = −0.0005. • The actual change is

f = f (a + x) − f (a) = √

1 101

−

1 = −0.000496281. 10

• The error in this estimate is |−0.0005 − (−0.000496281)| = 3.71902 × 10−6 .

15. sin(0.023) Hint: Estimate sin(0.023) − sin(0). 1 1 √ − SOLUTION 98 Let10f (x) = sin x, a = 0, and x = .023. Then f (x) = cos x and f (a) = f (0) = 1. • The Linear Approximation is f ≈ f (a)x = 1(.023) = .023. • The actual change is f = f (a + x) − f (a) = f (0.023) − f (0) = 0.02299797. • The error in this estimate is |.023 − .02299797| ≈ 2.03 × 10−6 .

17. The cube root of 27 is 3. How much larger is the cube root of 27.2? Estimate using the Linear Approximation. (15.8)1/4 1 1 SOLUTION Let f (x) = x 1/3 , a = 27, and x = .2. Then f (x) = 3 x −2/3 and f (a) = f (27) = 27 . The Linear Approximation is 1 f ≈ f (a)x = (.2) = .0074074 27 Approximation. Note: You must express θ in radians. 19. Estimate sin 61◦ − sin √ the√Linear √ √ 60◦ using Which is larger: 2.1 − 2 or 9.1 − 9? Explain using the Linear Approximation.

S E C T I O N 4.1

Linear Approximation and Applications

161

π 1 π SOLUTION Let f (x) = sin x, a = π 3 , and x = 180 . Then f (x) = cos x and f (a) = f ( 3 ) = 2 . Finally, the Linear Approximation is

1 π π f ≈ f (a)x = = ≈ .008727 2 180 360 21. Atmospheric pressure P at altitude h = 40,000 ft is 390 lb/ft2 . Estimate P at h = 41,000 if d P/dh = T = 25◦ C A thin silver wire has length L = 18 cm when the temperature is T = 30◦ C. Estimate the length whenh=40,000 3. −5◦ −1 −0.0188 lb/ft if the coefﬁcient of thermal expansion is k = 1.9 × 10 C (see Example 3). SOLUTION

Let P(h) by the pressure P at altitude h. By the Linear Approximation, P(41000) − P(40000) ≈

d P h = −.0188 lbs/ft3 (1000 ft) = −18.8 lbs/ft2 . dh h=40000

Hence, P(41000) ≈ 390 lbs/ft2 − 18.8 lbs/ft2 = 371.2 lbs/ft2 . 23. The side s of a square carpet is measured at 6 ft. Estimate ◦the maximum error in the area A of the carpet if s◦ is resistance a copper wire at temperature T = 20 C is R = 15 . Estimate the resistance at T = 22 C, accurateThe to within half R anof inch. assuming that d R/d T T =20 = 0.06 /◦ C. SOLUTION Let s be the length in feet of the side of the square carpet. Then A(s) = s 2 is the area of the carpet. With 1 (note that 1 inch equals 1 foot), an estimate of the size of the error in the area is given by the Linear a = 6 and s = 24 12 Approximation: 1 A ≈ A (6)s = 12 = .5 ft2 24 25. A stone tossed vertically in the air with velocitythe v ft/s reaches a maximum height of h= v 2 /64 ft. increases A spherical balloon has a radius of 6initial in. Estimate change in volume and surface area if the radius (a) Estimate by 0.3 in.h if v is increased from 25 to 26 ft/s. (b) Estimate h if v is increased from 30 to 31 ft/s. (c) In general, does a 1 ft/s increase in initial velocity cause a greater change in maximum height at low or high initial velocities? Explain. SOLUTION

A stone tossed vertically with initial velocity v ft/s attains a maximum height of h = v 2 /64 ft.

1 (25)(1) = .78125 ft. (a) If v = 25 and v = 1, then h ≈ h (v)v = 32

1 (30)(1) = .9375 ft. (b) If v = 30 and v = 1, then h ≈ h (v)v = 32 (c) A one foot per second increase in initial velocity v increases the maximum height by approximately v/32 ft. Accordingly, there is a bigger effect at higher velocities. 2 ft, 27. TheIfstopping automobile (aftertoapplying the brakes) F(s) = 1.1s the pricedistance of a bus for passanfrom Albuquerque Los Alamos is set atisx approximately dollars, a bus company takes+in0.054s a monthly where s is the in 1.5x mph.−Use the2Linear Approximation to estimate the change in stopping distance per additional (in thousands of dollars). revenue ofspeed R(x) = 0.01x mph when s = 35 and when s = 55. (a) Estimate the change in revenue if the price rises from $50 to $53. 2. SOLUTION Let that F(s)x==1.1s .054swill (b) Suppose 80.+How revenue be affected by a small increase in price? Explain using the Linear •Approximation. The Linear Approximation at s = 35 mph is

F ≈ F (35)s = (1.1 + .108 × 35)s = 4.88s ft The change in stopping distance per additional mph for s = 35 mph is approximately 4.88 ft. • The Linear Approximation at s = 55 mph is

F ≈ F (55)s = (1.1 + .108 × 55)s = 7.04s ft The change in stopping distance per additional mph for s = 55 mph is approximately 7.04 ft. 29. Estimate the weight loss per mile of altitude gained for a 130-lb pilot. At which altitude would she weigh 129.5 lb? Juan measures the circumference C of a spherical ball at 40 cm and computes the ball’s volume V . Estimate the See Example 4. maximum possible error in V if the error in C is at most 2 cm. Recall that C = 2π r and V = 43 π r 3 , where r is the SOLUTION From the discussion in the text, the weight loss W at altitude h (in miles) for a person weighing W0 at ball’s radius. the surface of the earth is approximately W ≈ −

1 W h 1980 0

If W0 = 130 pounds, then W ≈ −0.066h. Accordingly, the pilot loses approximately 0.066 pounds per mile of altitude gained. She will weigh 129.5 pounds at the altitude h such that −0.066h = −0.5, or h = 0.5/0.066 ≈ 7.6 miles. 31. The volume of a certain gas (in liters) is related to pressure P (in atmospheres) by the formula P V = 24. Suppose How much would a 160-lb astronaut weigh in a satellite orbiting the earth at an altitude of 2,000 miles? Estimate that V = 5 with a possible error of ±0.5 L. the astronaut’s weight loss per additional mile of altitude beyond 2,000.

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(a) Compute P and estimate the possible error. (b) Estimate the maximum allowable error in V if P must have an error of at most 0.5 atm. SOLUTION

(a) We have P = 24V −1 . For V = 5 L, P = 24/5 = 4.8 atm. If the error in computing the volume is ±0.5 liters, then the error in computing the pressure is approximately 24 12 |P| ≈ |P (V )V | = | − 24V −2 V | = − (±.5) = = .48 atm 25 25 (b) When the volume is V = 5 L, the error in computing the pressure is approximately 24 24 −2 |V | = .5 atm |P| ≈ |P (V )V | = | − 24V V | = − V = 25 25 Thus, the maximum allowable error in V is approximately V = ±.520833 L. In Exercises 33–38, ﬁnd the linearization at x = a. The dosage D of diphenhydramine for a dog of body mass w kg is D = kw 2/3 mg, where k is a constant. A cocker 33. y = cosspaniel x sin x,hasamass = 0 w = 10 kg according to a veterinarian’s scale. Estimate the maximum allowable error in w if the percentage error in the dosage1D must be less than 5%. SOLUTION Let f (x) = sin x cos x = 2 sin 2x. Then f (x) = cos 2x. The linearization at a = 0 is L(x) = f (a)(x − a) + f (a) = 1(x − 0) + 0 = x. 35. y = (1 + x)−1/2 , a = 0 π y = cos x sin x, a = 1 SOLUTION Let f (x) = (1 +4x)−1/2 . Then f (x) = − 2 (1 + x)−3/2 . The linearization at a = 0 is 1 L(x) = f (a)(x − a) + f (a) = − x + 1. 2 , a=0 37. y = (1 + x 2 )−1/2 y = (1 + x)−1/2 , a = 32 −1/2 SOLUTION Let f (x) = (1 + x ) . Then f (x) = −x(1 + x 2 )−3/2 , f (a) = 1 and f (a) = 0, so the linearization at a is L(x) = f (a)(x − a) + f (a) = 1. √ √ Estimate 16.2 39. π using the linearization L(x) of f (x) = x at a sin x y = determine , a if=the estimate is too large or too small. of axes and x 2 SOLUTION Let f (x) = x 1/2 , a = 16, and x = .2. Then f (x) = linearization to f (x) is

= 16. Plot f (x) and L(x) on the same set 1 x −1/2 and f (a) = f (16) = 1 . The 2 8

1 1 L(x) = f (a)(x − a) + f (a) = (x − 16) + 4 = x + 2. 8 8

√ Thus, we have 16.2 ≈ L(16.2) = 4.025. Graphs of f (x) and L(x) are shown below. Because the graph of L(x) lies above the graph of f (x), we expect that the estimate from the Linear Approximation is too large. y 5 4 3 2 1

L(x)

0

5

f (x) x 10

15

20

25

√ √ to compute the percentage error. In Exercises 41–46, approximate using linearization and use a calculator Estimate 1/ 15 using a suitable linearization of f (x) = 1/ x. Plot f (x) and L(x) on the same set of axes √ 41. and 17determine if the estimate is too large or too small. Use a calculator to compute the percentage error. SOLUTION

to f (x) is

Let f (x) = x 1/2 , a = 16, and x = 1. Then f (x) = 12 x −1/2 , f (a) = f (16) = 18 and the linearization

Thus, we have

√

1 1 L(x) = f (a)(x − a) + f (a) = (x − 16) + 4 = x + 2. 8 8 17 ≈ L(17) = 4.125. The percentage error in this estimate is √ 17 − 4.125 √ × 100% ≈ .046% 17

S E C T I O N 4.1

Linear Approximation and Applications

163

43. (17)1/4 1 √ 1 1 SOLUTION Let f (x) = x 1/4 , a = 16, and x = 1. Then f (x) = 4 x −3/4 , f (a) = f (16) = 32 and the linearization 17 to f (x) is 1 1 3 (x − 16) + 2 = x+ . L(x) = f (a)(x − a) + f (a) = 32 32 2 Thus, we have (17)1/4 ≈ L(17) = 2.03125. The percentage error in this estimate is (17)1/4 − 2.03125 × 100% ≈ .035% (17)1/4 45. (27.001)1/31/3 (27.03) 1 1 SOLUTION Let f (x) = x 1/3 , a = 27, and x = .001. Then f (x) = 3 x −2/3 , f (a) = f (27) = 27 and the linearization to f (x) is 1 1 L(x) = f (a)(x − a) + f (a) = (x − 27) + 3 = x + 2. 27 27 Thus, we have (27.001)1/3 ≈ L(27.001) ≈ 3.0000370370370. The percentage error in this estimate is (27.001)1/3 − 3.0000370370370 × 100% ≈ .000000015% (27.001)1/3

π on the same set of axes. 5/3 f (x) = tan x and its linearization L(x) at a = (1.2)Plot 4 (a) Does the linearization overestimate or underestimate f (x)? (b) Show, by graphing y = f (x) − L(x) and y = 0.1 on the same set of axes, that the error | f (x) − L(x)| is at most 0.1 for 0.55 ≤ x ≤ 0.95. (c) Find an interval of x-values on which the error is at most 0.05. 47.

SOLUTION

Let f (x) = tan x and a = π4 . Then f (x) = sec2 x, f (a) = 2 and

π L(x) = f (a) + f (a)(x − a) = 1 + 2 x − . 4

Graphs of f (x) and L(x) are shown below. y 2 x 1 −2

(a) From the ﬁgure above, we see that the graph of the linearization L(x) lies below the graph of f (x); thus, the linearization underestimates f (x). (b) The graph of y = f (x) − L(x) is shown below at the left. Below at the right, portions of the graphs of y = f (x) − L(x) and y = 0.1 are shown. From the graph on the right, we see that | f (x) − L(x)| ≤ 0.1 for 0.55 ≤ x ≤ 0.95.

y 4 3 2 1 x 0

x

1

0.5 0.6 0.7 0.8 0.9

1

(c) Portions of the graphs of y = f (x) − L(x) and y = 0.05 are shown below. From the graph, we see that | f (x) − L(x)| ≤ 0.05 roughly for 0.62 < x < 0.93.

x 0.6

0.7

0.8

0.9

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In Exercises 49–50, use the following fact derived from2 Newton’s Laws: An object released at an angle θ with initial Compute the linearization L(x) 1of 2f (x) = x − x 3/2 at a = 2. Then plot f (x) − L(x) and ﬁnd an interval velocity v ft/s travels a total distance s = 32 v sin 2θ ft (Figure 8). around a = 1 such that | f (x) − L(x)| ≤ 0.1. 49. A player located 18.1 ft from a basket launches a successful jump shot from a height of 10 ft (level with the rim of the basket), at an angle θ = 34◦ and initial velocity of v = 25 ft/s. y

q

x

FIGURE 8 Trajectory of an object released at an angle θ .

(a) Show that the distance s of the shot changes by approximately 0.255θ ft if the angle changes by an amount θ . Remember to convert the angles to radians in the Linear Approximation. (b) Is it likely that the shot would have been successful if the angle were off by 2◦ ? 1 v 2 sin 2t = Using Newton’s laws and the given initial velocity of v = 25 ft/s, the shot travels s = 32 625 sin 2t ft, where t is in radians. 32

SOLUTION

(a) If θ = 34◦ (i.e., t = 17 90 π ), then

s ≈ s (t)t =

625 π 17 17 625 π t = π θ · cos cos ≈ 0.255θ . 16 45 16 45 180

(b) If θ = 2◦ , this gives s ≈ 0.51 ft, in which case the shot would not have been successful, having been off half a foot. 51. Compute the linearization of f (x) = 3x − 4 at a = 0 and a = 2. Prove more generally that a linear function Estimate change inatthe distance of the shot if the angle changes from 50◦ to 51◦ for v = 25 ft/s and coincides with itsthe linearization x= a for alls a. v = 30 ft/s. Is the shot more sensitive to the angle when the velocity is large or small? Explain. SOLUTION Let f (x) = 3x − 4. Then f (x) = 3. With a = 0, f (a) = −4 and f (a) = 3, so the linearization of f (x) at a = 0 is L(x) = −4 + 3(x − 0) = 3x − 4 = f (x). With a = 2, f (a) = 2 and f (a) = 3, so the linearization of f (x) at a = 2 is L(x) = 2 + 3(x − 2) = 2 + 3x − 6 = 3x − 4 = f (x). More generally, let g(x) = bx + c be any linear function. The linearization L(x) of g(x) at x = a is L(x) = g (a)(x − a) + g(a) = b(x − a) + ba + c = bx + c = g(x); i.e., L(x) = g(x). 53. Show that the Linear Approximation to f (x) = tan x at x = π4 yields the estimate tan( π4 + h) − 1 ≈ 2h. Compute According to (3), error in the Linear Approximation is of “order two” in h. Show that the Linear Approxi√ √,the the error E for h = 10−n 1 ≤ n ≤ 4, and verify that E satisﬁes the Error Bound (3) with K = 6.2. mation to f (x) = x at x = 9 yields the estimate 9 + h − 3 ≈ 16 h. Then compute the error E for h = 10−n , SOLUTION Let = tan x. Then fthat ( π4 )E=≤1,0.006h f (x) 2=. sec2 x and f ( π4 ) = 2. Therefore, by the Linear Approxima1 ≤ n ≤ 4, andf (x) verify numerically tion,

π

π

π = tan f +h − f + h − 1 ≈ 2h. 4 4 4 From the following table, we see that for h = 10−n , 1 ≤ n ≤ 4, E ≤ 6.2h 2 . h

E = | tan( π4 + h) − 1 − 2h|

6.2h 2

10−1 10−2 10−3 10−4

2.305 × 10−2 2.027 × 10−4 2.003 × 10−6 2.000 × 10−8

6.20 × 10−2 6.20 × 10−4 6.20 × 10−6 6.20 × 10−8

S E C T I O N 4.2

Extreme Values

165

Further Insights and Challenges 55. (a) Show that f (x) = sin x and g(x) = tan x have the same linearization at a = 0. Show that forisany real numbermore k, (1accurately? + x)k ≈ 1 Explain + kx forusing smallax. Estimate (1.02) π ].0.7 and (1.02)−0.3 . (b) Which function approximated graph over [0, 6

(c) Calculate the error in these linearizations at x = π6 . Does the answer conﬁrm your conclusion in (b)? SOLUTION Let f (x) = sin x and g(x) = tan x. (a) The Linear Approximation of f (x) at a = 0 is L(x) = f (0)(x − 0) + f (0) = cos 0 · (x − 0) + sin 0 = x, whereas the Linear Approximation of g(x) at a = 0 is L(x) = f (0)(x − 0) + f (0) = sec2 0 · (x − 0) + tan 0 = x. (b) The linearization L(x) = x more closely approximates the function f (x) = sin x than g(x) = tan x, since the graph of x lies closer to that of sin x than to tan x. y

tan x x sin x

0.5 0.4 0.3 0.2 0.1 x 0

0.1 0.2 0.3 0.4 0.5

(c) The error in the Linear Approximation for f at x = π6 is sin( π6 ) − π6 ≈ −.02360; the error in the Linear Approximation for g at x = π6 is tan( π6 ) − π6 ≈ .05375. These results conﬁrm what the graphs of the curves told us in (b). error in the Linear Approximation at a = 1. Show directly that 57. Let f (x) = x −1 and let E = | f − f (1)h| be the 2 , and let E In = this | f case, − f (5)h| the error f (5 + h) −that f (5), where 1 . Hint: 1 ≤ 1be 3 . in the Linear ApproxiE = h 2Let /(1 + fh).=Then prove E≤ 2h 2 iff (x) − 12 = ≤ xh ≤ + h ≤ 2 in h). 2 mation. Verify directly that E satisﬁes (3) with K = 22 (thus E is of order two SOLUTION Let f (x) = x −1 . Then f = f (1 + h) − f (1) =

1 h −1=− 1+h 1+h

and

E = | f − f (1)h| = −

h h2 + h = . 1+h 1+h

1 ≤ 2. Thus, E ≤ 2h 2 for − 1 ≤ h ≤ 1 . If − 12 ≤ h ≤ 12 , then 12 ≤ 1 + h ≤ 32 and 23 ≤ 1+h 2 2

Consider the curve y 4 + x 2 + x y = 13. Calculate d y/d x at P = (3, 1) and ﬁnd the linearization y = L(x) at P. 4.2 (a)Extreme Values (b) Plot the curve and y = L(x) on the same set of axes. (c) Although y is only deﬁned implicitly as a function of x, we can use y = L(x) to approximate y for x near 3. Use Preliminary Questions this idea to ﬁnd an approximate solution of y 4 + 3.12 + 3.1y = 13. Based on your plot in (b), is your approximation 1. If f (x) is continuous on (0, 1), then (choose the correct statement): too large or too small? (a) f (x) has a minimum value on (0, 1). (d) Solve y 4 + 3.12 + 3.1y = 13 numerically using a computer algebra system and use the result to ﬁnd the (b) percentage f (x) has noerror minimum on (0, 1). in yourvalue approximation. (c) f (x) might not have a minimum value on (0, 1). SOLUTION The correct response is (c): f (x) might not have a minimum value on (0, 1). The key is that the interval (0, 1) is not closed. The function might have a minimum value but it does not have to have a minimum value.

2. If f (x) is not continuous on [0, 1], then (choose the correct statement): (a) f (x) has no extreme values on [0, 1]. (b) f (x) might not have any extreme values on [0, 1]. SOLUTION The correct response is (b): f (x) might not have any extreme values on [0, 1]. Although [0, 1] is closed, because f is not continuous, the function is not guaranteed to have any extreme values on [0, 1].

3. What is the deﬁnition of a critical point? A critical point is a value of the independent variable x in the domain of a function f at which either f (x) = 0 or f (x) does not exist.

SOLUTION

4. True or false: If f (x) is differentiable and f (x) = 0 has no solutions, then f (x) has no local minima or maxima. True. If f (x) is differentiable but f (x) = 0 has no solutions, then f (x) has no critical points. As critical points are the only candidates for local minima and local maxima, it follows that f (x) has no local extrema. SOLUTION

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5. Fermat’s Theorem does not claim that if f (c) = 0, then f (c) is a local extreme value (this is false). What does Fermat’s Theorem assert? SOLUTION

Fermat’s Theorem claims: If f (c) is a local extreme value, then either f (c) = 0 or f (c) does not exist.

6. If f (x) is continuous but has no critical points in [0, 1], then (choose the correct statement): (a) f (x) has no min or max on [0, 1]. (b) Either f (0) or f (1) is the minimum value on [0, 1]. SOLUTION The correct response is (b): either f (0) or f (1) is the minimum value on [0, 1]. Remember that extreme values occur either at critical points or endpoints. If a continuous function on a closed interval has no critical points, the extreme values must occur at the endpoints.

Exercises 1. The following questions refer to Figure 15. y 6 5 4 3

f(x)

2 1 x 1

2

3

4

5

6

7

8

FIGURE 15

(a) (b) (c) (d) (e)

How many critical points does f (x) have? What is the maximum value of f (x) on [0, 8]? What are the local maximum values of f (x)? Find a closed interval on which both the minimum and maximum values of f (x) occur at critical points. Find an interval on which the minimum value occurs at an endpoint.

SOLUTION

(a) f (x) has three critical points on the interval [0, 8]: at x = 3, x = 5 and x = 7. Two of these, x = 3 and x = 5, are where the derivative is zero and one, x = 7, is where the derivative does not exist. (b) The maximum value of f (x) on [0, 8] is 6; the function takes this value at x = 0. (c) f (x) achieves a local maximum of 5 at x = 5. (d) Answers may vary. One example is the interval [4, 8]. Another is [2, 6]. (e) Answers may vary. The easiest way to ensure this is to choose an interval on which the graph takes no local minimum. One example is [0, 2]. In Exercises 3–10, ﬁnd all critical points of the function. State whether f (x) = x −1 (Figure 16) has a minimum or maximum value on the following intervals: (b) (1, 2) (c) [1, 2] 3. (a) f (x)(0, =2) x 2 − 2x + 4 SOLUTION

Let f (x) = x 2 − 2x + 4. Then f (x) = 2x − 2 = 0 implies that x = 1 is the lone critical point of f .

9 3− 5. f (x)f (x) = x= 7x −x 22 − 54x + 2 2 Let f (x) = x 3 − 92 x 2 − 54x + 2. Then f (x) = 3x 2 − 9x − 54 = 3(x + 3)(x − 6) = 0 implies that x = −3 and x = 6 are the critical points of f . x 7. f (x) = 2 3 2 f (t) = x 8t + 1− t

SOLUTION

SOLUTION

x 1 − x2 . Then f (x) = 2 Let f (x) = 2 = 0 implies that x = ±1 are the critical points of f . x +1 (x + 1)2

9. f (x) = x 1/3 f (t) = 4t − t 2 + 1 1 SOLUTION Let f (x) = x 1/3 . Then f (x) = 3 x −2/3 . The derivative is never zero but does not exist at x = 0. Thus, x = 0 is the only critical point of f . 11. (a) (b) (c)

Let f (x) = x 2 2− 4x + 1. x = Findf (x) the critical point c of f (x) and compute f (c). x +1 Compute the value of f (x) at the endpoints of the interval [0, 4]. Determine the min and max of f (x) on [0, 4].

S E C T I O N 4.2

Extreme Values

167

(d) Find the extreme values of f (x) on [0, 1]. SOLUTION

Let f (x) = x 2 − 4x + 1.

(a) Then f (c) = 2c − 4 = 0 implies that c = 2 is the sole critical point of f . We have f (2) = −3. (b) f (0) = f (4) = 1. (c) Using the results from (a) and (b), we ﬁnd the maximum value of f on [0, 4] is 1 and the minimum value is −3. (d) We have f (1) = −2. Hence the maximum value of f on [0, 1] is 1 and the minimum value is −2. 13. Find the critical points of f (x) = sin x + cos x and determine the extreme values on [0, π2 ]. Find the extreme values of 2x 3 − 9x 2 + 12x on [0, 3] and [0, 2]. SOLUTION

• Let f (x) = sin x + cos x. Then on the interval 0, π , we have f (x) = cos x − sin x = 0 at x = π , the only 2 4 critical point of f in this interval. √ √ 2 and f (0) = f ( π2 ) = 0, the maximum value of f on 0, π2 is 2, while the minimum value is 0.

• Since f ( π ) = 4

√ Plot f (x) = 2 x − x on [0, 4] and determine the maximum value graphically. Then verify your answer 15. Compute the critical points of h(t) = (t 2 − 1)1/3 . Check that your answer is consistent with Figure 17. Then using calculus. ﬁnd the extreme values of h(t)√on [0, 1] and [0, 2]. SOLUTION The graph of y = 2 x − x over the interval [0, 4] is shown below. From the graph, we see that at x = 1, the function achieves its maximum value of 1. y 1 0.8 0.6 0.4 0.2 x 0

1

2

3

4

√ To verify the information obtained from the plot, let f (x) = 2 x − x. Then f (x) = x −1/2 − 1. Solving f (x) = 0 yields the critical points x = 0 and x = 1. Because f (0) = f (4) = 0 and f (1) = 1, we see that the maximum value of f on [0, 4] is 1. In Exercises 17–45, ﬁnd the maximum and minimum values of the function on the given interval. Plot f (x) = 2x 3 − 9x 2 + 12x on [0, 3] and locate the extreme values graphically. Then verify your answer 2 − 4x + 2, [0, 3] 17. using y = 2xcalculus. SOLUTION Let f (x) = 2x 2 − 4x + 2. Then f (x) = 4x − 4 = 0 implies that x = 1 is a critical point of f . On the interval [0, 3], the minimum value of f is f (1) = 0, whereas the maximum value of f is f (3) = 8. (Note: f (0) = 2.)

− 1, [−2, 2] 19. y = x 2 − 6x y = −x 2 + 10x + 43, [3, 8] SOLUTION Let f (x) = x 2 − 6x − 1. Then f (x) = 2x − 6 = 0 implies that x = 3 is a critical point of f . The minimum of f on the interval [−2, 2] is f (2) = −9, whereas its maximum is f (−2) = 15. (Note: The critical point x = 3 is outside the interval [−2, 2].) 21. y = −4x 22 + 3x + 4, [−1, 1] y = x − 6x − 1, [−2, 0] 3 SOLUTION Let f (x) = −4x 2 + 3x + 4. Then f (x) = −8x + 3 = 0 implies that x = 8 is a critical point of f . The 3 minimum of f on the interval [−1, 1] is f (−1) = −3, whereas its maximum is f ( 8 ) = 4.5625. (Note: f (1) = 3.) 23. y = x 3 −36x + 1, [−1, 1] y = x − 3x + 1, [0, 2] √ SOLUTION Let f (x) = x 3 − 6x + 1. Then f (x) = 3x 2 − 6 = 0 implies that x = ± 2 are the critical points of f . The minimum of f on the interval [−1, 1] is f (1) = −4, whereas its maximum is f (−1) = 6. (Note: The critical points √ x = ± 2 are not in the interval [−1, 1].) 25. y = x 3 +33x 2 − 9x + 2, [−1, 1] y = x − 12x 2 + 21x, [0, 2] SOLUTION Let f (x) = x 3 + 3x 2 − 9x + 2. Then f (x) = 3x 2 + 6x − 9 = 3(x + 3)(x − 1) = 0 implies that x = 1 and x = −3 are the critical points of f . The minimum of f on the interval [−1, 1] is f (1) = −3, whereas its maximum is f (−1) = 13. (Note: The critical point x = −3 is not in the interval [−1, 1].) 27. y = x 3 +33x 2 −29x + 2, [−4, 4] y = x + 3x − 9x + 2, [0, 2] SOLUTION Let f (x) = x 3 + 3x 2 − 9x + 2. Then f (x) = 3x 2 + 6x − 9 = 3(x + 3)(x − 1) = 0 implies that x = 1 and x = −3 are the critical points of f . The minimum of f on the interval [−4, 4] is f (1) = −3, whereas its maximum is f (4) = 78. (Note: f (−4) = 22 and f (−3) = 29.) 29. y = x 5 −53x 2 , [−1, 5] y = x − x, [0, 2]

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A P P L I C AT I O N S O F T H E D E R I VATI V E 1/3 ≈ 1.06 are SOLUTION Let f (x) = x 5 − 3x 2 . Then f (x) = 5x 4 − 6x = 0 implies that x = 0 and x = 1 5 (150) critical points of f . Over the interval [−1, 5], the minimum value of f is f (−1) = −4, whereas its maximum value is 9 (150)2/3 ≈ −2.03). f (5) = 3050. (Note: f (0) = 0 and f ( 15 (150)1/3 ) = − 125

x2 + 1 2 6] , 3s [5, 31. y =y = 2s 3 − + 3, x −4 SOLUTION

Let f (x) =

[−4, 4]

x2 + 1 . Then x −4 f (x) =

(x − 4) · 2x − (x 2 + 1) · 1 x 2 − 8x − 1 = =0 2 (x − 4) (x − 4)2

√ implies x = 4 ± 17 are critical points of f . x = 4 is not a critical point because x = 4 is not in the domain of f . On the interval [5, 6], the minimum of f is f (6) = 37 2 = 18.5, whereas the maximum of f is f (5) = 26. (Note: The critical √ points x = 4 ± 17 are not in the interval [5, 6].) 4x 33. y = x − 1 − x , [0, 3] y = x2 + 1 , [1, 4] x + 3x 4x SOLUTION Let f (x) = x − . Then x +1 f (x) = 1 −

4 (x − 1)(x + 3) = =0 (x + 1)2 (x + 1)2

implies that x = 1 and x = −3 are critical points of f . x = −1 is not a critical point because x = −1 is not in the domain of f . The minimum of f on the interval [0, 3] is f (1) = −1, whereas the maximum is f (0) = f (3) = 0. (Note: The critical point x = −3 is not in the interval [0, 3].) 35. y = (2 + x) 2 2 + (2 − x)2 , [0, 2] y = 2 x + 1 − x, [0, 2] SOLUTION Let f (x) = (2 + x) 2 + (2 − x)2 . Then

2(x − 1)2 2 + (2 − x)2 − (2 + x)(2 + (2 − x)2 )−1/2 (2 − x) = =0 2 + (2 − x)2 √ implies that x = 1 is the √ critical point of f . On the interval √ [0, 2], the minimum is f (0) = 2 6 ≈ 4.898979 and the maximum is f (2) = 4 2 ≈ 5.656854. (Note: f (1) = 3 3 ≈ 5.196152.) √ + x 2 − 2 x, [0, 4] 37. y = x y = 1 + x 2 − 2x, [3, 6] √ SOLUTION Let f (x) = x + x 2 − 2 x. Then f (x) =

√

1 1 + 2x − 2 f (x) = (x + x 2 )−1/2 (1 + 2x) − x −1/2 = √ √ 2

√

1+x

2 x 1+x

=0 √

implies that x = 0 and x = 23 are the critical points of f . Neither x = −1 nor x = − 23 is a critical point because

√ neither is in the domain of f . On the interval [0, 4], the minimum of f is f 23 ≈ −.589980 and the maximum is f (4) ≈ .472136. (Note: f (0) = 0.) 39. y = sin x cos x, [0, π2 ] y = (t − t 2 )1/3 , [−1, 2] π 1 π SOLUTION Let f (x) = sin x cos x = 2 sin 2x. On the interval 0, 2 , f (x) = cos 2x = 0 when x = 4 . The π π 1 minimum of f on this interval is f (0) = f ( 2 ) = 0, whereas the maximum is f ( 4 ) = 2 . √ π] 41. y =y =2xθ − −sin secx,θ , [0, [0,2π 3] √ √ π π SOLUTION Let f (θ ) = 2θ − sec θ . On the interval [0, 3 ], f (θ ) = 2 − sec θ tan θ = 0 at θ = 4 . The minimum √ value of f on this interval is f (0) = −1, whereas the maximum value over this interval is f ( π4 ) = 2( π4 − 1) ≈ √ −.303493. (Note: f ( π3 ) = 2 π3 − 2 ≈ −.519039.) 43. y = θ − 2 sin θ , [0, 2π ] y = cos θ + sin θ , [0, 2π ] SOLUTION Let g(θ ) = θ − 2 sin θ . On the interval [0, 2π ], g (θ ) = 1 − 2 cos θ = 0 at √ minimum of g on this interval is g( π3 ) = π3 − 3 ≈ −.685 and the maximum is g( 53 π ) = g(0) = 0 and g(2π ) = 2π ≈ 6.283.) 45. y = tan x − 2x, [0, 1] y = sin3 θ − cos2 θ ,

[0, 2π ]

θ = π3 and θ = 53 π . The √ 5 π + 3 ≈ 6.968. (Note: 3

S E C T I O N 4.2

Extreme Values

169

SOLUTION Let f (x) = tan x − 2x. Then on the interval [0, 1], f (x) = sec2 x − 2 = 0 at x = π 4 . The minimum of f is f ( π4 ) = 1 − π2 ≈ −.570796 and the maximum is f (0) = 0. (Note: f (1) = tan 1 − 2 ≈ −.442592.)

Find the critical points of f (x) = 2 cos 3x + 3 cos 2x. Check your answer against a graph of f (x). Let f (θ ) = 2 sin 2θ + sin 4θ . SOLUTION (x) θisisdifferentiable are2θlooking for points where f (x) = 0 only. Setting f (x) = a critical pointforif all cosx, 4θso = we − cos . (a) Show fthat −6 sin 3x − 6 sin 2x, we get sin 3x = − sin 2x. Looking at a unit circle, we ﬁnd the relationship between angles y and x (b) Show, using a unit circle, that cos θ1 = − cos θ2 if and only if θ1 = π ± θ2 + 2π k for an integer k. such that sin y = − sin x. This technique is also used in Exercise 46. (c) Show that cos 4θ = − cos 2θ if and only if θ = π /2 + π k or θ = π /6 + (π /3)k. (d) Find the six critical points of f (θ ) on [0, 2π ] and ﬁnd the extreme values of f (θ ) on this interval. (e) Check your results against a graph of f (θ ). 47.

From the diagram, we see that sin y = − sin x if y is either (i.) the point antipodal to x (y = π + x + 2π k) or (ii.) the point obtained by reﬂecting x through the horizontal axis (y = −x + 2π k). Since sin 3x = − sin 2x, we get either 3x = π + 2x + 2π k or 3x = −2x + 2π k. Solving each of these equations for x yields x = π + 2π k and x = 25π k, respectively. The values of x between 0 and 2π are 0, 25π , 45π , π , 65π , 85π , and 2π . The graph is shown below. As predicted, it has horizontal tangent lines at 25π k and at x = π2 . Each of these points is a local extremum. y 4 2 x 1

−2

2

3

4

5

6

−4

In Exercises 48–51, ﬁnd the critical points and the extreme values on [0, 3]. In Exercises 50 and 51, refer to Figure 18. y

y 1

30 20 10 x

−6

2

−π

π 2

2

π

3π 2

x

y = | cos x |

y = | x 2 + 4x − 12 |

FIGURE 18

49. y = |3x − 9| y = |x − 2| SOLUTION Let f (x) = |3x − 9| = 3|x − 3|. For x < 3, we have f (x) = −3. For x > 3, we have f (x) = 3. Now as f (x) − f (3) 3(3 − x) − 0 f (x) − f (3) 3(x − 3) − 0 x → 3−, we have = → −3; whereas as x → 3+, we have = → 3. x −3 x −3 x −3 x −3 f (x) − f (3) does not exist and the lone critical point of f is x = 3. Alternately, we examine Therefore, f (3) = lim x −3 x→3 the graph of f (x) = |3x − 9| shown below. To ﬁnd the extrema of f (x) on [0, 3], we test the values of f (x) at the critical point and the endpoints. f (0) = 9 and f (3) = 0, so f (x) takes its minimum value of 0 at x = 3, and its maximum value of 9 at x = 0. y 12 8 4 x 2

4

6

51. y = | cos x| y = |x 2 + 4x − 12| SOLUTION Let f (x) = | cos x|. There are two types of critical points: points of the form π n where the derivative is zero and points of the form n π + π /2 where the derivative does not exist.

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To ﬁnd points where f (x) does not exist, we look at points where the inside of the absolute value function is equal to zero, and take derivatives in either direction, to ﬁnd out whether they are the same. cos x = 0 at the points x = − π2 , x = π2 , and x = 32π , etc. Only one of these, x = π2 is in the interval [0, 3], so we check the one critical point and the endpoints. f (0) = 1, f (3) = | cos 3| ≈ .98992, and f ( π2 ) = 0, so f (x) takes its maximum value of 1 at x = 0 and its minimum of 0 at x = π2 . In Exercises 53–56, verify Rolle’s Theorem for the given interval. Let f (x) = 3x − x 3 . Check that f (−2) = f (1). What may we conclude from Rolle’s Theorem? Verify this 1 −1 53. conclusion. f (x) = x + x , [ , 2] 2

SOLUTION

Because f is continuous on [ 12 , 2], differentiable on ( 12 , 2) and 1 5 1 1 1 f = + 1 = = 2 + = f (2), 2 2 2 2 2

we may conclude from Rolle’s Theorem that there exists a c ∈ ( 12 , 2) at which f (c) = 0. Here, f (x) = 1 − x −2 = x 2 −1 , so we may take c = 1. 2 x

x2 π , 5] 3π 55. f (x)f (x) = = sin x, , [[3, 4 4 ] 8x − 15 SOLUTION Because f is continuous on [3, 5], differentiable on (3, 5) and f (3) = f (5) = 1, we may conclude from Rolle’s Theorem that there exists a c ∈ (3, 5) at which f (c) = 0. Here,

f (x) =

(8x − 15)(2x) − 8x 2 2x(4x − 15) = , (8x − 15)2 (8x − 15)2

so we may take c = 15 4 . 57. Use Rolle’s Theorem to prove that f (x) = x 5 + 2x 3 + 4x − 12 has at most one real root. f (x) = sin2 x − cos2 x, [ π4 , 34π ] SOLUTION We use proof by contradiction. Suppose f (x) = x 5 + 2x 3 + 4x − 12 has two real roots, x = a and x = b. Then f (a) = f (b) = 0 and Rolle’s Theorem guarantees that there exists a c ∈ (a, b) at which f (c) = 0. However, f (x) = 5x 4 + 6x 2 + 4 ≥ 4 for all x, so there is no c ∈ (a, b) at which f (c) = 0. Based on this contradiction, we conclude that f (x) = x 5 + 2x 3 + 4x − 12 cannot have more than one real root. bloodstream after t hours is 59. The concentration C(t) (in mg/cm3 ) of a drug in3 a patient’s x x2 Use Rolle’s Theorem to prove that f (x) = + + x + 1 has at most one real root. 6 = 2 0.016t C(t) t 2 + 4t + 4 Find the maximum concentration and the time at which it occurs. SOLUTION

C (t) =

.016(t 2 + 4t + 4) − (.016t (2t + 4)) −t 2 + 4 2−t = .016 2 = .016 . 2 2 (t + 4t + 4) (t + 4t + 4)2 (t + 2)3

C (t) exists for all t ≥ 0, so we are looking for points where C (t) = 0. C (t) = 0 when t = 2, so we check C(2) = .002 mg3 . Since C(0) = 0 and C(t) → 0 as t → ∞, the point (2, .002) is a local maximum of C(t). cm

Hence, the maximum concentration occurs 2 hours after being administered, and that concentration is .002

mg . cm3

61. Bees build honeycomb structures out of cells with a hexagonal base and three rhombus-shaped faces on top as in Maximum of a Resonance Curve size of the of aiscircuit or other oscillatory system to an input Figure 20. Using geometry, we can show thatThe the surface arearesponse of this cell signal of frequency ω (“omega”) is described in suitable units by the function 3 √ A(θ ) = 6hs + s 2 ( 3 csc 1 θ − cot θ ) φ (ω ) = 2 . 2 2 ω02 − ω 2 )2fact + 4D where h, s, and θ are as indicated in the ﬁgure. It is a (remarkable thatωbees “know” which angle θ minimizes the surface area (and therefore requires the least amount of wax). Both ω0 (the natural frequency of the system) and D (the damping factor) are positive constants. The graph of φ is (a) Show this anglecurve, is approximately 54.7◦frequency by ﬁndingω the>critical point of A(its θ )maximum for 0 < θ value, < π /2if(assume h and s called that a resonance and the positive 0 where φ takes such a positive r √ are constant). frequency exists, is the resonant frequency. Show that a resonant √

frequency exists if and only if 0 < D < ω0 / 2 Conﬁrm, by graphing f (θ ) = 3 csc θ − cot θ , that the critical point indeed minimizes the surface area. (b) (Figure 19). Then show that if this condition is satisﬁed, ωr = ω02 − 2D 2 .

S E C T I O N 4.2

Extreme Values

171

q

h

s

FIGURE 20 A cell in a honeycomb constructed by bees. SOLUTION

√ (a) Because h and s are constant relative to θ , we have A (θ ) = 32 s 2 (− 3 csc θ cot θ + csc2 θ ) = 0. From this, we get √ 3 csc θ cot θ = csc2 θ , or cos θ = √1 , whence θ = cos−1 √1 = .955317 radians = 54.736◦ . 3 3 √ (b) The plot of 3 csc θ − cot θ , where θ is given in degrees, is given below. We can see that the minimum occurs just below 55◦ . h 1.5 1.48 1.46 1.44 1.42 1.4 40 45 50 55 60 65

Degrees

63. FindMigrating the maximum of yto=swim x a −at x baon [0, 1] where < a < b.the In particular, ﬁnd theof maximum of According y = x 5 − xto10one on ﬁsh tend velocity v that 0minimizes total expenditure energy E. [0, 1]. 3 v model, E is proportional to f (v) = , where vr is the velocity of the river water. SOLUTION v − vr (a) Find the critical points of f (v). • Let f (x) = x a − x b . Then f (x) = ax a−1 − bx b−1 . Since a < b, f (x) = x a−1 (a − bx b−a ) = 0 implies (say, and plot (v).interval Conﬁrm[0,that a minimum the critical (b) Choose of vxr = a < value r = 10) , which is inf the 1] asf (v) a

1 1 1 1 2 1 1 1/5 f = = − = . − 2 2 2 2 4 4 In Exercises 64–66, plot the function using a graphing utility and ﬁnd its critical points and extreme values on [−5, 5]. 1 1 y= + 1 y = 1 + |x − 1| 1 + |x − 4| 1 + |x − 1| SOLUTION Let

65.

f (x) =

1 1 + . 1 + |x − 1| 1 + |x − 4|

The plot follows: 1.2 1 0.8 0.6 0.4 0.2 −5 −4 −3 −2 −1

1

2

3

4

5

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We can see on the plot that the critical points of f (x) lie at the cusps at x = 1 and x = 4 and at the location of the 5 5 4 7 horizontal tangent line at x = 52 . With f (−5) = 17 70 , f (1) = f (4) = 4 , f ( 2 ) = 5 and f (5) = 10 , it follows that the maximum value of f (x) on [−5, 5] is f (1) = f (4) = 54 and the minimum value is f (−5) = 17 70 . 67. (a) Use implicit differentiation to ﬁnd the critical points on the curve 27x 2 = (x 2 + y 2 )3 . x y = Plot|xthe 2 −curve (b) 1| + and |x 2 the − 4|horizontal tangent lines on the same set of axes. SOLUTION

(a) Differentiating both sides of the equation 27x 2 = (x 2 + y 2 )3 with respect to x yields dy 54x = 3(x 2 + y 2 )2 2x + 2y . dx Solving for d y/d x we obtain x(9 − (x 2 + y 2 )2 ) dy 27x − 3x(x 2 + y 2 )2 = . = 2 2 2 dx 3y(x + y ) y(x 2 + y 2 )2 Thus, the derivative is zero when x 2 + y 2 = 3. Substituting into the equation for the curve, this yields x 2 = 1, or x = ±1. There are therefore four points at which the derivative is zero: √ √ √ √ (−1, − 2), (−1, 2), (1, − 2), (1, 2). There are also critical points where the derivative does not exist. This occurs when y = 0 and gives the following points with vertical tangents: √ 4 (0, 0), (± 27, 0). (b) The curve 27x 2 = (x 2 + y 2 )3 and its horizontal tangents are plotted below. y 1 −2

x

−1

1

2

−1

69. Sketch the graph of a continuous function on (0, 4) having a local minimum but no absolute minimum. Sketch the graph of a continuous function on (0, 4) with a minimum value but no maximum value. SOLUTION Here is the graph of a function f on (0, 4) with a local minimum value [between x = 2 and x = 4] but no absolute minimum [since f (x) → −∞ as x → 0+]. y 10 x 1

2

3

−10

71. Sketch the graph of a function f (x) on [0, 4] with a discontinuity such that f (x) has an absolute minimum but no Sketch the graph of a function on [0, 4] having absolute maximum. (a) Two local maxima and one local minimum. SOLUTION Here is the graph of a function f on [0, 4] that (a) has a discontinuity [at x = 4] and (b) has an absolute (b) An absolute minimum that occurs at an endpoint, and an absolute maximum that occurs at a critical point. minimum [at x = 0] but no absolute maximum [since f (x) → ∞ as x → 4−]. y 4 3 2 1 x 0

1

2

3

4

The Mean Value Theorem and Monotonicity

S E C T I O N 4.3

173

Further Insights and Challenges on only positive values. More generally, ﬁnd 73. Show, by considering its minimum, that f (x) = x 2 − 2x + 3 takes 2 + b2 . Show that extreme values f (x) = a sinfunction x + b cosfx(x) are=±x 2a+ the conditions on rthe and s under whichofthe quadratic r x + s takes on only positive values. Give examples of r and s for which f takes on both positive and negative values. SOLUTION

• Observe that f (x) = x 2 − 2x + 3 = (x − 1)2 + 2 > 0 for all x. Let f (x) = x 2 + r x + s. Completing the square, we note that f (x) = (x + 12 r )2 + s − 14 r 2 > 0 for all x provided that s > 14 r 2 . • Let f (x) = x 2 − 4x + 3 = (x − 1)(x − 3). Then f takes on both positive and negative values. Here, r = −4 and

s = 3.

75. Generalize Exercise 74: Show that if the horizontal 2line y = c intersects the graph of f (x) = x 2 + r x + s at two Show that if the quadratic polynomial f (x) = x + r x + s takes on both positive and x 1negative + x 2 values, then its points (x1 , f (xvalue (x 2 , at f (x then f (x) takes its (Figure 21). minimum occurs the midpoint between theminimum two roots.value at the midpoint M = 1 )) and 2 )), 2 y f(x) c

y=c

x x1

M

x2

FIGURE 21

Suppose that a horizontal line y = c intersects the graph of a quadratic function f (x) = x 2 + r x + s in two points (x 1 , f (x 1 )) and (x 2 , f (x 2 )). Then of course f (x1 ) = f (x 2 ) = c. Let g(x) = f (x) − c. Then g(x1 ) = g(x 2 ) = 0. By Exercise 74, g takes on its minimum value at x = 12 (x 1 + x2 ). Hence so does f (x) = g(x) + c. SOLUTION

77. Find the minimum and maximum values of f (x) = x p (1 − x)q on [0, 1], where p and q are positive numbers. The graphs in Figure 22 show that a cubic polynomial may have a local min and max, or it may have neither. p (1 − x)q , 0 ≤ x ≤ 1, where p 1and 1 axpositive 2 + bx numbers. SOLUTION Let f (x) q are Then f has neither a local min + c that ensure Find conditions on = thexcoefﬁcients a and b of f (x) = 3 x 3 + 2 nor max. Hint: Apply Exercise 73 to f (x). f (x) = x p q(1 − x)q−1 (−1) + (1 − x)q px p−1 p = x p−1 (1 − x)q−1 ( p(1 − x) − qx) = 0 at x = 0, 1, p+q The minimum value of f on [0, 1] is f (0) = f (1) = 0, whereas its maximum value is p p pqq f . = p+q ( p + q) p+q Prove that if f is continuous and f (a) and f (b) are local minima where a < b, then there exists a value c between a and b such that f (c) is a local maximum. (Hint: Apply Theorem 1 to the interval [a, b].) Show that 4.3continuity The Mean Value Theorem and Monotonicity is a necessary hypothesis by sketching the graph of a function (necessarily discontinuous) with two local minima but no local maximum.

Preliminary Questions 1. Which value of m makes the following statement correct? If f (2) = 3 and f (4) = 9, where f (x) is differentiable, then the graph of f has a tangent line of slope m. SOLUTION

The Mean Value Theorem guarantees that the function must have a tangent line with slope equal to 9−3 f (4) − f (2) = = 3. 4−2 4−2

Hence, m = 3 makes the statement correct. 2. (a) (b) (c) (d)

Which of the following conclusions does not follow from the MVT (assume that f is differentiable)? If f has a secant line of slope 0, then f (c) = 0 for some value of c. If f (5) < f (9), then f (c) > 0 for some c ∈ (5, 9). If f (c) = 0 for some value of c, then there is a secant line whose slope is 0. If f (x) > 0 for all x, then every secant line has positive slope.

Conclusion (c) does not follow from the Mean Value Theorem. As a counterexample, consider the function f (x) = x 3 . Note that f (0) = 0, but no secant line has zero slope.

SOLUTION

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3. Can a function that takes on only negative values have a positive derivative? Sketch an example or explain why no such functions exist. SOLUTION

Yes. The ﬁgure below displays a function that takes on only negative values but has a positive derivative. y x

4. (a) Use the graph of f (x) in Figure 10 to determine whether f (c) is a local minimum or maximum. (b) Can you conclude from Figure 10 that f (x) is a decreasing function? y

x

c

FIGURE 10 Graph of derivative f (x). SOLUTION

(a) To the left of x = c, the derivative is positive, so f is increasing; to the right of x = c, the derivative is negative, so f is decreasing. Consequently, f (c) must be a local maximum. (b) No. The derivative is a decreasing function, but as noted in part (a), f (x) is increasing for x < c and decreasing for x > c.

Exercises In Exercises 1–6, ﬁnd a point c satisfying the conclusion of the MVT for the given function and interval. 1. y = x −1 , SOLUTION

[1, 4] Let f (x) = x −1 , a = 1, b = 4. Then f (x) = −x −2 , and by the MVT, there exists a c ∈ (1, 4) such that 1

1

− f (b) − f (a) 1 1 = 4 1 =− . − 2 = f (c) = b−a 4−1 4 c Thus c2 = 4 and c = ±2. Choose c = 2 ∈ (1, 4). 3. y = (x − √1)(x − 3), [1, 3] y = x, [4, 9] SOLUTION Let f (x) = (x − 1) (x − 3), a = 1, b = 3. Then f (x) = 2x − 4, and by the MVT, there exists a c ∈ (1, 3) such that f (b) − f (a) 0−0 2c − 4 = f (c) = = = 0. b−a 3−1 Thus 2c − 4 = 0 and c = 2 ∈ (1, 3). x 5. y =y = cos, x − [3,sin 6]x, [0, 2π ] x +1 SOLUTION

Let f (x) = x/ (x + 1), a = 3, b = 6. Then f (x) =

such that

1 , and by the MVT, there exists a c ∈ (3, 6) (x+1)2

6 3 (c) = f (b) − f (a) = 7 − 4 = 1 . = f b−a 6−3 28 (c + 1)2 √ √ Thus (c + 1)2 = 28 and c = −1 ± 2 7. Choose c = 2 7 − 1 ≈ 4.29 ∈ (3, 6).

1

7. Let = x 5 + x 2 . Check that the secant line between x = 0 and x = 1 has slope 2. By the MVT, 3 , f (x) [−3, y = x f (c) = 2 for some c ∈−1] (0, 1). Estimate c graphically as follows. Plot f (x) and the secant line on the same axes. Then plot the lines y = 2x + b for different values of b until you ﬁnd a value of b for which it is tangent to y = f (x). Zoom in on the point of tangency to ﬁnd its x-coordinate.

The Mean Value Theorem and Monotonicity

S E C T I O N 4.3 SOLUTION

175

Let f (x) = x 5 + x 2 . The slope of the secant line between x = 0 and x = 1 is 2−0 f (1) − f (0) = = 2. 1−0 1

A plot of f (x), the secant line between x = 0 and x = 1, and the line y = 2x − 0.764 is shown below at the left. The line y = 2x − 0.764 appears to be tangent to the graph of y = f (x). Zooming in on the point of tangency (see below at the right), it appears that the x-coordinate of the point of tangency is approximately 0.62. y

0.6

y = x5 + x 2

y

0.5

4

0.4

y = 2x − .764

2

x

0.3 0.52

1

x 0.56

0.6

0.64

9. Determine the intervals on which f (x) is increasing or decreasing, assuming that Figure 11 is the graph of the Determine derivative f (x). the intervals on which f (x) is positive and negative, assuming that Figure 11 is the graph of f (x). y

x 1

2

3

4

5

6

FIGURE 11

f (x) is increasing on every interval (a, b) over which f (x) > 0, and is decreasing on every interval over which f (x) < 0. If the graph of f (x) is given in Figure 11, then f (x) is increasing on the intervals (0, 2) and (4, 6), SOLUTION

and is decreasing on the interval (2, 4).

In Exercises 11–14, sketch the graph of a function f (x) whose derivative f (x) has the given description. Plot the derivative f (x) of f (x) = 3x 5 − 5x 3 and describe the sign changes of f (x). Use this to determine local f (x). > 0extreme for x >values 3 andoff (x)

Here is the graph of a function f for which f (x) > 0 for x > 3 and f (x) < 0 for x < 3. y 10 8 6 4 2 x 0

1

2

3

4

5

13. f (x) is negative on (1, 3) and positive everywhere else. f (x) > 0 for x < 1 and f (x) < 0 for x > 1. SOLUTION Here is the graph of a function f for which f (x) is negative on (1, 3) and positive elsewhere. y 8 6 4 2 x −2

1

2

3

4

In Exercises the First Derivative f (x)15–18, makes use the sign transitions +, −,Test +, to −.determine whether the function attains a local minimum or local maximum (or neither) at the given critical point. 15. y = 7 + 4x − x 2 ,

c=2

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A P P L I C AT I O N S O F T H E D E R I VATI V E SOLUTION Let f (x) = 7 + 4x − x 2 . Then f (c) = 4 − 2c = 0 implies c = 2 is a critical point of f . Since f makes the sign transition +, − as x increases through c = 2, we conclude that f (2) = 11 is a local maximum of f .

x2 17. y =y = x 3 ,− 27x c =+ 0 2, c = −3 x +1 SOLUTION

x 2 . Then Let f (x) = x+1

f (c) =

2c(c + 1) − c2 c(c + 2) = = 0 at c = 0. (c + 1)2 (c + 1)2

Since f makes the sign transition −, + as x increases through c = 0, we conclude that f (0) = 0 is a local minimum of f. 19. Assuming that Figure 11 πis the graph of the derivative f (x), state whether f (2) and f (4) are local minima or maxima.y = sin x cos x, c = 4 SOLUTION

• f (x) makes a transition from positive to negative at x = 2, so f (2) is a local maximum. • f (x) makes a transition from negative to positive at x = 4, so f (4) is a local minimum.

In Exercises 21–40, ﬁnd the and the intervals which thef function increasing or decreasing, and deterapply Figure 12 shows the critical graph ofpoints the derivative f (x) ofon a function (x). Findisthe critical points of f (x) and the First Testare to local each critical mineDerivative whether they minima,point. maxima, or neither. SOLUTION

Here is a table legend for Exercises 21–40. SYMBOL

MEANING

−

The entity is negative on the given interval.

0

The entity is zero at the speciﬁed point.

+

The entity is positive on the given interval.

U

The entity is undeﬁned at the speciﬁed point.

f is increasing on the given interval.

f is decreasing on the given interval.

M

f has a local maximum at the speciﬁed point.

m

f has a local minimum at the speciﬁed point.

¬

There is no local extremum here.

21. y = −x 2 + 7x − 17 SOLUTION

Let f (x) = −x 2 + 7x − 17. Then f (x) = 7 − 2x = 0 yields the critical point c = 72 . x

−∞, 72

7/2

7,∞ 2

f

+

0

−

f

M

2 23. y = x 3 − 6x y = 5x 2 + 6x − 4 SOLUTION Let f (x) = x 3 − 6x 2 . Then f (x) = 3x 2 − 12x = 3x(x − 4) = 0 yields critical points c = 0, 4.

25. y = 3x 4 + 8x 3 −3 6x 2 − 24x y = x(x + 1)

x

(−∞, 0)

0

(0, 4)

4

(4, ∞)

f

+

0

−

0

+

f

M

m

The Mean Value Theorem and Monotonicity

S E C T I O N 4.3 SOLUTION

177

Let f (x) = 3x 4 + 8x 3 − 6x 2 − 24x. Then f (x) = 12x 3 + 24x 2 − 12x − 24 = 12x 2 (x + 2) − 12(x + 2) = 12(x + 2)(x 2 − 1) = 12 (x − 1) (x + 1) (x + 2) = 0

yields critical points c = −2, −1, 1. x

(−∞, −2)

−2

(−2, −1)

−1

(−1, 1)

1

(1, ∞)

f

−

0

+

0

−

0

+

f

m

M

m

27. y = 13 x 3 2+ 32 x 2 + 2x 2+ 4 y = x + (10 − x) 1 3 SOLUTION Let f (x) = 3 x 3 + 2 x 2 + 2x + 4. Then f (x) = x 2 + 3x + 2 = (x + 1) (x + 2) = 0 yields critical points c = −2, −1. x

(−∞, −2)

−2

(−2, −1)

−1

(−1, ∞)

f

+

0

−

0

+

f

M

m

29. y = x 4 +5x 3 3 y = x +x +1 3 SOLUTION Let f (x) = x 4 + x 3 . Then f (x) = 4x 3 + 3x 2 = x 2 (4x + 3) yields critical points c = 0, − 4 . x

−∞, − 34

− 34

− 34 , 0

0

(0, ∞)

f

−

0

+

0

+

f

m

¬

1 31. y =y =2 x 2 − x 4 x +1 SOLUTION

−1

−2 Let f (x) = x 2 + 1 . Then f (x) = −2x x 2 + 1 = 0 yields critical point c = 0. x

(−∞, 0)

0

(0, ∞)

f

+

0

−

f

M

33. y = x +2x x −1 + 1 (x > 0) y= 2 −1 −2 = 0 yields the critical point c = 1. (Note: c = −1 SOLUTION xLet + 1f (x) = x + x for x > 0. Then f (x) = 1 − x is not in the interval under consideration.) x

(0, 1)

1

(1, ∞)

f

−

0

+

f

m

(x > 0) 35. y = x 5/2 4− x 2 3/2 y = x − 4x (x > 0) 5 5 16 SOLUTION Let f (x) = x 5/2 − x 2 . Then f (x) = 2 x 3/2 − 2x = x( 2 x 1/2 − 2) = 0, so the critical point is c = 25 . (Note: c = 0 is not in the interval under consideration.) x

(0, 16 25 )

16 25

( 16 25 , ∞)

f

−

0

+

f

m

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37. y = sin θ cos θ , [0, 2π ] y = x −2 − 4x −1 (x > 0) SOLUTION Let f (θ ) = sin θ cos θ . Then f (θ ) = − sin2 θ + cos2 θ = 0 implies that tan2 θ = 1. On the interval [0, 2π ], this yields c = π4 , 34π , 54π , 74π . x

(0, π4 )

π 4

( π4 , 34π )

3π 4

( 34π , 54π )

5π 4

( 54π , 74π )

7π 4

( 74π , 2π )

f

+

0

−

0

+

0

−

0

+

f

M

m

M

m

39. y = θ + cos θ , [0, 2π ] y = cos θ + sin θ , [0, 2π ] π SOLUTION Let f (θ ) = θ + cos θ . Then f (θ ) = 1 − sin θ = 0, which yields c = 2 on the interval [0, 2π ]. x

0, π2

π 2

π

2 , 2π

f

+

0

+

f

¬

b b 41. Show =θx, 2 + y =that θ −f (x) 2 cos [0,bx2π+] c is decreasing on (−∞, − 2 ) and increasing on (− 2 , ∞).

Let f (x) = x 2 + bx + c. Then f (x) = 2x + b = 0 yields the critical point c = − b2 .

• For x < − b , we have f (x) < 0, so f is decreasing on −∞, − b . 2 2

• For x > − b , we have f (x) > 0, so f is increasing on − b , ∞ . 2 2

SOLUTION

43. Find conditions on a and3 b that2 ensure that f (x) = x 3 + ax + b is increasing on (−∞, ∞). Show that f (x) = x − 2x + 2x is an increasing function. Hint: Find the minimum value of f (x). SOLUTION Let f (x) = x 3 + ax + b. • If a > 0, then f (x) = 3x 2 + a > 0 and f is increasing for all x. • If a = 0, then

f (x 2 ) − f (x 1 ) = (3x 23 + b) − (3x 13 + b) = 3(x 2 − x 1 )(x 22 + x 2 x 1 + x 12 ) > 0 whenever x2 > x1 . Thus, f is increasing for all x. • If a < 0, then f (x) = 3x 2 + a < 0 and f is decreasing for |x| <

− a3 .

In summary, f (x) = x 3 + ax + b is increasing on (−∞, ∞) whenever a ≥ 0. 45. Sam made two statements that Deborah found dubious. x(x 2 − 1) (a) “Although the average for my is trip 70 mph,ofatano point inf time did my 70describe mph.” the thewas derivative function (x). Plot h(x)speedometer and use the read plot to Suppose that h(x)velocity = x 2 me + 1going 70 mph, my speedometer never read 65 mph.” (b) “Although a policeman clocked local extrema and the increasing/decreasing behavior of f (x). Sketch a plausible graph for f (x) itself. In each case, which theorem did Deborah apply to prove Sam’s statement false: the Intermediate Value Theorem or the Mean Value Theorem? Explain. SOLUTION

(a) Deborah is applying the Mean Value Theorem here. Let s(t) be Sam’s distance, in miles, from his starting point, let a be the start time for Sam’s trip, and let b be the end time of the same trip. Sam is claiming that at no point was s (t) =

s(b) − s(a) . b−a

This violates the MVT. (b) Deborah is applying the Intermediate Value Theorem here. Let v(t) be Sam’s velocity in miles per hour. Sam started out at rest, and reached a velocity of 70 mph. By the IVT, he should have reached a velocity of 65 mph at some point. 47. Show that f (x) = 1 − |x| satisﬁes the conclusion of the MVT on [a, b] if both a and b are positive or negative, but 2 2 2 2 where not if a Determine < 0 and b > 0. f (x) = (1,000 − x) + x is increasing. Use this to decide which is larger: 1,000 or 998 + 22 . SOLUTION Let f (x) = 1 − |x|. • If a and b (where a < b) are both positive (or both negative), then f is continuous on [a, b] and differentiable on

(a, b). Accordingly, the hypotheses of the MVT are met and the theorem does apply. Indeed, in these cases, any point c ∈ (a, b) satisﬁes the conclusion of the MVT (since f is constant on [a, b] in these instances).

S E C T I O N 4.3

The Mean Value Theorem and Monotonicity

179

1 f (b) − f (a) 0 − (−1) = . Yet there is no point c ∈ (−2, 1) such that = b−a 1 − (−2) 3 f (c) = 13 . Indeed, f (x) = 1 for x < 0, f (x) = −1 for x > 0, and f (0) is undeﬁned. The MVT does not apply in this case, since f is not differentiable on the open interval (−2, 1).

• For a = −2 and b = 1, we have

a+b 49. Show that if f is aofquadratic then = intervalsatisﬁes conclusion the MVT on [a, b] Which values c satisfy polynomial, the conclusion ofthe themidpoint MVT on cthe [a, b] ifthe f (x) is a linearoffunction? 2 for any a and b. SOLUTION Let f (x) = px 2 + qx + r with p = 0 and consider the interval [a, b]. Then f (x) = 2 px + q, and by the MVT we have

pb2 + qb + r − pa 2 + qa + r f (b) − f (a) 2 pc + q = f (c) = = b−a b−a

=

(b − a) ( p (b + a) + q) = p (b + a) + q b−a

Thus 2 pc + q = p(a + b) + q, and c =

a+b . 2

51. Suppose that f (2) = −2 and f (x) ≥ 5. Show that f (4) ≥ 8. Suppose that f (0) = 4 and f (x) ≤ 2 for x > 0. Apply the MVT to the interval [0, 3] to prove that f (3) ≤ 10. SOLUTION The MVT, applied to the [2,all 4],xguarantees there exists a c ∈ (2, 4) such that Prove more generally that f (x) ≤ 4interval + 2x for > 0. f (c) =

f (4) − f (2) 4−2

or

f (4) − f (2) = 2 f (c).

Because f (x) ≥ 5, it follows that f (4) − f (2) ≥ 10, or f (4) ≥ f (2) + 10 = 8.

Further Insights and Challenges 53. Prove that if f (0) = g(0) and f (x) ≤ g (x) for x ≥ 0, then f (x) ≤ g(x) for all x ≥ 0. Hint: Show that f (x) − g(x) Show that the cubic function f (x) = x 3 + ax 2 + bx + c is increasing on (−∞, ∞) if b > a 2 /3. is nonincreasing. Let h(x) = f (x) − g(x). By the sum rule, h (x) = f (x) − g (x). Since f (x) ≤ g (x) for all x ≥ 0, h (x) ≤ 0 for all x ≥ 0. This implies that h is nonincreasing. Since h(0) = f (0) − g(0) = 0, h(x) ≤ 0 for all x ≥ 0 (as SOLUTION

h is nonincreasing, it cannot climb above zero). Hence f (x) − g(x) ≤ 0 for all x ≥ 0, and so f (x) ≤ g(x) for x ≥ 0.

55. Use Exercises 53 and 54 to establish the following assertions for all x ≥ 0 (each assertion follows from the previous Use Exercise 53 to prove that sin x ≤ x for x ≥ 0. one): (a) cos x ≥ 1 − 12 x 2 (b) sin x ≥ x − 16 x 3 1 x4 (c) cos x ≤ 1 − 12 x 2 + 24 (d) Can you guess the next inequality in the series? SOLUTION

(a) We prove this using Exercise 53: Let g(x) = cos x and f (x) = 1 − 12 x 2 . Then f (0) = g(0) = 1 and g (x) = − sin x ≥ −x = f (x) for x ≥ 0 by Exercise 54. Now apply Exercise 53 to conclude that cos x ≥ 1 − 12 x 2 for x ≥ 0.

(b) Let g(x) = sin x and f (x) = x − 16 x 3 . Then f (0) = g(0) = 0 and g (x) = cos x ≥ 1 − 12 x 2 = f (x) for x ≥ 0 by part (a). Now apply Exercise 53 to conclude that sin x ≥ x − 16 x 3 for x ≥ 0.

1 x 4 and f (x) = cos x. Then f (0) = g(0) = 1 and g (x) = −x + 1 x 3 ≥ − sin x = f (x) (c) Let g(x) = 1 − 12 x 2 + 24 6 1 x 4 for x ≥ 0. for x ≥ 0 by part (b). Now apply Exercise 53 to conclude that cos x ≤ 1 − 12 x 2 + 24

1 x 5 , valid for x ≥ 0. To construct (d) from (c), we note (d) The next inequality in the series is sin x ≤ x − 16 x 3 + 120 that the derivative of sin x is cos x, and look for a polynomial (which we currently must do by educated guess) whose 1 x 4 . We know the derivative of x is 1, and that a term whose derivative is − 1 x 2 should be of derivative is 1 − 12 x 2 + 24 2 1 x 4 should be of the form Dx 5 . the form C x 3 . ddx C x 3 = 3C x 2 = − 12 x 2 , so C = − 16 . A term whose derivative is 24 1 x 4 , so that 5D = 1 , or D = 1 . From this, ddx Dx 5 = 5Dx 4 = 24 24 120 exists Deﬁne = x 3and sin( f1x )(x) for = x = 0 and f (0) = 0.that f (x) = mx + b, where m = f (0) and b = f (0). 57. Assume that ff(x) 0 for all x. Prove (a) Show that f (x) is continuous at x = 0 and x = 0 is a critical point of f . Examine the graphs of f (x) and f (x). Can the First Derivative Test be applied? (b) (c) Show that f (0) is neither a local min nor max. SOLUTION

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A P P L I C AT I O N S O F T H E D E R I VATI V E

(a) Let f (x) = x 3 sin( 1x ). Then f (x) = 3x 2 sin

1 1 1 1 + x 3 cos (−x −2 ) = x 3x sin − cos . x x x x

This formula is not deﬁned at x = 0, but its limit is. Since −1 ≤ sin x ≤ 1 and −1 ≤ cos x ≤ 1 for all x, 1 1 3x sin 1 + cos 1 ≤ |x|(3|x| + 1) | f (x)| = |x| 3x sin − cos ≤ |x| x x x x so, by the Squeeze Theorem, lim | f (x)| = 0. But does f (0) = 0? We check using the limit deﬁnition of the derivative: x→0

1 f (x) − f (0) = lim x 2 sin = 0. x −0 x x→0 x→0

f (0) = lim

Thus f (x) is continuous at x = 0, and x = 0 is a critical point of f . (b) The ﬁgure below at the left shows f (x), and the ﬁgure below at the right shows f (x). Note how the two functions oscillate near x = 0, which implies that the First Derivative Test cannot be applied. y

−0.2

y

x 0.2

−0.2

x 0.2

(c) As x approaches 0 from either direction, f (x) alternates between positive and negative arbitrarily close to x = 0. This means that f (0) cannot be a local minimum (since f (x) gets lower than f (0) arbitrarily close to 0), nor can f (0) be a local maximum (since f (x) takes values higher than f (0) arbitrarily close to x = 0). Therefore f (0) is neither a local minimum nor a local maximum of f .

4.4 The Shape of a Graph Preliminary Questions 1. Choose the correct answer: If f is concave up, then f is: (a) increasing (b) decreasing The correct response is (a): increasing. If the function is concave up, then f is positive. Since f is the derivative of f , it follows that the derivative of f is positive and f must therefore be increasing. SOLUTION

2. If x 0 is a critical point and f is concave down, then f (x 0 ) is a local: (a) min (b) max (c) undetermined SOLUTION

By the Second Derivative Test, the correct response is (b): maximum.

In Questions 3–8, state whether true or false and explain. Assume that f (x) exists for all x. 3. If f (c) = 0 and f (c) < 0, then f (c) is a local minimum. SOLUTION

False. By the Second Derivative Test, the correct conclusion would be that f (c) is a local maximum.

4. A function that is concave down on (−∞, ∞) can have no minimum value. SOLUTION

True.

5. If f (c) = 0, then f must have a point of inﬂection at x = c. False. f has an inﬂection point at x = c provided concavity changes at x = c. Points where f (c) = 0 are simply candidates for inﬂection points. The function f (x) = x 4 provides a counterexample. Here, f (x) = 12x 2 . Thus, f (0) = 0 but f (x) does not change sign at x = 0. Therefore, f (x) = x 4 does not have an inﬂection point at x = 0. SOLUTION

6. If f has a point of inﬂection at x = c, then f (c) = 0. SOLUTION True. In general, if f has an inﬂection point at x = c, then either f (c) = 0 or f (c) does not exist. Because we are assuming that f (x) exists for all x, we must have f (c) = 0.

7. If f is concave up and f changes sign at x = c, then f changes sign from negative to positive at x = c. True. If f is concave up, then f is positive and f is increasing; therefore, f must go from negative to positive at x = c. SOLUTION

S E C T I O N 4.4

The Shape of a Graph

181

8. If f (c) is a local maximum, then f (c) must be negative. SOLUTION True. In general, it could be that either f (c) does not exist or f (c) is negative. Because we are assuming that f (x) exists for all x, we must have f (c) < 0.

9. Suppose that f (c) = 0 and f (x) changes sign from + to − at x = c. Which of the following statements are correct? (a) f (x) has a local maximum at x = a. (b) f (x) has a local minimum at x = a. (c) f (x) has a local maximum at x = a. (d) f (x) has a point of inﬂection at x = a. Statements (c) and (d) are correct. Because f (x) goes from positive to negative, it follows that f (x) goes from increasing to decreasing and so must have a local maximum at x = c. Also, because f (c) = 0 and f (x) changes sign at x = c, it follows that f has a point of inﬂection at x = c. SOLUTION

Exercises 1. Match the graphs in Figure 12 with the description: (a) f (x) < 0 for all x. (c) f (x) > 0 for all x.

(A)

(b) f (x) goes from + to −. (d) f (x) goes from − to +.

(B)

(C)

(D)

FIGURE 12 SOLUTION

(a) (b) (c) (d)

In C, we have f (x) < 0 for all x. In A, f (x) goes from + to −. In B, we have f (x) > 0 for all x. In D, f (x) goes from − to +.

3. Sketch the graph of an increasing function such that f (x) changes from + to − at x = 2 and from − to + at x = 4. Match each statement with a graph in Figure 13 that represents company proﬁts as a function of time. Do the same for a decreasing function. (a) The outlook is great: The growth rate keeps increasing. SOLUTION The graph shown below at the left is an increasing function which changes from concave up to concave (b) We’re losing money, but not as quickly as before. down at x = 2 and from concave down to concave up at x = 4. The graph shown below at the right is a decreasing (c) We’re money, it’s getting worse asdown time at goes function which losing changes from and concave up to concave x =on. 2 and from concave down to concave up at x = 4. (d) We’re doing well, but our growth rate is leveling off. y y (e) Business had been cooling off, but now it’s picking up. 2 6 (f) Business had been picking up, but now it’s cooling off. 4 1 2 x 2

4

x 2

4

5. If Figure 14 is the graph of the derivative f (x), where do the points of inﬂection of f (x) occur, and on which If Figure 14 is the graph of a function f (x), where do the points of inﬂection of f (x) occur, and on which interval interval is f (x) concave down? is f (x) concave down? SOLUTION Points of inﬂection occur when f (x) changes sign. Consequently, points of inﬂection occur when f (x) changes from increasing to decreasing or from decreasing to increasing. In Figure 14, this occurs at x = b and at x = e; therefore, f (x) has an inﬂection point at x = b and another at x = e. The function f (x) will be concave down when f (x) < 0 or when f (x) is decreasing. Thus, f (x) is concave down for b < x < e. In Exercises 7–14,14determine theof intervals on which the function concave uppoints or down and ﬁnd the of inﬂection. If Figure is the graph the second derivative f (x), is where do the of inﬂection of points f (x) occur, and on + 7x +is10f (x) concave down? 7. which y = x 2interval

SOLUTION Let f (x) = x 2 + 7x + 10. Then f (x) = 2x + 7 and f (x) = 2 > 0 for all x. Therefore, f is concave up everywhere, and there are no points of inﬂection.

9. y = x − 2 cos x y = t 3 − 3t 2 + 1

182

CHAPTER 4

A P P L I C AT I O N S O F T H E D E R I VATI V E SOLUTION Let f (x) = x − 2 cos x. Then f (x) = 1 + 2 sin x and f (x) = 2 cos x = 0 at x = 1 2 (2n + 1) π , where n is an integer. Now, f is concave up on the intervals

π π − + 2n π , + 2n π 2 2

where n is any integer since f (x) > 0 there. Moreover, f is concave down on the intervals 3π π + 2n π , + 2n π 2 2 where n is any integer since f (x) < 0 there. Finally, because f (x) changes sign at each x = 12 (2n + 1)π , there is a point of inﬂection at each of these locations. √ 11. y = x(x − 8 x) 5 y = 4x − 5x 4 √ SOLUTION Let f (x) = x(x − 8 x) = x 2 − 8x 3/2 . Then f (x) = 2x − 12x 1/2 and f (x) = 2 − 6x −1/2 . Now, f is concave down for 0 < x < 9 since f (x) < 0 there. Moreover, f is concave up for x > 9 since f (x) > 0 there. Finally, because f (x) changes sign at x = 9, f (x) has a point of inﬂection at x = 9. 1 13. y =y =2 (x − 2)(1 − x 3 ) x +3 SOLUTION

1 2x . Then f (x) = − 2 Let f (x) = 2 and x +3 (x + 3)2 f (x) = −

2(x 2 + 3)2 − 8x 2 (x 2 + 3) 6x 2 − 6 = . (x 2 + 3)4 (x 2 + 3)3

Now, f is concave up for |x| > 1 since f (x) > 0 there. Moreover, f is concave down for |x| < 1 since f (x) < 0 there. Finally, because f (x) changes sign at both x = −1 and x = 1, f (x) has a point of inﬂection at both x = −1 and x = 1. 15. Sketch the7/5 graph of f (x) = x 4 and state whether f has any points of inﬂection. Verify your conclusion by showing y = x that f (x) does not change sign. SOLUTION

From the plot of f (x) = x 4 below, it appears that f has no points of inﬂection. Indeed, f (x) = 4x 3 and

f (x) = 12x 2 = 0 at x = 0, but f (x) does not change sign as x increases through 0 because 12x 2 ≥ 0 for all x. y 1

x

−1

1

The her growth of a sunﬂower its ﬁrst bean 100 days is modeled by the logistic curve ybelow. = h(t)Inshown in 17. Through website, Leticia hasduring been selling bag chairs with well monthly sales as recorded a report Figure 15. Estimate growth ratereached at the point ofofinﬂection explain its signiﬁcance. Then make a rough sketch of to investors, she the states, “Sales a point inﬂectionand when I started using pay-per-click advertising.” In which the ﬁrst anddid second derivatives of h(t). month that occur? Explain. Height (cm)

Month Sales

300

1

250 2

2

3

4

5

6

7

8

20

30

35

38

44

60

90

200 150 100 50 t (days) 20

40

60

80

100

FIGURE 15 SOLUTION The point of inﬂection in Figure 15 appears to occur at t = 40 days. The graph below shows the logistic curve with an approximate tangent line drawn at t = 40. The approximate tangent line passes roughly through the points (20, 20) and (60, 240). The growth rate at the point of inﬂection is thus

220 240 − 20 = = 5.5 cm/day. 60 − 20 40 Because the logistic curve changes from concave up to concave down at t = 40, the growth rate at this point is the maximum growth rate for the sunﬂower plant.

The Shape of a Graph

S E C T I O N 4.4

183

Height (cm) 300 250 200 150 100 50 t (days) 20

40

60

80

100

Sketches of the ﬁrst and second derivative of h(t) are shown below at the left and at the right, respectively. h′

h′′

6 5

0.1

4

t

3

20

2

40

60

80

100

−0.1

1 t 20

40

60

80

100

In Exercises 19–28, ﬁnd the the graph critical of f (x) and Derivative Testof (ifinﬂection possible)of to fdetermine whether Figure 16 shows ofpoints the derivative f (x)use onthe [0, Second 1.2]. Locate the points (x) and the points eachwhere corresponds to a local minimum or maximum. the local minima and maxima occur. Determine the intervals on which f (x) has the following properties: (b) Decreasing 19. (a) f (x)Increasing = x 3 − 12x 2 + 45x (c) Concave up (d) Concave down SOLUTION Let f (x) = x 3 − 12x 2 + 45x. Then f (x) = 3x 2 − 24x + 45 = 3(x − 3)(x − 5), and the critical points are x = 3 and x = 5. Moreover, f (x) = 6x − 24, so f (3) = −6 < 0 and f (5) = 6 > 0. Therefore, by the Second Derivative Test, f (3) = 54 is a local maximum, and f (5) = 50 is a local minimum.

21. f (x) = 3x 4 − 8x 3 + 6x 2 f (x) = x 4 − 8x 2 + 1 SOLUTION Let f (x) = 3x 4 − 8x 3 + 6x 2 . Then f (x) = 12x 3 − 24x 2 + 12x = 12x(x − 1)2 = 0 at x = 0, 1 and f (x) = 36x 2 − 48x + 12. Thus, f (0) > 0, which implies f (0) is a local minimum; however, f (1) = 0, which is inconclusive. 23. f (x) = x 5 −5x 3 2 f (x) = x − x

3 SOLUTION Let f (x) = x 5 − x 3 . Then f (x) = 5x 4 − 3x 2 = x 2 (5x 2 − 3) = 0 at x = 0, x = ± 5 and f (x) = 3 3 3 2 20x − 6x = x(20x − 6). Thus, f − 35 < 0, 5 > 0, which implies f 5 is a local minimum, and f which implies that f − 35 is a local maximum; however, f (0) = 0, which is inconclusive. 1 25. f (x)f (x) = = sin2 x + cos x, [0, π ] cos x + 2 1 sin x SOLUTION Let f (x) = = 0 at x = 0, ±π , ±2π , ±3π , . . . , and . Then f (x) = cos x + 2 (cos x + 2)2 f (x) =

cos2 x + 2 cos x + 2 sin2 x . (cos x + 2)3

Thus, f (0) > 0, f (±2π ) > 0 and f (±4π ) > 0, which implies that f (x) has local minima when x is an even multiple of π . Similarly, f (±π ) < 0, f (±3π ) < 0, and f (±5π ) < 0, which implies that f (x) has local maxima when x is an odd multiple of π . 27. f (x) = 3x 3/2 − 1x 1/2 f (x) = 3/2 − x 1/2 . Then f (x) = 9 x 1/2 − 1 x −1/2 = 1 x −1/2 (9x − 1), so there are two critical SOLUTION Letx 2f − (x)x = + 23x 2 2 2 1 points: x = 0 and x = 9 . Now, f (x) =

9 −1/2 1 −3/2 1 + x = x −3/2 (9x + 1). x 4 4 4

Thus, f 19 > 0, which implies f 19 is a local minimum. f (x) is undeﬁned at x = 0, so the Second Derivative Test cannot be applied there. In Exercises 29–40, ﬁnd the intervals on which f is concave up or down, the points of inﬂection, and the critical points, f (x) = x 7/4 − x and determine whether each critical point corresponds to a local minimum or maximum (or neither).

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A P P L I C AT I O N S O F T H E D E R I VATI V E SOLUTION

Here is a table legend for Exercises 29–40. SYMBOL

MEANING

−

The entity is negative on the given interval.

0

The entity is zero at the speciﬁed point.

+

The entity is positive on the given interval.

U

The entity is undeﬁned at the speciﬁed point.

The function ( f , g, etc.) is increasing on the given interval.

The function ( f , g, etc.) is decreasing on the given interval.

The function ( f , g, etc.) is concave up on the given interval.

The function ( f , g, etc.) is concave down on the given interval.

M

The function ( f , g, etc.) has a local maximum at the speciﬁed point.

m

The function ( f , g, etc.) has a local minimum at the speciﬁed point.

I

The function ( f , g, etc.) has an inﬂection point here.

¬

There is no local extremum or inﬂection point here.

29. f (x) = x 3 − 2x 2 + x SOLUTION

Let f (x) = x 3 − 2x 2 + x.

• Then f (x) = 3x 2 − 4x + 1 = (x − 1)(3x − 1) = 0 yields x = 1 and x = 1 as candidates for extrema. 3 • Moreover, f (x) = 6x − 4 = 0 gives a candidate for a point of inﬂection at x = 2 . 3

x

(−∞, 13 )

1 3

( 13 , 1)

1

(1, ∞)

x

(−∞, 23 )

2 3

( 23 , ∞)

f

+

0

−

0

+

f

−

0

+

f

M

m

f

I

31. f (t) = t 2 − t23 f (x) = x (x − 4) SOLUTION Let f (t) = t 2 − t 3 . • Then f (t) = 2t − 3t 2 = t (2 − 3t) = 0 yields t = 0 and t = 2 as candidates for extrema. 3 • Moreover, f (t) = 2 − 6t = 0 gives a candidate for a point of inﬂection at t = 1 . 3

t

(−∞, 0)

0

(0, 23 )

2 3

( 23 , ∞)

t

(−∞, 13 )

1 3

( 13 , ∞)

f

−

0

+

0

−

f

+

0

−

f

m

M

f

I

33. f (x) = x 2 − x41/2 2 f (x) = 2x − 3x + 2 SOLUTION Let f (x) = x 2 − x 1/2 . Note that the domain of f is x ≥ 0.

2/3 • Then f (x) = 2x − 1 x −1/2 = 1 x −1/2 4x 3/2 − 1 = 0 yields x = 0 and x = 1 as candidates for extrema. 2 2 4 • Moreover, f (x) = 2 + 1 x −3/2 > 0 for all x ≥ 0, which means there are no inﬂection points. 4

1 x 35. f (t) = 2 f (x)t =+ 12 x +2

x

0

2/3 0, 14

2/3

f

U

−

0

+

f

M

m

1 4

2/3 1 4

,∞

The Shape of a Graph

S E C T I O N 4.4

185

1 . Let f (t) = 2 t +1 2t • Then f (t) = − 2 = 0 yields t = 0 as a candidate for an extremum. 2 t +1 2

2 − 2t 2 − 2 t 2 + 1 (2) − 2t · 2 t 2 + 1 (2t) 2 3t 2 − 1 8t • Moreover, f (t) = − = 4 3 = 3 = 0 gives candidates t2 + 1 t2 + 1 t2 + 1

for a point of inﬂection at t = ± 13 .

SOLUTION

t

(−∞, 0)

0

(0, ∞)

f

+

0

−

f

M

−∞, − 13

− 13

− 13 , 13

f

+

0

−

0

+

f

I

I

t

1 3

1,∞ 3

37. f (θ ) = θ + sin θ for 0 ≤ θ ≤ 2π 1 f (x) = SOLUTION Let 4f (θ ) = θ + sin θ on [0, 2π ]. x +1 • Then f (θ ) = 1 + cos θ = 0 yields θ = π as a candidate for an extremum. • Moreover, f (θ ) = − sin θ = 0 gives candidates for a point of inﬂection at θ = 0, at θ = π , and at θ = 2π .

θ

(0, π )

f

+

f

π

(π , 2π )

θ

0

(0, π )

π

(π , 2π )

2π

0

+

f

0

−

0

+

0

¬

f

¬

I

¬

39. f (x) = x − sin x for 0 ≤ x ≤ 2π f (t) = sin2 t for 0 ≤ t ≤ π SOLUTION Let f (x) = x − sin x on [0, 2π ]. • Then f (x) = 1 − cos x > 0 on (0, 2π ). • Moreover, f (x) = sin x = 0 gives a candidate for a point of inﬂection at x =

π.

(0, 2π )

x

(0, π )

π

(π , 2π )

f

+

f

+

0

−

f

f

I

x

π <x < π 41. infectious slowly at the beginning of an epidemic. The infection process accelerates until a f (x)An = tan x for ﬂu−spreads 2 2 majority of the susceptible individuals are infected, at which point the process slows down. (a) If R(t) is the number of individuals infected at time t, describe the concavity of the graph of R near the beginning and end of the epidemic. (b) Write a one-sentence news bulletin describing the status of the epidemic on the day that R(t) has a point of inﬂection. SOLUTION

(a) Near the beginning of the epidemic, the graph of R is concave up. Near the epidemic’s end, R is concave down. (b) “Epidemic subsiding: number of new cases declining.” 43. Water is pumped a sphere a variable in such wayh(t) that be thethe water level rises at a constant Water is pumped intointo a sphere at aatconstant raterate (Figure 17).a Let water level at time t. Sketchrate the c (Figure 17). Let V (t) be the volume of water at time t. Sketch the graph of V (t) (approximately, but with graph of h(t) (approximately, but with the correct concavity). Where does the point of inﬂection occur? the correct concavity). Where does the point of inﬂection occur?

h

FIGURE 17

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A P P L I C AT I O N S O F T H E D E R I VATI V E SOLUTION Because water is entering the sphere in such a way that the water level rises at a constant rate, we expect the volume to increase more slowly near the bottom and top of the sphere where the sphere is not as “wide” and to increase more rapidly near the middle of the sphere. The graph of V (t) should therefore start concave up and change to concave down when the sphere is half full; that is, the point of inﬂection should occur when the water level is equal to the radius of the sphere. A possible graph of V (t) is shown below. V

t

In Exercises 45–47, sketch the graph of a function f (x) satisfying all of the given conditions. (Continuation of Exercise 43) When the water level in a sphere of radius R is h, the volume of water is 2 − 1 3 ). Assume the level rises at a constant rate c = 1 (i.e., h = t). = π> (Rh 0 and < 0 for all x. 45. Vf (x) 3f h(x) (a) Find the inﬂection point of V (t). Does this agree with your conclusion in Exercise 43? SOLUTION Here is the graph of a function f (x) satisfying f (x) > 0 for all x and f (x) < 0 for all x. (b) Plot V (t) for R = 1. y

0.5 x 1

2

−0.5

47. (i) f (x) < 0 for x < 0 and f (x) > 0 for x > 0, and f (x) for > all2,x,and and f (x) > 0 for |x| < 2. (x) < 0>for0 |x| (ii) f(i) (ii) f (x) < 0 for x < 0 and f (x) > 0 for x > 0. SOLUTION

Interval

(−∞, −2)

(−2, 0)

(0, 2)

(2, ∞)

Direction

Concavity

One potential graph with this shape is the following: y

−10

x 10

Further Insights and Challenges In Exercises 48–50, assume that f (x) is differentiable. (x) > 0. Prove that the graph of f (x) “sits 49. that f Derivative (x) exists and above” its tangent ProofAssume of the Second Testf Let c be a critical point in (a, b) such that f (c) > 0 (the case f lines (c)

(a) Let c be any number. Then y = f (c)(x − c) + f (c) is the equation of the line tangent to the graph of f (x) at x = c and G(x) = f (x) − f (c)(x − c) − f (c) measures the amount by which the value of the function exceeds the value of the tangent line (see the ﬁgure below). Thus, to prove that the graph of f (x) “sits above” its tangent lines, it is sufﬁcient to prove that G(x) ≥ 0 for all c.

S E C T I O N 4.4

The Shape of a Graph

187

y

x

(b) Note that G(c) = f (c) − f (c)(c − c) − f (c) = 0, G (x) = f (x) − f (c) and G (c) = f (c) − f (c) = 0. Moreover, G (x) = f (x) > 0 for all x. Now, because G (c) = 0 and G (x) is increasing, it must be true that G (x) < 0 for x < c and that G (x) > 0 for x > c. Therefore, G(x) is decreasing for x < c and increasing for x > c. This implies that G(c) = 0 is a minimum; consequently G(x) > G(c) = 0 for x = c. 51. Let C(x) be the cost of producing x units of a certain good. Assume that the graph of C(x) is concave up. Assume that f (x) exists and let c be a point of inﬂection of f (x). (a) Show that the average cost A(x) = C(x)/x is minimized at that production level x 0 for which average cost equals (a) Use marginal cost.the method of Exercise 49 to prove that the tangent line at x = c crosses the graph (Figure 18). Hint: Show that G(x) sign at x(0, = 0) c. and (x , C(x )) is tangent to the graph of C(x). (b) Show that changes the line through 0 0 Let C(x) be the cost of producing x units of a commodity. Assume the graph of C is concave up. (a) Let A(x) = C(x)/x be the average cost and let x 0 be the production level at which average cost is minimized. x x C (x 0 ) − C(x 0for ) f (x) = graphing f (x) and the tangent line at each inﬂection point on ) = 0this conclusion = 0 implies x C 1(xby Then(b) A (x 0Verify 20+ 0 ) − C(x 0 ) = 0, whence C (x 0 ) = C(x 0 )/x 0 = A(x 0 ). In other 3x 2 x the same set of axes. 0 words, A(x0 ) = C (x 0 ) or average cost equals marginal cost at production level x 0 . To conﬁrm that x 0 corresponds to a local minimum of A, we use the Second Derivative Test. We ﬁnd SOLUTION

x 2 C (x 0 ) − 2(x 0 C (x 0 ) − C(x 0 )) C (x 0 ) A (x 0 ) = 0 = >0 3 x0 x0 because C is concave up. Hence, x0 corresponds to a local minimum. (b) The line between (0, 0) and (x 0 , C(x 0 )) is C(x 0 ) C(x 0 ) − 0 (x − x0 ) + C(x 0 ) = A(x 0 )(x − x 0 ) + C(x 0 ) (x − x 0 ) + C(x 0 ) = x0 − 0 x0 = C (x0 )(x − x 0 ) + C(x 0 ) which is the tangent line to C at x0 . 53. Critical Points and Inﬂection Points If f (c) = 0 and f (c) is neither a local min or max, must x = c be a point Let f (x) be a polynomial of degree n. of inﬂection? This is true of most “reasonable” examples (including the examples in this text), but it is not true in general. (a) Show that if n is odd and n > 1, then f (x) has at least one point of inﬂection. Let have a point of inﬂection if n is even. (b) Show by giving an example that f (x) need not x 2 sin 1x for x = 0 f (x) = 0 for x = 0 (a) Use the limit deﬁnition of the derivative to show that f (0) exists and f (0) = 0. (b) Show that f (0) is neither a local min nor max. (c) Show that f (x) changes sign inﬁnitely often near x = 0 and conclude that f (x) does not have a point of inﬂection at x = 0. x 2 sin (1/x) for x = 0 . SOLUTION Let f (x) = 0 for x = 0 1 f (x) − f (0) x 2 sin (1/x) (a) Now f (0) = lim = lim = lim x sin = 0 by the Squeeze Theorem: as x → 0 x −0 x x x→0 x→0 x→0 we have x sin 1 − 0 = |x| sin 1 → 0, x x since | sin u| ≤ 1. (b) Since sin( 1x ) oscillates through every value between −1 and 1 with increasing frequency as x → 0, in any open interval (−δ , δ ) there are points a and b such that f (a) = a 2 sin( a1 ) < 0 and f (b) = b2 sin( b1 ) > 0. Accordingly, f (0) = 0 can neither be a local minimum value nor a local maximum value of f . (c) In part (a) it was shown that f (0) = 0. For x = 0, we have 1 1 1 1 1 f (x) = x 2 cos − 2 + 2x sin = 2x sin − cos . x x x x x As x → 0, f (x) oscillates increasingly rapidly; consequently, f (x) changes sign inﬁnitely often near x = 0. From this we conclude that f (x) does not have a point of inﬂection at x = 0.

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A P P L I C AT I O N S O F T H E D E R I VATI V E

4.5 Graph Sketching and Asymptotes Preliminary Questions 1. Sketch an arc where f and f have the sign combination ++. Do the same for −+. SOLUTION An arc with the sign combination ++ (increasing, concave up) is shown below at the left. An arc with the sign combination −+ (decreasing, concave up) is shown below at the right. y

y

x

x

2. If the sign combination of f and f changes from ++ to +− at x = c, then (choose the correct answer): (a) f (c) is a local min (b) f (c) is a local max (c) c is a point of inﬂection SOLUTION

Because the sign of the second derivative changes at x = c, the correct response is (c): c is a point of

inﬂection. 3. What are the following limits? (a) lim x 3 x→∞

(b)

lim x 3

(c)

x→−∞

lim x 4

x→−∞

SOLUTION

(a) limx→∞ x 3 = ∞ (b) limx→−∞ x 3 = −∞ (c) limx→−∞ x 4 = ∞ 4. What is the sign of a2 if f (x) = a2 x 2 + a1 x + a0 satisﬁes

lim

x→−∞

f (x) = −∞?

The behavior of f (x) = a2 x 2 + a1 x + a0 as x → −∞ is controlled by the leading term; that is, limx→−∞ f (x) = limx→−∞ a2 x 2 . Because x 2 → +∞ as x → −∞, a2 must be negative to have limx→−∞ f (x) = −∞. SOLUTION

5. What is the sign of the leading coefﬁcient a7 if f (x) is a polynomial of degree 7 such that

lim

x→−∞

f (x) = ∞?

SOLUTION The behavior of f (x) as x → −∞ is controlled by the leading term; that is, lim x→−∞ f (x) = limx→−∞ a7 x 7 . Because x 7 → −∞ as x → −∞, a7 must be negative to have limx→−∞ f (x) = ∞.

6. The second derivative of the function f (x) = (x − 4)−1 is f (x) = 2(x − 4)−3 . Although f (x) changes sign at x = 4, f (x) does not have a point of inﬂection at x = 4. Why not? SOLUTION

The function f does not have a point of inﬂection at x = 4 because x = 4 is not in the domain of f.

Exercises 1. Determine the sign combinations of f and f for each interval A–G in Figure 18. y

y = f(x)

A

B C

D

E F

G

FIGURE 18 SOLUTION

• In A, f is decreasing and concave up, so f < 0 and f > 0. • In B, f is increasing and concave up, so f > 0 and f > 0.

• In C, f is increasing and concave down, so f > 0 and f < 0. • In D, f is decreasing and concave down, so f < 0 and f < 0. • In E, f is decreasing and concave up, so f < 0 and f > 0.

x

S E C T I O N 4.5

Graph Sketching and Asymptotes

189

• In F, f is increasing and concave up, so f > 0 and f > 0. • In G, f is increasing and concave down, so f > 0 and f < 0. f 19. takeExample: on the given sign combinations. In Exercises drawchange the graph of atransition function for which State 3–6, the sign at each point A–Gfinand Figure f (x) goes from + to − at A.

3. ++,

+−,

SOLUTION

x = 0.

−−

This function changes from concave up to concave down at x = −1 and from increasing to decreasing at −1

0

1

x

−1

y

5. −+, −−, −+ +−, −−, −+ SOLUTION The function is decreasing everywhere and changes from concave up to concave down at x = −1 and from concave down to concave up at x = − 12 . y

0.05

x

−1

0

7. Sketch graph +− of y = x 2 − 2x + 3. −+,the++, SOLUTION Let f (x) = x 2 − 2x + 3. Then f (x) = 2x − 2 and f (x) = 2. Hence f is decreasing for x < 1, is increasing for x > 1, has a local minimum at x = 1 and is concave up everywhere. y 5 4 3 2 1 x

−1

1

2

3

9. Sketch graph of the cubic f (x) = x 3 2− 3x 2 + 2. For extra accuracy, plot the zeros of f (x), which are x = 1 and √ the the Sketch of y2.73. = 3 + 5x − 2x . x = 1 ± 3 or x ≈graph −0.73, Let f (x) = x 3 − 3x 2 + 2. Then f (x) = 3x 2 − 6x = 3x(x − 2) = 0 yields x = 0, 2 and f (x) = 6x − 6. Thus f is concave down for x < 1, is concave up for x > 1, has an inﬂection point at x = 1, is increasing for x < 0 and for x > 2, is decreasing for 0 < x < 2, has a local maximum at x = 0, and has a local minimum at x = 2. SOLUTION

y 2 1 −1

x −1

1

2

3

−2

11. Extend the sketch of the graph of f (x) = cos x + 1 x over [0, π ] in Example 4 to the interval [0, 5π ]. Show that the cubic x 3 − 3x 2 + 6x has a point 2of inﬂection but no local extreme values. Sketch the graph. 1 1 π 5π 13π 17π SOLUTION Let f (x) = cos x + 2 x. Then f (x) = − sin x + 2 = 0 yields critical points at x = 6 , 6 , 6 , 6 , 25π , and 29π . Moreover, f (x) = − cos x so there are points of inﬂection at x = π , 3π , 5π , 7π , and 9π . 6 6 2 2 2 2 2

190

CHAPTER 4

A P P L I C AT I O N S O F T H E D E R I VATI V E y 6 4 2 x 0

2

4

6

8 10 12 14

In Exercises 13–30, sketch the graph of the function. Indicate the transition points (local extrema and points of inﬂection). Sketch the graphs of y = x 2/3 and y = x 4/3 . 13. y = x 3 + 32 x 2 Let f (x) = x 3 + 32 x 2 . Then f (x) = 3x 2 + 3x = 3x (x + 1) and f (x) = 6x + 3. This shows that f has critical points at x = 0 and x = −1 and a candidate for an inﬂection point at x = − 12 . SOLUTION

Interval

(−∞, −1)

(−1, − 12 )

(− 12 , 0)

(0, ∞)

+−

−−

−+

++

Signs of f and f

Thus, there is a local maximum at x = −1, a local minimum at x = 0, and an inﬂection point at x = − 12 . Here is a graph of f with these transition points highlighted as in the graphs in the textbook. y 2 1 −2

x

−1

−1

15. y = x 2 −34x 3 y = x − 3x + 5 SOLUTION Let f (x) = x 2 − 4x 3 . Then f (x) = 2x − 12x 2 = 2x(1 − 6x) and f (x) = 2 − 24x. Critical points are 1 . at x = 0 and x = 16 , and the sole candidate point of inﬂection is at x = 12 Interval Signs of f and f

(−∞, 0)

1 ) (0, 12

1 , 1) ( 12 6

( 16 , ∞)

−+

++

+−

−−

1 . Here is the graph Thus, f (0) is a local minimum, f ( 16 ) is a local maximum, and there is a point of inﬂection at x = 12 of f with transition points highlighted as in the textbook: y 0.04 x

−0.2

0.2 −0.04

17. y = 4 −12x32 + 162x 4 y = 3 x + x + 3x

1 2 2 SOLUTION Let f (x) = 6 x 4 − 2x 2 + 4. Then f (x) = 3 x 3 − 4x = 3 x x 2 − 6 and f (x) = 2x 2 − 4. This shows √ √ that f has critical points at x = 0 and x = ± 6 and has candidates for points of inﬂection at x = ± 2. Interval Signs of f and f

√ (−∞, − 6)

√ √ (− 6, − 2)

√ (− 2, 0)

−+

++

+−

(0,

√

2)

−−

√ √ ( 2, 6)

√ ( 6, ∞)

−+

++

√ √ Thus, f has local minima at x = ± 6, a local maximum at x = 0, and inﬂection points at x = ± 2. Here is a graph of f with transition points highlighted.

S E C T I O N 4.5

Graph Sketching and Asymptotes

191

y 10 5 −2

2

x

19. y = x 5 + 5x y = 7x 4 − 6x 2 + 1 SOLUTION Let f (x) = x 5 + 5x. Then f (x) = 5x 4 + 5 = 5(x 4 + 1) and f (x) = 20x 3 . f (x) > 0 for all x, so the graph has no critical points and is always increasing. f (x) = 0 at x = 0. Sign analyses reveal that f (x) changes from negative to positive at x = 0, so that the graph of f (x) has an inﬂection point at (0, 0). Here is a graph of f with transition points highlighted. y 40 20 −2

−1

x 1

−20

2

−40

21. y = x 4 −53x 3 +34x y = x − 5x SOLUTION Let f (x) = x 4 − 3x 3 + 4x. Then f (x) = 4x 3 − 9x 2 + 4 = (4x 2 − x − 2)(x − 2) and f (x) = √ 1 ± 33 2 and candidate points of 12x − 18x = 6x(2x − 3). This shows that f has critical points at x = 2 and x = 8 √ 3 inﬂection at x = 0 and x = 2 . Sign analyses reveal that f (x) changes from negative to positive at x = 1−8 33 , from √

√

positive to negative at x = 1+8 33 , and again from negative to positive at x = 2. Therefore, f ( 1−8 33 ) and f (2) are √ local minima of f (x), and f ( 1+ 8 33 ) is a local maximum. Further sign analyses reveal that f (x) changes from positive to negative at x = 0 and from negative to positive at x = 32 , so that there are points of inﬂection both at x = 0 and x = 32 . Here is a graph of f (x) with transition points highlighted. y 6 4 2 −1

x 1

−2

2

23. y = 6x 7 − 7x 6 y = x 2 (x − 4)2 SOLUTION Let f (x) = 6x 7 − 7x 6 . Then f (x) = 42x 6 − 42x 5 = 42x 5 (x − 1) and f (x) = 252x 5 − 210x 4 = 42x 4 (6x − 5). Critical points are at x = 0 and x = 1, and candidate inﬂection points are at x = 0 and x = 56 . Sign analyses reveal that f (x) changes from positive to negative at x = 0 and from negative to positive at x = 1. Therefore f (0) is a local maximum and f (1) is a local minimum. Also, f (x) changes from negative to positive at x = 56 . Therefore, there is a point of inﬂection at x = 56 . Here is a graph of f with transition points highlighted. y 2 1 −0.5

x −1

0.5

1

√ 25. y = x − x y = x 6 − 3x 4 √ 1 SOLUTION Let f (x) = x − x = x − x 1/2 . Then f (x) = 1 − 2 x −1/2 . This shows that f has critical points at x = 0 1 (where the derivative does not exist) and at x = 4 (where the derivative is zero). Because f (x) < 0 for 0 < x < 14

and f (x) > 0 for x > 14 , f 14 is a local minimum. Now f (x) = 14 x −3/2 > 0 for all x > 0, so the graph is always concave up. Here is a graph of f with transition points highlighted.

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A P P L I C AT I O N S O F T H E D E R I VATI V E y 0.8 0.6 0.4 0.2 x 0.5

−0.2

1

1.5

2

√ √ + 9−x 27. y = x√ y = x +2 √ √ SOLUTION Let f (x) = x + 9 − x = x 1/2 + (9 − x)1/2 . Note that the domain of f is [0, 9]. Now, f (x) = 1 x −1/2 − 1 (9 − x)−1/2 and f (x) = − 1 x −3/2 − 1 (9 − x)−3/2 . Thus, the critical points are x = 0, x = 9 and 2 2 4 4 2 x = 9. Sign analysis reveals that f (x) > 0 for 0 < x < 92 and f (x) < 0 for 92 < x < 9, so f has a local maximum at x = 92 . Further, f (x) < 0 on (0, 9), so the graph is always concave down. Here is a graph of f with the transition point highlighted. y 4 3 2 1 x 0

1 2 3 4 5 6 7 8 9

29. y = (x 2 − x)1/3 1/3 y = x(1 − x) SOLUTION Let f (x) = (x 2 − x)1/3 . Then f (x) = and 1 f (x) =

3

1 (2x − 1)(x 2 − x)−2/3 3

2 2(x 2 − x)−2/3 − (2x − 1)(2x − 1)(x 2 − x)−5/3

3

2 2 = (x 2 − x)−5/3 3(x 2 − x) − (2x − 1)2 = − (x 2 − x)−5/3 (x 2 − x + 1). 9 9

Critical points of f (x) are points where the numerator 2x − 1 = 0 or where f (x) doesn’t exist, that is, at x = 12 and points where x 2 − x = 0 so that x = 1 or x = 0. Candidate points of inﬂection lie at points where f (x) = 0 (of which there are none), and points where f (x) does not exist (at x = 0 and x = 1). Sign analyses reveal that f (x) < 0 for x < 0 and for x > 1, while f (x) > 0 for 0 < x < 1. Therefore, the graph of f (x) has points of inﬂection at x = 0 and x = 1. Since (x 2 − x)−2/3 is positive wherever it is deﬁned, the sign of f (x) depends solely on the sign of 2x − 1. Hence, f (x) does not change sign at x = 0 or x = 1, and goes from negative to positive at x = 12 . f ( 12 ) is, in that case, a local minimum. Here is a graph of f (x) with the transition points indicated. y 1.5 1 0.5 −1

x −0.5

1

2

31. Sketch the3graph of f (x) = 18(x − 3)(x − 1)2/3 using the following formulas: y = (x − 4x)1/3 3 30(x − 95 ) (x) = 20(x − 5 ) , f f (x) = (x − 1)1/3 (x − 1)4/3 SOLUTION

f (x) =

30(x − 95 ) (x − 1)1/3

S E C T I O N 4.5

Graph Sketching and Asymptotes

193

yields critical points at x = 95 , x = 1. f (x) =

20(x − 35 ) (x − 1)4/3

yields potential inﬂection points at x = 35 , x = 1. Interval

signs of f and f

(−∞, 35 )

+−

( 35 , 1) (1, 95 ) ( 95 , ∞)

++ −+ ++

The graph has an inﬂection point at x = 35 , a local maximum at x = 1 (at which the graph has a cusp), and a local minimum at x = 95 . The sketch looks something like this. y −2

40 −1 20

1

2

3

−20 −40 −60 −80

x

In Exercises 32–37, sketch the graph over the given interval. Indicate the transition points. 33. y = sin x + cos x, [0, 2π ] y = x + sin x, [0, 2π ] SOLUTION Let f (x) = sin x + cos x. Setting f (x) = cos x − sin x = 0 yields sin x = cos x, so that tan x = 1, and x = π4 , 54π . Setting f (x) = − sin x − cos x = 0 yields sin x = − cos x, so that − tan x = 1, and x = 34π , x = 74π . Interval

signs of f and f

(0, π4 )

+−

( π4 , 34π )

−−

( 34π , 54π )

−+

( 54π , 74π )

++

( 74π , 2π )

+−

The graph has a local maximum at x = π4 , a local minimum at x = 54π , and inﬂection points at x = 34π and x = 74π . Here is a sketch of the graph of f (x): y 1 x 1

2

3

4

5

6

−1

35. y = sin x + 12 x, [0,2 2π ] y = 2 sin x − cos x, [0, 2π ] 1 1 2π 4π SOLUTION Let f (x) = sin x + 2 x. Setting f (x) = cos x + 2 = 0 yields x = 3 or 3 . Setting f (x) = − sin x = 0 yields potential points of inﬂection at x = 0, π , 2π .

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Interval

signs of f and f

(0, 23π )

+−

( 23π , π )

−−

(π , 43π )

−+

( 43π , 2π )

++

The graph has a local maximum at x = 23π , a local minimum at x = 43π , and an inﬂection point at x = π . Here is a graph of f without transition points highlighted. y 3 2 1 x 0

1

2

3

4

5

6

1 √ ] π] [0, π[0, 37. y =y sin x −x +sin 32x, = sin cos x, 2 Hint for Exercise 37: We ﬁnd numerically that there is one point of inﬂection in the interval, occurring at x ≈ 1.3182. Let f (x) = sin x − 12 sin 2x. Setting f (x) = cos x − cos 2x = 0 yields cos 2x = cos x. Using the double angle formula for cosine, this gives 2 cos2 x − 1 = cos x or (2 cos x + 1)(cos x − 1) = 0. Solving for x ∈ [0, π ], we ﬁnd x = 0 or 23π . Setting f (x) = − sin x + 2 sin 2x = 0 yields 4 sin x cos x = sin x, so sin x = 0 or cos x = 14 . Hence, there are potential points of inﬂection at x = 0, x = π and x = cos−1 41 ≈ 1.31812. SOLUTION

Interval

Sign of f and f

(0, cos−1 41 )

++

(cos−1 14 , 23π )

+−

( 23π , π )

−−

The graph of f (x) has a local maximum at x = 23π and a point of inﬂection at x = cos−1 41 . y

1

x 0

1

2

3

39. Suppose that f is twice differentiable satisfying (i) f (0) = 1, (ii) f (x) > 0 for all x = 0, and (iii) f (x) < 0 for all > sign transitions possible? x < 0 andAre f (x) 0 for x > 0. Let g(x) =Explain f (x 2 ). with a sketch why the transitions ++ → −+ and −− → +− do not occur if the function is differentiable. (See Exercise 83 for a proof.) (a) Sketch a possible graph of f (x). (b) Prove that g(x) has no points of inﬂection and a unique local extreme value at x = 0. Sketch a possible graph of g(x). SOLUTION

(a) To produce a possible sketch, we give the direction and concavity of the graph over every interval. Interval

(−∞, 0)

(0, ∞)

Direction

Concavity

A sketch of one possible such function appears here:

Graph Sketching and Asymptotes

S E C T I O N 4.5

195

y

2 −2

x

−1

1

2

(b) Let g(x) = f (x 2 ). Then g (x) = 2x f (x 2 ). If g (x) = 0, either x = 0 or f (x 2 ) = 0, which implies that x = 0 as well. Since f (x 2 ) > 0 for all x = 0, g (x) < 0 for x < 0 and g (x) > 0 for x > 0. This gives g(x) a unique local extreme value at x = 0, a minimum. g (x) = 2 f (x 2 ) + 4x 2 f (x 2 ). For all x = 0, x 2 > 0, and so f (x 2 ) > 0 and f (x 2 ) > 0. Thus g (x) > 0, and so g (x) does not change sign, and can have no inﬂection points. A sketch of g(x) based on the sketch we made for f (x) follows: indeed, this sketch shows a unique local minimum at x = 0. y

2 −1

x 1

In Exercises following limits theofnumerator and Explain. denominator by the highest power of x Which41–50, of the calculate graphs in the Figure 20 cannot be (divide the graph a polynomial? appearing in the denominator). 41. lim

x

x→∞ x + 9

SOLUTION

1 x x −1 (x) 1 = = lim −1 = 1. = lim x→∞ x + 9 x→∞ x (x + 9) x→∞ 1 + 9 1+0 x lim

3x 22 + 20x 43. lim 3x + 20x 4 x→∞ lim 2x + 3x 3 − 29 x→∞ 4x 2 + 9 SOLUTION

3 + 20 0 3x 2 + 20x x −4 (3x 2 + 20x) x2 x3 = = 0. = lim = lim x→∞ 2x 4 + 3x 3 − 29 x→∞ x −4 (2x 4 + 3x 3 − 29) x→∞ 2 + 3 − 29 2 4 x

lim

x

7x − 9 45. lim 4 x→∞ 4x + 3 lim x→∞ x + 5 SOLUTION

7− 7x − 9 x −1 (7x − 9) = lim −1 = lim x→∞ 4x + 3 x→∞ x (4x + 3) x→∞ 4 + lim

47.

9 7 x = . 3 4 x

7x 2 2− 9 lim 7x − 9 x→−∞ lim 4x + 3 x→∞ 4x + 3

SOLUTION

7x − 9x 7x 2 − 9 x −1 (7x 2 − 9) = −∞. = lim = lim x→−∞ 4x + 3 x→−∞ x −1 (4x + 3) x→−∞ 4 + 3 x lim

49.

x2 − 1 limlim 5x − 9 x→−∞ +34 x→∞x4x +1

SOLUTION

x− x2 − 1 x −1 (x 2 − 1) = lim = lim x→−∞ x + 4 x→−∞ x −1 (x + 4) x→−∞ 1 + lim

1 x = −∞. 4 x

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In Exercises 51–56, calculate the limit. 2x 5 + 3x 4 − 31x lim 41 x→∞x 8x 2+ − 31x 2 + 12 51. lim x→∞ x + 1 SOLUTION

√

√

Looking at only the highest powers in the top and bottom of

x 2 +1 x+1 , it seems this quotient behaves like

x2 x −1 yields x = x . Multiplying top and bottom by x

x2 + 1 = x +1

1 + 12 x

. 1 + 1x

From this we get: lim 1 + 1/x 2 1 x2 + 1 x→∞ lim = = = 1. x→∞ x + 1 lim 1 + 1/x 1

x→∞

x +1 53. lim 4 x→∞ 3 x 2 +x1 + 1 lim x→−∞ x 3 + 1 SOLUTION We can see that the denominator behaves like x 2/3 . Dividing top and bottom by x 2/3 yields: lim x 1/3 + 1/x 2/3 x +1 ∞ x→∞ = = = ∞. lim 3 x→∞ 3 x 2 + 1 2 1 lim 1 + 1/x x→∞

55.

x

lim 4/3 +1/3 4x 1/3 x→−∞ (xx6 + 1) lim x→∞ x 5/4 − 2x

SOLUTION

Looking at highest exponents of x, we see the denominator behaves like x 2 . Dividing top and bottom by

x 2 yields: x = x→−∞ (x 6 + 1)1/3

lim 1/x

x→−∞

lim

lim (1 + 1/x 6 )1/3

=

x→−∞

0 = 0. 1

4 4xFigure − 9 21 is the graph of f (x) = 2x − 1 ? Explain on the basis of horizontal asymptotes. 57. Which curve in lim 1 + x4 x→−∞ (3x 4 − 2)1/3 y

y

2

−4

2

x

−2

2

4

−4

−1.5

x

−2

2

4

−1.5

(A)

(B)

FIGURE 21 SOLUTION

Since 2 2x 4 − 1 = · lim 1 = 2 x→±∞ 1 + x 4 1 x→±∞ lim

the graph has left and right horizontal asymptotes at y = 2, so the left curve is the graph of f (x) =

2x 4 − 1 . 1 + x4

59. Match the functions with their graphs in Figure 23. 3x 2 3x 2 1 the graphs in Figure 22 with the two functions y = x and y = Match . Explain. (a) y = 2 (b) yx 2=− 12 x2 − 1 x −1 x +1 x 1 (d) y = 2 (c) y = 2 x +1 x −1

S E C T I O N 4.5 y

Graph Sketching and Asymptotes

197

y

x

x (A)

(B)

y

y x x

(C)

(D)

FIGURE 23 SOLUTION

1 should have a horizontal asymptote at y = 0 and vertical asymptotes at x = ±1. Further, the x 2 −1 graph should consist of positive values for |x| > 1 and negative values for |x| < 1. Hence, the graph of 21 is (D). x −1 2 (b) The graph of 2x should have a horizontal asymptote at y = 1 and no vertical asymptotes. Hence, the graph of x +1 x 2 is (A). 2 x +1 (c) The graph of 21 should have a horizontal asymptote at y = 0 and no vertical asymptotes. Hence, the graph of x +1 1 is (B). 2 x +1 (d) The graph of 2x should have a horizontal asymptote at y = 0 and vertical asymptotes at x = ±1. Further, the x −1

(a) The graph of

graph should consist of positive values for −1 < x < 0 and x > 1 and negative values for x < 1 and 0 < x < 1. Hence, the graph of 2x is (C). x −1

x 61. Sketch the graph of f (x) = 2 2x −using 1 the formulas x Sketch the graph of f (x) = + 1 . x +1 1 − x2 f (x) = , (1 + x 2 )2 SOLUTION

• Because

f (x) =

2x(x 2 − 3) . (x 2 + 1)3

x . Let f (x) = 2 x +1 lim

x→±∞

f (x) = 11 ·

lim x −1 = 0, y = 0 is a horizontal asymptote for f .

x→±∞

1 − x2 2 is negative for x < −1 and x > 1, positive for −1 < x < 1, and 0 at x = ±1. x2 + 1 Accordingly, f is decreasing for x < −1 and x > 1, is increasing for −1 < x < 1, has a local minimum value at x = −1 and a local maximum value at x = 1. • Moreover,

2x x 2 − 3 f (x) = 3 . x2 + 1 • Now f (x) =

Here is a sign chart for the second derivative, similar to those constructed in various exercises in Section 4.4. (The legend is on page 184.)

√

√ √

√ √ √ −∞, − 3 − 3, 0 0, 3 3 3, ∞ x − 3 0 f

−

0

+

0

−

0

+

f

I

I

I

• Here is a graph of f (x) =

x x2 + 1

.

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CHAPTER 4

A P P L I C AT I O N S O F T H E D E R I VATI V E y 0.5 −10

−5

x 5

10

−0.5

In Exercises 62–77, sketch the graph of the function. Indicate the asymptotes, local extrema, and points of inﬂection. x 63. y = 1 y 2x = −1 2x − 1 SOLUTION Let f (x) =

x −1 , so that f is decreasing for all x = 12 . Moreover, f (x) = . Then f (x) = 2x − 1 (2x − 1)2 4 x 1 , so that f is concave up for x > 12 and concave down for x < 12 . Because lim = , f has a x→±∞ 2x − 1 2 (2x − 1)3 horizontal asymptote at y = 12 . Finally, f has a vertical asymptote at x = 12 with lim

x→ 12 −

x = −∞ 2x − 1

and

lim

x→ 12 +

x = ∞. 2x − 1

y 4 2 −1

x 1

−2

2

3

−4

x +3 65. y = x +1 y x=− 2 2x − 1 x +3 −5 SOLUTION Let f (x) = , so that f is decreasing for all x = 2. Moreover, f (x) = . Then f (x) = x −2 (x − 2)2 x +3 10 , so that f is concave up for x > 2 and concave down for x < 2. Because lim = 1, f has a horizontal x→±∞ x − 2 (x − 2)3 asymptote at y = 1. Finally, f has a vertical asymptote at x = 2 with lim

x +3

x→2− x − 2

= −∞

and

lim

x +3

x→2+ x − 2

= ∞.

y 10 5 −10

−5

x 5

−5

10

−10

1 1 67. y = + 1 x −1 y x= x + x 1 2x 2 − 2x + 1 1 SOLUTION Let f (x) = , so that f is decreasing for all x = 0, 1. Moreover, + . Then f (x) = − x x− x 2 (x − 1)2 1

2 2x 3 − 3x 2 + 3x − 1 , so that f is concave up for 0 < x < 12 and x > 1 and concave down for x < 0 f (x) = x 3 (x − 1)3 1 1 and 12 < x < 1. Because lim + = 0, f has a horizontal asymptote at y = 0. Finally, f has vertical x→±∞ x x −1 asymptotes at x = 0 and x = 1 with 1 1 1 1 lim + = −∞ and lim + =∞ x −1 x −1 x→0− x x→0+ x and

1 1 + x −1 x→1− x lim

= −∞

and

1 1 + x −1 x→1+ x lim

= ∞.

S E C T I O N 4.5

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199

y 5 x

−1

1

−5

2

4 3 69. y = 1 −1 + 13 y = x− x x x −1 3 4 SOLUTION Let f (x) = 1 − + 3 . Then x x 3 12 3(x − 2)(x + 2) , f (x) = 2 − 4 = x x x4 so that f is increasing for |x| > 2 and decreasing for −2 < x < 0 and for 0 < x < 2. Moreover, 6 48 6(8 − x 2 ) , f (x) = − 3 + 5 = x x x5 √ √ √ so that f is √ concave down for −2 2 < x < 0 and for x > 2 2, while f is concave up for x < −2 2 and for 0 < x < 2 2. Because 4 3 lim 1 − + 3 = 1, x→±∞ x x f has a horizontal asymptote at y = 1. Finally, f has a vertical asymptote at x = 0 with 4 4 3 3 and lim 1 − + 3 = ∞. lim 1 − + 3 = −∞ x x x→0− x→0+ x x y 6 4 2 x

−6 −4 −2

−2

2

4

6

−4 −6

1 1 71. y = 2 + 1 2 y x= 2 (x − 2) x − 6x + 8 1 1 SOLUTION Let f (x) = + . Then 2 x (x − 2)2 f (x) = −2x −3 − 2 (x − 2)−3 = −

4(x − 1)(x 2 − 2x + 4) , x 3 (x − 2)3

so that f is increasing for x < 0 and for 1 < x < 2, is decreasing for 0 < x < 1 and for x > 2, and has a local minimum at x = 1. Moreover, f (x) = 6x −4 + 6 (x − 2)−4 , so that f is concave up for all x = 0, 2. Because 1 1 + lim = 0, f has a horizontal asymptote at y = 0. Finally, f has vertical asymptotes at x = 0 and x→±∞ x 2 (x − 2)2 x = 2 with 1 1 1 1 + + lim = ∞ and lim =∞ x→0− x 2 x→0+ x 2 (x − 2)2 (x − 2)2 and

1 1 + x→2− x 2 (x − 2)2 lim

=∞

1 1 + x→2+ x 2 (x − 2)2

and

lim

y

2 −2 −1

x 1

2

3

4

= ∞.

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4 73. y = 2 1 1 y x= −29− x (x − 2)2 4 8x . Then f (x) = − SOLUTION Let f (x) = 2 , so that f is increasing for x < −3 and for −3 < x < 0, 2 x2 − 9 x −9

24 x 2 + 3 is decreasing for 0 < x < 3 and for x > 3, and has a local maximum at x = 0. Moreover, f (x) = 3 , so x2 − 9 4 = 0, f that f is concave up for x < −3 and for x > 3 and is concave down for −3 < x < 3. Because lim 2 x→±∞ x − 9 has a horizontal asymptote at y = 0. Finally, f has vertical asymptotes at x = −3 and x = 3, with 4 4 lim = ∞ and lim = −∞ x→−3− x 2 − 9 x→−3+ x 2 − 9 and

4 x→3− x 2 − 9

lim

= −∞

4 x→3+ x 2 − 9

and

lim

= ∞.

y 2 1 x

−5

5

−1 −2

x12 75. y =y =2 2 (x (x −21)(x + 1)2+ 1) SOLUTION Let f (x) =

x2 (x 2 − 1)(x 2 + 1)

.

Then f (x) = −

2x(1 + x 4 ) (x − 1)2 (x + 1)2 (x 2 + 1)2

,

so that f is increasing for x < −1 and for −1 < x < 0, is decreasing for 0 < x < 1 and for x > 1, and has a local maximum at x = 0. Moreover, f (x) =

2 + 24x 4 + 6x 8 , (x − 1)3 (x + 1)3 (x 2 + 1)3

so that f is concave up for |x| > 1 and concave down for |x| < 1. Because

lim

x2

x→±∞ (x 2 − 1)(x 2 + 1)

horizontal asymptote at y = 0. Finally, f has vertical asymptotes at x = −1 and x = 1, with lim

x2

x→−1− (x 2 − 1)(x 2 + 1)

=∞

and

x2

lim

x→−1+ (x 2 − 1)(x 2 + 1)

= −∞

and lim

x2

x→1− (x 2 − 1)(x 2 + 1)

= −∞

and

lim

y 2 1 −2

x

−1

1 −1 −2

x2

x→1+ (x 2 − 1)(x 2 + 1)

2

= ∞.

= 0, f has a

S E C T I O N 4.5

Graph Sketching and Asymptotes

201

x 77. y = 1 2 y =x + 1 2 x +1 SOLUTION Let f (x) =

x x2 + 1

.

Then f (x) = (x 2 + 1)−3/2

and

f (x) =

−3x . (x 2 + 1)5/2

Thus, f is increasing for all x, is concave up for x < 0, is concave down for x > 0, and has a point of inﬂection at x = 0. Because x x lim =1 and lim = −1, x→∞ x 2 + 1 x→−∞ x 2 + 1 f has horizontal asymptotes of y = −1 and y = 1. There are no vertical asymptotes. y 1

x

−5

5 −1

Further Insights and Challenges In Exercises 78–82, we explore functions whose graphs approach a nonhorizontal line as x → ∞. A line y = ax + b is called a slant asymptote if lim ( f (x) − (ax + b)) = 0

x→∞

or lim ( f (x) − (ax + b)) = 0.

x→−∞

If f (x) =x 2P(x)/Q(x), where P and Q are polynomials of degrees m + 1 and m, then by long division, we 79. . Verify the following: Let f (x) = can write x −1 (a) f is concave down on (−∞, 1) andf concave up+onb)(1, as in Figure 24. (x) = (ax + ∞) P (x)/Q(x) 1

where P1 is a polynomial of degree < m. Show that y = ax + b is the slant asymptote of f (x). Use this procedure to ﬁnd the slant asymptotes of the functions: (b) f (0)2 is a local max and f (2) a local min. x x3 + x (a) y(c)= lim f (x) = −∞ and lim f (x) = ∞. (b) y = 2 x +2 x +x +1 x→1− x→1+ (d) y = x + 1 is a slant asymptote of f (x) as x → ±∞. SOLUTION Since deg(P1 ) < deg(Q), (e) The slant asymptote lies above the graph of f (x) for x < 1 and below the graph for x > 1. P1 (x) lim = 0. x→±∞ Q(x) Thus lim ( f (x) − (ax + b)) = 0

x→±∞

and y = ax + b is a slant asymptote of f . x2 4 x2 (a) = x −2+ ; hence y = x − 2 is a slant asymptote of . x +2 x +2 x +2 3 x +x x +1 x3 + x (b) 2 = (x − 1) + 2 ; hence, y = x − 1 is a slant asymptote of 2 . x +x +1 x −1 x +x +1 81. Show that y = 3x is a slant asymptote for f (x) = 3x + x −2 . Determine whether f (x) approaches the slant asympx2 tote from abovethe or graph belowof andf (x) make of the graph. . Proceed as in the previous exercise to ﬁnd the slant asymptote. Sketch = a sketch x +1

202

CHAPTER 4

A P P L I C AT I O N S O F T H E D E R I VATI V E SOLUTION

Let f (x) = 3x + x −2 . Then lim ( f (x) − 3x) =

x→±∞

lim (3x + x −2 − 3x) =

x→±∞

lim x −2 = 0

x→±∞

which implies that 3x is the slant asymptote of f (x). Since f (x) − 3x = x −2 > 0 as x → ±∞, f (x) approaches the slant asymptote from above in both directions. Moreover, f (x) = 3 − 2x −3 and f (x) = 6x −4 . Sign analyses reveal

−1/3 a local minimum at x = 32 ≈ .87358 and that f is concave up for all x = 0. Limit analyses give a vertical asymptote at x = 0. y 5 −4

x

−2

2

4

−5

83. Assume that f (x) and f (x) exist for all x and let c be a critical point of f (x). Show that f (x) cannot make a − x 2Apply the MVT to f (x). transition from the ++graph to −+ x ==c. 1Hint: Sketch ofatf (x) . 2−x SOLUTION Let f (x) be a function such that f (x) > 0 for all x and such that it transitions from ++ to −+ at a critical point c where f (c) is deﬁned. That is, f (c) = 0, f (x) > 0 for x < c and f (x) < 0 for x > c. Let g(x) = f (x). The previous statements indicate that g(c) = 0, g(x 0 ) > 0 for some x 0 < c, and g(x1 ) < 0 for some x 1 > c. By the Mean Value Theorem, g(x 1 ) − g(x 0 ) = g (c0 ), x1 − x0 for some c0 between x 0 and x1 . Because x1 > c > x 0 and g(x 1 ) < 0 < g(x 0 ), g(x 1 ) − g(x 0 ) < 0. x1 − x0 But, on the other hand g (c0 ) = f (c0 ) > 0, so there is a contradiction. This means that our assumption of the existence of such a function f (x) must be in error, so no function can transition from ++ to −+. If we drop the requirement that f (c) exist, such a function can be found. The following is a graph of f (x) = −x 2/3 . f (x) > 0 wherever f (x) is deﬁned, and f (x) transitions from positive to negative at x = 0. y −1

−0.5

x 0.5

1

−0.8

Assume that f (x) exists and f (x) > 0 for all x. Show that f (x) cannot be negative for all x. Hint: Show f (b) = 0 for some b and use the result of Exercise 49 in Section 4.4.

4.6 Applied Optimization Preliminary Questions

1. The problem is to ﬁnd the right triangle of perimeter 10 whose area is as large as possible. What is the constraint equation relating the base b and height h of the triangle? The perimeter of a right triangle is the sum of the lengths of the base, the height and the hypotenuse. If the 2 2 base has length b and the height is h, then the length of the hypotenuse is b + h and the perimeter of the triangle is 2 2 P = b + h + b + h . The requirement that the perimeter be 10 translates to the constraint equation b + h + b2 + h 2 = 10. SOLUTION

2. What are the relevant variables if the problem is to ﬁnd a right circular cone of surface area 20 and maximum volume?

S E C T I O N 4.6 SOLUTION

Applied Optimization

203

The relevant variables are the radius r and the height h of the cone. We are asked to maximize the volume V =

1 2 πr h 3

subject to the constraint S = π r r 2 + h 2 + π r 2 = 20. 3. Does a continuous function on an open interval always have a maximum value? SOLUTION No, it is possible for a continuous function on an open interval to have no maximum value. As an example, consider the function f (x) = x −1 on the open interval (0, 1). Because f is a decreasing function on (0, 1) and

lim f (x) = lim

1

x→0+ x

x→0+

= ∞,

it follows that f does not have a maximum value on (0, 1). 4. Describe a way of showing that a continuous function on an open interval (a, b) has a minimum value. If the function tends to inﬁnity at the endpoints of the interval, then the function must take on a minimum value at a critical point. SOLUTION

Exercises 1. (a) (b) (c) (d)

Find the dimensions of the rectangle of maximum area that can be formed from a 50-in. piece of wire. What is the constraint equation relating the lengths x and y of the sides? Find a formula for the area in terms of x alone. Does this problem require optimization over an open interval or a closed interval? Solve the optimization problem.

SOLUTION

(a) The perimeter of the rectangle is 50 inches, so 50 = 2x + 2y, which is equivalent to y = 25 − x. (b) Using part (a), A = x y = x(25 − x) = 25x − x 2 . (c) This problem requires optimization over the closed interval [0, 25], since both x and y must be non-negative. 25 (d) A (x) = 25 − 2x = 0, which yields x = 25 2 and consequently, y = 2 . Because A(0) = A(25) = 0 and 25 25 2 A( 2 ) = 156.25, the maximum area 156.25 in is achieved with x = y = 2 inches. 3. Find the positive number x such that the sum of x and its reciprocal is as small as possible. Does this problem require A 100-in. piece of wire is divided into two pieces and each piece is bent into a square. How should this be done optimization over an open interval or a closed interval? in order to minimize the sum of the areas of the two squares? SOLUTION Let x > 0 and f (x) = x + x −1 . Here we require optimization over the open interval (0, ∞). Solve (a) Express the sum of the areas of the squares in terms of the lengths x and y of the two pieces. f (x) = 1 − x −2 = 0 for x > 0 to obtain x = 1. Since f (x) → ∞ as x → 0+ and as x → ∞, we conclude that f has (b) What is the constraint equation relating x and y? an absolute minimum of f (1) = 2 at x = 1. (c) Does this problem require optimization over an open or closed interval? 5. Find positive numbers x, y such that x y = 16 and x + y is as small as possible. legs a right triangle have lengths a and b satisfying a + b = 10. Which values of a and b maximize the (d) The Solve theofoptimization problem. −1 SOLUTION x, y > 0. Now x y = 16 implies y = 16 area of theLet triangle? x . Let f (x) = x + y = x + 16x . Solve f (x) = −2 = 0 for x > 0 to obtain x = 4 and, consequently, y = 4. Since f (x) → ∞ as x → 0+ and as x → ∞, we 1 − 16x conclude that f has an absolute minimum of f (4) = 8 at x = y = 4. 7. (a) (b)

Let S be the set of all rectangles with area 100. A 20-in. piece of wire is bent into an L-shape. Where should the bend be made to minimize the distance between What areends? the dimensions of the rectangle in S with the least perimeter? the two Is there a rectangle in S with the greatest perimeter? Explain.

SOLUTION

Consider the set of all rectangles with area 10.

(a) Let x, y > 0 be the lengths of the sides. Now x y = 100, so that y = 100/x. Let p(x) = 2x + 2y = 2x + 200x −1 be the perimeter. Solve p (x) = 2 − 200x −2 = 0 for x > 0 to obtain x = 10. Since p(x) → ∞ as x → 0+ and as x → ∞, the least perimeter is p (10) = 40 when x = 10 and y = 10. (b) There is no rectangle in this set with greatest perimeter. For as x → 0+ or as x → ∞, we have p(x) = 2x + 200x −1 → ∞. 9. Suppose that 600 ft of fencing are used to enclose a corral in the shape of a rectangle with a semicircle whose A box has a square base of side x and height y. diameter is a side of the rectangle as in Figure 10. Find the dimensions of the corral with maximum area. (a) Find the dimensions x, y for which the volume is 12 and the surface area is as small as possible. (b) Find the dimensions for which the surface area is 20 and the volume is as large as possible.

FIGURE 10

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A P P L I C AT I O N S O F T H E D E R I VATI V E SOLUTION Let x be the width of the corral and therefore the diameter of the semicircle, and let y be the height of the rectangular section. Then the perimeter of the corral can be expressed by the equation 2y + x + π2 x = 2y + (1 + π )x = 600 ft or equivalently, y = 1 600 − (1 + π )x . Since x and y must both be nonnegative, it follows that x must 2 2 2 600 ]. The area of the corral is the sum of the area of the rectangle and semicircle, be restricted to the interval [0, 1+ π /2

A = x y + π8 x 2 . Making the substitution for y from the constraint equation, A(x) =

1

1

π π π 2 π 2 x 600 − (1 + )x + x 2 = 300x − 1+ x + x . 2 2 8 2 2 8

Now, A (x) = 300 − 1 + π2 x + π4 x = 0 implies x = 300π ≈ 168.029746 feet. With A(0) = 0 ft2 , 1+ 4 600 300 and A ≈ 25204.5 ft2 ≈ 21390.8 ft2 , A 1 + π /4 1 + π /2 it follows that the corral of maximum area has dimensions x=

300 ft 1 + π /4

y=

and

150 ft. 1 + π /4

11. A landscape architect wishes to enclose a rectangular garden on one side by a brick wall costing $30/ft and on the Find the rectangle of maximum area that can be inscribed in a right triangle with legs of length 3 and 4 if the other three sides by a metal fence costing $10/ft. If the area of the garden is 1,000 ft2 , ﬁnd the dimensions of the garden sides of the rectangle are parallel to the legs of the triangle, as in Figure 11. that minimize the cost. SOLUTION

Let x be the length of the brick wall and y the length of an adjacent side with x, y > 0. With x y = 1000

or y = 1000 x , the total cost is C(x) = 30x + 10 (x + 2y) = 40x + 20000x −1 . √ −2 = 0 for x > 0 to obtain x = 10 5. Since C(x) → ∞ as x → 0+ and as x → ∞, the Solve C (x) = 40 − 20000x √ √ √ √ minimum cost is C(10 5) = 800 5 ≈ $1788.85 when x = 10 5 ≈ 22.36 ft and y = 20 5 ≈ 44.72 ft. 13. FindFind the the point P on x 2 closest the point point onthe theparabola line y = yx = closest to thetopoint (1, 0).(3, 0) (Figure 12). y

y = x2 P x 3

FIGURE 12 SOLUTION

With y = x 2 , let’s equivalently minimize the square of the distance, f (x) = (x − 3)2 + y 2 = x 4 + x 2 − 6x + 9.

Then f (x) = 4x 3 + 2x − 6 = 2(x − 1)(2x 2 + 2x + 3), so that f (x) = 0 when x = 1 (plus two complex solutions, which we discard). Since f (x) → ∞ as x → ±∞, P = (1, 1) is the point on y = x 2 closest to (3, 0). 15. Find the dimensions of the rectangle of maximum area that can be inscribed in a circle of radius r (Figure 13). A box is constructed out of two different types of metal. The metal for the top and bottom, which are both square, costs $1/ft2 and the metal for the sides costs $2/ft2 . Find the dimensions that minimize cost if the box has a volume of 20 ft3 . r

FIGURE 13

Place the center of the circle at the origin with the sides of the rectangle (of lengths 2x > 0 and 2y > 0) parallel to the coordinate axes. By the Pythagorean Theorem, x 2 + y 2 = r 2 , so that y = r 2 − x 2 . Thus the area of the rectangle is A(x) = 2x · 2y = 4x r 2 − x 2 . To guarantee both x and y are real and nonnegative, we must restrict x to the interval [0, r ]. Solve SOLUTION

4x 2 A (x) = 4 r 2 − x 2 − =0 r2 − x2

S E C T I O N 4.6

Applied Optimization

205

√ for x > 0 to obtain x = √r . Since A(0) = A(r ) = 0 and A(r/ 2) = 2r 2 , the rectangle of maximum area has dimensions x = y = √r .

2

2

17. Find the angle θ that maximizes the area of the isosceles triangle whose legs have length (Figure 14). Problem of Tartaglia (1500–1557) Among all positive numbers a, b whose sum is 8, ﬁnd those for which the product of the two numbers and their difference is largest. Hint: Let x = a − b and express abx in terms of x alone. q

FIGURE 14 SOLUTION

The area of the triangle is A(θ ) =

1 2 sin θ , 2

where 0 ≤ θ ≤ π . Setting A (θ ) =

1 2 cos θ = 0 2

yields θ = π2 . Since A(0) = A(π ) = 0 and A( π2 ) = 12 2 , the angle that maximizes the area of the isosceles triangle is θ = π2 . 19. Rice production requires both labor and capital investment in equipment and land. Suppose that if x dollars per acre 2 + h 2 . Find the dimensions of the r 2 h andinitsequipment surface area S = πthen r rthe The volume a right circular is π3invested are invested in laborof and y√dollars per cone acre are andis land, yield P of rice per acre is given √ cone with surface area 1x and maximal by the formula P = 100 + 150 y. If volume a farmer(Figure invests15). $40/acre, how should he divide the $40 between labor and capital investment in order to maximize the amount of rice produced? √ √ √ 50 √ SOLUTION Since x + y = 40, we have P(x) = 100 x + 150 y = 100 x + 150 40 − x. Solve P (x) = √ − x √ √ 75 160 = 0 to obtain x = 13 ≈ $12.31. Since P(0) = 300 10 ≈ 948.68 and P(40) = 200 10 ≈ 632.46, we have √ 40 − x

√ √ 160 360 that the maximum yield is P 160 13 = 100 13 10 ≈ 1140.18 when x = 13 ≈ $12.31 and y = 13 ≈ $27.69. 21. Find the dimensions x and y of the rectangle inscribed in a circle of radius r that maximizes the quantity x y 2 . Kepler’s Wine Barrel Problem The following problem was stated and solved in the work Nova stereometria SOLUTION the center the circle of radius r at the originpublished with the sides of the rectangle (of lengths x >Kepler 0 and doliorum Place vinariorum (NewofSolid Geometry of a Wine Barrel), in 1615 by the astronomer Johannes y inscribed x )2can 2 = r 2 , whence 2 = 4r 2 radius What are the dimensions the cylinderTheorem, of largestwe volume be in the sphere of y > (1571–1630). 0) parallel to the coordinate axes. By theofPythagorean have (that + ( ) y − x 2. 2 2 − x22 ), where 2 2 3 2 2 π x(R x is one-half the height of the R? Hint: Show that the volume of an inscribed cylinder is 2 for Let f (x) = x y = 4xr − x . Allowing for degenerate rectangles, we have 0 ≤ x ≤ 2r . Solve √ f 3(x) = 4r − 3x 2r 2r 16 2r cylinder. x ≥ 0 to obtain x = √ . Since f (0) = f (2r ) = 0, the maximal value of f is f ( √ ) = 9 3r when x = √ and 3 3 3

2 y = 2 3r. 23. Consider a rectangular industrial warehouse consisting of three separate spaces of equal size as in Figure 17. Assume Find the angle θ that maximizes the area of the trapezoid with a base of length 4 and sides of length 2, as in that the wall materials cost $200 per linear ft and the company allocates $2,400,000 for the project. Figure 16. (a) Which dimensions maximize the total area of the warehouse? (b) What is the area of each compartment in this case?

FIGURE 17

Let the dimensions of one compartment be height x and width y. Then the perimeter of the warehouse is given by P = 4x + 6y and the constraint equation is cost = 2,400,000 = 200(4x + 6y), which gives y = 2000 − 23 x.

(a) Area is given by A = 3x y = 3x 2000 − 23 x = 6000x − 2x 2 , where 0 ≤ x ≤ 3000. Then A (x) = 6000 − 4x = 0 yields x = 1500 and consequently y = 1000. Since A(0) = A(3000) = 0 and A(1500) = 4, 500, 000, the area of the warehouse is maximized with a height of 1500 feet and a total width of 3000 feet. (b) The area of one compartment is 1500 · 1000 = 1, 500, 000 square feet. SOLUTION

2 . Suppose that 25. TheSuppose, amount of lightprevious reachingexercise, a point that at a the distance r fromconsists a lightof source A of intensity A is Isize. A /r Find in the warehouse n separate spaces of Iequal a formula a second lightofsource B of intensitypossible I B = 4Iarea located 10 ft from A. Find the point on the segment joining A and B A isof in terms n for the maximum the warehouse. where the total amount of light is at a minimum.

206

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A P P L I C AT I O N S O F T H E D E R I VATI V E SOLUTION Place the segment in the x y-plane with A at the origin and B at (10, 0). Let x be the distance from A. Then 10 − x is the distance from B. The total amount of light is IA IB 4 1 f (x) = 2 + = I + . A x x2 (10 − x)2 (10 − x)2

Solve f (x) = I A

2 − 3 3 x (10 − x) 8

=0

for 0 ≤ x ≤ 10 to obtain (10 − x)3 = 4= x3

3 10 −1 x

or

x=

10 √ ≈ 3.86 ft. 1+ 3 4

Since f (x) → ∞ as x → 0+ and x → 10− we conclude that the minimal amount of light occurs 3.86 feet from A. 27. According to postal regulations, a carton is classiﬁed as “oversized” if the sum of its height and girth (the perimeter 4−x of its base) in.largest Find the dimensions of abecarton withinsquare base bounded that is not Findexceeds the area108 of the rectangle that can inscribed the region byoversized the graphand of yhas = maximum and 2 +x volume. the coordinate axes (Figure 18). SOLUTION Let h denote the height of the carton and s denote the side length of the square base. Clearly the volume will be maximized when the sum of the height and girth equals 108; i.e., 4s + h = 108, whence h = 108 − 4s. Allowing for degenerate cartons, the carton’s volume is V (s) = s 2 h = s 2 (108 − 4s), where 0 ≤ s ≤ 27. Solve V (s) = 216s − 12s 3 = 0 for s to obtain s = 0 or s = 18. Since V (0) = V (27) = 0, the maximum volume is V (18) = 11664 in3 when s = 18 in and h = 36 in. 29. What is the area of the largest rectangle that can be circumscribed around a rectangle of sides L and H ? Hint: Find the maximum area of a triangle formed in the ﬁrst quadrant by the x-axis, y-axis, and a tangent line to the Express the area of the circumscribed rectangle in terms of the angle θ (Figure 19). graph of y = (x + 1)−2 .

q H L

FIGURE 19 SOLUTION Position the L × H rectangle in the ﬁrst quadrant of the x y-plane with its “northwest” corner at the origin. Let θ be the angle the base of the circumscribed rectangle makes with the positive x-axis, where 0 ≤ θ ≤ π2 . Then the area of the circumscribed rectangle is A = L H + 2 · 12 (H sin θ )(H cos θ ) + 2 · 12 (L sin θ )(L cos θ ) = L H + 12 (L 2 + H 2 ) sin 2θ , which has a maximum value of L H + 12 (L 2 + H 2 ) when θ = π4 because sin 2θ achieves its maximum when θ = π4 .

31. An 8-billion-bushel corn crop brings a price of $2.40/bushel. A commodity broker uses the following rule of thumb: Optimal Priceby Let r be thethen monthly rent increases per unit inby an10x apartment building with 100results units. in A maximum survey reveals that If the crop is reduced x percent, the price cents. Which crop size revenue all units canprice be rented when rHint: = $900 and that one unit becomes with each $10 increase in rent. Suppose that and what is the per bushel? Revenue is equal to price timesvacant crop size. the average monthly maintenance per occupied unit is $100/month. SOLUTION Let x denote the percentage reduction in crop size. Then the price for corn is 2.40 + 0.10x, the crop size is (a) Show that the number of units rented is n = 190 − r/10 for 900 ≤ r ≤ 1,900. 8(1 − 0.01x) and the revenue (in billions of dollars) is (b) Find a formula for the net cash intake (revenue minus maintenance) and determine the rent r that maximizes intake. R(x) = (2.4 + .1x)8(1 − .01x) = 8(−.001x 2 + .076x + 2.4), where 0 ≤ x ≤ 100. Solve R (x) = −.002x + .076 = 0 to obtain x = 38 percent. Since R(0) = 19.2, R(38) = 30.752, and R(100) = 0, revenue is maximized when x = 38. So we reduce the crop size to 8(1 − .38) = 4.96 billion bushels. The price would be $2.40 + .10(38) = 2.40 + 3.80 = $6.20. 33. Let P = (a, b) be a point in the ﬁrst quadrant. Given n numbers x1 , . . . , x n , ﬁnd the value of x minimizing the sum of the squares: (a) Find the slope of the line through P such that the triangle bounded by this line and the axes in the ﬁrst quadrant has minimal area. (x − x 1 )2 + (x − x 2 )2 + · · · + (x − xn )2 First solve the problem for n = 2, 3 and then solve it for arbitrary n.

Applied Optimization

S E C T I O N 4.6

207

(b) Show that P is the midpoint of the hypotenuse of this triangle. SOLUTION Let P(a, b) be a point in the ﬁrst quadrant (thus a, b > 0) and y − b = m(x − a), −∞ < m < 0, be a line through P that cuts the positive x- and y-axes. Then y = L(x) = m(x − a) + b.

b , 0 . Hence the area of the triangle (a) The line L(x) intersects the y-axis at H (0, b − am) and the x-axis at W a − m is 1 b 1 1 = ab − a 2 m − b2 m −1 . A(m) = (b − am) a − 2 m 2 2

Solve A (m) = 12 b2 m −2 − 12 a 2 = 0 for m < 0 to obtain m = − ab . Since A → ∞ as m → −∞ or m → 0−, we conclude that the minimal triangular area is obtained when m = − ab . (b) For m = −b/a, we have H (0, 2b) and W (2a, 0). The midpoint of the line segment connecting H and W is thus P(a, b). 35. Figure 20 shows a rectangular plot of size 100 × 200 feet. Pipe is to be laid from A to a point P on side BC and A truck gets 10 mpg (miles per gallon) traveling along an interstate highway at 50 mph, and this is reduced by from there to C. The cost of laying pipe through the lot is $30/ft (since it must be underground) and the cost along the 0.15 mpg for each mile per hour increase above 50 mph. side of the plot is $15/ft. (a) If the truck driver is paid $30/hour and diesel fuel costs P = $3/gal, which speed v between 50 and 70 mph (a) Let f (x) be the total cost, where x is the distance from P to B. Determine f (x), but note that f is discontinuous at will minimize the cost of a trip along the highway? Notice that the actual cost depends on the length of the trip but x = 0 (when x = 0, the cost of the entire pipe is $15/ft). the optimal speed does not. (b) What is the most economical way to lay the pipe? What if the cost along the sides is $24/ft? (b) Plot cost as a function of v (choose the length arbitrarily) and verify your answer to part (a). (c) Do you expect the optimal speed v to Bincrease or decrease P C if fuel costs go down to P = $2/gal? Plot the 200same − x axis and verify your conclusion. graphs of cost as a function of v for P = 2 and P =x 3 on the 100 A

200

FIGURE 20 SOLUTION

(a) Let x be the distance from P to B. If x > 0, then the length of the underground pipe is of the pipe along the side of the plot is 200 − x. The total cost is f (x) = 30 1002 + x 2 + 15(200 − x).

1002 + x 2 and the length

If x = 0, all of the pipe is along the side of the plot and f (0) = 15(200 + 100) = $4500. (b) To locate the critical points of f , solve 30x f (x) = − 15 = 0. 1002 + x 2

√ We ﬁnd√x = ±100/ 3. Note that only the positive value is in the domain of the problem. Because f (0) = $4500, f (100/ 3) = $5598.08 and f (200) = $6708.20, the most economical way to lay the pipe is to place the pipe along the side of the plot. If the cost of laying the pipe along the side of the plot is $24 per foot, then f (x) = 30 1002 + x 2 + 24(200 − x) and 30x f (x) = − 24. 1002 + x 2 The only critical point in the domain of the problem is x = 400/3. Because f (0) = $7200, f (400/3) = $6600 and f (200) = $6708.20, the most economical way to lay the pipe is place the underground pipe from A to a point 133.3 feet to the right of B and continuing to C along the side of the plot. 37. In Example 6 in this section, ﬁnd the x-coordinate of the point P where the light beam strikes the mirror if h 1 = 10, the=dimensions of a cylinder of volume 1 m3 of minimal cost if the top and bottom are made of material that h 2 = 5,Find and L 20. costs twice as much as the material for the side. SOLUTION Substitute h 1 = 10 feet, h 2 = 5 feet, and L = 20 feet into

L−x = x 2 + h 21 (L − x)2 + h 22 x

Solving for x gives x = 40 3 feet.

to obtain

x x 2 + 102

=

20 − x (20 − x)2 + 52

.

208

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A P P L I C AT I O N S O F T H E D E R I VATI V E

In Exercises 38–40, a box (with no top) is to be constructed from a piece of cardboard of sides A and B by cutting out squares of length h from the corners and folding up the sides (Figure 21).

B h A

FIGURE 21

39. Which value of h maximizes the volume if A = B? Find the value of h that maximizes the volume of the box if A = 15 and B = 24. What are the dimensions of the SOLUTION When A = B, the volume of the box is resulting box? V (h) = hx y = h ( A − 2h)2 = 4h 3 − 4 Ah 2 + A2 h, A 2 2 where 0 ≤ h ≤ A 2 (allowing for degenerate boxes). Solve V (h) = 12h − 8 Ah + A = 0 for h to obtain h = 2 or A A A 2 A 3 h = 6 . Because V (0) = V ( 2 ) = 0 and V ( 6 ) = 27 A , volume is maximized when h = 6 .

41. The monthly output P of a light bulb factory is given by the formula P = 350L K , where L is the amount invested cardboard ABto=produce 144). Which that a box invested of heightinh equipment = 3 in. is constructed using 144 in.2 Ifofthe in laborSuppose and K the amount (in thousands of dollars). company(i.e., needs 10,000values units A and Bhow maximize the investment volume? be divided among labor and equipment to minimize the cost of production? The per month, should the cost of production is L + K . 200 . Accordingly, the cost of production is SOLUTION Since P = 10000 and P = 350L K , we have L = 7K C(K ) = L + K = K + 200 for K ≥ 0 to obtain K = 7K 2 minimum cost of production is achieved for K = L = Solve C (K ) = 1 −

200 . 7K

√

10 14. Since C(K ) → ∞ as K → 0+ and as K → ∞, the 7 √ 10 14 ≈ 5.345 or $5435 invested in both labor and equipment. 7

43. Janice can swim 3 mph and run 8 mph. She is standing at one bank of a river that is 300 ft wide and wants to reach Use calculus to show that among right trianglesaswith hypotenuse ofswim length 1, the isosceles triangle has a point located 200 ft downstream on the otherallside as quickly possible. She will diagonally across the river and maximum area. Can you see more directly why this must be true by reasoning from Figure 22? then jog along the river bank. Find the best route for Janice to take. Let lengths be in feet, times in seconds, and speeds in ft/s. Let x be the distance from the point directly opposite Janice on the other shore to the point where she starts to run after having swum. Then x 2 + 3002 is the 176 distance she swims and 200 − x is the distance she runs. Janice swims at 22 5 ft/s (i.e., 3 mph) and runs at 15 ft/s (i.e., 8 mph). The total time she travels is 375 200 − x 5 2 15 x 2 + 3002 f (x) = x + 90000 + + = − x, 0 ≤ x ≤ 200. 22/5 176/15 22 22 176 SOLUTION

Solve 5 x 15 =0 − 22 x 2 + 90000 176 √ √ 4875 750 to obtain x = 180 11 55. Since f (0) = 22 ≈ 221.59 and f (200) = 11 13 ≈ 245.83, weconclude that the minimum √ √ √ 375 375 480 2 2 amount of time f 180 11 55 = 44 55 + 22 ≈ 80.35 s occurs when Janice swims x + 300 = 11 55 ≈ √ 323.62 ft and runs 200 − x = 200 − 180 11 55 ≈ 78.64 ft. f (x) =

45. (a) Find the radius and height of a cylindrical can of total surface area A whose volume is as large as possible. Delivery Schedule gas station gallons total of gasoline (b) CanOptimal you design a cylinder with totalAsurface area sells A andQminimal volume?per year, which is delivered N times per year in equal shipments of Q/N gallons. The cost of each delivery is d dollars and the yearly storage costs are SOLUTION Let a closed cylindrical can be of radius r and height h. s QT , where T is the length shipments and s is a constant. Show that costs √ of time (a fraction of a year) between A .) Find the optimal number of deliveries are minimized for N = s Q/d. (Hint: Express T in terms of N 2 2 (a) Its total surface area is S = 2π r + 2π r h = A, whence h = − r. Its volume is thus V (r ) = π r 2 h =if 12QAr= − r million gal, d = $8,000, and s = 30 cents/gal-yr. Your answer2πshould be a whole number, so compare costs for the

A integer nearest the) = optimal A .NSolve 1 A −value. A (r 2 for r > 0 to obtain r = π r 3 ,two where 0 < values r ≤ of V 3 π r . Since V (0) = V ( 2π 2 2π ) = 0 and 6π √ A 6 A3/2 = V √ , 6π 18 π the maximum volume is achieved when

r=

A 6π

and

1 h= 3

6A . π

Applied Optimization

S E C T I O N 4.6

209

(b) For a can of total surface area A, there are cans of arbitrarily small volume since lim V (r ) = 0. r →0+

47. A billboard of height b is mounted on the side of a building with its bottom edge at a distance h from the street. At Find the area of the largest isosceles triangle that can be inscribed in a circle of radius r . what distance x should an observer stand from the wall to maximize the angle of observation θ (Figure 23)? (a) Find x using calculus. You may wish to use the addition formula cot(a − b) =

1 + cot a cot b . cot b − cot a

(b) Solve the problem again using geometry (without any calculation!). There is a unique circle passing through points B and C which is tangent to the street. Let R be the point of tangency. Show that θ is maximized at the point R. Hint: The two angles labeled ψ are, in fact, equal because they subtend equal arcs on the circle. Let A be the intersection of the circle with PC and show that ψ = θ + P B A > θ . (c) Prove that the two answers in (a) and (b) agree.

C b

P

A

h

q x

y

B y

q

P

R

FIGURE 23 SOLUTION

(a) From the leftmost diagram in Figure 23 and the addition formula for the cotangent function, we see that 1+ x x x 2 + h(b + h) cot θ = x b+hx h = , bx h − b+h where b and h are constant. Now, differentiate with respect to x and solve − csc2 θ

x 2 − h(b + h) dθ = =0 dx bx 2

to obtain x = bh + h 2 . Since this is the only critical point, and since θ → 0 as x → 0+ and θ → 0 as x → ∞, θ (x) reaches its maximum at x = bh + h 2 . (b) Following the directions, and mindful of the diagram in Figure 23, let C be the point at the top of the painting along the wall. For every radius r , there is a unique circle of radius r through B and C with center to the front of the billboard. Only one of these is tangent to the street (all the others are either too big or too small). Draw such a circle (call it D) , as shown in the right half of Figure 23, and let R be the point where the circle touches the street. Let P be any other such point along the street. We will prove that the angle of observation θ at P is less than the angle of observation ψ at R, thus proving that R is the point with maximum angle of observation along the ﬂoor. P, C, and B form a triangle. Let A be the topmost point of intersection of the triangle and circle D as shown. Because the two angles both subtend the arc BC, the measure of angle C AB is equal to the measure of angle C R B. By supplementarity, ψ + P AB = 180◦ . Because they form a triangle, θ + P AB + P B A = 180◦ . Thus, ψ = θ + P B A > θ . (c) To show that the two answers agree, let O be the center of the circle. One observes that if d is the distance from R to the wall, then O has coordinates (−d, b2 + h). This is because the height of the center is equidistant from points B and C and because the center must lie directly above R if the circle is tangent to the ﬂoor. Now we can solve for d. The radius of the circle is clearly b2 + h, by the distance formula:

2

O B = d2 +

2 2 b b +h−h = +h 2 2

This gives d2 = or d =

2 2 b b = bh + h 2 +h − 2 2

bh + h 2 as claimed.

49. Snell’s Law, derived in Exercise 48, explains why it is impossible to see above the water if the angle of vision θ2 is 24 represents the surface of a swimming pool. A light beam travels from point A located such thatSnell’s sin θ2 Law > v2 /vFigure 1. above the pool to point B located underneath the water. Let v1 be the velocity of light in air and let v2 be the velocity (a) Show that if this inequality holds, then there is no angle θ1 for which Snell’s Law holds. of light in water (it is a fact that v1 > v2 ). Prove Snell’s Law of Refraction according to which the path from A to B that takes the least time satisﬁes the relation sin θ

sin θ

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(b) What will you see if you look through the water with an angle of vision θ2 such that sin θ2 > v2 /v1 ? SOLUTION

v sin θ1 sin θ2 v v v (a) Suppose sin θ2 > 2 . By Snell’s Law, = , whence sin θ1 = 1 sin θ2 > 1 · 2 = 1, a contradiction, v1 v1 v2 v2 v2 v1 since sin θ ≤ 1 for all real values of θ . Accordingly, there is no real value θ1 for which Snell’s Law holds. (b) The underwater observer sees what appears to be a silvery mirror due to total internal reﬂection. 51. Vascular Branching A small blood vessel of radius r branches off at an angle θ from a larger vessel of radius R to blank margins oftowidth 6 in. onLaw, the top and bottom andto4 blood in. onﬂow the sides. Find the A poster of aarea ft2 has supply blood along path6 from A to B. According Poiseuille’s the total resistance is proportional to dimensions that maximize the printed area. a − b cot θ b csc θ T = + R4 r4 where a and b are as in Figure 25. Show that the total resistance is minimized when cos θ = (r/R)4 . B r b R

q

A a

FIGURE 25

SOLUTION

a − b cot θ b csc θ + . Set R4 r4 b csc2 θ b csc θ cot θ = 0. − T (θ ) = R4 r4

With a, b, r, R > 0 and R > r , let T (θ ) =

Then

b r 4 − R 4 cos θ R 4 r 4 sin2 θ

so that cos θ =

r 4 cos θ = . R

r 4 R

. Since

lim T (θ ) = ∞ and

θ →0+

= 0,

lim T (θ ) = ∞, the minimum value of T (θ ) occurs when

θ →π −

53. Let (a, b) be a ﬁxed point in the ﬁrst quadrant and let S(d) be the sum of the distances from (d, 0) to the points Find the minimum length of a beam that can clear a fence of height h and touch a wall located b ft behind the (0, 0), (a, b), and (a, −b). √ √ fence (Figure 26). (a) Find the value of √ d for which S(d) is minimal. The answer depends on whether b < 3a or b ≥ 3a. Hint: Show that d = 0 when b ≥ 3a. √ Let a = 1. Plot S(d) for b = 0.5, 3, 3 and describe the position of the minimum. (b) SOLUTION

(a) If d < 0, then the distance from (d, 0) to the other three points can all be reduced by increasing the value of d. Similarly, if d > a, then the distance from (d, 0) to the other three points can all be reduced by decreasing the value of d. It follows that the minimum of S(d) must occur for 0 ≤ d ≤ a. Restricting attention to this interval, we ﬁnd

S(d) = d + 2 (d − a)2 + b2 . Solving 2(d − a) S (d) = 1 + =0 (d − a)2 + b2 √ √ √ yields the critical point d = √ a − b/ 3. If b < 3a, then d = a − b/ 3 > 0 and the minimum occurs at this value of d. On the other hand, if b ≥ 3a, then the minimum occurs at the endpoint d = 0. √ (b) Let a = 1. Plots of S(d) for b = 0.5,√ b = 3 and b = 3 are shown below. For b = 0.5, the results√of (a) indicate the minimum should occur for d = 1 − 0.5/ 3 ≈ 0.711, and this is conﬁrmed in the plot. For both b = 3 and b = 3, the results of (a) indicate that the minimum should occur at d = 0, and both of these conclusions are conﬁrmed in the plots.

S E C T I O N 4.6 y

Applied Optimization

211

y b = 0.5

2.1 2 1.9 1.8 1.7 1.6 1.5

x 0

0.2 0.4 0.6 0.8

b = √3

4.4 4.3 4.2 4.1 4

1

x 0

0.2 0.4 0.6 0.8

1

y b=3 6.8 6.6 6.4 x 0

0.2 0.4 0.6 0.8

1

55. In the of Exercise 54, show any f the minimal force required is proportional to 1/ 1 to + f 2. Thesetting minimum force required to that drivefor a wedge of angle α into a block (Figure 27) is proportional k SOLUTION Let F(α ) = , where k > 0 is a proportionality constant and 0 ≤ α ≤ π2 . Solve 1 sin α + f cos α F(α ) = sin α + f cos α k f sin α − cos α ) ( (α ) = = 0 is required, assuming f = 0.4. where f is a positive constant. Find theFangle α for which the least2 force (sin α + f cos α ) for 0 ≤ α ≤ π2 to obtain tan α = 1f . From the diagram below, we note that when tan α = 1f , 1 sin α = 1+ f2

f cos α = . 1+ f2

and

Therefore, at the critical point the force is k 2 √1 +√f 2 1+ f 2 1+ f

k = . 1+ f2

Since F(0) = 1f k > √ k 2 and F( π2 ) = k > √ k 2 , we conclude that the minimum force is proportional to 1+ f 1+ f 1/ 1 + f 2 .

√1 + f 2 1

α f

57. Find the maximum length of a pole that can be carried horizontally around a corner joining corridors of widths 8 ft The problem is to put a “roof” of side s on an attic room of height h and width b. Find the smallest length s for and 4 ft (Figure 29). which this is possible. See Figure 28. 4

8

FIGURE 29 SOLUTION In order to ﬁnd the length of the longest pole that can be carried around the corridor, we have to ﬁnd the shortest length from the left wall to the top wall touching the corner of the inside wall. Any pole that does not ﬁt in this shortest space cannot be carried around the corner, so an exact ﬁt represents the longest possible pole. Let θ be the angle between the pole and a horizontal line to the right. Let c1 be the length of pole in the 8 ft corridor and let c2 be the length of pole in the 4 foot corridor. By the deﬁnitions of sine and cosine,

4 = sin θ c2

and

8 = cos θ , c1

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so that c1 = cos8 θ , c2 = sin4 θ . What must be minimized is the total length, given by f (θ ) =

8 4 + . cos θ sin θ

Setting f (θ ) = 0 yields 4 cos θ 8 sin θ − =0 cos2 θ sin2 θ 8 sin θ 4 cos θ = cos2 θ sin2 θ 8 sin3 θ = 4 cos3 θ As θ < π2 (the pole is being turned around a corner, after all), we can divide both sides by cos3 θ , getting tan3 θ = 12 .

This implies that tan θ = 12

1/3

(tan θ > 0 as the angle is acute).

1/3 Since f (θ ) → ∞ as θ → 0+ and as θ → π2 −, we can tell that the minimum is attained at θ0 where tan θ0 = 12 . Because tan θ0 =

opposite 1 = 1/3 , adjacent 2

we draw a triangle with opposite side 1 and adjacent side 21/3 .

c 1 q 21/3

By Pythagoras, c =

1 + 22/3 , so 1 sin θ0 = 1 + 22/3

and

21/3 cos θ0 = . 1 + 22/3

From this, we get f (θ0 ) =

8 4 8 + = 1/3 1 + 22/3 + 4 1 + 22/3 = 4 1 + 22/3 22/3 + 1 . cos θ0 sin θ0 2

59. Find the isosceles triangle of smallest area that circumscribes a circle of radius 1 (from Thomas Simpson’s The Redo Exercise 57 for corridors of arbitrary widths a and b. Doctrine and Application of Fluxions, a calculus text that appeared in 1750). See Figure 30.

q

1

FIGURE 30 SOLUTION From the diagram, we see that the height h and base b of the triangle are h = 1 + csc θ and b = 2h tan θ = 2(1 + csc θ ) tan θ . Thus, the area of the triangle is

A(θ ) =

1 hb = (1 + csc θ )2 tan θ , 2

where 0 < θ < π . We now set the derivative equal to zero: A (θ ) = (1 + csc θ )(−2 csc θ + sec2 θ (1 + csc θ )) = 0. The ﬁrst factor gives θ = 3π /2 which is not in the domain of the problem. To ﬁnd the roots of the second factor, multiply through by cos2 θ sin θ to obtain −2 cos2 θ + sin θ + 1 = 0,

S E C T I O N 4.6

Applied Optimization

213

or 2 sin2 θ + sin θ − 1 = 0. This is a quadratic equation in sin θ with roots sin θ = −1 and sin θ = 1/2. Only the second solution is relevant and gives us θ = π /6. Since A(θ ) → ∞ as θ → 0+ and as θ → π −, we see that the minimum area occurs when the triangle is an equilateral triangle. basketballand player stands d feet from the basket. Let h and α be as in Figure 31. Using physics, one can show Further AInsights Challenges

that if the player releases the ball at an angle θ , then the initial velocity required to make the ball go through the 61. Bird Migration Power P is the rate at which energy E is consumed per unit time. Ornithologists have found that basket satisﬁes the power consumed by a certain pigeon ﬂying at velocity v m/s is described well by the function P(v) = 17v −1 + 4 16denergy as body fat. 2 10 10−3 v 3 J/s. Assume that the pigeon can store 5v× usable = J of . 2 θ (tan θ − tan α ) cos (a) Find the velocity vpmin that minimizes power consumption. (b) Show that a why pigeon at velocity v and usingfor allαof

(e) For a given α , the optimal angle for shooting the basket is θ0 because it minimizes v 2 and therefore minimizes Minimum to power Power (J/s) the energy required to make the shot (energy is proportional v 2 ). Show that the velocity vopt at the optimal angle consumption θ0 satisﬁes 4 Maximum

2 = vopt

32d cos α 32 d2 distance = . traveled 1 − sin α −h + d 2 + h 2 5

10

15

Velocity (m/s) 2 is an increasing function (f) Show with a graph that for ﬁxed d (say, d = 15 ft, the distance of a free throw), vopt FIGURE 32 and why it can help to jump while shooting. of h. Use this to explain why taller players have an advantage

SOLUTION

17 1/4 ≈ 8.676247. (a) Let P(v) = 17v −1 + 10−3 v 3 . Then P (v) = −17v −2 + 0.003v 2 = 0 implies vpmin = .003 This critical point is a minimum, because it is the only critical point and P(v) → ∞ both as v → 0+ and as v → ∞. 5 · 104 joules 5 · 104 sec (b) Flying at a velocity v, the birds will exhaust their energy store after T = = . The total P(v) joules/sec P(v) 4 5 · 10 v . distance traveled is then D(v) = vT = P(v) P(v) · 5 · 104 − 5 · 104 v · P (v) P(v) − v P (v) = 5 · 104 = 0 implies P(v) − v P (v) = 0, or P (v) = (c) D (v) = 2 (P(v)) (P(v))2 P(v) . Since D(v) → 0 as v → 0 and as v → ∞, the critical point determined by P (v) = P(v)/v corresponds to a v maximum. Graphically, the expression P(v) P(v) − 0 = v v−0 is the slope of the line passing through the origin and (v, P(v)). The condition P (v) = P(v)/v which deﬁnes vdmax therefore indicates that vdmax is the velocity component of the point where a line through the origin is tangent to the graph of P(v). P(v) (d) Using P (v) = gives v −17v −2 + .003v 2 =

17v −1 + .001v 3 = 17v −2 + .001v 2 , v

which simpliﬁes to .002v 4 = 34 and thus vdmax ≈ 11.418583. The maximum total distance is given by D(vdmax ) = 5 · 104 · vdmax = 191.741 kilometers. P(vdmax )

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Seismic Prospecting Exercises 62–64 are concerned with determining the thickness d of a layer of soil that lies on top of a rock formation. Geologists send two sound pulses from point A to point D separated by a distance s. The ﬁrst pulse travels directly from A to D along the surface of the earth. The second pulse travels down to the rock formation, then along its surface, and then back up to D (path ABCD), as in Figure 33. The pulse travels with velocity v1 in the soil and v2 in the rock. s

A q

D q

Soil B

d

C

Rock

FIGURE 33

63. In this exercise, assume that v2 /v1 ≥ 1 + 4(d/s)2 . (a) Show that the time required for the ﬁrst pulse to travel from A to D is t1 = s/v1 . (a) Show that inequality (1) holds if sin θ = v1 /v2 . (b) Show that the time required for the second pulse is (b) Show that the minimal time for the second pulse is 2d s − 2d tan θ = 2d (1 sec θk 2+)1/2 + s − tt22 = vv2 vv1 1

2

provided that where k = v1 /v 2.

t 2d(1 − k 2 )1/2 (c) Conclude that 2 = + k. t1 s

tan θ ≤

s 2d

SOLUTION

(Note: If this inequality is not satisﬁed, then point B does not lie to the left of C.) v1 (a) If θ = that v2 , then (c)sinShow t2 is minimized when sin θ = v1 /v2 . v 1 . = tan θ = 1 2 2 v2 2 v2 − v1 −1 v1

Because vv2 ≥ 1 + 4( ds )2 , it follows that 1

v2 2 −1≥ v1

1+4

2 d 2d −1= . s s

s as required. Hence, tan θ ≤ 2d v v v and tan θ = 1 . (b) For the time-minimizing choice of θ , we have sin θ = 1 from which sec θ = 2 v2 2 2 2 v2 − v1 v2 − v12 Thus

2d s − 2d tan θ 2d v

2 sec θ + = + t2 = v1 v2 v1 v 2 − v 2 2 1 ⎛ ⎞ v12 v2 2d ⎝ ⎠+ s

= − v1 v2 v2 − v2 v v2 − v2 2

2

1

2

s − 2d v1

v22 −v12

v2

1

⎛ ⎞ 2 2 s s 2d ⎝ v22 − v12 ⎠ 2d ⎝ v2 − v1 ⎠

+ + = = v1 v v 2 − v 2 v2 v1 v 2 2 v2 2 2 1 ⎛

⎞

2d = v1 (c) Recall that t1 =

1−

1/2

2d 1 − k 2 v1 2 s s + = + . v2 v2 v1 v2

s . We therefore have v1 1/2 2d 1−k 2 + vs2 t2 v1 = s t1 v1

=

1/2

2d 1 − k 2 s

v + 1 = v2

1/2

2d 1 − k 2 s

+ k.

S E C T I O N 4.6

Applied Optimization

215

65. Three towns A, B, and C are to be joined by an underground ﬁber cable as illustrated in Figure 34(A). Assume that Continue with the assumption of the previous exercise. C is located directly below the midpoint of AB. Find the junction point P that minimizes the total amount of cable used. (a) Find the thickness of the soil layer, assuming that v1 = 0.7v2 , t2 /t1 = 1.3, and s = 1,500 ft. (a) First show that P must lie directly above C. Show that if the junction is placed at a point Q as in Figure 34(B), then t2 are measured experimentally. equation in Exercise 63(c) shows that tmay is a linear func(b) reduce The times t1 andlength 2 /t1 want we can the cable by moving Q horizontallyThe over to the point P lying above C. You to use the of 1/s. What resulttion of Example 6. might you conclude if experiments were formed for several values of s and the points (1/s, t2 /t1 ) did not lie on a straight line? D A

B

A

B

x Cable

x L

P

Q

C

P C (B)

(A)

FIGURE 34

(b) With x as in Figure 34(A), let f (x) be the total length √ of cable used. Show that f (x) has a unique critical point c. Compute c and show that 0 ≤ c ≤ L if and only if D ≤ 2 3 L. (c) Find the minimum of f (x) on [0, L] in two cases: D = 2, L = 4 and D = 8, L = 2. SOLUTION

(a) Look at diagram 34(B). Let T be the point directly above Q on AB. Let s = AT and D = AB so that T B = D − s. Let be the total length of cable from A to Q and B to Q. By the Pythagorean Theorem applied to AQT and B QT , we get: 2 = s 2 + x 2 + (D − s)2 + x 2 = 2x 2 + D 2 − 2Ds + 2s 2 = 2x 2 + D 2 + 2s 2 − 2Ds = 2x 2 + D 2 + 2(s − D/2)2 − D 2 /2. We wish to ﬁnd the value of s producing the minimum value of 2 . Since D and x are not being changed, this clearly occurs where s = D/2, that is, where Q = P. Since it is obvious that PC ≤ QC (QC is the hypotenuse of the triangle P QC), it follows that total cable length is minimized at Q = P. (b) Let f (x) be the total cable length. From diagram 34(A), we get:

f (x) = (L − x) + 2 x 2 + D 2 /4. Then f (x) = −1 +

2x x 2 + D 2 /4

=0

gives 2x =

x 2 + D 2 /4

or 4x 2 = x 2 + D 2 /4 and the critical point is √ c = D/2 3. This is the only critical point of f . It lies in the interval [0, L] if and only if c ≤ L, or √ D ≤ 2 3L . √ (c) The minimum of f will depend on whether D ≤ 2 3L. √ √ √ √ • D = 2, L = 4; 2 3L = 8 3 > D, so c = D/(2 3) = 3/3 (0) = L + D = 6, f (L) =

∈ [0, L]. f √ √ √ 1 2 2 2 L + D /4 = 2 17 ≈ 8.24621, and f (c) = 4 − ( 3/3) + 2 3 + 1 = 4 + 3 ≈ 5.73204. Therefore, the √ total length is minimized where √ x = c = 3/3. √ √ • D = 8, L = 2; 2 3L = 4 3 < D, so c does not lie in the interval [0, L]. f (0) = 2 + 2 64/4 = 10, and √ √ √ f (L) = 0 + 2 4 + 64/4 = 2 20 = 4 5 ≈ 8.94427. Therefore, the total length is minimized were x = L, or where P = C. A jewelry designer plans to incorporate a component made of gold in the shape of a frustum of a cone of height 1 cm and ﬁxed lower radius r (Figure 35). The upper radius x can take on any value between 0 and r . Note that

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4.7 Newton’s Method Preliminary Questions 1. How many iterations of Newton’s Method are required to compute a root if f (x) is a linear function? SOLUTION Remember that Newton’s Method uses the linear approximation of a function to estimate the location of a root. If the original function is linear, then only one iteration of Newton’s Method will be required to compute the root.

2. What happens in Newton’s Method if your initial guess happens to be a zero of f ? SOLUTION

If x0 happens to be a zero of f, then f (x ) x1 = x0 − 0 = x0 − 0 = x0 ; f (x 0 )

in other words, every term in the Newton’s Method sequence will remain x0 . 3. What happens in Newton’s Method if your initial guess happens to be a local min or max of f ? SOLUTION Assuming that the function is differentiable, then the derivative is zero at a local maximum or a local minimum. If Newton’s Method is started with an initial guess such that f (x 0 ) = 0, then Newton’s Method will fail in the sense that x 1 will not be deﬁned. That is, the tangent line will be parallel to the x-axis and will never intersect it.

4. Is the following a reasonable description of Newton’s Method: “A root of the equation of the tangent line to f (x) is used as an approximation to a root of f (x) itself”? Explain. SOLUTION Yes, that is a reasonable description. The iteration formula for Newton’s Method was derived by solving the equation of the tangent line to y = f (x) at x 0 for its x-intercept.

Exercises In Exercises 1–4, use Newton’s Method with the given function and initial value x 0 to calculate x1 , x 2 , x3 . 1. f (x) = x 2 − 2, SOLUTION

x0 = 1

Let f (x) = x 2 − 2 and deﬁne x n+1 = x n −

f (xn ) x n2 − 2 − . = x n f (x n ) 2xn

With x 0 = 1, we compute n

1

2

3

xn

1.5

1.416666667

1.414215686

3. f (x) = x 3 −25, x 0 = 1.6 f (x) = x − 7, x 0 = 2.5 SOLUTION Let f (x) = x 3 − 5 and deﬁne x n+1 = x n −

f (xn ) x3 − 5 = xn − n 2 . f (x n ) 3x n

With x 0 = 1.6 we compute n

1

2

3

xn

1.717708333

1.710010702

1.709975947

3 5. UsefFigure 6 toxchoose (x) = cos − x, an x0 initial = 0.8guess x0 to the unique real root of x + 2x + 5 = 0. Then compute the ﬁrst three iterates of Newton’s Method. y

−2

−1

x 1

2

FIGURE 6 Graph of y = x 3 + 2x + 5.

S E C T I O N 4.7 SOLUTION

Newton’s Method

217

Let f (x) = x 3 + 2x + 5 and deﬁne f (x n ) x 3 + 2x n + 5 . = xn − n 2 f (x n ) 3xn + 2

x n+1 = x n −

We take x 0 = −1.4, based on the ﬁgure, and then calculate n

1

2

3

xn

−1.330964467

−1.328272820

−1.328268856

In Exercises 7–10, useMethod Newton’s to approximate thecos root decimal[0, places compare the value π ] toand Use Newton’s to Method ﬁnd a solution of sin x = 2xtointhree the interval three decimalwith places. Then 2 obtained from a calculator. guess the exact solution and compare with your approximation. √ 7. 10 SOLUTION

Let f (x) = x 2 − 10, and let x 0 = 3. Newton’s Method yields: n

1

2

3

xn

3.16667

3.162280702

3.16227766

A calculator yields 3.16227766. 9. 51/3 1/4 7 SOLUTION

Let f (x) = x 3 − 5, and let x0 = 2. Here are approximations to the root of f (x), which is 51/3 . n

1

2

3

4

xn

1.75

1.710884354

1.709976429

1.709975947

A calculator yields 1.709975947. 11. Use Newton’s Method to approximate the largest positive root of f (x) = x 4 − 6x 2 + x + 5 to within an error of at −1/2 2 −4 most 10 . Refer to Figure 5. SOLUTION

Figure 5 from the text suggests the largest positive root of f (x) = x 4 − 6x 2 + x + 5 is near 2. So let

f (x) = x 4 − 6x 2 + x + 5 and take x 0 = 2. n

1

2

3

4

xn

2.111111111

2.093568458

2.093064768

2.093064358

The largest positive root of x 4 − 6x 2 + x + 5 is approximately 2.093064358. Use a graphing calculator to3 choose an initial guess for the unique positive root of x 4 + x 2 − 2x − 1 = 0. 13. + 1 and use Newton’s Method to approximate the largest positive root to Sketch graph of f of (x)Newton’s = x − 4x Calculate the ﬁrstthe three iterates Method. −3 . within an error of at most 10 SOLUTION Let f (x) = x 4 + x 2 − 2x − 1. The graph of f (x) shown below suggests taking x 0 = 1. Starting from x 0 = 1, the ﬁrst three iterates of Newton’s Method are: n

1

2

3

xn

1.25

1.189379699

1.184171279

y 30 20 10 −1

x 1

2

3

sin θ the smallest positive solution of π . Use = 0.9 to three decimal places. π Use graphing calculator The Estimate ﬁrst positive solution of sin x = 0 is x = Newton’s Method to calculate to afour decimal places. to θ choose the initial guess.

15.

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A P P L I C AT I O N S O F T H E D E R I VATI V E SOLUTION

Let f (θ ) =

sin θ − .9. θ

We use the following plot to obtain an initial guess: y 1

x

0.5

−0.1 −0.2 −0.3 −0.4

1.5

θ = 0.8 seems a good ﬁrst guess for the location of a zero of the function. A series of approximations based on θ0 = 0.8 follows: n

1

2

3

θn

0.7867796879

0.7866830771

0.7866830718

A three digit approximation for the point where sinθ θ = .9 is 0.787. Plugging sin(0.787)/0.787 into a calculator yields 0.900 to three decimal places. 17. Let x 1 , x 2 be the estimates to a root obtained by applying Newton’s Method with x0 = 1 to the function graphed in In 1535, the mathematician Antonio Fior challenged his rival Niccolo Tartaglia to solve this problem: A tree Figure 7. Estimate the numerical values of x1 and x2 , and draw the tangent lines used to obtain them. stands 12 braccia high; it is broken into two parts at such a point that the height of the part left standing is the cube root of the length of the part cut away. What is ythe height of the part left standing? Show that this is equivalent to solving x 3 + x = 12 and ﬁnd the height to three decimal places. Tartaglia, who had discovered the secrets of cubic equations, was able to determine the exact answer: x −1 1 3

2 √ 3 3 3 2,919 + 54 − 2,919 − 54 9 x= FIGURE 7 SOLUTION The graph with tangent lines drawn on it appears below. The tangent line to the curve at (x 0 , f (x 0 )) has an x-intercept at approximately x 1 = 3.0. The tangent line to the curve at (x 1 , f (x 1 )) has an x-intercept at approximately x 2 = 2.2. y

x

−1

1

2

3

19. Find the x-coordinate to two decimal places of the ﬁrst point in the region x > 0 where y = x intersects y = tan x Find the coordinates to two decimal places of the point P in Figure 8 where the tangent line to y = cos x passes (draw a graph). through the origin. SOLUTION Here is a plot of tan x and x on the same axes: y 5 x 1

2

3

4

−5

The ﬁrst intersection with x > 0 lies on the second “branch” of y = tan x, between x = 54π and x = 32π . Let f (x) = tan x − x. The graph suggests an initial guess x 0 = 54π , from which we get the following table: n

1

2

3

4

xn

6.85398

21.921

4480.8

7456.27

This is clearly leading nowhere, so we need to try a better initial guess. Note: This happens with Newton’s Method—it is sometimes difﬁcult to choose an initial guess. We try the point directly between 54π and 32π , x 0 = 118π :

Newton’s Method

S E C T I O N 4.7

n

1

2

3

4

5

6

7

xn

4.64662

4.60091

4.54662

4.50658

4.49422

4.49341

4.49341

219

The ﬁrst point where y = x and y = tan x cross is at approximately x = 4.49341, which is approximately 1.4303π . Newton’s Method is often used to determine interest rates in ﬁnancial calculations. In Exercises 20–22, r denotes a yearly interest rate expressed as a decimal (rather than as a percent). 21. If you borrow L dollars for N years at a yearly interest rate r , your monthly payment of P dollars is calculated using If P dollars are deposited every month in an account earning interest at the yearly rate r , then the value S of the the equation account after N years is 1 − b−12N r L=P where b = 1 + .r b12N +1 − b b−1 12 . S=P where b = 1 + b−1 12 (a) What is the monthly payment if L = $5,000, N = 3, and r = 0.08 (8%)? decideda to deposit 100 dollars month. (b) You have are offered loan of L P==$5,000 to beper paid back over 3 years with monthly payments of P = $200. Use Newton’s Method tovalue compute ﬁndif the interestrate rateisrrof=this (a) What is the afterb5and years the implied yearly interest 0.07loan. (i.e.,Hint: 7%)?Show that (L/P)b12N +1 − (1 + 12N L/P)b + 1that = 0.to save $10,000 after 5 years, you must earn interest at a rate r determined by the equation b61 − (b) Show SOLUTION 101b +

100 = 0. Use Newton’s Method to solve for b (note that b = 1 is a root, but you want the root satisfying > (1 1).+ Then ﬁnd r=. 1.00667 (a) b = .08/12) b−1 1.00667 − 1 P=L = 5000 ≈ $156.69 1 − b−12N 1 − 1.00667−36

(b) Starting from

L=P

1 − b−12N b−1

,

divide by P, multiply by b − 1, multiply by b12N and collect like terms to arrive at (L/P)b12N +1 − (1 + L/P)b12N + 1 = 0. Since L/P = 5000/200 = 25, we must solve 25b37 − 26b36 + 1 = 0. Newton’s Method gives b ≈ 1.02121 and r = 12(b − 1) = 12(.02121) ≈ .25452 So the interest rate is around 25.45%. 23. Kepler’s Problem Although planetary motion is beautifully described by Kepler’s three laws (see Section 14.6), If you deposit P dollars in a retirement fund every year for N years with the intention of then withdrawing there is no simple formula for the position of a planet P along its elliptical orbit as a function of time. Kepler developed a Q dollars per year for M years, you must earn interest at a rate r satisfying P(b N − 1) = Q(1 − b−M ), where method for locating P at time t by drawing the auxiliary dashed circle in Figure 9 and introducing the angle θ (note that b = 1 + r . Assume that $2,000 is deposited each year for 30 years and the goal is to withdraw $8,000 per year for P determines θ , which is the central angle of the point B on the circle). Let a = O A and e = O S/O A (the eccentricity 20 years. Use Newton’s Method to compute b and then ﬁnd r . of the orbit). (a) Show that sector BSA has area (a 2 /2)(θ − e sin θ ). (b) It follows from Kepler’s Second Law that the area of sector BSA is proportional to the time t elapsed since the planet passed point A. More precisely, since the circle has area π a 2 , BSA has area (π a 2 )(t/T ), where T is the period of the orbit. Deduce that 2π t = θ − e sin θ . T (c) The eccentricity of Mercury’s orbit is approximately e = 0.2. Use Newton’s Method to ﬁnd θ after a quarter of Mercury’s year has elapsed (t = T /4). Convert θ to degrees. Has Mercury covered more than a quarter of its orbit at t = T /4? Auxiliary circle

B P

q O

Elliptical orbit

FIGURE 9

S Sun

A

220

CHAPTER 4

A P P L I C AT I O N S O F T H E D E R I VATI V E SOLUTION

(a) The sector S AB is the slice O AB with the triangle O P S removed. O AB is a central sector with arc θ and radius 2 O A = a, and therefore has area a 2θ . O P S is a triangle with height a sin θ and base length O S = ea. Hence, the area of the sector is 1 a2 a2 θ − ea 2 sin θ = (θ − e sin θ ). 2 2 2 (b) Since Kepler’s second law indicates that the area of the sector is proportional to the time t since the planet passed point A, we get

π a 2 (t/T ) = a 2 /2 (θ − e sin θ ) 2π

t = θ − e sin θ . T

(c) If t = T /4, the last equation in (b) gives:

π = θ − e sin θ = θ − .2 sin θ . 2 Let f (θ ) = θ − .2 sin θ − π2 . We will use Newton’s Method to ﬁnd the point where f (θ ) = 0. Since a quarter of the year on Mercury has passed, a good ﬁrst estimate θ0 would be π2 . n

1

2

3

4

xn

1.7708

1.76696

1.76696

1.76696

From the point of view of the Sun, Mercury has traversed an angle of approximately 1.76696 radians = 101.24◦ . Mercury has therefore traveled more than one fourth of the way around (from the point of view of central angle) during this time. 25. What happens when you apply Newton’s Method to the equation x 3 − 20x = 1/3 0 with the unlucky initial guess x 0 = 2?What happens when you apply Newton’s Method to ﬁnd a zero of f (x) = x ? Note that x = 0 is the only zero. SOLUTION Let f (x) = x 3 − 20x. Deﬁne x n+1 = x n −

f (x n ) x n3 − 20x n . − = x n f (x n ) 3x n2 − 20

Take x 0 = 2. Then the sequence of iterates is −2, 2, −2, 2, . . . , which diverges by oscillation.

Further Insights and Challenges 27. The roots of f (x) = 13 x 3 − 4x + 1 to three decimal places are −3.583, 0.251, and 3.332 (Figure 10). Determine Let c be a positive number and let f (x) = x −1 − c. the root to which Newton’s Method converges for the initial choices x0 = 1.85, 1.7, and 1.55. The answer shows that a 2x − on cx 2the . Thus, Newton’s MethodMethod. provides a way of computing reciprocals Showinthat x −have ( f (x)/ f (x)) = effect small(a) change x 0 can a signiﬁcant outcome of Newton’s without performing division. y with c = 10.324 and the two initial guesses x = 0.1 and (b) Calculate the ﬁrst three iterates of Newton’s Method 0 x0 = 0.5. 4 (c) Explain graphically why x0 = 0.5 does not yield a sequence of approximations to the reciprocal 1/10.324. 0.2513 3.332 −3.583

x

−4

FIGURE 10 Graph of f (x) = 13 x 3 − 4x + 1. SOLUTION

Let f (x) = 13 x 3 − 4x + 1, and deﬁne xn+1 = x n −

1 x 3 − 4x + 1 f (x n ) n 3 n − = x . n 2 f (xn ) xn − 4

• Taking x 0 = 1.85, we have

n

1

2

3

4

5

6

7

xn

−5.58

−4.31

−3.73

−3.59

−3.58294362

−3.582918671

−3.58291867

• Taking x 0 = 1.7, we have

Antiderivatives

S E C T I O N 4.8

n

1

2

3

4

5

6

7

8

9

xn

−2.05

−33.40

−22.35

−15.02

−10.20

−7.08

−5.15

−4.09

−3.66

n

10

11

12

13

xn

−3.585312288

−3.582920989

−3.58291867

−3.58291867

221

• Taking x 0 = 1.55, we have

n

1

2

3

4

5

6

xn

−0.928

0.488

0.245

0.251320515

0.251322863

0.251322863

In Exercises 28–29, consider a metal rod of length L inches that is fastened at both ends. If you cut the rod and weld on an additional m inches of rod, leaving the ends ﬁxed, the rod will bow up into a circular arc of radius R (unknown), as indicated in Figure 11. 29. Let L = 3 and m = 1. Apply Newton’s Method to Eq. (2) to estimate θ and use this to estimate h. Let h be the maximum vertical displacement of the rod. SOLUTION andθmand = 1. We want the solution of: conclude that (a) ShowWe thatletL L==2R3 sin h=

sin L(1θ− cos θL) = . θ 2 sin θL + m

L sin θ − =0 θ L +m 3 sin θ − = 0. θ 4 sin θ L = . θ L +m

(b) Show L + m = 2R θ and then prove Let f (θ ) = sinθ θ − 34 . y 0.2 0.1

x 0.5

−0.2 −0.2

1

1.5

The ﬁgure above suggests that θ0 = 1.5 would be a good initial guess. The Newton’s Method approximations for the solution follow: n

1

2

3

4

θn

1.2854388

1.2757223

1.2756981

1.2756981

L is approximately 1.2757. Hence The angle where sinθ θ = L+m

h=L

Quadratic Convergence to Square Roots

1 − cos θ ≈ 1.11181. 2 sin θ √ Let f (x) = x 2 − c and let en = x n − c be the error in x n .

1

Show that xn+1 = 2 (x n + c/xn ) and that en+1 = en2 /2xn . 4.8 (a)Antiderivatives √ √

(b) Show that if x0 > c, then xn > c for all n. Explain this graphically. √ √ (c) Assuming that x0 > c, show that en+1 ≤ en2 /2 c. Preliminary Questions 1. Find an antiderivative of the function f (x) = 0. SOLUTION

Since the derivative of any constant is zero, any constant function is an antiderivative for the function

f (x) = 0. ! 2. What is the difference, if any, between ﬁnding the general antiderivative of a function f (x) and evaluating f (x) d x? SOLUTION

No difference. The indeﬁnite integral is the symbol for denoting the general antiderivative.

3. Jacques happens to know that f (x) and g(x) have the same derivative, and he would like to know if f (x) = g(x). Does Jacques have sufﬁcient information to answer his question?

222

CHAPTER 4

A P P L I C AT I O N S O F T H E D E R I VATI V E SOLUTION No. Knowing that the two functions have the same derivative is only good enough to tell Jacques that the functions may differ by at most an additive constant. To determine whether the functions are equal for all x, Jacques needs to know the value of each function for a single value of x. If the two functions produce the same output value for a single input value, they must take the same value for all input values.

4. Write any two antiderivatives of cos x. Which initial conditions do they satisfy at x = 0? SOLUTION

Antiderivatives for cos x all have the form sin x + C for some constant C. At x = 0, sin x + C has the

value C. 5. (a) (b) (c)

Suppose that F (x) = f (x) and G (x) = g(x). Are the following statements true or false? Explain. If f = g, then F = G. If F and G differ by a constant, then f = g. If f and g differ by a constant, then F = G.

SOLUTION

(a) False. Even if f (x) = g(x), the antiderivatives F and G may differ by an additive constant. (b) True. This follows from the fact that the derivative of any constant is 0. (c) False. If the functions f and g are different, then the antiderivatives F and G differ by a linear function: F(x) − G(x) = ax + b for some constants a and b. 6. Determine if y = x 2 is a solution to the differential equation with initial condition dy = 2x, dx

y(0) = 1

d SOLUTION Although d x x 2 = 2x, x 2 takes the value 0 when x = 0, so y = x 2 is not a solution of the indicated initial value problem.

Exercises In Exercises 1–6, ﬁnd the general antiderivative of f (x) and check your answer by differentiating. 1. f (x) = 12x SOLUTION

"

" 12x d x = 12

x d x = 12 ·

1 2 x + C = 6x 2 + C. 2

As a check, we have d (6x 2 + C) = 12x dx as needed. 3. f (x) = x 2 +23x + 2 f (x) = x

SOLUTION

"

" (x 2 + 3x + 2) d x = =

" x 2d x + 3

" x1 dx + 2

x0 dx =

1 3 1 1 x + 3 · x2 + 2 · x1 + C 3 2 1

1 3 3 2 x + x + 2x + C. 3 2

As a check, we have d dx

1 3 3 2 x + x + 2x + C 3 2

= x 2 + 3x + 2

as needed. 5. f (x) = 8x −42 f (x) = x + 1

SOLUTION

"

8x −4 d x = 8

"

x −4 d x = 8 ·

As a check, we have d dx as needed.

1 −3 8 x + C = − x −3 + C. −3 3

8 − x −3 + C 3

= 8x −4

S E C T I O N 4.8

Antiderivatives

223

In Exercises f (x)7–10, = cosmatch x + 3 the sin xfunction with its antiderivative (a)–(d). (a) F(x) = cos(1 − x) (b) F(x) = − cos x (c) F(x) = − 12 cos(x 2 ) (d) F(x) = sin x − x cos x 7. f (x) = sin x SOLUTION

An antiderivative of sin x is − cos x, which is (b). As a check, we have ddx (− cos x) = − (− sin x) = sin x.

9. f (x) = sin(1 − x) f (x) = x sin(x 2 ) An antiderivative of sin (1 − x) is cos (1 − x) or (a). As a check, we have ddx cos(1 − x) = − sin(1 − x) · (−1) = sin(1 − x). SOLUTION

In Exercises the indeﬁnite integral. f (x)11–36, = x sinevaluate x " 11. (x + 1) d x " 1 (x + 1) d x = x 2 + x + C. SOLUTION 2 " "5 13. (t + 3t + 2) dt (9 − 5x) d x " 1 3 SOLUTION (t 5 + 3t + 2) dt = t 6 + t 2 + 2t + C. 6 2 " "−9/5 15. t dt 8s −4 ds " 5 SOLUTION t −9/5 dt = − t −4/5 + C. 4 " " 17. 2 dx 3 (5x − x −2 − x 3/5 ) d x " 2 d x = 2x + C. SOLUTION " " 19. (5t −19) dt √ dx x" 5 (5t − 9) dt = t 2 − 9t + C. SOLUTION 2 " "−2 21. x d3x (x + 4x −2 ) d x " 1 −1 1 SOLUTION x +C =− +C x −2 d x = −1 x " " 23. (x +√3)−2 d x x dx " 1 1 SOLUTION (x + 3)−1 + C = − + C. (x + 3)−2 d x = −1 x +3 " "3 dz 25. z 5 (4t − 9)−3 dt SOLUTION

" "

3 dz = z5

"

1 3z −5 dz = 3 − z −4 4

3 + C = − z −4 + C. 4

√ "

x(x3− 1) d x dx x 3/2 √ SOLUTION To perform this indeﬁnite integral, we need to distribute x = x 1/2 over (x − 1): √ x(x − 1) = x 1/2 (x − 1) = x 3/2 − x 1/2 .

27.

Written in this form, we can take the indeﬁnite integral: "

" √ 1 1 2 2 x 3/2 − x 1/2 d x = x(x − 1) d x = x 5/2 − x 3/2 + C = x 5/2 − x 3/2 + C. 5/2 3/2 5 3

224

CHAPTER 4

A P P L I C AT I O N S O F T H E D E R I VATI V E

" 29.

t"− 7 √ dt −1 (x t + x )(3x 2 − 5x) d x

SOLUTION

To proceed, we ﬁrst distribute √1 = t −1/2 over (t − 7): t

t −7 √ = (t − 7)t −1/2 = t 1/2 − 7t −1/2 . t From this, we get: " " 1 1/2 2 t −7 1 3/2 1/2 −1/2 t t − 7t ) dt = −7 + C = t 3/2 − 14t 1/2 + C. √ dt = (t 3/2 1/2 3 t "

" sin2x − 3 cos x) d x (4 x + 2x − 3 dx " x4 (4 sin x − 3 cos x) d x = −4 cos x − 3 sin x + C. SOLUTION " " 33. cos(6t + 4) dt sin 9x d x SOLUTION Using the integral formula for cos(kt + b), we get: " 1 cos(6t + 4) dt = sin(6t + 4) + C. 6

31.

"

" cos(3 − 4t) dt (4θ + cos 8θ ) d θ SOLUTION From the integral formula for cos(kt + b) with k = −4, b = 3, we get: " 1 1 cos(3 − 4t) dt = sin(3 − 4t) + C = − sin(3 − 4t) + C. −4 4

35.

37. In Figure " 2, which of (A) or (B) is the graph of an antiderivative of f (x)? 18 sin(3z + 8) dz y y y x x x f(x)

(A)

(B)

FIGURE 2

Let F(x) be an antiderivative of f (x). By deﬁnition, this means F (x) = f (x). In other words, f (x) provides information as to the increasing/decreasing behavior of F(x). Since, moving left to right, f (x) transitions from − to + to − to + to − to +, it follows that F(x) must transition from decreasing to increasing to decreasing to increasing to decreasing to increasing. This describes the graph in (A)! SOLUTION

39. Use the formulas for the derivatives of f (x) = tan x and f (x) = sec x to evaluate the integrals. " In Figure 3, which of (A), (B), (C) is not the graph of an antiderivative " of f (x)? Explain. (a) sec2 (3x) d x (b) sec(x + 3) tan(x + 3) d x Recall that ddx (tan x) = sec2 x and ddx (sec x) = sec x tan x. ! (a) Accordingly, we have sec2 (3x) d x = 13 tan(3x) + C. ! (b) Moreover, we see that sec(x + 3) tan(x + 3) d x = sec(x + 3) + C. SOLUTION

In Exercises 41–52, solvefor thethe differential equation initial condition. Use the formulas derivatives of f (x)with = cot x and f (x) = csc x to evaluate the integrals. " " d y 2 (a) csc x d x (b) csc x cot x d x = cos 2x, y(0) = 3 41. dx SOLUTION

Since dd xy = cos 2x, we have

" y=

cos 2x d x =

1 sin 2x + C. 2

Thus 3 = y(0) = so that C = 3. Therefore, y = 12 sin 2x + 3.

1 sin (2 · 0) + C = C, 2

S E C T I O N 4.8

dy =5 d=y x, y(0) dx = x 3 , y(0) = 2 dx dy SOLUTION Since d x = x, we have

Antiderivatives

225

43.

" y=

x dx =

1 2 x + C. 2

Thus 5 = y(0) = 0 + C, so that C = 5. Therefore, y = 12 x 2 + 5. dy 45. d=y 5 − 2t 2 , y(1) = 2 dt = 0, y(3) = 5 dt dy SOLUTION Since dt = 5 − 2t 2 , we have " 2 y = (5 − 2t 2 ) dt = 5t − t 3 + C. 3 Thus, 2 2 = y(1) = 5(1) − (13 ) + C, 3 so that C = −3 + 23 = − 73 . Therefore y = 5t − 23 t 3 − 73 . dy d=y 4t + 9,3 y(0)2 = 1 dt = 8x + 3x − 3, y(1) = 1 dx dy SOLUTION Since dt = 4t + 9, we have

47.

" (4t + 9) dt = 2t 2 + 9t + C.

y=

Thus 1 = y(0) = 0 + C, so that C = 1. Therefore, y = 2t 2 + 9t + 1.

π dy =1 49. x, y d=y sin √ dx 2 =1 = t, y(1) dt dy SOLUTION Since d x = sin x, we have " y = sin x d x = − cos x + C. Thus 1=y

π 2

= 0 + C,

so that C = 1. Therefore, y = 1 − cos x. dy π 3 d=y cos 5x, y(π ) = dx =4 = sin 2z, y dz 4 dy SOLUTION Since d x = cos 5x, we have

51.

" y=

cos 5x d x =

1 sin 5x + C. 5

Thus 3 = y(π ) = 0 + C, so that C = 3. Therefore, y = 3 + 15 sin 5x.

πf and then ﬁnd f . In Exercises ﬁrst ﬁnd d y 53–58, = sec2 3x, y =2 d x = x, f (0) = 1,4 f (0) = 0 53. f (x) Let g(x) = f (x). g (x) = x and g(0) = 1, so g(x) = 12 x 2 + C with g(0) = 1 = C. Hence f (x) = g(x) = 12 x 2 + 1. f (x) = 12 x 2 + 1 and f (0) = 0, so f (x) = 12 ( 13 x 3 ) + x + C = 16 x 3 + x + C, and f (0) = 0 = C. Therefore f (x) = 16 x 3 + x. SOLUTION

55. f (x) = x 3 −32x + 1, f (1) = 0, f (1) = 4 f (x) = x − 2x + 1, f (0) = 1, f (0) = 0

226

CHAPTER 4

A P P L I C AT I O N S O F T H E D E R I VATI V E SOLUTION

Let g(x) = f (x). The problem statement gives us g (x) = x 3 − 2x + 1, g(0) = 0. From g (x), we get

g(x) = 14 x 4 − x 2 + x + C, and from g(1) = 0, we get 0 = 14 − 1 + 1 + C, so that C = − 14 . This gives f (x) = g(x) = 1 x 4 − x 2 + x − 1 . From f (x), we get f (x) = 1 ( 1 x 5 ) − 1 x 3 + 1 x 2 − 1 x + C = 1 x 5 − 1 x 3 + 1 x 2 − 1 x + C. 4 4 4 5 3 2 4 20 3 2 4

From f (1) = 4, we get

1 1 1 1 − + − + C = 4, 20 3 2 4 so that C = 121 30 . Hence, 1 5 1 3 1 2 1 121 x − x + x − x+ . 20 3 2 4 30

f (x) =

π = 1, f π = 6 = cos θ , f 57. f (fθ)(t) −3/2 =t , f 2(4) = 1, f (4) 2 =4 SOLUTION

Let g(θ ) = f (θ ). The problem statement gives g (θ ) = cos θ ,

g

π 2

= 1.

From g (θ ) we get g(θ ) = sin θ + C. From g( π2 ) = 1 we get 1 + C = 1, so C = 0. Hence f (θ ) = g(θ ) = sin θ . From f (θ ) we get f (θ ) = − cos θ + C. From f ( π2 ) = 6 we get C = 6, so f (θ ) = − cos θ + 6. 59. Show that f (x) = tan2 x and g(x) = sec2 x have the same derivative. What can you conclude about the relation f (t) = t − cos t, f (0) = 2, f (0) = −2 between f and g? Verify this conclusion directly. SOLUTION

Let f (x) = tan2 x and g(x) = sec2 x. Then f (x) = 2 tan x sec2 x and g (x) = 2 sec x · sec x tan x =

2 tan x sec2 x; hence f (x) = g (x). Accordingly, f (x) and g(x) must differ by a constant; i.e., f (x) − g(x) = tan2 x − sec2 x = C for some constant C. To see that this is true directly, divide the identity sin2 x + cos2 x = 1 by cos2 x. This yields tan2 x + 1 = sec2 x, so that tan2 x − sec2 x = −1. 61. A particle located at the origin at t = 0 begins along the x-axis with velocity v(t) = 12 t 2 − t ft/s. Let s(t) 1 cos 2 x moving =− 2x + C for some constant C. Find C bys(t). setting x = 0. Show, by computing that sin 2 initial be its position at time t. Statederivatives, the differential equation with condition satisﬁed by s(t) and ﬁnd SOLUTION

Given that v(t) = ds dt and that the particle starts at the origin, the differential equation is 1 ds = t 2 − t, dt 2

s(0) = 0.

Accordingly, we have " s(t) =

1 2 t −t 2

dt =

1 2

"

" t 2 dt −

t dt =

1 1 3 1 2 1 1 · t − t + C = t 3 − t 2 + C. 2 3 2 6 2

Thus 0 = s(0) = 16 (0)3 − 12 (0)2 + C, so that C = 0. Therefore, the position is s(t) = 16 t 3 − 12 t 2 . 2 63. A particle along velocity v(t) = 25t be the position at time t. Repeat moves Exercise 61, the but x-axis replacewith the initial condition s(0)−=t 0 ft/s. with Let s(2)s(t) = 3. (a) Find s(t), assuming that the particle is located at x = 5 at time t = 0. (b) Find s(t), assuming that the particle is located at x = 5 at time t = 2. 2 The differential equation is v(t) = ds dt = 25t − t . (a) Suppose s(0) = 5. Then " " " s(t) = (25t − t 2 ) dt = 25 t dt − t 2 dt

SOLUTION

1 1 25 2 1 3 t − t + C. = 25 · t 2 − t 3 + C = 2 3 2 3 1 2 3 Thus 5 = s(0) = 25 2 (0) − 3 (0) + C, so that C = 5. Therefore, the position is

s(t) =

25 2 1 3 t − t + 5. 2 3

1 3 25 1 142 2 2 3 (b) From (a), we originally ascertained that s(t) = 25 2 t − 3 t + C. Thus 5 = s(2) = 2 (2) − 3 (2) + C = 3 + C, 127 so that C = 5 − 142 3 = − 3 . Therefore, the position is

s(t) =

25 2 1 3 127 t − t − . 2 3 3

S E C T I O N 4.8

Antiderivatives

227

the car come 65. A car traveling 84 ft/s begins to decelerate at a constant rate of 14 ft/s2 . After how many seconds does particle located at the at t = 0before movesstopping? in a straight line with acceleration a(t) = 4 − 12 t ft/s2 . Let v(t) to a stopAand how far will the carorigin have traveled be the velocity and s(t) the position at time t. SOLUTION Since the acceleration of the car is a constant −14ft/s2 , v is given by the differential equation: (a) State and solve the differential equation for v(t) assuming that the particle is at rest at t = 0. (b) Find s(t). dv = −14, v(0) = 84. dt ! ft From dv dt , we get v(t) = −14 dt = −14t + C. Since v(0) = 84, C = 84. From this, v(t) = −14t + 84 s . To ﬁnd the time until the car stops, we must solve v(t) = 0: −14t + 84 = 0 14t = 84 t = 84/14 = 6 s. Now we have a differential equation for s(t). Since we want to know how far the car has traveled from the beginning of its deceleration at time t = 0, we have s(0) = 0 by deﬁnition, so: ds = v(t) = −14t + 84, dt

s(0) = 0.

! From this, s(t) = (−14t + 84) dt = −7t 2 + 84t + C. Since s(0) = 0, we have C = 0, and s(t) = −7t 2 + 84t. At stopping time t = 6 s, the car has traveled s(6) = −7(36) + 84(6) = 252 ft. 67. A 900-kg rocket is released from a spacecraft. As the rocket burns fuel, its mass decreases and its velocity increases. Beginning at rest, an object moves in a straight line with constant acceleration a, covering 100 ft in 5 s. Find a. Let v(m) be the velocity (in meters per second) as a function of mass m. Find the velocity when m = 500 if dv/dm = −50m −1/2 . Assume that v(900) = 0. ! dv SOLUTION Since dm = −50m −1/2 , we have v(m) = −50m −1/2 dm = −100m 1/2 + C. Thus 0 = v(900) = √ √ −100 900 + C = −3000 + C, and C = 3000. Therefore, v(m) = 3000 − 100 m. Accordingly, √ √ v(500) = 3000 − 100 500 = 3000 − 1000 5 ≈ 764 meters/sec. 69. Find constants c1 and c2 such that F(x) = c1 x sin x + c2 cos x is an antiderivative of f (x) = x cos x. As water ﬂows through a tube of radius R = 10 cm, the velocity of an individual water particle depends on dv SOLUTION Let F(x) = c1 x sin x + c2 cos x. If F(x) is to be an antiderivative of f (x) = x cos x, we must have = −0.06r . Determine v(r ), assuming that its distance r from the center of the tube according to the formula F (x) = f (x) for all x. Hence c1 (x cos x + sin x) − c2 sin x = x cos x for dr all x. Equating coefﬁcients on the left- and particles at the velocity. right-hand sides, wewalls have of c1 the = 1tube (i.e.,have the zero coefﬁcients of x cos x are equal) and c1 − c2 = 0 (i.e., the coefﬁcients of sin x are equal). Thus c1 = c2 = 1 and hence F(x) = x sin x + cos x. As a check, we have F (x) = x cos x + sin x − sin x = x cos x = f (x), as required. 71. Verify the linearity properties of the indeﬁnite integral stated in Theorem 3. Find the general antiderivative of (2x + 9)10 . SOLUTION To verify the Sum Rule, let F(x) and G(x) be any antiderivatives of f (x) and g(x), respectively. Because d d d (F(x) + G(x)) = F(x) + G(x) = f (x) + g(x), dx dx dx it follows that F(x) + G(x) is an antiderivative of f (x) + g(x); i.e., " " " ( f (x) + g(x)) d x = f (x) d x + g(x) d x. To verify the Multiples Rule, again let F(x) be any antiderivative of f (x) and let c be a constant. Because d d (cF(x)) = c F(x) = c f (x), dx dx it follows that cF(x) is and antiderivative of c f (x); i.e., " " (c f (x)) d x = c f (x) d x.

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Further Insights and Challenges 73. (a) (b)

Suppose that F (x) = f (x). Suppose that F (x) = f (x) and G (x) = g(x). Is it true that F(x)G(x) is an antiderivative of f (x)g(x)? Conﬁrm Show that 12 F(2x) is an antiderivative of f (2x). or provide a counterexample. Find the general antiderivative of f (kx) for any constant k.

Let F (x) = f (x). (a) By the Chain Rule, we have

SOLUTION

d dx

1 1 F(2x) = F (2x) · 2 = F (2x) = f (2x). 2 2

Thus 12 F(2x) is an antiderivative of f (2x). (b) For nonzero constant k, the Chain Rules gives d 1 1 F (kx) = F (kx) · k = F (kx) = f (kx). dx k k Thus k1 F(kx) is an antiderivative of f (kx). Hence the general antiderivative of f (kx) is 1k F(kx) + C, where C is a constant. −1 75. TheFind Power for antiderivatives does an Rule antiderivative for f (x) = |x|.not apply to f (x) = x . Which of the graphs in Figure 4 could plausibly −1 represent an antiderivative of f (x) = x ? y

y

y x

x 1

3

5

1

3

5

x 1 (A)

3 (B)

5 (C)

FIGURE 4 SOLUTION Let F(x) be an antiderivative of f (x) = x −1 . Then for x > 0, we have F (x) = f (x) = x −1 > 0. In other words, the graph of F(x) is increasing for x > 0. The only graph for which this is true is (A). Accordingly, neither graph (B) nor graph (C) is the graph of F(x).

CHAPTER REVIEW EXERCISES In Exercises 1–6, estimate using the Linear Approximation or linearization and use a calculator to compute the error. 1. 8.11/3 − 2 1 1 SOLUTION Let f (x) = x 1/3 , a = 8 and x = 0.1. Then f (x) = 3 x −2/3 , f (a) = 12 and, by the Linear Approximation,

1 f = 8.11/3 − 2 ≈ f (a)x = (0.1) = 0.00833333. 12 Using a calculator, 8.11/3 − 2 = 0.00829885. The error in the Linear Approximation is therefore |0.00829885 − 0.00833333| = 3.445 × 10−5 . 3. 6251/41 − 62411/4 √ − 1 −3/4 , f (a) = 1 and, by the Linear 1/4 SOLUTION 4.1 Let2 f (x) = x , a = 625 and x = −1. Then f (x) = 4 x 500 Approximation, 1 f = 6241/4 − 6251/4 ≈ f (a)x = (−1) = −0.002. 500 Thus 6251/4 − 6241/4 ≈ 0.002. Using a calculator, 6251/4 − 6241/4 = 0.00200120. The error in the Linear Approximation is therefore |0.00200120 − (0.002)| = 1.201 × 10−6 .

Chapter Review Exercises

5.

229

1√ 1.02 101

Let f (x) = x −1 and a = 1. Then f (a) = 1, f (x) = −x −2 and f (a) = −1. The linearization of f (x) at a = 1 is therefore SOLUTION

L(x) = f (a) + f (a)(x − a) = 1 − (x − 1) = 2 − x, 1 ≈ L(1.02) = 0.98. Using a calculator, 1 = 0.980392, so the error in the Linear Approximation is and 1.02 1.02

|0.980392 − 0.98| = 3.922 × 10−4 . √ In Exercises 7–10, ﬁnd the linearization at the point indicated. 5 33 √ 7. y = x, a = 25 √ 1 1 SOLUTION Let y = x and a = 25. Then y(a) = 5, y = 2 x −1/2 and y (a) = 10 . The linearization of y at a = 25 is therefore L(x) = y(a) + y (a)(x − 25) = 5 +

1 (x − 25). 10

9. A(r ) = 43 π r 3 , a = 3 v(t) = 32t − 4t 2 , a = 2 4 SOLUTION Let A(r ) = 3 π r 3 and a = 3. Then A(a) = 36π , A (r ) = 4π r 2 and A (a) = 36π . The linearization of A(r ) at a = 3 is therefore L(r ) = A(a) + A (a)(r − a) = 36π + 36π (r − 3) = 36π (r − 2). In Exercises the Linear V (h)11–15, = 4h(2use − h)(4 − 2h),Approximation. a=1 11. The position of an object in linear motion at time t is s(t) = 0.4t 2 + (t + 1)−1 . Estimate the distance traveled over the time interval [4, 4.2]. Let s(t) = 0.4t 2 + (t + 1)−1 , a = 4 and t = 0.2. Then s (t) = 0.8t − (t + 1)−2 and s (a) = 3.16. Using the Linear Approximation, the distance traveled over the time interval [4, 4.2] is approximately SOLUTION

s = s(4.2) − s(4) ≈ s (a)t = 3.16(0.2) = 0.632. 13. A store sells 80 MP3 players per week when the players are priced at P = $75. Estimate the number N sold if P is bondassuming that paysthat $10,000 offered Nforifsale at a price P. Thetopercentage yield Y of the bond is raised toA$80, d N /dinP 6=years −4. is Estimate the price is lowered $69. SOLUTION If P is raised to $80, then P = 5. With the assumption 10,000 1/6that d N /d P = −4, we estimate, using the Linear Y = 100 −1 . Approximation, that P dN Verify that if P = $7,500, then Y = 4.91%. the(−4)(5) drop in = yield if the price rises to $7,700. P = −20; N ≈ Estimate dP therefore, we estimate that only 60 MP3 players will be sold per week when the price is $80. On the other hand, if the price is lowered to $69, then P = −6 and N ≈ (−4)(−6) = 24. We therefore estimate that 104 MP3 players will be sold per week when the price is $69. √ b 15. Show a 2 + b ≈ of a +a sphere is measured small. Useatthis to 100 estimate 26 and the ﬁndmaximum the error using a calculator. Thethat circumference C = cm. Estimate percentage error in V if the 2a if b is error in C is at most 3 cm. 2 SOLUTION Let a > 0 and let f (b) = a + b. Then 1 f (b) = . 2 a2 + b By the Linear Approximation, f (b) ≈ f (0) + f (0)b, so

To estimate

√ 26, let a = 5 and b = 1. Then √

b a2 + b ≈ a + . 2a

26 =

1 52 + 1 ≈ 5 + = 5.1. 10

√ The error in this estimate is | 26 − 5.1| = 9.80 × 10−4 .

Use the Intermediate Value Theorem to prove that sin x − cos x = 3x has a solution and use Rolle’s Theorem to show that this solution is unique.

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x 17. Show that f (x) = x + 2 has precisely one real root. x +1 Let f (x) = x + 2x and note that f (0) = 0. Thus, f (x) has at least one real root. Now, suppose f (x) x +1 has two real roots, say a and b. Because f is continuous on [a, b], differentiable on (a, b) and f (a) = f (b) = 0, Rolle’s Theorem guarantees there exists c ∈ (a, b) such that f (c) = 0. However, SOLUTION

f (x) = 1 +

x 2 + 1 − 2x 2 x4 + x2 + 2 = >0 2 2 (x + 1) (x 2 + 1)2

for all x. We have reached a contradiction. Consequently, f (x) must have precisely one real root. 19. Suppose that f (1) = 5 and f (x) ≥ 2 for x ≥ 1. Use the MVT to show that f (8) ≥ 19. Verify the MVT for f (x) = x −1/2 on [1, 4]. SOLUTION Because f is continuous on [1, 8] and differentiable on (1, 8), the Mean Value Theorem guarantees there exists a c ∈ (1, 8) such that f (8) − f (1) 8−1

f (c) =

f (8) = f (1) + 7 f (c).

or

Now, we are given that f (1) = 5 and that f (x) ≥ 2 for x ≥ 1. Therefore, f (8) ≥ 5 + 7(2) = 19. In Exercises 21–24, the local they arethen minima, neither. Use the MVTﬁnd to prove thatextrema if f (x)and ≤ 2determine for x > 0whether and f (0) = 4, f (x) maxima, ≤ 2x + 4orfor all x ≥ 0. 21. f (x) = x 3 − 4x 2 + 4x Let f (x) = x 3 − 4x 2 + 4x. Then f (x) = 3x 2 − 8x + 4 = (3x − 2)(x − 2), so that x = 23 and x = 2 are critical points. Next, f (x) = 6x − 8, so f ( 23 ) = −4 < 0 and f (2) = 4 > 0. Therefore, by the Second Derivative Test, f ( 23 ) is a local maximum while f (2) is a local minimum. SOLUTION

23. f (x) = x 2 (x + 2)3 s(t) = t 4 − 8t 2 SOLUTION Let f (x) = x 2 (x + 2)3 . Then f (x) = 3x 2 (x + 2)2 + 2x(x + 2)3 = x(x + 2)2 (3x + 2x + 4) = x(x + 2)2 (5x + 4), so that x = 0, x = −2 and x = − 45 are critical points. The sign of the ﬁrst derivative on the intervals surrounding the critical points is indicated in the table below. Based on this information, f (−2) is neither a local maximum nor a local minimum, f (− 45 ) is a local maximum and f (0) is a local minimum. Interval

(−∞, −2)

(−2, − 45 )

(− 45 , 0)

(0, ∞)

+

+

−

+

Sign of f

In Exercises 25–30, ﬁnd the extreme values on the interval. f (x) = x 2/3 (1 − x) 25. f (x) = x(10 − x),

[−1, 3]

Let f (x) = x(10 − x) = 10x − x 2 . Then f (x) = 10 − 2x, so that x = 5 is the only critical point. As this critical point is not in the interval [−1, 3], we only need to check the value of f at the endpoints to determine the extreme values. Because f (−1) = −11 and f (3) = 21, the maximum value of f (x) = x(10 − x) on the interval [−1, 3] is 21 while the minimum value is −11. SOLUTION

27. g(θ ) = sin2 θ 4− cos θ6 , [0, 2π ] f (x) = 6x − 4x , [−2, 2] SOLUTION Let g(θ ) = sin2 θ − cos θ . Then g (θ ) = 2 sin θ cos θ + sin θ = sin θ (2 cos θ + 1) = 0 when θ = 0, 23π , π , 43π , 2π . The table below lists the value of g at each of the critical points and the endpoints of the interval [0, 2π ]. Based on this information, the minimum value of g(θ ) on the interval [0, 2π ] is −1 and the maximum value is 54 .

θ g(θ ) 1/3 , [−1, 3] 29. f (x) = x 2/3 − 2x t , [0, 3] R(t) = 2 t +t +1

0

2π /3

π

4π /3

2π

−1

5/4

1

5/4

−1

Chapter Review Exercises

231

Let f (x) = x 2/3 − 2x 1/3 . Then f (x) = 23 x −1/3 − 23 x −2/3 = 23 x −2/3 (x 1/3 − 1), so that the critical √ √ points are x = 0 and x = 1. With f (−1) = 3, f (0) = 0, f (1) = −1 and f (3) = 3 9 − 2 3 3 ≈ −0.804, it follows that the minimum value of f (x) on the interval [−1, 3] is −1 and the maximum value is 3. SOLUTION

31. Find the critical points and extreme values of f (x) = |x − 1| + |2x − 6| in [0, 8]. f (x) = x − tan x, [−1, 1] SOLUTION Let ⎧ ⎪ ⎨7 − 3x, f (x) = |x − 1| + |2x − 6| = 5 − x, ⎪ ⎩ 3x − 7,

x <1 1≤x <3. x ≥3

The derivative of f (x) is never zero but does not exist at the transition points x = 1 and x = 3. Thus, the critical points of f are x = 1 and x = 3. With f (0) = 7, f (1) = 4, f (3) = 2 and f (8) = 17, it follows that the minimum value of f (x) on the interval [0, 8] is 2 and the maximum value is 17. In Exercises 33–36, ﬁnd the points of inﬂection. 1 . Where on the interval [1, 4] does f (x) take on its maximum A function f (x) has derivative f (x) = 4 3 2 x +1 33. y = x − 4x + 4x value? SOLUTION Let y = x 3 − 4x 2 + 4x. Then y = 3x 2 − 8x + 4 and y = 6x − 8. Thus, y > 0 and y is concave up for x > 43 , while y < 0 and y is concave down for x < 43 . Hence, there is a point of inﬂection at x = 43 . x2 35. y =y =2 x − 2 cos x x +4 SOLUTION

4 x2 8x =1− 2 . Then y = 2 Let y = 2 and x +4 x +4 (x + 4)2 y =

(x 2 + 4)2 (8) − 8x(2)(2x)(x 2 + 4) 8(4 − 3x 2 ) = . (x 2 + 4)4 (x 2 + 4)3

Thus, y > 0 and y is concave up for 2 2 −√ < x < √ , 3 3 while y < 0 and y is concave down for 2 |x| ≥ √ . 3 Hence, there are points of inﬂection at 2 x = ±√ . 3 37. (a) (b) (c)

Match the description of f (x) with the graph of its derivative f (x) in Figure 1. x y = f (x) is increasing and concave up. (x 2 − 4)1/3 f (x) is decreasing and concave up. f (x) is increasing and concave down. y

y

y

x

x

x (i)

(ii)

(iii)

FIGURE 1 Graphs of the derivative. SOLUTION

(a) If f (x) is increasing and concave up, then f (x) is positive and increasing. This matches the graph in (ii). (b) If f (x) is decreasing and concave up, then f (x) is negative and increasing. This matches the graph in (i). (c) If f (x) is increasing and concave down, then f (x) is positive and decreasing. This matches the graph in (iii). In Exercises thefor limit. Draw39–52, a curveevaluate y = f (x) which f and f have signs as indicated in Figure 2. 39. lim (9x 4 − 12x 3 ) x→∞

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Because the leading term in the polynomial has a positive coefﬁcient, lim (9x 4 − 12x 3 ) = ∞.

x→∞

x 3 + 2x 41. lim lim 2 (7x 2 − 9x 5 ) x→∞ x→−∞ 4x − 9 SOLUTION

x 3 + 2x (x 3 + 2x)x −2 x + 2x −1 = lim = lim = ∞. 2 2 −2 x→∞ 4x − 9 x→∞ (4x − 9)x x→∞ 4 − 9x −2 lim

x 3 + 2x x 3 + 2x x→∞lim 4x 3 − 92 x→−∞ 4x − 9

43. lim

SOLUTION

lim

x 3 + 2x

x→∞ 4x 3 − 9

= lim

(x 3 + 2x)x −3

x→∞ (4x 3 − 9)x −3

= lim

1 + 2x −2

x→∞ 4 − 9x −3

=

1 . 4

x 2 − 9x3 45. lim x + 2x x→∞limx 2 + 4x x→−∞ 4x 5 − 40x 3 √ SOLUTION For x > 0, x −2 = |x|−1 = x −1 . Then, x 2 − 9x (x 2 − 9x)x −1 x −9 = lim lim = lim = ∞. √ x→∞ x→∞ x 2 + 4x x 2 + 4x x −2 1 + 4x −1

x→∞

x 1/212x + 1 47. lim lim √ x→∞ 4x 92 x→−∞ −4x + 4x SOLUTION

lim √

x→∞

49.

x 1/2 4x − 9

= lim √ x→∞

1 x 1/2 x −1/2 1 = lim = . √ x→∞ −1 −1 2 4x − 9 x 4 − 9x

1 − 2x

lim x 3/2 x→3+ limx − 3 6 x→∞ (16x − 9x 4 )1/4

SOLUTION

As x → 3+, 1 − 2x → −5 and x − 3 → 0+, so 1 − 2x = −∞. x→3+ x − 3 lim

x −5 lim 1 limx − 2 x→2+ x→2− x 2 − 4 SOLUTION As x → 2+, x − 5 → −3 and x − 2 → 0+, so

51.

lim

x −5

x→2+ x − 2

= −∞.

In Exercises 53–62, 1sketch the graph, noting the transition points and asymptotic behavior. lim (x 2+ 3)3 53. y =x→−3+ 12x − 3x Let y = 12x − 3x 2 . Then y = 12 − 6x and y = −6. It follows that the graph of y = 12x − 3x 2 is increasing for x < 2, decreasing for x > 2, has a local maximum at x = 2 and is concave down for all x. Because SOLUTION

lim (12x − 3x 2 ) = −∞,

x→±∞

the graph has no horizontal asymptotes. There are also no vertical asymptotes. The graph is shown below.

Chapter Review Exercises

233

y 10 5 x

−1 −5

1

2

3

4

5

−10

2+3 55. y = x 3 − 2x y = 8x 2 − x 4 SOLUTION Let y = x 3 − 2x 2 + 3. Then y = 3x 2 − 4x and y = 6x − 4. It follows that the graph of y = x 3 − 2x 2 + 3 is increasing for x < 0 and x > 43 , is decreasing for 0 < x < 43 , has a local maximum at x = 0, has a local minimum at x = 43 , is concave up for x > 23 , is concave down for x < 23 and has a point of inﬂection at x = 23 . Because

lim (x 3 − 2x 2 + 3) = −∞

lim (x 3 − 2x 2 + 3) = ∞,

and

x→−∞

x→∞

the graph has no horizontal asymptotes. There are also no vertical asymptotes. The graph is shown below. y 10 5 −1

x 1

−5

2

−10

x 57. y = 3 3/2 y x= 4x + 1− x SOLUTION

x Let y = 3 . Then x +1 y =

x 3 + 1 − x(3x 2 ) 1 − 2x 3 = 3 3 2 (x + 1) (x + 1)2

and (x 3 + 1)2 (−6x 2 ) − (1 − 2x 3 )(2)(x 3 + 1)(3x 2 ) 6x 2 (2 − x 3 ) =− . 3 4 (x + 1) (x 3 + 1)3

x is increasing for x < −1 and −1 < x < 3 12 , is decreasing for x > 3 12 , has a It follows that the graph of y = 3 x +1

√ √ 3 3 3 1 local maximum at x = 2 , is concave up for x < −1 and x > 2, is concave down for −1 < x < 0 and 0 < x < 2 √ 3 and has a point of inﬂection at x = 2. Note that x = −1 is not an inﬂection point because x = −1 is not in the domain of the function. Now, y =

x

lim

x→±∞ x 3 + 1

= 0,

so y = 0 is a horizontal asymptote. Moreover, lim

x

x→−1− x 3 + 1

=∞

and

x

lim

x→−1+ x 3 + 1

so x = −1 is a vertical asymptote. The graph is shown below. y 4 2 −3 −2 −1

x −2 −4

1 x 59. y = y |x = + 2| + 1 2/3 2 (x − 4)

1

2

3

= −∞,

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SOLUTION

Let y =

1 . Because |x + 2| + 1 1

lim

= 0,

x→±∞ |x + 2| + 1

the graph of this function has a horizontal asymptote of y = 0. The graph has no vertical asymptotes as |x + 2| + 1 ≥ 1 for all x. The graph is shown below. From this graph we see there is a local maximum at x = −2. y 1 0.8 0.6 0.4 0.2 x

−8 −6 − 4 −2

2

4

61. y = 2 sin x − cos x on [0, 2π ] y = 2 −yx 3= √3 sin x − cos x. Then y = √3 cos x + sin x and y = −√3 sin x + cos x. It follows that the SOLUTION Let √ graph of y = 3 sin x − cos x is increasing for 0 < x < 5π /6 and 11π /6 < x < 2π , is decreasing for 5π /6 < x < 11π /6, has a local maximum at x = 5π /6, has a local minimum at x = 11π /6, is concave up for 0 < x < π /3 and 4π /3 < x < 2π , is concave down for π /3 < x < 4π /3 and has points of inﬂection at x = π /3 and x = 4π /3. The graph is shown below. y 1 −1

4 1

2

x

3

5

6

63. Find the maximum volume of a right-circular cone placed upside-down in a right-circular cone of radius R and y = 2x − tan x on [0, 2π ] height H (Figure 3). The volume of a cone of radius r and height h is 43 π r 2 h.

h

r

FIGURE 3 SOLUTION

Let r denote the radius and h the height of the upside down cone. By similar triangles, we obtain the

relation H −h H = r R

so

r h = H 1− R

and the volume of the upside down cone is

4 2 4 r3 2 V (r ) = π r h = π H r − 3 3 R

for 0 ≤ r ≤ R. Thus,

3r 2 4 dV = π H 2r − , dr 3 R

and the critical points are r = 0 and r = 2R/3. Because V (0) = V (R) = 0 and 2R 4R 2 4 8R 2 16 2 V π R H, = πH − = 3 3 9 27 81 the maximum volume of a right-circular cone placed upside down in a right-circular cone of radius R and height H is 16 2 π R H. 81

Chapter Review Exercises

235

65. Show that the maximum area of a parallelogram ADEF inscribed in a triangle ABC, as in Figure 4, is equal to On a certain farm, the corn yield is one-half the area of ABC. Y = −0.118x 2 + 8.5x + B12.9 (bushels per acre) where x is the number of corn plants per acre (in thousands). Assume that corn seed costs $1.25 (per thousand seeds) and that corn can be sold for $1.50/bushel. D E (a) Find the value x 0 that maximizes yield Y . Then compute the proﬁt (revenue minus the cost of seeds) at planting level x 0 . (b) Compute the proﬁt P(x) as a function ofA x and ﬁndF the valueCx 1 that maximizes proﬁt. 4 yield lead to maximum proﬁt? maximum (c) Compare the proﬁt at levels x0 and x 1 . Does aFIGURE SOLUTION Let θ denote the measure of angle B AC. Then the area of the parallelogram is given by AD · AF sin θ . Now, suppose that

B E/BC = x. Then, by similar triangles, AD = (1 − x) AB, AF = D E = x AC, and the area of the parallelogram becomes AB · AC x(1 − x) sin θ . The function x(1 − x) achieves its maximum value of 14 when x = 12 . Thus, the maximum area of a parallelogram inscribed in a triangle ABC is 1 1 1 1 AB · AC sin θ = AB · AC sin θ = (area of ABC) . 4 2 2 2 67. Let f (x) be a function whose graph does not pass through the x-axis and let Q = (a, 0). Let P = (x0 , f (x 0 )) be the 1 r 2 (θ − sin θ ). What is the maximum possible area of this region if of closest the shaded in 6). Figure 5 isthat point onThe the area graph to Qregion (Figure Prove 2 P Q is perpendicular to the tangent line to the graph of x 0 . Hint: the length the circular Let q(x) be theofdistance fromarc (x,isf 1? (x)) to (a, 0) and observe that x 0 is a critical point of q(x). y

y = f(x) P = (x 0 , f(x 0))

Q = (a, 0)

x

FIGURE 6 SOLUTION Let P = (a, 0) and let Q = (x 0 , f (x 0 )) be the point on the graph of y = f (x) closest to P. The slope of the segment joining P and Q is then

f (x 0 ) . x0 − a Now, let q(x) =

(x − a)2 + ( f (x))2 ,

the distance from the arbitrary point (x, f (x)) on the graph of y = f (x) to the point P. As (x 0 , f (x 0 )) is the point closest to P, we must have q (x 0 ) =

2(x 0 − a) + 2 f (x 0 ) f (x 0 ) = 0. (x 0 − a)2 + ( f (x 0 ))2

Thus, x −a f (x 0 ) = − 0 =− f (x 0 )

f (x 0 ) −1 . x0 − a

In other words, the slope of the segment joining P and Q is the negative reciprocal of the slope of the line tangent to the graph of y = f (x) at x = x 0 ; hence; the two lines are perpendicular. √ 3 to four decimalaplaces. 69. UseTake Newton’s Method to of estimate a circular piece paper of 25 radius R, remove sector of angle θ (Figure 7), and fold the remaining piece 3 SOLUTION Let f (x) = x − 25 and deﬁne into a cone-shaped cup. Which angle θ produces the cup of largest volume? x n+1 = x n − With x 0 = 3, we ﬁnd

f (x n ) x 3 − 25 = xn − n 2 . f (x n ) 3xn

236

CHAPTER 4

A P P L I C AT I O N S O F T H E D E R I VATI V E

Thus, to four decimal places

√ 3

n

1

2

3

xn

2.925925926

2.924018982

2.924017738

25 = 2.9240.

In Exercises 71–80, calculate the indeﬁnite integral. Use Newton’s Method to ﬁnd a root of f (x) = x 2 − x − 1 to four decimal places. " 3 4x − 2x 2 d x 71. " 2 SOLUTION (4x 3 − 2x 2 ) d x = x 4 − x 3 + C. 3 " " − 8) d θ 73. sin(θ9/4 dx x " SOLUTION sin(θ − 8) d θ = − cos(θ − 8) + C. "

" (4t −3 − 12t −4 ) dt cos(5 − 7θ ) d θ " SOLUTION (4t −3 − 12t −4 ) dt = −2t −2 + 4t −3 + C.

75.

"

" dx sec2 x−2/3 + 4t 7/3 ) dt (9t " SOLUTION sec2 x d x = tan x + C.

77.

"

" (y + 2)4 d y tan 3θ sec 3θ d θ " 1 (y + 2)4 d y = (y + 2)5 + C. SOLUTION 5

79.

" In Exercises 81–84, solve the differential equation with initial condition. 3x 3 − 9 dx 2 dy 81. = 4x x3 , y(1) = 4 dx SOLUTION

Let dd xy = 4x 3 . Then

" y(x) =

4x 3 d x = x 4 + C.

Using the initial condition y(1) = 4, we ﬁnd y(1) = 14 + C = 4, so C = 3. Thus, y(x) = x 4 + 3. dy d=y x −1/22, y(1) = 1 dx = 3t + cos t, y(0) = 12 dt dy SOLUTION Let d x = x −1/2 . Then

83.

" y(x) =

x −1/2 d x = 2x 1/2 + C.

√ Using the initial condition y(1) = 1, we ﬁnd y(1) = 2 1 + C = 1, so C = −1. Thus, y(x) = 2x 1/2 − 1. 85. Find f (t), assuming that f (t) = 1 − 2t, f (0) = 2, and f (0) = −1. dy π4 ) = = 12 − 2t. Then = sec2 x, y( SOLUTION d x Suppose f (t) " " f (t) dt = (1 − 2t) dt = t − t 2 + C. f (t) = Using the initial condition f (0) = −1, we ﬁnd f (0) = 0 − 02 + C = −1, so C = −1. Thus, f (t) = t − t 2 − 1. Now, " " 1 1 f (t) = f (t) dt = (t − t 2 − 1) dt = t 2 − t 3 − t + C. 2 3 Using the initial condition f (0) = 2, we ﬁnd f (0) = 12 02 − 13 03 − 0 + C = 2, so C = 2. Thus, f (t) =

1 2 1 3 t − t − t + 2. 2 3

The driver of an automobile applies the brakes at time t = 0 and comes to a halt after traveling 500 ft. Find the automobile’s velocity at t = 0 assuming that the rate of deceleration was a constant −10 ft/s2 .

5 THE INTEGRAL 5.1 Approximating and Computing Area Preliminary Questions 1. Suppose that [2, 5] is divided into six subintervals. What are the right and left endpoints of the subintervals? 1 If the interval [2, 5] is divided into six subintervals, the length of each subinterval is 5−2 6 = 2 . The right 5 7 9 5 7 9 endpoints of the subintervals are then 2 , 3, 2 , 4, 2 , 5, while the left endpoints are 2, 2 , 3, 2 , 4, 2 . SOLUTION

2. If f (x) = x −2 on [3, 7], which is larger: R2 or L 2 ? SOLUTION On [3, 7], the function f (x) = x −2 is a decreasing function; hence, for any subinterval of [3, 7], the function value at the left endpoint is larger than the function value at the right endpoint. Consequently, L 2 must be larger than R2 . 3. Which of the following pairs of sums are not equal? 4 4 # # (a) i, (c)

i=1 4 #

j,

j=1

=1 5 #

(i − 1)

(b)

(d)

i=2

4 # j=1 4 #

j 2,

5 #

k2

k=2

i(i + 1),

i=1

5 #

( j − 1) j

j=2

SOLUTION

(a) Only the name of the index variable has been changed, so these two sums are the same. (b) These two sums are not the same; the second squares the numbers two through ﬁve while the ﬁrst squares the numbers one through four. (c) These two sums are the same. Note that when i ranges from two through ﬁve, the expression i − 1 ranges from one through four. (d) These two sums are the same. Both sums are 1 · 2 + 2 · 3 + 3 · 4 + 4 · 5. 4. Explain why

100 $

j is equal to

j=1 SOLUTION

100 $ j=0

The ﬁrst term in the sum

j but $100

100 $

1 is not equal to

j=1

1.

j=0

j=0 j is equal to zero, so it may be dropped. More speciﬁcally, 100 #

j =0+

j=0

On the other hand, the ﬁrst term in

100 $

100 #

j=

j=1

100 #

j.

j=1

$100

j=0 1 is not zero, so this term cannot be dropped. In particular, 100 # j=0

1=1+

100 #

1 =

j=1

100 #

1.

j=1

5. We divide the interval [1, 5] into 16 subintervals. (a) What are the left endpoints of the ﬁrst and last subintervals? (b) What are the right endpoints of the ﬁrst two subintervals? 1 Note that each of the 16 subintervals has length 5−1 16 = 4 . (a) The left endpoint of the ﬁrst subinterval is 1, and the left endpoint of the last subinterval is 5 − 14 = 19 4 .

(b) The right endpoints of the ﬁrst two subintervals are 1 + 14 = 54 and 1 + 2 14 = 32 . SOLUTION

6. (a) (b) (c)

Are the following statements true or false? The right-endpoint rectangles lie below the graph if f (x) is increasing. If f (x) is monotonic, then the area under the graph lies between R N and L N . If f (x) is constant, then the right-endpoint rectangles all have the same height.

SOLUTION

(a) False. If f is increasing, then the right-endpoint rectangles lie above the graph. (b) True. If f (x) is increasing, then the area under the graph is larger than L N but smaller than R N ; on the other hand, if f (x) is decreasing, then the area under the graph is larger than R N but smaller than L N . (c) True. The height of the right-endpoint rectangles is given by the value of the function, which, for a constant function, is always the same.

THE INTEGRAL

Exercises 1. An athlete runs with velocity 4 mph for half an hour, 6 mph for the next hour, and 5 mph for another half-hour. Compute the total distance traveled and indicate on a graph how this quantity can be interpreted as an area. SOLUTION The ﬁgure below displays the velocity of the runner as a function of time. The area of the shaded region equals the total distance traveled. Thus, the total distance traveled is (4) (0.5) + (6) (1) + (5) (0.5) = 10.5 miles. 6 5 4 3 2 1

Velocity (mph)

CHAPTER 5

0.5 1 1.5 Time (hours)

2

3. A rainstorm hit Portland, Maine, in October 1996, resulting in record rainfall. The rainfall rate R(t) on October 21 Figure 14 shows the velocity of an object over a 3-min interval. Determine the distance traveled over the intervals is recorded, in inches per hour, in the following table, where t is the number of hours since midnight. Compute the total [0, 3] and [1, 2.5] (remember to convert from miles per hour to miles per minute). rainfall during this 24-hour period and indicate on a graph how this quantity can be interpreted as an area.

SOLUTION

t

0–2

2–4

4–9

9–12

12–20

20–24

R(t)

0.2

0.1

0.4

1.0

0.6

0.25

Over each interval, the total rainfall is the time interval in hours times the rainfall in inches per hour. Thus R = 2(.2) + 2(.1) + 5(.4) + 3(1.0) + 8(.6) + 4(.25) = 11.4 inches.

The ﬁgure below is a graph of the rainfall as a function of time. The area of the shaded region represents the total rainfall. 1 Rainfall (inches per hour)

238

0.8 0.6 0.4 0.2 5

10 15 Time (hours)

20

25

5. Compute R6 , L 6 , and M3 to estimate the distance traveled over [0, 3] if the velocity at half-second intervals is as The velocity of an object is v(t) = 32t ft/s. Use Eq. (2) and geometry to ﬁnd the distance traveled over the time follows: intervals [0, 2] and [2, 5].

SOLUTION

t (s)

0

0.5

1

1.5

2

2.5

3

v (ft/s)

0

12

18

25

20

14

20

3−0 For R6 and L 6 , t = 3−0 6 = .5. For M3 , t = 3 = 1. Then

R6 = 0.5 sec (12 + 18 + 25 + 20 + 14 + 20) ft/sec = .5(109) ft = 54.5 ft, L 6 = 0.5 sec (0 + 12 + 18 + 25 + 20 + 14) ft/sec = .5(89) ft = 44.5 ft, and M3 = 1 sec (12 + 25 + 14) ft/sec = 51 ft. 7. (a) (b)

Consider f (x) = 2x + 3 on [0, 3]. Use the following table of values to estimate the area under the graph of f (x) over [0, 1] by computing the Compute L 6 over [0, 3]. 6 and average ofRR 5 and L 5 . Find the error in these approximations by computing the area exactly using geometry.

SOLUTION

Let f (x) = 2x + 3 on [0, 3]. x

0 0.2 0.4 0.6 0.8 1 % & (a) We partition [0, 3] into 6 equally-spaced subintervals. The left 44 endpoints of40 the subintervals are 0, 12 , 1, 32 , 2, 52 f (x) 50 48 46 42 & % whereas the right endpoints are 12 , 1, 32 , 2, 52 , 3 .

S E C T I O N 5.1

Approximating and Computing Area

239

• Let a = 0, b = 3, n = 6, x = (b − a) /n = 1 , and x k = a + kx, k = 0, 1, . . . , 5 (left endpoints). Then 2

L6 =

5 #

f (x k )x = x

k=0

5 #

f (x k ) =

k=0

1 (3 + 4 + 5 + 6 + 7 + 8) = 16.5. 2

• With x k = a + kx, k = 1, 2, . . . , 6 (right endpoints), we have

R6 =

6 #

f (x k )x = x

k=1

6 #

f (x k ) =

k=1

1 (4 + 5 + 6 + 7 + 8 + 9) = 19.5. 2

(b) Via geometry (see ﬁgure below), the exact area is A = 12 (3) (6) + 32 = 18. Thus, L 6 underestimates the true area (L 6 − A = −1.5), while R6 overestimates the true area (R6 − A = +1.5). y 9 6 3 x 0.5

1

1.5

2

2.5

3

9. Estimate R6 , L 6 , and M6 over [0, 1.5] for the function in Figure 15. Let f (x) = x 2 + x − 2. y (a) Calculate R3 and L 3 over [2, 5]. 3 (b) Sketch the graph of f and the rectangles that make up each approximation. Is the area under the graph larger or smaller than R3 ? Than L 3 ? 2 1 x 0.5

1

1.5

FIGURE 15 SOLUTION

& % Let f (x) on [0, 32 ] be given by Figure 15. For n = 6, x = ( 32 − 0)/6 = 14 , {x k }6k=0 = 0, 14 , 12 , 34 , 1, 54 , 32 .

Therefore L6 =

5 1# 1 f (x k ) = (2.4 + 2.35 + 2.25 + 2 + 1.65 + 1.05) = 2.925, 4 k=0 4

R6 =

6 1# 1 f (x k ) = (2.35 + 2.25 + 2 + 1.65 + 1.05 + 0.65) = 2.4875, 4 k=1 4

6 1# 1 1 f x k − x = (2.4 + 2.3 + 2.2 + 1.85 + 1.45 + 0.8) = 2.75. M6 = 4 k=1 2 4 1 . Sketch the graph of f (x) and draw the rectangles whose area is represented by 11. LetEstimate f (x) = R x,2M + 1, and and Lxfor = the 3 graph in Figure 16. 2 3 6 $6 the sum i=1 f (1 + ix)x. SOLUTION Because the summation index runs from i = 1 through i = 6, we will treat this as a right-endpoint approximation to the area under the graph of y = x 2 + 1. With x = 13 , it follows that the right endpoints of the subintervals are x1 = 43 , x2 = 53 , x3 = 2, x 4 = 73 , x5 = 83 and x 6 = 3. The sketch of the graph with the rectangles $6 represented by the sum i=1 f (1 + ix)x is given below. y 3 2.5 2 1.5 1 0.5 x 0.5

1

1.5

2

2.5

3

In Exercises 13–24, theshaded approximation forinthe given17. function interval. do these rectangles represent? Calculate thecalculate area of the rectangles Figure Whichand approximation

240

CHAPTER 5

THE INTEGRAL

13. R8 ,

f (x) = 7 − x,

SOLUTION

[3, 5]

% & Let f (x) = 7 − x on [3, 5]. For n = 8, x = (5 − 3)/8 = 14 , and {x k }8k=0 = 3, 3 14 , 3 12 , 3 34 , 4, 4 14 , 4 12 , 4 34 , 5 .

Therefore R8 =

8 1# 1 1 (7 − x k ) = (3.75 + 3.5 + 3.25 + 3 + 2.75 + 2.5 + 2.25 + 2) = (23) = 5.75. 4 k=1 4 4

= x=2 , 7 − [0,x,1] [3, 5] 15. M4 ,M ,f (x)f (x) 4 ' (3 1 SOLUTION Let f (x) = x 2 on [0, 1]. For n = 4, x = (1 − 0)/4 = 4 and x k∗ = {.125, .375, .625, .875}. k=0 Therefore M4 =

3 1# 1 (x ∗ )2 = (0.1252 + 0.3752 + 0.6252 + 0.8752 ) = .328125. 4 k=0 k 4

17. R6 , f (x) = 2x 2 √ − x + 2, [1, 4] M6 , f (x) = x, [2, 5] % & 1 1 1 1 SOLUTION Let f (x) = 2x 2 − x + 2 on [1, 4]. For n = 6, x = (4 − 1)/6 = 2 , {x k }6 k=0 = 1, 1 2 , 2, 2 2 , 3, 3 2 , 4 . Therefore R6 =

6 1# 1 (2x 2 − x k + 2) = (5 + 8 + 12 + 17 + 23 + 30) = 47.5. 2 k=1 k 2

19. L 5 , f (x) = x −1 , 2 [1, 2] L 6 , f (x) = 2x − x + 2, [1, 4] % & (2−1) 1 6 7 8 9 5 SOLUTION Let f (x) = x −1 on [1, 2]. For n = 5, x = 5 = 5 , {x k }k=0 = 1, 5 , 5 , 5 , 5 , 2 . Therefore L5 =

4 1# 1 5 5 5 5 (x k )−1 = 1+ + + + ≈ .745635. 5 k=0 5 6 7 8 9

21. L 4 , f (x) = cos x, [ π4 , π2 ] M4 , f (x) = x −2 , [1, 3] π π SOLUTION Let f (x) = cos x on [ 4 , 2 ]. For n = 4, x =

π (π /2 − π /4) = 4 16

and

{x k }4k=0 =

π 5π 3π 7π π , , , , . 4 16 8 16 2

Therefore L4 =

3 π # cos x k ≈ .361372. 16 k=0

1 π ,5]3π ] = =2sin x,, [[1, 23. M4 ,R , f (x) f (x) 5 4 4 x +1 SOLUTION

Let f (x) =

1 . For n = 4, x = 5−1 = 1, and {x ∗ }3 k k=0 = {1.5, 2.5, 3.5, 4.5}. Therefore 4 x 2 +1

M4 = 1

3 #

1 = ∗ (x )2 + 1 k=0 k

4 4 4 4 + + + 13 29 53 85

≈ .568154.

In Exercises 25–28, use2the Graphical Insight on page 254 to obtain bounds on the area. L 5 , f (x) = x + 3|x|, [−3, 2] √ 25. Let A be the area under the graph of f (x) = x over [0, 1]. Prove that 0.51 ≤ A ≤ 0.77 by computing R4 and L 4 . Explain your reasoning. SOLUTION

1 1 1 3 4 For n = 4, x = 1−0 4 = 4 and {xi }i=0 = {0 + ix} = {0, 4 , 2 , 4 , 1}. Therefore, √ √ 4 # 1 1 2 3 f (xi ) = + + + 1 ≈ .768 R4 = x 4 2 2 2 i=1

√ √ 1 2 3 1 f (xi ) = L 4 = x 0+ + + ≈ .518. 4 2 2 2 i=0 3 #

Approximating and Computing Area

S E C T I O N 5.1

241

In the plot below, you can see the rectangles whose area is represented by L 4 under the graph and the top of those whose area is represented by R4 above the graph. The area A under the curve is somewhere between L 4 and R4 , so .518 ≤ A ≤ .768.

L 4 , R4 and the graph of f (x). 27. Use R4 and L 4 to show that the area A under the graph of y = sin x over [0, π /2] satisﬁes 0.79 ≤ A ≤ 1.19. Use R6 and L 6 to show that the area A under y = x −2 over [10, 12] satisﬁes 0.0161 ≤ A ≤ 0.0172. SOLUTION Let f (x) = sin x. f (x) is increasing over the interval [0, π /2], so the Insight on page 254 applies, which

4 4 = π8 and {xi }i=0 = {0 + ix}i=0 = {0, π8 , π4 , 38π , π2 }. From indicates that L 4 ≤ A ≤ R4 . For n = 4, x = π /2−0 4 this,

L4 =

3 π# f (xi ) ≈ .79, 8 i=0

R4 =

4 π# f (xi ) ≈ 1.18. 8 i=1

Hence A is between .79 and 1.19.

Left and Right endpoint approximations to A. 29. Show that the area A in Exercise 25 satisﬁes L−1 N ≤ A ≤ R N for all N . Then use a computer algebra system ShowL that the area A under the graph of f (x) = x over [1, 8] satisﬁes to calculate N and R N for N = 100 and 150. Which of these calculations allows you to conclude that A ≈ 0.66 to two decimal places? 1 1 1 1 1 1 1 1 1 1 1 1 1 + √+ + + + + ≤ A ≤ 1 + + + + + + 2 =3 x 4is an 5increasing 6 7function; 8 7 . Now, SOLUTION On [0, 1], f (x) therefore,2 L N3≤ A4≤ R5N for6 all N L 100 = .6614629

R100 = .6714629

L 150 = .6632220

R150 = .6698887

Using the values obtained with N = 150, it follows that .6632220 ≤ A ≤ .6698887. Thus, to two decimal places, A ≈ .66. 31. Calculate the following sums: Show that the area A in Exercise 265 satisﬁes R N ≤ A ≤ L N for all N . Use 5 4 a computer algebra system to # # # L N and R N for N sufﬁciently (a) calculate 3 (b) large3to determine A to within an error(c)of at most k 3 10−4 . i=1 4 #

π (d) sin j 2 j=3

(e)

i=0 4 #

1 k − 1 k=2

(f)

k=2 3 #

3j

j=0

SOLUTION

(a)

(b)

(c)

5 # i=1 5 # i=0 4 #

3 = 3 + 3 + 3 + 3 + 3 = 15. Alternatively,

5 #

3=3

i=1

3 = 3 + 3 + 3 + 3 + 3 + 3 = 18. Alternatively,

5 #

1 = (3)(5) = 15.

i=1 5 # i=0

3=3

5 #

= (3)(6) = 18.

i=0

k 3 = 23 + 33 + 43 = 99. Alternatively,

k=2 4 # k=2

k3 =

4 #

k=1

k3 −

1 #

k=1

k3

=

44 43 42 + + 4 2 4

−

14 13 12 + + 4 2 4

= 99.

242

CHAPTER 5

THE INTEGRAL

(d)

4 #

sin

j=3 4 #

jπ 2

= sin

3π 2

+ sin

4π 2

= −1 + 0 = −1.

1 1 11 1 =1+ + = . k − 1 2 3 6 k=2 3 # 3 j = 1 + 3 + 32 + 33 = 40. (f)

(e)

j=0 200 # = 1, b3 = it17, b4 = −17. Let b1 = 3, b2j by 33. Calculate writing asand a difference of Calculate two sums the andsums. using formula (3). 4 2 3 # # # j=101 bk b j (a) bi (b) (b j + 2 ) (c) b SOLUTION i=2 j=1 k=1 k+1 200 #

j=

j=101

200 #

j−

j=1

100 #

j=

j=1

2002 200 + 2 2

−

1002 100 + 2 2

= 20100 − 5050 = 15050.

In Exercises 34–39, write the sum in summation notation. 35. (22 + 2) + (32 + 3) + (42 + 4) + (52 + 5) 47 + 57 + 67 + 77 + 87 SOLUTION The ﬁrst term is 22 + 2, and the last term is 52 + 5, so it seems that the sum limits are 2 and 5, and the kth term is k 2 + k. Therefore, the sum is: 5 #

(k 2 + k).

k=2

1 +213 + 2 +323 + · · · +4 n + n 3 5 (2 + 2) + (2 + 2) + (2 + 2) + (2 + 2) SOLUTION The ﬁrst term is 1 + 13 and the last term is n + n 3 , so it seems the summation limits are 1 through n, and the k-th term is k + k 3 . Therefore, the sum is 37.

n #

k + k3.

k=1

π

π π 39. sin(π1) + sin 2 + sin + ·n· · + sin + 2 + ··· + 3 n+1 2·3 3·4 (n + 1)(n + 2)

π π SOLUTION The ﬁrst summand is sin 1 = sin 0+1 hence, sin(π ) + sin

π 2

+ sin

π 3

+ · · · + sin

π n+1

=

n # i=0

sin

π . i +1

In Exercises 40–47, use linearity and formulas (3)–(5) to rewrite and evaluate the sums. 41.

20 # 15 + 1) # (2k 12 j 3 k=1 j=1

20 #

SOLUTION

(2k + 1) = 2

k=1

43.

20 # k=1

k+

20 # k=1

1=2

202 20 + 2 2

+ 20 = 440.

200 # 150k 3 # (2k + 1) k=100 k=51

SOLUTION

By rewriting the sum as a difference of two power sums,

200 #

k3 =

k=100

30 2 # 4 j 10 #6 j + 45. (3 − 322 ) j=2 =1

200 # k=1

k3 −

99 # k=1

k3 =

2004 2003 2002 + + 4 2 4

−

994 993 992 + + 4 2 4

= 379507500.

Approximating and Computing Area

S E C T I O N 5.1 SOLUTION

30 # j=2

4 j2 6j + 3

⎛ ⎞ ⎛ ⎞ 30 30 1 30 1 # # # # 4# 4 2 2 2 j+ j = 6⎝ j− j⎠ + ⎝ j − j ⎠ =6 3 j=2 3 j=1 j=2 j=1 j=1 j=1 30 4 303 302 30 302 + −1 + + + −1 =6 2 2 3 3 2 6 30 #

= 6 (464) +

47.

37816 46168 4 = . (9454) = 2784 + 3 3 3

30 # 50 2 − 4s − 1) # (3s j ( j − 1) s=1 j=0

SOLUTION

30 #

(3s 2 − 4s − 1) = 3

s=1

30 #

s2 − 4

s=1

30 #

s−

s=1

30 #

1=3

s=1

303 302 30 + + 3 2 6

In Exercises 48–51, calculate the sum, assuming that a1 = −1,

10 #

ai = 10, and

i=1

49.

10 #

302 30 + 2 2

− 30 = 26475.

bi = 7.

i=1

10 # 10 − b ) # (a i i 2ai i=1 i=1

10 #

SOLUTION

(ai − bi ) =

i=1

51.

−4

10 #

ai −

i=1

10 #

bi = 10 − 7 = 3.

i=1

10 # # a10i

(3a + 4b ) 10 10 # # SOLUTION ai = ai − a1 = 10 − (−1) = 11. i=2 =1

i=2

i=1

In Exercises 52–55, use formulas (3)–(5) to evaluate the limit. 53.

N 3 # Nj # i N →∞ limj=1 N 4 2 N →∞ N i=1

lim

SOLUTION

Let s N =

N # j3 . Then N4 j=1 N 1 # 1 j3 = 4 sN = 4 N j=1 N

Therefore, lim s N = N →∞

N4 N3 N2 + + 4 2 4

=

1 1 1 . + + 4 2N 4N 2

1 . 4

N # i 32 20 N # 55. lim i −i +1 N N →∞ lim N4 i=1 N →∞ N3 i=1 N # 20 i3 SOLUTION Let s N = − . Then N N4 i=1 N N 1 # 20 # 1 i3 − 1= 4 sN = 4 N i=1 N i=1 N

Therefore, lim s N = N →∞

1 79 − 20 = − . 4 4

N4 N3 N2 + + 4 2 4

− 20 =

1 1 1 − 20. + + 4 2N 4N 2

243

244

CHAPTER 5

THE INTEGRAL

In Exercises 56–59, calculate the limit for the given function and interval. Verify your answer by using geometry. 57.

lim L N , f (x) = 5x, [1, 3] lim R N , f (x) = 5x, [0, 3] N →∞ N →∞

SOLUTION Let f (x) = 5x on [1, 3]. Let N > 0 be an integer, and set a = 1, b = 3, and x = (b − a)/N = 2/N . Also, let x k = a + kx = 1 + 2k N , k = 0, 1, . . . N − 1 be the left endpoints of the N subintervals of [1, 3]. Then

−1 2 N# 2k 10 5 1+ = N N N k=0 k=0 10 20 (N − 1)2 N −1 = N+ 2 + = 20 − N 2 2 N

L N = x

N −1 #

f (xk ) =

N −1 #

1+

k=0

−1 20 N# k N k=0

30 20 + 2. N N

The area under the graph is lim L N = 20.

N →∞

The region under the curve is a trapezoid with base width 2 and heights 5 and 15. Therefore the area is 12 (2)(5 + 15) = 20, which agrees with the value obtained from the limit of the left-endpoint approximations. 59.

lim M N , f (x) = x, [0, 1] lim L N , f (x) = 6 − 2x, N →∞ N →∞

[0, 2]

Let f (x) = x on [0, 1]. Let N > 0 be an integer and set a = 0, b = 1, and x = (b − a)/N = N1 . Also, let x k∗ = 0 + (k − 12 )x = 2k−1 2N , k = 1, 2, . . . N be the midpoints of the N subintervals of [0, 1]. Then SOLUTION

N N 1 # 2k − 1 1 # = (2k − 1) N k=1 2N 2N 2 k=1 k=1 N # 1 1 N 1 1 N2 = k−N = 2 + − = . 2 2 2 2 2N 2 2N N k=1

M N = x

N #

f (x k∗ ) =

The area under the curve over [0, 1] is lim M N =

N →∞

1 . 2

The region under the curve over [0, 1] is a triangle with base and height 1, and thus area 12 , which agrees with the answer obtained from the limit of the midpoint approximations. In Exercises 60–69, ﬁnd a formula for R N for the given function and interval. Then compute the area under the graph as a limit. 61. f (x) = x 3 , 2[0, 1] f (x) = x , [0, 1] SOLUTION

1−0 1 Let f (x) = x 3 on the interval [0, 1]. Then x = = and a = 0. Hence, N N N N 1 # 1 1 # 3 f (0 + jx) = j3 j = R N = x N j=1 N3 N 4 j=1 j=1 N4 1 N3 N2 1 1 1 = 4 + + = + + 4 2 4 4 2N N 4N 2 N #

and

1 1 1 + + 2N N →∞ 4 4N 2

lim R N = lim

N →∞

1] 63. f (x) = 1 − x33 , [0, f (x) = x + 2x 2 , [0, 3] SOLUTION

Let f (x) = 1 − x 3 on the interval [0, 1]. Then x = R N = x

N # j=1

f (0 + jx) =

N 1 1 # 1 − j3 3 N j=1 N

=

1 . 4

1−0 1 = and a = 0. Hence, N N

Approximating and Computing Area

S E C T I O N 5.1

N N 1 # 1 # 1 1 = 1− 4 j3 = N − 4 N j=1 N N j=1 N

and

N4 N3 N2 + + 4 2 4

1 1 3 − − 2N N →∞ 4 4N 2

lim R N = lim

N →∞

=

=1−

1 1 1 − − 4 2N 4N 2

3 . 4

65. f (x) = 3x 2 −2x + 4, [1, 5] f (x) = 3x − x + 4, [0, 1] 5−1 4 SOLUTION Let f (x) = 3x 2 − x + 4 on the interval [1, 5]. Then x = = and a = 1. Hence, N N N N N N N # 4 # 20 80 # 24 # 48 192 # f (1 + jx) = j2 + 2 j+ 1 R N = x +6 = 3 j2 2 + j N j=1 N N j=1 N N j=1 N j=1 j=1 192 = 3 N

N3 N2 N + + 3 2 6

80 + 2 N

and

N2 N + 2 2

+

40 96 32 24 N = 64 + + 2 + 40 + + 24 N N N N

136 32 128 + + 2 = 128. N N →∞ N

lim R N = lim

N →∞

2 , [2, 4] 67. f (x)f (x) = x= 2x + 7, [3, 6] 4−2 2 SOLUTION Let f (x) = x 2 on the interval [2, 4]. Then x = N = N and a = 2. Hence, N N N N N # 2 # 16 # 8 # 4 8 8 # 2 R N = x f (2 + jx) = 1 + j + j2 4+ j + j = N j=1 N N j=1 N2 N 2 j=1 N 3 j=1 j=1

8 16 = N+ 2 N N

N2 N + 2 2

8 + 3 N

N3 N2 N + + 3 2 6

and

56 12 4 + + N N →∞ 3 3N 2

lim R N = lim

N →∞

8 8 4 4 + + + N 3 N 3N 2

=8+8+

=

56 . 3

2 , [a, b] (a, b constants with a < b) 69. f (x)f (x) = x= 2x + 1, [a, b] (a, b constants with a < b)

b−a Let f (x) = x 2 on the interval [a, b]. Then x = . Hence, N N N # (b − a) # (b − a) (b − a)2 2 2 a + 2a j f (a + jx) = R N = x +j N N N2 j=1 j=1

SOLUTION

=

N N N a 2 (b − a) # 2a(b − a)2 # (b − a)3 # 1+ j + j2 2 3 N N N j=1 j=1 j=1

a 2 (b − a) 2a(b − a)2 = N+ N N2 = a 2 (b − a) + a(b − a)2 + and

lim R N = lim

N →∞

N →∞

N N2 + 2 2

N2 N N3 + + 3 2 6

a(b − a)2 (b − a)3 (b − a)3 (b − a)3 + + + N 3 2N 6N 2

(b − a)3 1 1 = b3 − a 3 . 3 3 3

In Exercises 70–73, describe the area represented by the limits.

lim

(b − a)3 + N3

a(b − a)2 (b − a)3 (b − a)3 (b − a)3 a 2 (b − a) + a(b − a)2 + + + + N 3 2N 6N 2

= a 2 (b − a) + a(b − a)2 +

N 4 1 # j

245

246

CHAPTER 5

THE INTEGRAL

71.

N 3 # 3j 4 2+ N N →∞ N j=1 lim

SOLUTION

The limit N 3 # 3 4 2+ j · N N →∞ N j=1

lim R N = lim

N →∞

represents the area between the graph of f (x) = x 4 and the x-axis over the interval [2, 5]. N jπ π # π −1 73. lim sin + j 4 5 N# 3 + 2N N →∞ lim2N −2 5 N N →∞ Nj=1 j=0 SOLUTION The limit N π # π jπ sin + 3 2N N →∞ 2N j=1 lim

represents the area between the graph of f (x) = sin x and the x axis over the interval [ π3 , 56π ]. In Exercises 75–80, use the approximation 2indicated (in summation notation) to express the area under the graph as a N j 1 # limit butEvaluate do not evaluate. lim 1− by interpreting it as the area of part of a familiar geometric ﬁgure. N N →∞ N j=1 [0, π ] 75. R N , f (x) = sin x over SOLUTION

Let f (x) = sin x over [0, π ] and set a = 0, b = π , and x = (b − a) /N = π /N . Then R N = x

N #

f (x k ) =

k=1

N kπ π # sin . N k=1 N

Hence N kπ π # sin N N →∞ N k=1

lim R N = lim

N →∞

is the area between the graph of f (x) = sin x and the x-axis over [0, π ]. 77. M N , f (x) = tan x−1 over [ 1 , 1] R N , f (x) = x over2[1, 7] SOLUTION

1− 1

1 and a = 1 . Hence Let f (x) = tan x over the interval [ 12 , 1]. Then x = N 2 = 2N 2 N N # 1 1 1 1 # 1 1 M N = x f tan + j− x = + j− 2 2 2N j=1 2 2N 2 j=1

and so N 1 1 1 1 # tan + j− 2 2N 2 N →∞ 2N j=1

lim M N = lim

N →∞

is the area between the graph of f (x) = tan x and the x-axis over [ 12 , 1]. 79. L N , f (x) = cos x over [ π8 , π ] M N , f (x) = x −2 over [3, 5] SOLUTION

π − π8 7π Let f (x) = cos x over the interval π8 , π . Then x = = and a = π8 . Hence, N 8N N −1

−1 # π π 7π N# 7π f cos + jx = +j L N = x 8 8N j=0 8 8N j=0

and −1 7π N# π 7π cos +j 8 8N N →∞ 8N j=0

lim L N = lim

N →∞

is the area between the graph of f (x) = cos x and the x-axis over [ π8 , π ]. LN,

f (x) = cos x over [ π8 , π4 ]

S E C T I O N 5.1

Approximating and Computing Area

247

In Exercises 81–83, let f (x) = x 2 and let R N , L N , and M N be the approximations for the interval [0, 1]. 1 1 1 1 1 . Interpret the quantity as the area of a region. + + + 2 3 2N 2N 6N 6N 2 1 Let f (x) = x 2 on [0, 1]. Let N > 0 be an integer and set a = 0, b = 1 and x = 1−0 N = N . Then N N # 1 # 1 1 N2 N 1 1 1 N3 2 R N = x f (0 + jx) = j = 3 . + + = + + 2 N 3 2 6 3 2N N N 6N 2 j=1 j=1

Show that R N =

81. SOLUTION

The quantity 6 1 + 2 2N N

in

RN =

1 1 1 + + 3 2N 6N 2

represents the collective area of the parts of the rectangles that lie above the graph of f (x). It is the error between R N and the true area A = 13 . y 1 0.8 0.6 0.4 0.2 x 0.2

0.4

0.6

0.8

1

83. For each of R N , L N , and M N , ﬁnd the smallest integer N for which the error is less than 0.001. Show that SOLUTION

1

1

1

L Nwhen: = − , + • For R N , the error is less than .001 3 2N 6N 2

MN =

1 1 − 3 12N 2

1 1 Then rank the three approximations R N , L N , and M+N in order of increasing accuracy (use the formula for R N in < .001. 2N 6N 2 Exercise 81). We ﬁnd an adequate solution in N : 1 1 < .001 + 2N 6N 2 3N + 1 < .006(N 2 ) 0 < .006N 2 − 3N − 1,

√ 9.024 = 500.333. Hence R in particular, if N > 3+ .012 501 is within .001 of A. • For L N , the error is less than .001 if

1 1 − + < .001. 2N 6N 2

We solve this equation for N :

1 1 < .001 − 2N 6N 2 3N − 1 6N 2 < .001 3N − 1 < .006N 2 0 < .006N 2 − 3N + 1,

√

9−.024 = 499.666. Therefore, L 500 is within .001 units of A. which is satisﬁed if N > 3+ .012 1 • For M N , the error is given by − 2 , so the error is less than .001 if 12N

1 < .001 12N 2 1000 < 12N 2 9.13 < N Therefore, M10 is within .001 units of the correct answer.

248

CHAPTER 5

THE INTEGRAL

Further Insights and Challenges 85. Draw the graph of a positive continuous function on an interval such that R2 and L 2 are both smaller than the exact Although the accuracy of R N generally improves as N increases, this need not be true for small values of N . area under the graph. Can such a function be monotonic? Draw the graph of a positive continuous function f (x) on an interval such that R1 is closer than R2 to the exact area SOLUTION the plot area under the saw-tooth function f (x) is 3, whereas L 2 = R2 = 2. Thus L 2 and R2 under the In graph. Canbelow, such athe function be monotonic? are both smaller than the exact area. Such a function cannot be monotonic; if f (x) is increasing, then L N underestimates and R N overestimates the area for all N , and, if f (x) is decreasing, then L N overestimates and R N underestimates the area for all N . Left/right-endpoint approximation, n = 2 2

1

1

2

Assume f (x) isstatement monotonic. Prove thatThe M Nendpoint lies between R N and Lare M N iswhen closerf to 87. N and Explain thethat following graphically: approximations lessthat accurate (x)the is actual area under the graph than both R N and L N . Hint: Argue from Figure 18; the part of the error in R N due to the ith large. rectangle is the sum of the areas A + B + D, and for M N it is |B − E|. A B C

D E F

x i − 1 midpoint x i

x

FIGURE 18 SOLUTION

Suppose f (x) is monotonic increasing on the interval [a, b], x =

b−a , N

N = {a, a + x, a + 2x, . . . , a + (N − 1)x, b} {x k }k=0

and ' ∗ ( N −1 xk k=0 =

(a + (N − 1)x) + b a + (a + x) (a + x) + (a + 2x) , ,..., . 2 2 2

Note that xi < xi∗ < xi+1 implies f (xi ) < f (xi∗ ) < f (xi+1 ) for all 0 ≤ i < N because f (x) is monotone increasing. Then −1 −1 N b − a N# b − a N# b−a # ∗ f (x k ) < M N = f (xk ) < R N = f (xk ) LN = N k=0 N k=0 N k=1 Similarly, if f (x) is monotone decreasing, −1 −1 N b − a N# b − a N# b−a # ∗ LN = f (x k ) > M N = f (xk ) > R N = f (xk ) N k=0 N k=0 N k=1 Thus, if f (x) is monotonic, then M N always lies in between R N and L N . Now, as in Figure 18, consider the typical subinterval [xi−1 , xi ] and its midpoint xi∗ . We let A, B, C, D, E, and F be the areas as shown in Figure 18. Note that, by the fact that xi∗ is the midpoint of the interval, A = D + E and F = B + C. Let E R represent the right endpoint approximation error ( = A + B + D), let E L represent the left endpoint approximation error ( = C + F + E) and let E M represent the midpoint approximation error ( = |B − E|). • If B > E, then E M = B − E. In this case,

E R − E M = A + B + D − (B − E) = A + D + E > 0, so E R > E M , while E L − E M = C + F + E − (B − E) = C + (B + C) + E − (B − E) = 2C + 2E > 0, so E L > E M . Therefore, the midpoint approximation is more accurate than either the left or the right endpoint approximation.

Approximating and Computing Area

S E C T I O N 5.1

249

• If B < E, then E M = E − B. In this case,

E R − E M = A + B + D − (E − B) = D + E + D − (E − B) = 2D + B > 0, so that E R > E M while E L − E M = C + F + E − (E − B) = C + F + B > 0, so E L > E M . Therefore, the midpoint approximation is more accurate than either the right or the left endpoint approximation. • If B = E, the midpoint approximation is exactly equal to the area. Hence, for B < E, B > E, or B = E, the midpoint approximation is more accurate than either the left endpoint or the right endpoint approximation. thisfor exercise, we prove that lim R N and lim L N exist and are equal if f (x) is positive and ProveInthat any function f (x) onthe [a,limits b], N →∞ N →∞ increasing [the case of f (x) decreasing is similar]. We usebthe − aconcept of a least upper bound discussed in Appendix B. = 1. ( f (b) − f (a)). N − N ≥ (a) Explain with a graph why L N ≤ R M forRall N ,LM N (b) By part (a), the sequence {L N } is bounded by R M for any M, so it has a least upper bound L. By deﬁnition, L is the smallest number such that L N ≤ L for all N . Show that L ≤ R M for all M. (c) According to part (b), L N ≤ L ≤ R N for all N . Use Eq. (8) to show that lim L N = L and lim R N = L. 89.

N →∞

N →∞

SOLUTION

(a) Let f (x) be positive and increasing, and let N and M be positive integers. From the ﬁgure below at the left, we see that L N underestimates the area under the graph of y = f (x), while from the ﬁgure below at the right, we see that R M overestimates the area under the graph. Thus, for all N , M ≥ 1, L N ≤ R M . y

y

x

x

(b) Because the sequence {L N } is bounded above by R M for any M, each R M is an upper bound for the sequence. Furthermore, the sequence {L N } must have a least upper bound, call it L. By deﬁnition, the least upper bound must be no greater than any other upper bound; consequently, L ≤ R M for all M. (c) Since L N ≤ L ≤ R N , R N − L ≤ R N − L N , so |R N − L| ≤ |R N − L N |. From this, lim |R N − L| ≤ lim |R N − L N |.

N →∞

N →∞

By Eq. (8), lim |R N − L N | = lim

N →∞

1

N →∞ N

|(b − a)( f (b) − f (a))| = 0,

so lim |R N − L| ≤ |R N − L N | = 0, hence lim R N = L. N →∞

N →∞

Similarly, |L N − L| = L − L N ≤ R N − L N , so

|L N − L| ≤ |R N − L N | =

(b − a) ( f (b) − f (a)). N

This gives us that lim |L N − L| ≤ lim

N →∞

1

N →∞ N

|(b − a)( f (b) − f (a))| = 0,

so lim L N = L. N →∞

This proves lim L N = lim R N = L . N →∞

N →∞

− the A| <area 10−4 for its thegraph given over function andUse interval. In Exercises 91–92,that use fEq. to ﬁnd aand value of N suchand thatlet |RA N be Assume (x)(9) is positive monotonic, under [a, b]. Eq. (8) to √ show that 91. f (x) = x, [1, 4] √ b and −a SOLUTION Let f (x) = x on [1, 4]. Then b = 4, a = 1, |R N − A| ≤ | f (b) − f (a)| N 4−1 3 3 |R N − A| ≤ ( f (4) − f (1)) = (2 − 1) = . N N N √ We need N3 < 10−4 , which gives N > 30000. Thus |R30001 − A| < 10−4 for f (x) = x on [1, 4]. f (x) =

9 − x 2,

[0, 3]

250

CHAPTER 5

THE INTEGRAL

5.2 The Definite Integral Preliminary Questions " b

1. What is a

d x [here the function is f (x) = 1]?

" b

SOLUTION

a

dx =

" b a

1 · d x = 1(b − a) = b − a.

2. Are the following statements true or false [assume that f (x) is continuous]? " b f (x) d x is the area between the graph and the x-axis over [a, b]. (a) a " b (b) f (x) d x is the area between the graph and the x-axis over [a, b] if f (x) ≥ 0. a " b f (x) d x is the area between the graph of f (x) and the x-axis over [a, b]. (c) If f (x) ≤ 0, then − a

SOLUTION

! (a) False. ab f (x) d x is the signed area between the graph and the x-axis. (b) True. (c) True. " π cos x d x = 0. 3. Explain graphically why 0

Because cos(π − x) = − cos x, the “negative” area between the graph of y = cos x and the x-axis over [ π2 , π ] exactly cancels the “positive” area between the graph and the x-axis over [0, π2 ]. " −1 8 d x negative? 4. Is SOLUTION

−5

No, the integrand is the positive constant 8, so the value of the integral is 8 times the length of the integration interval (−1 − (−5) = 4), or 32. " 6 5. What is the largest possible value of f (x) d x if f (x) ≤ 13 ? SOLUTION

SOLUTION

Because f (x) ≤ 13 ,

" 6 0

0

f (x) d x ≤

1 (6 − 0) = 2. 3

Exercises In Exercises 1–10, draw a graph of the signed area represented by the integral and compute it using geometry. " 3 1.

−3

2x d x

The region bounded by the graph of y = 2x and the x-axis over the interval [−3, 3] consists of two right triangles. One has area 12 (3)(6) = 9 below the axis, and the other has area 12 (3)(6) = 9 above the axis. Hence, SOLUTION

" 3 −3

2x d x = 9 − 9 = 0. y 6 4 2

−3

3.

" 1 " 3 (3x + 4) d x (2x + 4) d x −2 −2

−2

−1 −2 −4 −6

x 1

2

3

S E C T I O N 5.2

The Deﬁnite Integral

251

SOLUTION The region bounded by the graph of y = 3x + 4 and the x-axis over the interval [−2, 1] consists of two right triangles. One has area 12 ( 23 )(2) = 23 below the axis, and the other has area 12 ( 73 )(7) = 49 6 above the axis. Hence,

" 1 −2

49 2 15 − = . 6 3 2

(3x + 4) d x =

y 8 6 4 2 −2

x

−1

1

−2

" 8 " 1 5. (7 − x) d x 4 dx 6 −2

The region bounded by the graph of y = 7 − x and the x-axis over the interval [6, 8] consists of two right triangles. One triangle has area 12 (1)(1) = 12 above the axis, and the other has area 12 (1)(1) = 12 below the axis. Hence, SOLUTION

" 8 6

(7 − x) d x =

1 1 − = 0. 2 2

y 1 0.5 x 2

−0.5

4

6

8

−1

7.

" 5 " 3π /2 2 25 − x d x sin x d x 0

The region bounded by the graph of y = 25 − x 2 and the x-axis over the interval [0, 5] is one-quarter of a circle of radius 5. Hence, " 5 1 25π 25 − x 2 d x = π (5)2 = . 4 4 0 π /2

SOLUTION

y 5 4 3 2 1 x 1

9.

2

3

4

5

" 2 " 3 (2 − |x|) d x |x| d x −2 −2

The region bounded by the graph of y = 2 − |x| and the x-axis over the interval [−2, 2] is a triangle above the axis with base 4 and height 2. Consequently, " 2 1 (2 − |x|) d x = (2)(4) = 4. 2 −2 SOLUTION

y 2 1

−2

" 1

(2x − |x|) d x

−1

x 1

2

252

CHAPTER 5

THE INTEGRAL

" 6 11. Calculate 0

(4 − x) d x in two ways:

(a) As the limit lim R N N →∞

(b) By sketching the relevant signed area and using geometry ! ! Let f (x) = 4 − x over [0, 6]. Consider the integral 06 f (x) d x = 06 (4 − x) d x. (a) Let N be a positive integer and set a = 0, b = 6, x = (b − a) /N = 6/N . Also, let x k = a + kx = 6k/N , k = 1, 2, . . . , N be the right endpoints of the N subintervals of [0, 6]. Then N N N N # # 6 # 6 # 6 6k f (x k ) = 1 − k = 4 4− R N = x N k=1 N N N k=1 k=1 k=1 6 6 N2 N 18 = 4N − + =6− . N N 2 2 N SOLUTION

18 = 6. N N →∞ N →∞ (b) The region bounded by the graph of y = 4 − x and the x-axis over the interval [0, 6] consists of two right triangles. One triangle has area 12 (4)(4) = 8 above the axis, and the other has area 12 (2)(2) = 2 below the axis. Hence, Hence lim R N = lim

6−

" 6 0

(4 − x) d x = 8 − 2 = 6.

y 4 2 x 1

2

3

4

5

6

−2

13. Evaluate the integrals for f (x) shown in Figure 13. " 5 " " 2 Calculate (2x + 1) d x in two ways: As the limit lim R 6N and using geometry. f (x) d x 2 (b) f (x) d x (a) N →∞ 0 0 " 6 " 4 f (x) d x (d) | f (x)| d x (c) 1

1

y

y = f(x)

x 2

4

6

FIGURE 13 The two parts of the graph are semicircles.

Let f (x) be given by Figure 13. ! (a) The deﬁnite integral 02 f (x) d x is the signed area of a semicircle of radius 1 which lies below the x-axis. Therefore,

SOLUTION

" 2 0

1 π f (x) d x = − π (1)2 = − . 2 2

! (b) The deﬁnite integral 06 f (x) d x is the signed area of a semicircle of radius 1 which lies below the x-axis and a semicircle of radius 2 which lies above the x-axis. Therefore, " 6 1 1 3π f (x) d x = π (2)2 − π (1)2 = . 2 2 2 0 ! (c) The deﬁnite integral 14 f (x) d x is the signed area of one-quarter of a circle of radius 1 which lies below the x-axis and one-quarter of a circle of radius 2 which lies above the x-axis. Therefore, " 4 1 1 3 f (x) d x = π (2)2 − π (1)2 = π . 4 4 4 1

S E C T I O N 5.2

The Deﬁnite Integral

253

! (d) The deﬁnite integral 16 | f (x)| d x is the signed area of one-quarter of a circle of radius 1 and a semicircle of radius 2, both of which lie above the x-axis. Therefore, " 6 1 9π 1 | f (x)| d x = π (2)2 + π (1)2 = . 2 4 4 1 In Exercises 14–15, refer to Figure 14. y

y = g (t)

2 1

t 1

−1

2

3

4

5

−2

FIGURE 14

" a " c " c3 such that " g(t) 5 dt and 15. Find a, b, and g(t) dt are as large as possible. Evaluate g(t) dt and0 g(t) dt. b 0 3" a SOLUTION To make the value of g(t) dt as large as possible, we want to include as much positive area as possible. 0 " c g(t) dt as large as possibe, we want to make sure to This happens when we take a = 4. Now, to make the value of b

include all of the positive area and only the positive area. This happens when we take b = 1 and c = 4. In Exercises 17–20, the Psigned area by the integral. the regions of positive and negative area. Describe thesketch partition and the setrepresented of intermediate points C forIndicate the Riemann sum shown in Figure 15. Compute " the2 value of the Riemann sum. 17. (x − x 2 ) d x 0

SOLUTION

! Here is a sketch of the signed area represented by the integral 02 (x − x 2 ) d x. y 0.5 +

x 1

−0.5

−

2

−1 −1.5 −2

19.

" 2π " 3 sin x d x 2 (2x − x ) d x π 0

SOLUTION

! Here is a sketch of the signed area represented by the integral π2π sin x d x. y 0.4 x −0.4 −0.8

1

2

3

4

5

6

7

−

−1.2

" 3π 21–24, determine the sign of the integral without calculating it. Draw a graph if necessary. In Exercises sin x d x " 1 21.

04

−2

x dx

SOLUTION The integrand is always positive. The integral must therefore be positive, since the signed area has only positive part. " " 1 2π x sin x d x 23. x03 d x

−2

As you can see from the graph below, the area below the axis is greater than the area above the axis. Thus, the deﬁnite integral is negative. SOLUTION

254

CHAPTER 5

THE INTEGRAL y 0.2

− 0.2

+ 1

x

2

3

4

5

6

7

−

− 0.4 − 0.6

" 225–28, In Exercises π sin xcalculate the Riemann sum R( f, P, C) for the given function, partition, and choice of intermediate points. Also, sketch the d xgraph of f and the rectangles corresponding to R( f, P, C). x 0 25. f (x) = x, P = {1, 1.2, 1.5, 2}, C = {1.1, 1.4, 1.9} SOLUTION

Let f (x) = x. With P = {x 0 = 1, x 1 = 1.2, x3 = 1.5, x 4 = 2}

C = {c1 = 1.1, c2 = 1.4, c3 = 1.9},

and

we get R( f, P, C) = x1 f (c1 ) + x 2 f (c2 ) + x 3 f (c3 ) = (1.2 − 1)(1.1) + (1.5 − 1.2)(1.4) + (2 − 1.5)(1.9) = 1.59. Here is a sketch of the graph of f and the rectangles. y 2 1.5 1 0.5 x 0.5

1

1.5

2

2.5

27. f (x) = x + 1, P = {−2, −1.6, −1.2, −0.8, −0.4, 0}, (x) = −1.3, x 2 + x, P −0.5, = {2, 3, C = f{−1.7, −0.9, 0} 4.5, 5}, C = {2, 3.5, 5} SOLUTION

Let f (x) = x + 1. With P = {x 0 = −2, x1 = −1.6, x3 = −1.2, x4 = −.8, x5 = −.4, x 6 = 0}

and C = {c1 = −1.7, c2 = −1.3, c3 = −.9, c4 = −.5, c5 = 0}, we get R( f, P, C) = x 1 f (c1 ) + x 2 f (c2 ) + x 3 f (c3 ) + x 4 f (c4 ) + x 5 f (c5 ) = (−1.6 − (−2))(−.7) + (−1.2 − (−1.6))(−.3) + (−0.8 − (−1.2))(.1) + (−0.4 − (−0.8))(.5) + (0 − (−0.4))(1) = .24. Here is a sketch of the graph of f and the rectangles. y 1 0.5 −2

−1

x − 0.5 −1

In Exercises the=basic integral f (x) 29–36, = sin x,use P {0, π6properties , π3 , π2 }, of C the = {0.4, 0.7,and 1.2}the formulas in the summary to calculate the integrals. " 4 x2 dx 29. 0

" 4

SOLUTION

By formula (4), 0

" 4

x2 dx

x2 dx =

1 3 64 (4) = . 3 3

The Deﬁnite Integral

S E C T I O N 5.2

" 3 31. 0

255

(3t + 4) dt " 3

SOLUTION

0

(3t + 4) dt = 3

" 3 0

" 1 " 32 (u − 2u) du 33. (3x + 4) d x 0

t dt + 4

" 3 0

1 51 . (3)2 + 4(3 − 0) = 2 2

1 dt = 3 ·

−2

SOLUTION

" 1 0

(u 2 − 2u) du =

" 1 0

u 2 du − 2

" 1 0

u du =

1 2 1 3 1 (1) − 2 (1)2 = − 1 = − . 3 2 3 3

" 1 " 32 (x + 2x) d x −a (6y + 7y + 1) d y 0 !b !b !b 1 1 SOLUTION First, 0 (x 2 + x) d x = 0 x 2 d x + 0 x d x = 3 b3 + 2 b2 . Therefore " 1 " 0 " 1 " 1 " −a (x 2 + x) d x = (x 2 + x) d x + (x 2 + x) d x = (x 2 + x) d x − (x 2 + x) d x

35.

−a

−a

=

0

1 3 1 2 ·1 + ·1 − 3 2

0

1 1 (−a)3 + (−a)2 3 2

0

=

1 3 1 2 5 a − a + . 3 2 6

37. Prove computing the limit of right-endpoint approximations: " aby 2 " b x2 dx b4 a x3 dx = . 4 0

7

Let f (x) = x 3 , a = 0 and x = (b − a)/N = b/N . Then

SOLUTION

N N N3 N2 b4 b4 b # b3 b4 # b4 N 4 b4 3 3 + + + + R N = x f (xk ) = k . k · 3 = 4 = 4 = N k=1 4 2 4 4 2N N N N 4N 2 k=1 k=1 " b b4 b4 b4 b4 Hence x 3 d x = lim R N = lim + + . = 2 2N 4 N →∞ N →∞ 4 4N 0 N #

In Exercises 38–45, use the formulas in the summary and Eq. (7) to evaluate the integral. 39.

" 2 " 32 (x +2 2x) d x x dx 0 0

Applying the linearity of the deﬁnite integral and the formulas from Examples 4 and 5,

SOLUTION

" 2 0

41.

(x 2 + 2x) d x =

0

x2 dx + 2

" 2 0

x dx =

1 3 1 20 (2) + 2 · (2)2 = . 3 2 3

" 2 " 3 3 (x − 3x ) d x x dx 0 0

Applying the linearity of the deﬁnite integral, the formula from Example 5 and Eq. (7):

SOLUTION

" 2 0

43.

" 2

(x − x 3 ) d x =

" 2 0

x dx −

" 2 0

x3 dx =

1 2 1 4 (2) − (2) = −2. 2 4

" 0 " 1 (2x − 35) d x −3 (2x − x + 4) d x 0

Applying the linearity of the deﬁnite integral, reversing the limits of integration, and using the formulas for the integral of x and of a constant: SOLUTION

" 0 −3

" 3 1

(2x − 5) d x = 2

x3 dx

" 0 −3

x dx −

" 0 −3

5 d x = −2

" −3 0

x dx −

" 0

1 5 d x = −2 · (−3)2 − 15 = −24. 2 −3

256

CHAPTER 5

THE INTEGRAL

" 2 45. 1

(x − x 3 ) d x

SOLUTION

Applying the linearity and the additivity of the deﬁnite integral: " 2 1

(x − x 3 ) d x =

" 2

=

1

x dx −

" 2 1

x3 dx =

1 2 1 (2 ) − (12 ) − 2 2

" 2 0

x dx −

1 4 1 4 (2) − (1) 4 4

In Exercises 46–50, calculate the integral, assuming that " 5 f (x) d x = 5,

" 5

0

0

" 1 0

=

" x dx −

0

2

x3 dx −

" 1

x3 dx

0

9 3 15 − =− . 2 4 4

g(x) d x = 12

" 5 " 5 ( f (x) + 4g(x)) d x ( f (x) + g(x)) d x 0 0 " 5 " 5 " 5 SOLUTION ( f (x) + 4g(x)) d x = f (x) d x + 4 g(x) d x = 5 + 4(12) = 53.

47.

0

0

0

" 5 " 0 49. (3 f (x) − 5g(x)) d x g(x) d x 0 5 " 5 " 5 " 5 SOLUTION (3 f (x) − 5g(x)) d x = 3 f (x) d x − 5 g(x) d x = 3(5) − 5(12) = −45. 0

0

0

In Exercises 51–54, calculate the"integral, assuming that 5 Is it possible to calculate g(x) f (x) d x from " " the information given? " 1

0

" 4 51. 0

2

0

f (x) d x = 1,

0

f (x) d x = 4,

4

1

f (x) d x = 7

f (x) d x " 4

SOLUTION

0

f (x) d x =

" 1 0

f (x) d x +

" 4 1

f (x) d x = 1 + 7 = 8.

" 1 " 2 f (x) d x f (x) d x 4 1 " 1 " 4 SOLUTION f (x) d x = − f (x) d x = −7.

53.

4

1

" 4 55–58, express each integral as a single integral. In Exercises f (x) d x" 7 " 3 2 f (x) d x + f (x) d x 55. 0

" 3

SOLUTION

0

3

f (x) d x +

" 7 3

f (x) d x =

" 7 0

f (x) d x.

" 5 " 9 " 9 " 9 f (x) d x − f (x) d x f (x) d x − f (x) d x 2 2 " " " " 5 " 9 " 9 2 4 9 5 5 SOLUTION f (x) d x − f (x) d x = f (x) d x + f (x) d x − f (x) d x = f (x) d x.

57.

2

2

2

5

2

5

" b " 3 " 9 In Exercises f59–62, calculate the integral, assuming that f is an integrable function such that f (x) d x = 1 − b−1 (x) d x + f (x) d x 7 0. for all b > " 3 59. f (x) d x 1

" 3

SOLUTION

1

61.

f (x) d x = 1 − 3−1 =

" 4 " 4 (4 f (x) − 2) d x f (x) d x 1 2

1

3

2 . 3

The Deﬁnite Integral

S E C T I O N 5.2

" 4 SOLUTION

1

(4 f (x) − 2) d x = 4

" 4

f (x) d x − 2

1

" 4 1

257

1 d x = 4(1 − 4−1 ) − 2(4 − 1) = −3.

" b " b " 1 Explain the difference in graphical interpretation between f (x) d x and | f (x)| d x. f (x) d x a a 1/2 !b SOLUTION When f (x) takes on both positive and negative values on [a, b], a f (x) d x represents the signed area !b between f (x) and the x-axis, whereas a | f (x)| d x represents the total (unsigned) area between f (x) and the x-axis. ! ! Any negatively signed areas that were part of ab f (x) d x are regarded as positive areas in ab | f (x)| d x. Here is a graphical example of this phenomenon. 63.

Graph of | f (x)|

Graph of f (x) 10 −4

−2

30 x 2

−10

20

4

10

−20 −30

−4

x

−2

2

4

In Exercises 65–68," calculate the integral. " 2π 2π sin2 x d x and J = cos2 x d x. Use the following trick to prove that I = J = π : First show " 6 Let I = 0 0 " 2π 65. |3 − x| d x 0 a graph that I = J and then prove I + J = with d x. 0 SOLUTION Over the interval, the region between the curve and the interval [0, 6] consists of two triangles above the x axis, each of which has height 3 and width 3, and so area 92 . The total area, hence the deﬁnite integral, is 9. y 3 2 1 x 1

2

3

4

5

6

Alternately, " 6 0

|3 − x| d x =

" 3

=3

0

(3 − x) d x +

" 3 0

dx −

" 3 0

" 6 3

(x − 3) d x

x dx +

" 6 0

x dx −

" 3 x dx 0

−3

" 6 dx 3

1 1 1 = 9 − 32 + 62 − 32 − 9 = 9. 2 2 2 67.

" 1 " 33 |x | d x |2x − 4| d x −1 1

SOLUTION

|x 3 | =

x3 −x 3

x ≥0 x < 0.

Therefore, " 1 −1

|x 3 | d x =

" 0 −1

−x 3 d x +

" 1 0

x3 dx =

" −1 0

69. Use"the Comparison Theorem to show that 2 " 1 " 1 |x 2 − 1| d x 5 0 x dx ≤ x 4 d x, 0

0

x3 dx +

" 2 1

" 1 0

x3 dx =

x4 dx ≤

" 2 1

1 1 1 (−1)4 + (1)4 = . 4 4 2

x5 dx

258

CHAPTER 5

THE INTEGRAL SOLUTION

On the interval [0, 1], x 5 ≤ x 4 , so, by Theorem 5, " 1 0

x5 dx ≤

" 1

x 4 d x.

0

On the other hand, x 4 ≤ x 5 for x ∈ [1, 2], so, by the same Theorem, " 2 " 2 x4 dx ≤ x 5 d x. 1

1

" " 0.3 71. Prove that 0.0198 1 ≤ 6 1 sin x d1x ≤ 0.0296. Hint: Show that 0.198 ≤ sin x ≤ 0.296 for x in [0.2, 0.3]. Prove that ≤ 0.2 d x ≤ . 3 2 4 x π d SOLUTION For 0 ≤ x ≤ 6 ≈ 0.52, we have d x (sin x) = cos x > 0. Hence sin x is increasing on [0.2, 0.3]. Accordingly, for 0.2 ≤ x ≤ 0.3, we have m = 0.198 ≤ 0.19867 ≈ sin 0.2 ≤ sin x ≤ sin 0.3 ≈ 0.29552 ≤ 0.296 = M Therefore, by the Comparison Theorem, we have " 0.3 " 0.3 " 0.3 m dx ≤ sin x d x ≤ M d x = M(0.3 − 0.2) = 0.0296. 0.0198 = m(0.3 − 0.2) = 0.2

Prove that

73.

Prove that 0.277 ≤

Hint: Graph y = SOLUTION

" π /4 π /8

0.2

0.2

√ sin x 2 dx ≤ x 2 π /4

cos x d x ≤ 0.363. " π /2

sin x and observe that it is decreasing on [ π4 , π2 ]. x

Let f (x) =

sin x . x

As we can see in the sketch √ below, f (x) is decreasing on the interval [π /4, π /2]. Therefore f (x) ≤ f (π /4) for all x in [π /4, π /2]. f (π /4) = 2 π 2 , so:

√ √ " π /2 " π /2 √ sin x 2 2 π2 2 2 dx = = dx ≤ . x π 4 π 2 π /4 π /4 y y = sin x

2 2/p

x

2/p

p /4

p/2

x

" b " b " Suppose that f (x) ≤ g(x) on 1[a, b]. f (x) d x ≤ g(x) d x. Is it also d xBy the Comparison Theorem, Find upper and lower bounds for . a a 3 0 give x a+counterexample. 4 true that f (x) ≤ g (x) for x ∈ [a, b]? If not,

75.

The assertion f (x) ≤ g (x) is false. Consider a = 0, b = 1, f (x) = x, g(x) = 2. f (x) ≤ g(x) for all x in the interval [0, 1], but f (x) = 1 while g (x) = 0 for all x. SOLUTION

whether trueChallenges or false. If false, sketch the graph of a counterexample. Further State Insights and "" ab

(x)ddxx=>00.if f (x) is an odd function. (a) If f (x) > 0, then 77. Explain graphically: f f(x) a −a " b SOLUTION is danx odd then f (−x) = − f (x) for all x. Accordingly, for every positively signed area in the (b) If Iff f(x) > 0,function, then f (x) > 0. a where f is above the x-axis, there is a corresponding negatively signed area in the left half-plane where right half-plane f is below the x-axis. Similarly, for every negatively signed area in the right half-plane where f is below the x-axis, there is a corresponding positively signed area in the left half-plane where f is above the x-axis. We conclude that the net area between the graph of f and the x-axis over [−a, a] is 0, since the positively signed areas and negatively signed areas cancel each other out exactly.

The Fundamental Theorem of Calculus, Part I

S E C T I O N 5.3

259

y 4 2 −1 −2

x 1

−2

2

−4

79. Let k and b be" positive. Show, by comparing the right-endpoint approximations, that 1

Compute

" 1 sin(sin(x))(sin2 (x) + 1)" dbx. x k d x = bk+1 xk dx

−1

0

0

! Let k and b be any positive numbers. Let f (x) = x k on [0, b]. Since f is continuous, both 0b f (x) d x ! and 01 f (x) d x exist. Let N be a positive integer and set x = (b − 0) /N = b/N . Let x j = a + jx = bj/N , j = 1, 2, . . . , N be the right endpoints of the N subintervals of [0, b]. Then the right-endpoint approximation to !b k !b 0 f (x) d x = 0 x d x is ⎛ ⎞ N N k N # # 1 b # bj f (x j ) = = bk+1 ⎝ k+1 jk⎠ . R N = x N N N j=1 j=1 j=1 SOLUTION

! ! In particular, if b = 1 above, then the right-endpoint approximation to 01 f (x) d x = 01 x k d x is N N N k # j 1 # 1 # 1 f (x j ) = = k+1 j k = k+1 R N S N = x N N N b j=1 j=1 j=1 In other words, R N = bk+1 S N . Therefore, " b " 1 x k d x = lim R N = lim bk+1 S N = bk+1 lim S N = bk+1 x k d x. N →∞

0

N →∞

N →∞

0

81. Show that Eq. (4) holds for b ≤ 0. Verify by interpreting the integral as an area: SOLUTION Let c = −b. Since b < 0, c > 0, so by Eq. (4), " b 1 1 1 "− cx 2 d x = b1 1 − b2 + θ 2 3 2 2 0 x dx = c . 3 0 Here, 0 ≤ b ≤ 1 and θ is the angle between 0 and π2 such that sin θ = b. Furthermore, x 2 is an even function, so symmetry of the areas gives " 0 −c

x2 dx =

" c

x 2 d x.

0

Finally, " b 0

x2 dx =

" −c 0

x2 dx = −

" 0 −c

x2 dx = −

" c 0

1 1 x 2 d x = − c3 = b3 . 3 3

Theorem 4 remains true without the assumption a ≤ b ≤ c. Verify this for the cases b < a < c and c < a < b.

5.3 The Fundamental Theorem of Calculus, Part I Preliminary Questions 1. Assume that f (x) ≥ 0. What is the area under the graph of f (x) over [0, 2] if f (x) has an antiderivative F(x) such that F(0) = 3 and F(2) = 7? SOLUTION

Because f (x) ≥ 0, the area under the graph of y = f (x) over the interval [0, 2] is " 2 0

f (x) d x = F(2) − F(0) = 7 − 3 = 4.

2. Suppose that F(x) is an antiderivative of f (x). What is the graphical interpretation of F(4) − F(1) if f (x) takes on both positive and negative values?

260

CHAPTER 5

THE INTEGRAL

! Because F(x) is an antiderivative of f (x), it follows that F(4) − F(1) = 14 f (x) d x. Hence, F(4) − F(1) represents the signed area between the graph of y = f (x) and the x-axis over the interval [1, 4]. " 7 " 7 f (x) d x and f (x) d x, assuming that f (x) has an antiderivative F(x) with values from the follow3. Evaluate SOLUTION

0

2

ing table:

SOLUTION

0

2

7

F(x)

3

7

9

Because F(x) is an antiderivative of f (x), " 7 0

and

" 7 2

4. (a) (b) (c)

x

f (x) d x = F(7) − F(0) = 9 − 3 = 6

f (x) d x = F(7) − F(2) = 9 − 7 = 2.

Are the following statements true or false? Explain. The FTC I is only valid for positive functions. To use the FTC I, you have to choose the right antiderivative. If you cannot ﬁnd an antiderivative of f (x), then the deﬁnite integral does not exist.

SOLUTION

(a) False. The FTC I is valid for continuous functions. (b) False. The FTC I works for any antiderivative of the integrand. (c) False. If you cannot ﬁnd an antiderivative of the integrand, you cannot use the FTC I to evaluate the deﬁnite integral, but the deﬁnite integral may still exist. " 9 5. What is the value of f (x) d x if f (x) is differentiable and f (2) = f (9) = 4? 2

SOLUTION

" 9

Because f is differentiable, 2

f (x) d x = f (9) − f (2) = 4 − 4 = 0.

Exercises In Exercises 1–4, sketch the region under the graph of the function and ﬁnd its area using the FTC I. 1. f (x) = x 2 ,

[0, 1]

SOLUTION y 1 0.8 0.6 0.4 0.2 x 0.2

0.4

0.6

0.8

1

We have the area A=

" 1 0

x2 dx =

1 3 1 1 x = . 3 0 3

3. f (x) = sin x, [0, π /2] f (x) = 2x − x 2 , [0, 2]

SOLUTION

y 1 0.8 0.6 0.4 0.2 x 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

S E C T I O N 5.3

The Fundamental Theorem of Calculus, Part I

Let A be the area indicated. Then A=

" π /2 0

π /2 sin x d x = − cos x = 0 − (−1) = 1. 0

In Exercises f (x) 5–34, = cosevaluate x, [0, πthe /2]integral using the FTC I. " 6 x dx 5. 3

" 6

SOLUTION

3

x dx =

" 2 " 92 u du 7. 2 dx −3 0 " 2 SOLUTION

−3

1 2 6 1 1 27 x = (6)2 − (3)2 = . 2 3 2 2 2

u 2 du =

1 3 2 1 1 35 u = (2)3 − (−3)3 = . 3 −3 3 3 3

" 3 " 31 (t − t 2 ) dt 9. (x − x 2 ) d x 1 0 " 3 1 4 1 3 3 1 4 1 3 34 1 1 SOLUTION (t 3 − t 2 ) dt = t − t = (3) − (3) − − = . 4 3 4 3 4 3 3 1 1 " 4 " 12 (x + 2) d x4 11. −3 (4 − 5u ) du 0

SOLUTION

" 4 −3

4 1 3 1 3 1 133 x + 2x = (4) + 2(4) − (−3)3 + 2(−3) = . 3 3 3 3 −3

(x 2 + 2) d x =

" 2 " 4 9 (10x 5+ 3x 52) d x −2 (3x + x − 2x) d x 0 " 2 1 6 2 1 6 1 6 9 5 10 10 10 SOLUTION (10x + 3x ) d x = x + x = 2 + 2 − 2 + 2 = 0. 2 2 2 −2 −2 " 3 " 13/2 (4t + t 7/2 ) dt 15. (5u 4 − 6u 2 ) du 1 13.

−1

SOLUTION

" 3 1

(4t 3/2 + t 7/2 ) dt =

√ √ √ 8 5/2 2 9/2 3 72 3 8 2 162 3 82 + t t + 18 3 − + = − . = 5 9 5 5 9 5 45 1

" 4 " 12 dt2 −2 1 t 2 (x − x ) d x 1 " 4 " 4 4

1 −2 dt = −t −1 = −(4)−1 − −(1)−1 = 3 . SOLUTION dt = t 4 1 t2 1 1 " 27 " 41/3 19. x √ dx y dy 1 0 27 " 27 3 3 3 SOLUTION x 1/3 d x = x 4/3 = (81) − = 60. 4 4 4 1 1 " 9 " −1/2 t 4 −4dt 21. x dx 1 1 9 " 9 SOLUTION t −1/2 dt = 2t 1/2 = 2(9)1/2 − 2(1)1/2 = 4. 17.

1

" −1 " 91 23. 8d x −2 x 3 3 d x x 4

1

261

262

CHAPTER 5

THE INTEGRAL

" −1 1 −2 −1 1 1 3 1 d x = − = − (−1)−2 + (−2)−2 = − . x 3 2 2 2 8 −2 x −2

SOLUTION

25.

" 27 " t4 + 1 √ 2 dt πt d x 1 2

SOLUTION

27 " 27 " 27 2 3/2 t +1 (t 1/2 + t −1/2 ) dt = + 2t 1/2 t √ dt = 3 t 1 1 1 √ √ √ 2 2 8 = (81 3) + 6 3 − + 2 = 60 3 − . 3 3 3 " π /2 " π /2 cos x d x cos θ d θ −π /2 0 π /2 " π /2 SOLUTION cos x d x = sin x = 1 − (−1) = 2.

27.

−π /2

" 3π /4 " 2π sin θ d θ 29. cos t dt π /4 0 " 3π /4 SOLUTION

π /4

−π /2

√ √ 3π /4 2 2 √ sin θ d θ = − cos θ = + = 2. 2 2 π /4

" π /4 " 4π 2 31. sec t dt sin x d x 0 2π " π /4 π /4 π 2 SOLUTION sec t dt = tan t = tan − tan 0 = 1. 4 0 0 " π /3 " π /4 33. csc x cot x d x sec θ tan θ d θ π /6 0 π /3

" π /3 π

π 2√ − − csc =2− SOLUTION csc x cot x d x = (− csc x) = − csc 3. 3 6 3 π /6 π /6 " π /235–40, write the integral as a sum of integrals without absolute values and evaluate. In Exercises csc2 y d y " 1 35.

π /6

−2

|x| d x

SOLUTION

1 2 0 1 2 1 1 1 5 |x| d x = (−x) d x + x d x = − x + x = 0 − − (4) + = . 2 2 2 2 2 −2 −2 0 −2 0

" 1

37.

" 0

" 1

" 3 " 53 |x | d x |3 − x| d x −2 0

SOLUTION

" 3 −2

|x 3 | d x =

" 0 −2

(−x 3 ) d x +

1 0 1 3 x 3 d x = − x 4 + x 4 4 −2 4 0 0

" 3

1 1 97 = 0 + (−2)4 + 34 − 0 = . 4 4 4 39.

" π "|cos 3 x| d x |x 2 − 1| d x 0 0

SOLUTION

" π 0

" 5

|cos x| d x =

" π /2 0

|x 2 − 4x + 3| d x

cos x d x +

" π π /2

π /2 π (− cos x) d x = sin x − sin x 0

π /2

= 1 − 0 − (−1 − 0) = 2.

The Fundamental Theorem of Calculus, Part I

S E C T I O N 5.3

263

In Exercises 41–44, evaluate the integral in terms of the constants. " b 41. x3 dx 1

" b

SOLUTION

1

" b " 5a 43. x d x4 x dx 1 b " b

x3 dx =

1 4 b 1 1 1 4 b − 1 for any number b. x = b4 − (1)4 = 4 1 4 4 4

1 6 b 1 1 1 x = b6 − (1)6 = (b6 − 1) for any number b. 6 1 6 6 6 1 " 1 " x x n d x = 0 if n is an odd whole number. Explain graphically. 45. Use the FTC I to show that (t 3 + t) dt −1 SOLUTION

x5 dx =

−x

SOLUTION

We have " 1 −1

xn dx =

x n+1 1 (1)n+1 (−1)n+1 = − . n + 1 −1 n+1 n+1

Because n is odd, n + 1 is even, which means that (−1)n+1 = (1)n+1 = 1. Hence (−1)n+1 1 1 (1)n+1 − = − = 0. n+1 n+1 n+1 n+1 Graphically speaking, for an odd function such as x 3 shown here, the positively signed area from x = 0 to x = 1 cancels the negatively signed area from x = −1 to x = 0. y 1 0.5 − 0.5 −1

x − 0.5

0.5

1

−1

47. Show that the area of a parabolic arch (the shaded region in Figure 5) is equal to four-thirds the area of the triangle What is the area (a positive number) between the x-axis and the graph of f (x) on [1, 3] if f (x) is a negative shown. function whose antiderivative F has the values F(1) = 7 and F(3) = 4? y

a

a+b 2

x b

FIGURE 5 Graph of y = (x − a)(b − x). SOLUTION

We ﬁrst calculate the area of the parabolic arch: " b a

(x − a)(b − x) d x = −

" b a

(x − a)(x − b) d x = −

" b a

(x 2 − ax − bx + ab) d x

b 1 3 a 2 b 2 x − x − x + abx 3 2 2 a

b 1 2x 3 − 3ax 2 − 3bx 2 + 6abx =− a 6 1 3 = − (2b − 3ab2 − 3b3 + 6ab2 ) − (2a 3 − 3a 3 − 3ba 2 + 6a 2 b) 6 1

= − (−b3 + 3ab2 ) − (−a 3 + 3a 2 b) 6 1 1

= − a 3 + 3ab2 − 3a 2 b − b3 = (b − a)3 . 6 6

=−

264

CHAPTER 5

THE INTEGRAL

The indicated triangle has a base of length b − a and a height of a+b b−a 2 a+b . −a b− = 2 2 2 Thus, the area of the triangle is 1 b−a 2 1 = (b − a)3 . (b − a) 2 2 8 Finally, we note that 4 1 1 (b − a)3 = · (b − a)3 , 6 3 8 as required.

" 3 " 1 f (x) d x, where 49. Calculate Does −2 x n d x get larger or smaller as n increases? Explain graphically. 0 12 − x 2 for x ≤ 2 f (x) = for x > 2 x3 SOLUTION

" 3 −2

f (x) d x =

" 2 −2

f (x) d x +

" 3 2

f (x) d x =

" 2 −2

(12 − x 2 ) d x +

" 3

x3 dx

2

2 1 1 3 12x − x 3 + x 4 3 4 2 −2 1 1 1 1 = 12(2) − (2)3 − 12(−2) − (−2)3 + 34 − 24 3 3 4 4 =

=

128 65 707 + = . 3 4 12

the function f (x) = sin 3x − x. Find the positive root of Further Plot Insights and Challenges

f (x) to three places and use it to ﬁnd the area under the graph of f (x) in the ﬁrst quadrant. 51. In this exercise, we generalize the result of Exercise 47 by proving the famous result of Archimedes: For r < s, the area of the shaded region in Figure 6 is equal to four-thirds the area of triangle AC E, where C is the point on the parabola at which the tangent line is parallel to secant line AE. (a) Show that C has x-coordinate (r + s)/2. (b) Show that AB D E has area (s − r )3 /4 by viewing it as a parallelogram of height s − r and base of length C F. (c) Show that AC E has area (s − r )3 /8 by observing that it has the same base and height as the parallelogram. (d) Compute the shaded area as the area under the graph minus the area of a trapezoid and prove Archimedes’s result. y B

C

D

A

F

E

r

r+s 2

x

s

FIGURE 6 Graph of f (x) = (x − a)(b − x). SOLUTION

(a) The slope of the secant line AE is (s − a)(b − s) − (r − a)(b − r ) f (s) − f (r ) = = a + b − (r + s) s −r s −r and the slope of the tangent line along the parabola is f (x) = a + b − 2x. If C is the point on the parabola at which the tangent line is parallel to the secant line AE, then its x-coordinate must satisfy a + b − 2x = a + b − (r + s)

or

x=

r +s . 2

S E C T I O N 5.3

The Fundamental Theorem of Calculus, Part I

265

(b) Parallelogram AB D E has height s − r and base of length C F. Since the equation of the secant line AE is y = [a + b − (r + s)] (x − r ) + (r − a)(b − r ), the length of the segment C F is r +s r +s r +s (s − r )2 −a b− − [a + b − (r + s)] − r − (r − a)(b − r ) = . 2 2 2 4 )3 Thus, the area of AB D E is (s−r 4 .

(c) Triangle AC E is comprised of AC F and C E F. Each of these smaller triangles has height s−r 2 and base of

)2 length (s−r 4 . Thus, the area of AC E is

1 s − r (s − r )2 1 s − r (s − r )2 (s − r )3 · + · = . 2 2 4 2 2 4 8 (d) The area under the graph of the parabola between x = r and x = s is " s 1 1 3 s 2 (x − a)(b − x) d x = −abx + (a + b)x − x 2 3 r r 1 1 1 1 = −abs + (a + b)s 2 − s 3 + abr − (a + b)r 2 + r 3 2 3 2 3 1 1 = ab(r − s) + (a + b)(s − r )(s + r ) + (r − s)(r 2 + r s + s 2 ), 2 3 while the area of the trapezoid under the shaded region is 1 (s − r ) [(s − a)(b − s) + (r − a)(b − r )] 2 1 = (s − r ) −2ab + (a + b)(r + s) − r 2 − s 2 2 1 1 = ab(r − s) + (a + b)(s − r )(r + s) + (r − s)(r 2 + s 2 ). 2 2 Thus, the area of the shaded region is 1 1 1 2 1 1 1 1 2 1 1 r + r s + s 2 − r 2 − s 2 = (s − r ) r − r s + s 2 = (s − r )3 , (r − s) 3 3 3 2 2 6 3 6 6 which is four-thirds the area of the triangle AC E. 53. Use the method of Exercise 52 to prove that (a) Apply the Comparison Theorem (Theorem 5 in Section 5.2) to the inequality sin x ≤ x (valid for x ≥ 0) to x2 x4 x2 prove ≤ cos x ≤ 1 − + 1− 2 2 24 x2 3 3 1 − ≤ cos x x x x 5≤ 1. x− ≤ sin x ≤ x 2− + (for x ≥ 0) 6 6 120 (b) Apply it again to prove Verify these inequalities for x = 0.1. Why have we speciﬁed x ≥ 0 for sin x but not cos x? 3 3 SOLUTION By Exercise 52, t − 1 ≤ tx for≤t sin > 0. Integrating inequality over the interval [0, x], and then 6 t ≤ sin xt − x≤ x (for xthis ≥ 0). 6 solving for cos x, yields: (c) Verify these inequalities for x 1= 20.3. 1 4 x − x ≤ 1 − cos x ≤ 2 24 1 1 − x 2 ≤ cos x ≤ 1 − 2

1 2 x 2 1 2 1 4 x + x . 2 24

2 2 x 4 are all even functions, they also apply for These inequalities apply for x ≥ 0. Since cos x, 1 − x2 , and 1 − x2 + 24 x ≤ 0. Having established that

1−

t2 t2 t4 ≤ cos t ≤ 1 − + , 2 2 24

for all t ≥ 0, we integrate over the interval [0, x], to obtain: x−

x3 x3 x5 ≤ sin x ≤ x − + . 6 6 120

266

CHAPTER 5

THE INTEGRAL 1 x 5 are all odd functions, so the inequalities are reversed for x < 0. The functions sin x, x − 16 x 3 and x − 16 x 3 + 120 Evaluating these inequalities at x = .1 yields

0.995000000 ≤ 0.995004165 ≤ 0.995004167 0.0998333333 ≤ 0.0998334166 ≤ 0.0998334167, both of which are true. 55. Assume that | f (x)| ≤ K for x ∈ [a, b]. Use FTC I to prove that | f (x) − f (a)| ≤ K |x − a| for x ∈ [a, b]. Calculate the next pair of inequalities for sin x and cos x by integrating the results of Exercise 53. Can you guess SOLUTION Let a > b be real numbers, and let f (x) be such that | f (x)| ≤ K for x ∈ [a, b]. By FTC, the general pattern? " x f (t) dt = f (x) − f (a). a

Since f (x) ≥ −K for all x ∈ [a, b], we get: f (x) − f (a) =

" x a

f (t) dt ≥ −K (x − a).

Since f (x) ≤ K for all x ∈ [a, b], we get: f (x) − f (a) =

" x a

f (t) dt ≤ K (x − a).

Combining these two inequalities yields −K (x − a) ≤ f (x) − f (a) ≤ K (x − a), so that, by deﬁnition, | f (x) − f (a)| ≤ K |x − a|. (a) Prove that | sin a − sin b| ≤ |a − b| for all a, b (use Exercise 55). (b) Let f (x) = sin(x + a) − sin x. Use part (a) to show that the graph of f (x) lies between the horizontal lines 5.4 y The Fundamental Theorem of Calculus, Part II = ±a. (c) Produce a graph of f (x) and verify part (b) for a = 0.5 and a = 0.2. Preliminary Questions " x 1. What is A(−2), where A(x) = f (t) dt? −2

SOLUTION

By deﬁnition, A(−2) =

2. Let G(x) =

" −2

" x t 3 + 1 dt.

−2

f (t) dt = 0.

4

(a) Is the FTC needed to calculate G(4)? (b) Is the FTC needed to calculate G (4)? SOLUTION

! (a) No. G(4) = 44 t 3 + 1 dt = 0. √ (b) Yes. By the FTC II, G (x) = x 3 + 1, so G (4) = 65. 3. Which of the following deﬁnes an antiderivative F(x) of f (x) = x 2 satisfying F(2) = 0? " x " 2 " x (a) 2t dt (b) t 2 dt (c) t 2 dt 2 SOLUTION

" x The correct answer is (c):

0

2

t 2 dt.

2

4. True or false? Some continuous functions do not have antiderivatives. Explain. " x SOLUTION False. All continuous functions have an antiderivative, namely f (t) dt. 5. Let G(x) =

a

" x3 sin t dt. Which of the following statements are correct? 4

(a) G(x) is the composite function sin(x 3 ).

S E C T I O N 5.4

(b) G(x) is the composite function A(x 3 ), where A(x) = (c) (d) (e) (f)

The Fundamental Theorem of Calculus, Part II

267

" x sin(t) dt. 4

G(x) is too complicated to differentiate. The Product Rule is used to differentiate G(x). The Chain Rule is used to differentiate G(x). G (x) = 3x 2 sin(x 3 ).

Statements (b), (e), and (f) are correct. " 3 t 3 dt at x = 2. 6. Trick question: Find the derivative of

SOLUTION

1

! Note that the deﬁnite integral 13 t 3 dt does not depend on x; hence the derivative with respect to x is 0 for any value of x. SOLUTION

Exercises 1. Write the area function of f (x) = 2x + 4 with lower limit a = −2 as an integral and ﬁnd a formula for it. SOLUTION

Let f (x) = 2x + 4. The area function with lower limit a = −2 is " x " x A(x) = f (t) dt = (2t + 4) dt. −2

a

Carrying out the integration, we ﬁnd x " x (2t + 4) dt = (t 2 + 4t) −2

−2

= (x 2 + 4x) − ((−2)2 + 4(−2)) = x 2 + 4x + 4

or (x + 2)2 . Therefore, A(x) = (x + 2)2 . " x (t 2for − the 2) dt. 3. LetFind G(x)a = formula area function of f (x) = 2x + 4 with lower limit a = 0. 1

(a) What is G(1)? (b) Use FTC II to ﬁnd G (1) and G (2). (c) Find a formula for G(x) and use it to verify your answers to (a) and (b). !x SOLUTION Let G(x) = 1 (t 2 − 2) dt. ! (a) Then G(1) = 11 (t 2 − 2) dt = 0.

(b) Now G (x) = x 2 − 2, so that G (1) = −1 and G (2) = 2. (c) We have x " x 1 3 1 3 1 3 1 5 (t 2 − 2) dt = t − 2t = x − 2x − (1) − 2(1) = x 3 − 2x + . 3 3 3 3 3 1 1 Thus G(x) = 13 x 3 − 2x + 53 and G (x) = x 2 − 2. Moreover, G(1) = 13 (1)3 − 2(1) + 53 = 0, as in (a), and G (1) = −1 and G (2) = 2, as in (b). " x (π /4), where G(x) = " x tan t dt. 5. Find G(1), G (0), and G Find F(0), F (0), and F (3), where F(x) = 1 t 2 + t dt. 0 !1 π SOLUTION By deﬁnition, G(1) = 1 tan t dt = 0. By FTC, G (x) = tan x, so that G (0) = tan 0 = 0 and G ( 4 ) = π tan 4 = 1. " x In Exercises 7–14, ﬁnd formulas for the functions represented du by the integrals. (−2), where H (x) = . Find H (−2) and H " x 2 −2 u + 1 3 u du 7. 2 SOLUTION

F(x) =

" x 2

u 3 du =

1 4 x 1 u = x 4 − 4. 4 2 4

" x"2 x 9. t dtsin u du 1

0

SOLUTION

" x

F(x) =

(t 2 − t) dt

" x2 1

t dt =

2 1 2 x 1 1 t = x4 − . 2 1 2 2

268

CHAPTER 5

THE INTEGRAL

" 5 11. x

(4t − 1) dt

SOLUTION

F(x) =

" x " x 2 sec θ d θ 13. −π /4 cos u du π /4

SOLUTION

F(x) =

" 5 x

5 (4t − 1) dt = (2t 2 − t) = 45 + x − 2x 2 . x

" x −π /4

x sec2 θ d θ = tan θ

−π /4

= tan x − tan(−π /4) = tan x + 1.

In Exercises " √x15–18, express the antiderivative F(x) of f (x) satisfying the given initial condition as an integral. t 3 dt 15. f (x)2= x 4 + 1, F(3) = 0 " x SOLUTION The antiderivative F(x) of f (x) = x 4 + 1 satisfying F(3) = 0 is F(x) = t 4 + 1 dt. 3

17. f (x) = sec x, F(0) = 0 x +1 " x , F(7) = 0 f (x) = 2 x antiderivative +9 SOLUTION The F(x) of f (x) = sec x satisfying F(0) = 0 is F(x) = sec t dt. 0

In Exercises 19–22, calculate the derivative. f (x) = sin(x 3 ), F(−π ) = 0 " x d 19. (t 3 − t) dt dx 0 " x d SOLUTION By FTC II, (t 3 − t) dt = x 3 − x. dx 0 " t d " x d cos 21. 5x d x dt 100 sin(t 2 ) dt dx 1 " t d SOLUTION By FTC II, cos(5x) d x = cos 5t. dt 100 " x " 23. Sketch f (t) dt for each of the functions shown in Figure 10. d thes graph of 1A(x) = du 0 tan ds −2 1 + u2 y

y 2 1 0 −1

2 1 0 −1

x 1

2

3

4

x 1

2

3

4

(B)

(A)

FIGURE 10 SOLUTION

• Remember that A (x) = f (x). It follows from Figure 10(A) that A (x) is constant and consequently A(x) is linear

on the intervals [0, 1], [1, 2], [2, 3] and [3, 4]. With A(0) = 0, A(1) = 2, A(2) = 3, A(3) = 2 and A(4) = 2, we obtain the graph shown below at the left. • Since the graph of y = f (x) in Figure 10(B) lies above the x-axis for x ∈ [0, 4], it follows that A(x) is increasing over [0, 4]. For x ∈ [0, 2], area accumulates more rapidly with increasing x, while for x ∈ [2, 4], area accumulates more slowly. This suggests A(x) should be concave up over [0, 2] and concave down over [2, 4]. A sketch of A(x) is shown below at the right. y

y

3 2.5 2 1.5 1 0.5

4 3 2 1 x 1

Let A(x) =

" x 0

2

3

4

x 1

2

3

4

f (t) dt for f (x) shown in Figure 11. Calculate A(2), A(3), A (2), and A (3). Then ﬁnd a

formula for A(x) (actually two formulas, one for 0 ≤ x ≤ 2 and one for 2 ≤ x ≤ 4) and sketch the graph of A(x).

The Fundamental Theorem of Calculus, Part II

S E C T I O N 5.4

269

25. Make a rough sketch of the graph of the area function of g(x) shown in Figure 12. y y = g(x)

x 1

2

3

4

FIGURE 12 SOLUTION The graph of y = g(x) lies above the x-axis over the interval [0, 1], below the x-axis over [1, 3], and above the x-axis over [3, 4]. The corresponding area function should therefore be increasing on (0, 1), decreasing on (1, 3) and increasing on (3, 4). Further, it appears from Figure 12 that the local minimum of the area function at x = 3 should be negative. One possible graph of the area function is the following. y 4 3 2 1 x 1

−1 −2 −3

2

3

4

" x3 " x 27. FindShow G (x), where G(x) = dt. Hint: Consider x ≥ 0 and x ≤ 0 separately. that |t| dt is equal totan12 tx|x|. 3

0

SOLUTION

By combining the FTC and the chain rule, we have G (x) = tan(x 3 ) · 3x 2 = 3x 2 tan(x 3 ).

In Exercises 29–36, calculate the derivative. " x2 t 3 + 3 dt. Find " x 2 G (1), where G(x) = d 0 2 29. sin t dt dx 0 " x2 SOLUTION Let G(x) = sin2 t dt. By applying the Chain Rule and FTC, we have 0

G (x) = sin2 (x 2 ) · 2x = 2x sin2 (x 2 ). " cos s d " 1/x (u 4 − 3u) du ds d−6 sin(t 2 ) dt dx 1 " s SOLUTION Let G(s) = (u 4 − 3u) du. Then, by the chain rule,

31.

−6

" cos s d d (u 4 − 3u) du = G(cos s) = − sin s(cos4 s − 3 cos s). ds −6 ds 33.

" 0 d " d sin0 2 t dt d x x3 sin2 t dt dx x

SOLUTION

Let F(x) =

" x 0

sin2 t dt. Then

" 0 x3

sin2 t dt = −

" x3 0

sin2 t dt = −F(x 3 ). From this,

" 0 d d sin2 t dt = (−F(x 3 )) = −3x 2 F (x 3 ) = −3x 2 sin2 x 3 . d x x3 dx 35.

" x2 d " tan4 t dt d x d√x x √ t dt d x x2 Hint for Exercise 34:F(x) = A(x 4 ) − A(x 2 ), where A(x) =

" x √

t dt

270

CHAPTER 5

THE INTEGRAL SOLUTION

Let " x2

G(x) = √ tan t dt = x

" x2 0

" √x

tan t dt −

tan t dt. 0

Applying the Chain Rule combined with FTC twice, we have √ √ 1 tan( x) G (x) = tan(x 2 ) · 2x − tan( x) · x −1/2 = 2x tan(x 2 ) − √ . 2 2 x " x " x " 3u+9 d 37–38, In Exercises let 2A(x) = f (t) dt and B(x) = f (t) dt, with f (x) as in Figure 13. x + 1 dx 0 2 du −u y 2 1 0 −1 −2

y = f(x) x 1

2

3

4

5

6

FIGURE 13

37. Find the min and max of A(x) on [0, 6]. SOLUTION The minimum values of A(x) on [0, 6] occur where A (x) = f (x) goes from negative to positive. This occurs at one place, where x = 1.5. The minimum value of A(x) is therefore A(1.5) = −1.25. The maximum values of A(x) on [0, 6] occur where A (x) = f (x) goes from positive to negative. This occurs at one place, where x = 4.5. The maximum value of A(x) is therefore A(4.5) = 1.25. " x ffor (t)dt, with (x) asvalid in Figure 39. LetFind A(x)formulas = A(x) andf B(x) on [2,14. 4].

0

(a) (b) (c) (d)

Does A(x) have a local maximum at P? Where does A(x) have a local minimum? Where does A(x) have a local maximum? True or false? A(x) < 0 for all x in the interval shown. y R P

S

x

y = f(x) Q

FIGURE 14 Graph of f (x). SOLUTION

(a) In order for A(x) to have a local maximum, A (x) = f (x) must transition from positive to negative. As this does not happen at P, A(x) does not have a local maximum at P. (b) A(x) will have a local minimum when A (x) = f (x) transitions from negative to positive. This happens at R, so A(x) has a local minimum at R. (c) A(x) will have a local maximum when A (x) = f (x) transitions from positive to negative. This happens at S, so A(x) has a local maximum at S. (d) It is true that A(x) < 0 on I since the signed area from 0 to x is clearly always negative from the ﬁgure. " x Find 41–42, the smallest positive point of f (x) is continuous. In Exercises let A(x) = critical f (t) dt, where a " x cos(t 3/2 ) dt F(x) = 41. Area Functions and Concavity Explain why0 the following statements are true. Assume f (x) is differentiable. and determine whether it is a local min or max. (a) If c is an inﬂection point of A(x), then f (c) = 0. (b) A(x) is concave up if f (x) is increasing. (c) A(x) is concave down if f (x) is decreasing. SOLUTION

(a) If x = c is an inﬂection point of A(x), then A (c) = f (c) = 0. (b) If A(x) is concave up, then A (x) > 0. Since A(x) is the area function associated with f (x), A (x) = f (x) by FTC II, so A (x) = f (x). Therefore f (x) > 0, so f (x) is increasing.

S E C T I O N 5.4

The Fundamental Theorem of Calculus, Part II

271

(c) If A(x) is concave down, then A (x) < 0. Since A(x) is the area function associated with f (x), A (x) = f (x) by FTC II, so A (x) = f (x). Therefore, f (x) < 0 and so f (x) is decreasing. " x f (t) dt, (x) as Figure 15. Determine: 43. LetMatch A(x) = the property of with A(x) fwith theincorresponding property of the graph of f (x). Assume f (x) is differentiable. 0

(a) (b) (c) (d)

The intervals on which A(x) is increasing and decreasing Areavalues function A(x) A(x) has a local min or max The x where (a) A(x) is decreasing. The inﬂection points of A(x) (b) intervals A(x) has where a localA(x) maximum. The is concave up or concave down (c) A(x) is concave up. y (d) A(x) goes from concave up to concave down. y = f(x) Graph of f(x) (i) Lies below the x-axis. (ii) Crosses the x-axis from positive to negative. 2 4 6 8 (iii) Has a local maximum. FIGURE 15 (iv) f (x) is increasing.

x 10

12

SOLUTION

(a) A(x) is increasing when A (x) = f (x) > 0, which corresponds to the intervals (0, 4) and (8, 12). A(x) is decreasing when A (x) = f (x) < 0, which corresponds to the intervals (4, 8) and (12, ∞). (b) A(x) has a local minimum when A (x) = f (x) changes from − to +, corresponding to x = 8. A(x) has a local maximum when A (x) = f (x) changes from + to −, corresponding to x = 4 and x = 12. (c) Inﬂection points of A(x) occur where A (x) = f (x) changes sign, or where f changes from increasing to decreasing or vice versa. Consequently, A(x) has inﬂection points at x = 2, x = 6, and x = 10. (d) A(x) is concave up when A (x) = f (x) is positive or f (x) is increasing, which corresponds to the intervals (0, 2) and (6, 10). Similarly, A(x) is concave down when f (x) is decreasing, which corresponds to the intervals (2, 6) and (10, ∞). " x " f (t) dt are decreasing. 45. Sketch the graph 2of an increasing function fx(x) such that both f (x) and A(x) = f (t) dt. Let f (x) = x − 5x − 6 and F(x) = 0 " x 0 (x) is points (x) must whether (a) Find the of F(x) andf determine they are local minima or maxima. SOLUTION If fcritical decreasing, then be negative. Furthermore, if A(x) = f (t) dt is decreasing, 0 (b) Find the points of inﬂection of F(x) and determine whether the concavity changes from up to down or vice then A (x) = f (x) must also be negative. Thus, we need a function which is negative but increasing and concave down. versa. The graph of one such function is shown below. (c) Plot f (x) and F(x) on the same set of axes and conﬁrm your answers to (a) and (b). y x

47.

Find the smallest positive inﬂection point of " x Figure 16 shows the graph of f (x) = x sin x. "Letx F(x) = t sin t dt. 0 cos(t 3/2 ) dt F(x) = 0

3/2 ) to determine whether the concavity changes from up to down or vice versa at this point of Use a(a)graph of ythe = local cos(xmaxima Locate and absolute maximum of F(x) on [0, 3π ]. inﬂection. (b) Justify graphically that F(x) has precisely one zero in the interval [π , 2π ]. SOLUTION points F(x) where F (x) = 0. By FTC, F (x) = cos(x 3/2 ), so F (x) = ]? (c) How Candidate many zerosinﬂection does F(x) have of in [0, 3πoccur 1/2 3/2 −(3/2)x ). Finding theof smallest positive F (x) 0, we get: the concavity changes from up to and, forofeach one,=state whether (d) Findsin(x the inﬂection points F(x) on [0, 3π ]solution down or vice versa. −(3/2)x 1/2 sin(x 3/2 ) = 0

sin(x 3/2 ) = 0 x 3/2 =

(since x > 0)

π

x = π 2/3 ≈ 2.14503. From the plot below, we see that F (x) = cos(x 3/2 ) changes from decreasing to increasing at π 2/3 , so F(x) changes from concave down to concave up at that point.

272

CHAPTER 5

THE INTEGRAL y 1 0.5 x 1

− 0.5

2

3

−1

49. Determine the function g(x) and all"values of c such that x 2 " Determine f (x), assuming that f (t) dt x is equal to x + x. 2 0 g(t) dt = x + x − 6 c

SOLUTION

By the FTC II we have g(x) =

d 2 (x + x − 6) = 2x + 1 dx

and therefore, " x c

g(t) dt = x 2 + x − (c2 + c)

We must choose c so that c2 + c = 6. We can take c = 2 or c = −3.

Further Insights and Challenges

" b Proof of FTC proof that in the textf (t) assumes that f−(x) is increasing. it for allFTC continuous functions, 51. Proof of FTC I II FTCThe I asserts dt = F(b) F(a) if F (x) =Tof prove (x). Assume II and give a new a " x and maximum of f (x) on [x, x + h] (Figure 17). The continuity of f (x) let m(h) and M(h) denote the minimum m(h)Set = lim (x). dt.Show that for h > 0, proofimplies of FTCthat I aslim follows. A(x)M(h) = = f f(t) h→0

h→0

a

(a) Show that F(x) = A(x) + C for some constant. hm(h) " ≤ b A(x + h) − A(x) ≤ h M(h) f (t) dt. (b) Show that F(b) − F(a) = A(b) − A(a) = For h < 0, the inequalities are reversed. Provea that A (x) = f (x). ! x SOLUTION Let F (x) = f (x) and A(x) = a f (t) dt. (a) Then by the FTC, Part II, A (x) = f (x) and thus A(x) and F(x) are both antiderivatives of f (x). Hence F(x) = A(x) + C for some constant C. (b) F(b) − F(a) = ( A(b) + C) − ( A(a) + C) = A(b) − A(a) " b " a " b " b = f (t) dt − f (t) dt = f (t) dt − 0 = f (t) dt a

a

a

a

which proves the FTC, Part I. " b " x 53. Find the values a ≤ b such that (x 2 − 9) d x has minimal value. f (t) dt is an antiderivative Can Every Antiderivative Bea Expressed as an Integral? The area function a !x 2 2 of f (x) for every value of a. However, not all antiderivatives are obtained in this way. The general antiderivative of SOLUTION Let a be given, and let Fa (x) = a (t − 9) dt. Then Fa (x) = x − 9, and the critical points are x = ±3. 1 x 2 +C. Show that F(x) is an area function if C ≤ 0 but not if C > 0. f (x) = x is F(x) = Because Fa (−3) = −6 2and Fa (3) = 6, we see that Fa (x) has a minimum at x = 3. Now, we ﬁnd a minimizing !3 2 !3 2 2 a (x − 9) d x. Let G(x) = x (x − 9) d x. Then G (x) = −(x − 9), yielding critical points x = 3 or x = −3. With x = −3, 3 " 3 1 3 G(−3) = (x 2 − 9) d x = x − 9x = −36. 3 −3 −3 With x = 3, G(3) = Hence a = −3 and b = 3 are the values minimizing

" 3 3

" b a

(x 2 − 9) d x = 0.

(x 2 − 9) d x.

S E C T I O N 5.5

Net or Total Change as the Integral of a Rate

273

5.5 Net or Total Change as the Integral of a Rate Preliminary Questions 1. An airplane makes the 350-mile trip from Los Angeles to San Francisco in 1 hour. Assuming that the plane’s velocity " 1 at time t is v(t) mph, what is the value of the integral v(t) dt? 0

!1

SOLUTION The deﬁnite integral 0 v(t) dt represents the total distance traveled by the airplane during the one hour ! ﬂight from Los Angeles to San Francisco. Therefore the value of 01 v(t) dt is 350 miles.

2. A hot metal object is submerged in cold water. The rate at which the object cools (in degrees per minute) is a function " T f (t) dt? f (t) of time. Which quantity is represented by the integral 0

! The deﬁnite integral 0T f (t) dt represents the total drop in temperature of the metal object in the ﬁrst T minutes after being submerged in the cold water. SOLUTION

3. (a) (b) (c) (d)

Which of the following quantities would be naturally represented as derivatives and which as integrals? Velocity of a train Rainfall during a 6-month period Mileage per gallon of an automobile Increase in the population of Los Angeles from 1970 to 1990

SOLUTION Quantities (a) and (c) involve rates of change, so these would naturally be represented as derivatives. Quantities (b) and (d) involve an accumulation, so these would naturally be represented as integrals.

4. Two airplanes take off at t = 0 from the same place and in the same direction. Their velocities are v1 (t) and v2 (t), respectively. What is the physical interpretation of the area between the graphs of v1 (t) and v2 (t) over an interval [0, T ]? SOLUTION The area between the graphs of v1 (t) and v2 (t) over an interval [0, T ] represents the difference in distance traveled by the two airplanes in the ﬁrst T hours after take off.

Exercises 1. Water ﬂows into an empty reservoir at a rate of 3,000 + 5t gal/hour. What is the quantity of water in the reservoir after 5 hours? SOLUTION

The quantity of water in the reservoir after ﬁve hours is 5 2 5 30125 (3000 + 5t) dt = 3000t + t = = 15,062.5 gallons. 2 2 0 0

" 5

2 perv(t) day.=Find insect after 3 3. A population of insects increases at a moving rate of 200 10t +line 0.25t Find the displacement of a particle in a+ straight withinsects velocity 4t −the 3 ft/s overpopulation the time interval days,[2, assuming that there are 35 insects at t = 0. 5]. SOLUTION

The increase in the insect population over three days is " 3

1 200 + 10t + t 2 dt = 4 0

200t + 5t 2 +

1 3 3 2589 t = = 647.25. 12 4 0

Accordingly, the population after 3 days is 35 + 647.25 = 682.25 or 682 insects. 2 − t bicycles per week (t in weeks). How many bicycles were 5. A factory produces bicycles at a rate of 95 +is0.1t A survey shows that a mayoral candidate gaining votes at a rate of 2,000t + 1,000 votes per day, where t is produced from day 8 to 21? the number of days since she announced her candidacy. How many supporters will the candidate have after 60 days,

assuming The that rate she had no supporters at t = = 95 0? + 10 t 2 SOLUTION of production is r (t) 1

− t bicycles per week and the period between days 8 and 21 corresponds to the second and third weeks of production. Accordingly, the number of bikes produced between days 8 and 21 is " 3 " 3 1 3 1 2 3 2803 1 2 r (t) dt = ≈ 186.87 95 + t − t dt = 95t + t − t = 10 30 2 15 1 1 1 or 187 bicycles. 7. A cat falls from a tree (with zero initial velocity) at time t = 0. How far does the cat fall between t = 0.5 and Find the displacement over the time interval [1, 6] of a helicopter whose (vertical) velocity at time t is v(t) = t = 1 s? Use Galileo’s formula v(t) = −32t ft/s. 0.02t 2 + t ft/s.

274

CHAPTER 5

THE INTEGRAL

Given v(t) = −32 ft/s, the total distance the cat falls during the interval [ 12 , 1] is

SOLUTION

" 1

|v(t)| dt =

1/2

" 1 1/2

1 32t dt = 16t 2

= 16 − 4 = 12 ft.

1/2

In Exercises 9–12, assume that with a particle in avelocity straight100 linem/s. withUse given total−displacement and A projectile is released initial moves (vertical) thevelocity. formulaFind v(t) the = 100 9.8t for velocity totaltodistance traveled over the time interval, and draw a motion diagram like Figure 3 (with distance and time labels). determine the distance traveled during the ﬁrst 15 s. 9. 12 − 4t ft/s,

[0, 5]

" 5

Total displacement is given by

SOLUTION

0

" 5 0

|12 − 4t| dt =

" 3 0

5 (12 − 4t) dt = (12t − 2t 2 ) = 10 ft, while total distance is given by 0

(12 − 4t) dt +

" 5 3

3 5 (4t − 12) dt = (12t − 2t 2 ) + (2t 2 − 12t) = 26 ft. 0

3

The displacement diagram is given here. t =5 t =3 t =0 Distance 0

11. t −2 − 1 m/s, [0.5, 2] 32 − 2t 2 ft/s, [0, 6] SOLUTION

10

" 2

Total displacement is given by

.5

18

2 (t −2 − 1) dt = (−t −1 − t) = 0 m, while total distance is given by .5

1 2 " 2 " 2 " 1 −2 −2 −2 −1 −1 (t − 1) dt + (1 − t ) dt = (−t − t) + (t + t ) = 1 m. t − 1 dt = .5

.5

.5

1

1

The displacement diagram is given here. t =2 t =1 t =0 Distance 0

0.5

13. The rate (in liters per minute) at which water drains from a tank is recorded at half-minute intervals. Use the average cos t m/s, [0, 4π ] of the left- and right-endpoint approximations to estimate the total amount of water drained during the ﬁrst 3 min.

SOLUTION

t (min)

0

0.5

1

1.5

2

2.5

3

l/min

50

48

46

44

42

40

38

Let t = .5. Then R N = .5(48 + 46 + 44 + 42 + 40 + 38) = 129.0 liters L N = .5(50 + 48 + 46 + 44 + 42 + 40) = 135.0 liters

The average of R N and L N is 12 (129 + 135) = 132 liters.

" t2 of a car is recorded at half-second intervals (in feet per second). the average thechange left- and 15. LetThe a(t)velocity be the acceleration of an object in linear motion at time t. Explain why Use a(t) dt is theofnet in right-endpoint approximations to estimate the total distance traveled during the ﬁrst 4t1s. velocity over [t1 , t2 ]. Find the net change in velocity over [1, 6] if a(t) = 24t − 3t 2 ft/s2 . t 0 of 0.5 1 in 1.5 2.5at time 3 t. 3.5 4 be the velocity of the object. SOLUTION Let a(t) be the acceleration an object linear 2motion Let v(t) We know that v (t) = a(t). By FTC, v(t) 0 12 20 29 38 44 32 35 30 t2 " t2 a(t) dt = (v(t) + C) = v(t2 ) + C − (vt1 + C) = v(t2 ) − v(t1 ), t1

t1

which is the net change in velocity over [t1 , t2 ]. Let a(t) = 24t − 3t 2 . The net change in velocity over [1, 6] is 6 " 6 (24t − 3t 2 ) dt = (12t 2 − t 3 ) = 205 ft/s. 1

1

Show that if acceleration a is constant, then the change in velocity is proportional to the length of the time interval.

Net or Total Change as the Integral of a Rate

S E C T I O N 5.5

275

17. The trafﬁc ﬂow rate past a certain point on a highway is q(t) = 3,000 + 2,000t − 300t 2 , where t is in hours and t = 0 is 8 AM. How many cars pass by during the time interval from 8 to 10 AM? SOLUTION

The number of cars is given by " 2 0

q(t) dt =

" 2 0

2 (3000 + 2000t − 300t 2 ) dt = 3000t + 1000t 2 − 100t 3 0

= 3000(2) + 1000(4) − 100(8) = 9200 cars. 19. Carbon Tax To encourage manufacturers to reduce pollution, a carbon tax on each ton of CO2 released into the 0.6x the + 350 dollars. What is the cost Suppose that the marginal of producing video is 0.001x 2 −study atmosphere has been proposed. Tocost model the effectsx of suchrecorders a tax, policymakers marginal cost of abatement ofdeﬁned producing 300cost unitsofifincreasing the setup cost $20,000 (see 4)? production 300 units,tons—Figure what is the cost fromExample x to x + 1 If tons (in units isofset tenatthousand 4). B(x), as the CO2isreduction " 3 of producing 20 additional units? B(t) dt? Which quantity is represented by 0 y Dollars/ton

100 75 50 25 x 1 2 3 Tons reduced (in ten thousands)

FIGURE 4 Marginal cost of abatement B(x).

" 3 SOLUTION

The quantity 0

sphere by 3 tons.

B(t) dt represents the total cost of reducing the amount of CO2 released into the atmo-

Figure shows the migration rate per M(t) of time. Ireland during theofperiod This×is10the rate at Figure which 21. 9 J/hour. Power is the 6rate of energy consumption unit A megawatt power1988–1998. is 106 W or 3.6 people (in thousands persupplied year) move in California or out of the country. 5 shows the"power by the power grid over a typical 1-day period. Which quantity is represented 1991 by the area under the graph? (a) What does M(t) dt represent? 1988

Migration (in thousands)

(b) Did migration over the 11-year period 1988–1998 result in a net inﬂux or outﬂow of people from Ireland? Base your answer on a rough estimate of the positive and negative areas involved. (c) During which year could the Irish prime minister announce, “We are still losing population but we’ve hit an inﬂection point—the trend is now improving.” 30 20 10 0 −10 −20 −30 −40 −50

1994 1988

1990

1992

1996

1998

2000

FIGURE 6 Irish migration rate (in thousands per year). SOLUTION

" 1991

(a) The amount

M(t) dt represents the net migration in thousands of people during the period from 1988–1991. 1988

(b) Via linear interpolation and using the midpoint approximation with n = 10, the migration (in thousands of people) over the period 1988 – 1998 is estimated to be 1 · (−43 − 33.5 − 12 + 0.5 − 2.5 − 6 − 3.5 + 3 + 11.5 + 19) = −66.5 That is, there was a net outﬂow of 66,500 people from Ireland during this period. (c) “The trend is now improving” implies that the population is decreasing, but that the rate of decrease is approaching zero. The population is decreasing with an improving trend in part of the years 1989, 1990, 1991, 1993, and 1994. “We’ve hit an inﬂection point” implies that the rate of population has changed from decreasing to increasing. There are two years in which the trend improves after it was getting worse: 1989 and 1993. During only one of these, 1989, was the population declining for the entire previous year. 23. Heat Capacity The heat capacity C(T ) of a substance is the amount of energy (in joules) required to raise the Figure 7 shows the graph of Q(t), the rate of retail truck sales in the United States (in thousands sold per year). temperature of 1 g by 1◦ C at temperature T . (a) What does the area under the graph over the interval [1995, 1997] represent? (b) Express the total number of trucks sold in the period 1994–1997 as an integral (but do not compute it). (c) Use the following data to compute the average of the right- and left-endpoint approximations as an estimate for

276

CHAPTER 5

THE INTEGRAL

(a) Explain why the energy required to raise the temperature from T1 to T2 is the area under the graph of C(T ) over [T1 , T2 ]. √ (b) How much energy is required to raise the temperature from 50 to 100◦ C if C(T ) = 6 + 0.2 T ? SOLUTION

(a) Since C(T ) is the energy required to raise the temperature of one gram of a substance by one degree when its temperature is T , the total energy required to raise the temperature from T1 to T2 is given by the deﬁnite integral " T2 C(T ) d T . As C(T ) > 0, the deﬁnite integral also represents the area under the graph of C(T ). T1 √ (b) If C(T ) = 6 + .2 T = 6 + 15 T 1/2 , then the energy required to raise the temperature from 50◦ C to 100◦ C is ! 100 50 C(T ) d T or " 100 50

6+

1 1/2 T 5

2 2 3/2 100 3/2 − 6(50) + 2 (50)3/2 = 6(100) + T (100) 15 15 15 50 √ 1300 − 100 2 = ≈ 386.19 Joules 3

dT =

6T +

In Exercises 24 and 25, consider the following. Paleobiologists have studied the extinction of marine animal families during the phanerozoic period, which began 544 million years ago. A recent study suggests that the extinction rate r (t) may be modeled by the function r (t) = 3,130/(t + 262) for 0 ≤ t ≤ 544. Here, t is time elapsed (in millions of years) since the beginning of the phanerozoic period. Thus, t = 544 refers to the present time, t = 540 is 4 million years ago, etc. the total number of extinct families from t = 0 to the present, using M N with N = 544. Use REstimate N or L N with N = 10 (or their average) to estimate the total number of families that became extinct in the periods 100 t≤ 150 and 350 ≤ t ≤ 400. SOLUTION We≤are estimating

25.

" 544 0

using M N with N = 544. If N = 544, t = M N = t

3130 dt (t + 262)

544 − 0 = 1 and {ti∗ }i=1,...N = it − (t/2) = i − 12 . 544

N #

r (ti∗ ) = 1 ·

i=1

544 #

3130 = 3517.3021. 261.5 +i i=1

Thus, we estimate that 3517 families have become extinct over the past 544 million years. output is the rate R of volume of blood pumped by the heart per unit time (in liters per minute). Doctors Further Cardiac Insights and Challenges measure R by injecting A mg of dye into a vein leading into the heart at t = 0 and recording the concentration c(t) 27. A particle located at the origin at t = 0 moves along the x-axis with velocity v(t) = (t + 1)−2 . Show that the of dye (in milligrams per liter) pumped out at short regular time intervals (Figure 8). particle will never pass the point x = 1.

SOLUTION The particle’s velocity is v(t) = s (t) = (t + 1)−2 , an antiderivative for which is F(t) = −(t + 1)−1 . Hence its position at time t is " tout in a small timetinterval [t, t + t] is approximately Rc(t)t. Explain why. (a) The quantity of dye pumped 1 " T s (u) du = F(u) = F(t) − F(0) = 1 − <1 s(t) = t + c(t) dt, (b) Show that A = R 0 where T is large 0enough that all of the dye is 1pumped through the heart but not

0

sotlarge returnswill by never recirculation. for all ≥ 0.that Thusthethedye particle pass the point x = 1. (c) Use the following data to estimate R, assuming that A = 5 mg: A particle located at the origin at t = 0 moves along the x-axis with velocity v(t) = (t + 1)−1/2 . Will the particle be at the point x = 1 at any time t? If so, ﬁnd t. t (s) 0 1 2 3 4 5 5.6 Substitution Method c(t)

0

0.4

2.8

6.5

9.8

8.9

Preliminary Questions

t (s) 6 7 8 9 10 1. Which of the following integrals is a candidate for the Substitution Method? " " " c(t) 6.15 4 2.3 1.1 0 (b) sin x cos x d x (c) x 5 sin x d x (a) 5x 4 sin(x 5 ) d x

The function in (c): x 5 sin x is not of the form g(u(x))u (x). The function in (a) meets the prescribed pattern with g(u) = sin u and u(x) = x 5 . Similarly, the function in (b) meets the prescribed pattern with g(u) = u 5 and u(x) = sin x. SOLUTION

2. Write each of the following functions in the form cg(u(x))u (x), where c is a constant. (a) x(x 2 + 9)4 (b) x 2 sin(x 3 ) (c) sin x cos2 x

S E C T I O N 5.6

Substitution Method

277

SOLUTION

(a) x(x 2 + 9)4 = 12 (2x)(x 2 + 9)4 ; hence, c = 12 , g(u) = u 4 , and u(x) = x 2 + 9. (b) x 2 sin(x 3 ) = 13 (3x 2 ) sin(x 3 ); hence, c = 13 , g(u) = sin u, and u(x) = x 3 . (c) sin x cos2 x = −(− sin x) cos2 x; hence, c = −1, g(u) = u 2 , and u(x) = cos x. " 2 x 2 (x 3 + 1) d x for a suitable substitution? 3. Which of the following is equal to 0 " " " 9 1 2 1 9 (a) u du (b) u du (c) u du 3 0 3 1 0 ! 2 1 !9 SOLUTION With the substitution u = x 3 + 1, the deﬁnite integral 0 x 2 (x 3 + 1) d x becomes 3 1 u du. The correct answer is (c).

Exercises In Exercises 1–6, calculate du for the given function. 1. u = 1 − x 2 SOLUTION

Let u = 1 − x 2 . Then du = −2x d x.

3. u =u x=3 − sin2x SOLUTION Let u = x 3 − 2. Then du = 3x 2 d x. 5. u = cos(x 24) u = 2x + 8x SOLUTION Let u = cos(x 2 ). Then du = − sin(x 2 ) · 2x d x = −2x sin(x 2 ) d x. In Exercises 7–20, u = tan x write the integral in terms of u and du. Then evaluate. " 7. (x − 7)3 d x, u = x − 7 SOLUTION

" 9.

" (x + 1)−22d x, u = x + 1 2 2x x + 1 d x, u = x + 1

SOLUTION

" 11.

Let u = x − 7. Then du = d x. Hence " " 1 1 (x − 7)3 d x = u 3 du = u 4 + C = (x − 7)4 + C. 4 4

Let u = x + 1. Then du = d x. Hence " " (x + 1)−2 d x = u −2 du = −u −1 + C = −(x + 1)−1 + C = −

1 + C. x +1

" sin(2x − 4) d9 x, u = 2x − 4 x(x + 1) d x, u = x + 1

SOLUTION

Let u = 2x − 4. Then du = 2 d x or 12 du = d x. Hence " " 1 1 1 sin u du = − cos u + C = − cos(2x − 4) + C. sin(2x − 4) d x = 2 2 2

"

" x + 13 d x, u = x 2 + 2x x 3 (x 2 + 2x) d x, u = x 4 + 1 (x 4 + 1)4 2 1 SOLUTION Let u = x + 2x. Then du = (2x + 2) d x or 2 du = (x + 1) d x. Hence " " 1 1 −1 1 1 −2 1 x +1 d x = du = + C. − u + C = − (x 2 + 2x)−2 + C = 2 2 2 4 (x 2 + 2x)3 u3 4(x 2 + 2x)2

13.

" √ "

u = 4x − 1 d x, u = 8x + 5 (8x + 5)3 1 SOLUTION Let u = 4x − 1. Then du = 4 d x or 4 du = d x. Hence " " √ 1 1 2 3/2 1 u 1/2 du = 4u − 1 d x = u + C = (4x − 1)3/2 + C. 4 4 3 6

15.

"

4x − 1x d x,

√ x 4x − 1 d x,

u = 4x − 1

278

CHAPTER 5

THE INTEGRAL

" 17.

√ x 2 4x − 1 d x,

SOLUTION

u = 4x − 1

Let u = 4x − 1. Then x = 14 (u + 1) and du = 4 d x or 14 du = d x. Hence 2 " " " √ 1 1 1 (u 5/2 + 2u 3/2 + u 1/2 ) du x 2 4x − 1 d x = (u + 1) u 1/2 du = 4 4 64 1 2 7/2 1 2 5/2 1 2 3/2 = u u u + + +C 64 7 64 5 64 3 =

" 19.

1 1 1 (4x − 1)7/2 + (4x − 1)5/2 + (4x − 1)3/2 + C. 224 160 96

" sin2 x cos x2d x, u = sin2x x cos(x ) d x, u = x

SOLUTION

Let u = sin x. Then du = cos x d x. Hence " " 1 1 sin2 x cos x d x = u 2 du = u 3 + C = sin3 x + C. 3 3

" In Exercises 21–24, show that each of the following integrals is equal to a multiple of sin(u(x)) + C for an appropriate sec2 x tan x d x, u = tan x choice of u(x). " 21. x 3 cos(x 4 ) d x SOLUTION

Let u = x 4 . Then du = 4x 3 d x or 14 du = x 3 d x. Hence " " 1 1 cos u du = sin u + C, x 3 cos(x 4 ) d x = 4 4

which is a multiple of sin(u(x)). " " ) dx 23. x 1/2 2cos(x 3/2 x cos(x 3 + 1) d x SOLUTION

Let u = x 3/2 . Then du = 32 x 1/2 d x or 23 du = x 1/2 d x. Hence " " 2 2 x 1/2 cos(x 3/2 ) d x = cos u du = sin u + C, 3 3

which is a multiple of sin(u(x)). " In Exercises 25–51, evaluate the indeﬁnite integral. cos x cos(sin x) d x " (4x + 3)4 d x

25.

SOLUTION

" 27.

" 1 √ 2 3d x 3 xx−(x7 + 1) d x

SOLUTION

" 29.

Let u = 4x + 3. Then du = 4 d x or 14 du = d x. Hence " " 1 1 1 5 1 (4x + 3)4 d x = u 4 du = u +C = (4x + 3)5 + C. 4 4 5 20

Let u = x − 7. Then du = d x. Hence " " √ (x − 7)−1/2 d x = u −1/2 du = 2u 1/2 + C = 2 x − 7 + C.

" x x2 − 4 dx sin(x − 7) d x

SOLUTION

Let u = x 2 − 4. Then du = 2x d x or 12 du = x d x. Hence " " 1 √ 1 2 3/2 1 x x2 − 4 dx = u du = u + C = (x 2 − 4)3/2 + C. 2 2 3 3

" (2x + 1)(x 2 + x)3 d x

S E C T I O N 5.6

" 31.

Substitution Method

dx (x + 9)2

SOLUTION

Let u = x + 9, then du = d x. Hence " " 1 1 dx du = =− +C =− + C. 2 2 u x + 9 (x + 9) u

" " 2x 2 + x x dx 3 + 3x 2 )2d x (4x x2 + 9 1 SOLUTION Let u = 4x 3 + 3x 2 . Then du = (12x 2 + 6x) d x or 6 du = (2x 2 + x) d x. Hence " " 1 1 1 u −2 du = − u −1 + C = − (4x 3 + 3x 2 )−1 + C. (4x 3 + 3x 2 )−2 (2x 2 + x) d x = 6 6 6

33.

35.

" " 4 5x + 2x dx3 2 2 2 )31)(x + x) d x (x 5(3x + x+

SOLUTION

Let u = x 5 + x 2 . Then du = (5x 4 + 2x) d x. Hence "

" 37.

"

"

1 1 1 1 1 du = − 2 + C = − + C. 3 5 2u 2 (x + x 2 )2 u

" (3x +2 9)310 d x 4 x (x + 1) d x

SOLUTION

39.

5x 4 + 2x dx = (x 5 + x 2 )3

Let u = 3x + 9. Then du = 3 d x or 13 du = d x. Hence " " 1 1 1 11 1 u 10 du = (3x + 9)10 d x = +C = u (3x + 9)11 + C. 3 3 11 33

" x x(x + 1)1/4 d10 x(3x + 9) d x

SOLUTION

Let u = x + 1. Then u − 1 = x and du = d x. Hence " " x(x + 1)1/4 d x = (u − 1)u 1/4 du " = =

" 41.

1 2

1 = 2

"

" (u + 1)u 3/2 du =

1 2

" (u 5/2 + u 3/2 ) du

1 2 7/2 1 2 5/2 1 u u + + C = (x 2 − 1)7/2 + (x 2 − 1)5/2 + C. 7 2 5 7 5

" sin5 x2 cos x 3d x x sin(x ) d x

SOLUTION

45.

4 4 (x + 1)9/4 − (x + 1)5/4 + C. 9 5

Let u = x 2 − 1. Then u + 1 = x 2 and du = 2x d x or 12 du = x d x. Hence " " x 3 (x 2 − 1)3/2 d x = x 2 · x(x 2 − 1)3/2 d x =

43.

4 9/4 4 5/4 − u +C u 9 5

" x 3 (x22 − 1)3/27 d x x (x + 1) d x

SOLUTION

"

(u 5/4 − u 1/4 ) du =

Let u = sin x. Then du = cos x d x. Hence " " 1 1 sin5 x cos x d x = u 5 du = u 6 + C = sin6 x + C. 6 6

" + 9) d x sec2 (4x x 2 sin(x 3 + 1) d x

279

280

CHAPTER 5

THE INTEGRAL SOLUTION

" 47.

" cos 2x 2 x tan24 dx xd x secsin (1 + 2x)

SOLUTION

" 49.

Let u = 4x + 9. Then du = 4 d x or 14 du = d x. Hence " " 1 1 1 sec2 (4x + 9) d x = sec2 u du = tan u + C = tan(4x + 9) + C. 4 4 4

Let u = 1 + sin 2x. Then du = 2 cos 2x or 12 du = cos 2x d x. Hence " " 1 1 1 (1 + sin 2x)−2 cos 2x d x = u −2 du = − u −1 + C = − (1 + sin 2x)−1 + C. 2 2 2

" √x − 1) d x cos x(3 sin sin 4x cos 4x + 1 d x

SOLUTION

Let u = 3 sin x − 1. Then du = 3 cos x d x or 13 du = cos x d x. Hence " " 1 1 2 1 1 u du = (3 sin x − 1) cos x d x = u + C = (3 sin x − 1)2 + C. 3 3 2 6

"

" 2 x(4√tan3 x − 3 tan2 x) d x sec cos x dx √ x SOLUTION Let u = tan x. Then du = sec2 x d x. Hence " " sec2 x(4 tan3 x − 3 tan2 x) d x = (4u 3 − 3u 2 ) du = u 4 − u 3 + C = tan4 x − tan3 x + C.

51.

" " 1)1/4 x 5 d x. 53. Evaluate (x 3 +5 Evaluate x x 3 + 1 d x using u = x 3 + 1. Hint: x 5 d x = x 3 · x 2 d x and x 3 = u − 1. SOLUTION

Let u = x 3 + 1. Then x 3 = u − 1 and du = 3x 2 d x or 13 du = x 2 d x. Hence " " " 1 1 x 5 (x 3 + 1)1/4 d x = u 1/4 (u − 1) du = (u 5/4 − u 1/4 ) du 3 3 4 4 3 1 4 9/4 4 5/4 − u +C = = u (x + 1)9/4 − (x 3 + 1)5/4 + C. 3 9 5 27 15 "

55. Evaluate sin x cos x d x using substitution in two different ways: ﬁrst using u = sin x and then u = cos x. Recon" Can They Both Be Right? Hannah uses the substitution u = tan x and Akiva uses u = sec x to evaluate cile thetan twox different 2 sec x d x.answers. Show that they obtain different answers and explain the apparent contradiction. SOLUTION

First, let u = sin x. Then du = cos x d x and " " 1 1 sin x cos x d x = u du = u 2 + C1 = sin2 x + C1 . 2 2

Next, let u = cos x. Then du = − sin x d x or −du = sin x d x. Hence, " " 1 1 sin x cos x d x = − u du = − u 2 + C2 = − cos2 x + C2 . 2 2 To reconcile these two seemingly different answers, recall that any two antiderivatives of a speciﬁed function differ by a constant. To show that this is true here, note that ( 12 sin2 x + C1 ) − (− 12 cos2 x + C2 ) = 12 + C1 − C2 , a constant. Here we used the trigonometric identity sin2 x + cos2 x = 1. " π π to the integral sin(3x + π ) d x? 57. What are the new limits of integration if we apply the substitution u = 3x + Some Choices Are Better Than Others Evaluate SOLUTION

" The new limits of integration are u(0) = 3sin · 0x+cos π= 2 xπdand x u(π ) = 3π + π = 4π .

0

" 8 In Exercises 59–70, use the Change of Variables Formula to evaluate the deﬁnite integral. twice. First use u = sin x to show that (4x − 9)20 d x? " 3Which of the following is the result of applying the substitution u = 4x − 9 to the integral " " 2 3 " 8 2 "+ (x 59. 8 2) d x sin x cos2 x d x = u1 1 − u du 1 u 20 du (b) u 20 du (a) 4 2 2 " 23 that u = cos x is a better choice. " 23 the integral on the right by a further substitution. Then and evaluate 1 show 20 (c) 4 u du (d) u 20 du 4 −1 −1

S E C T I O N 5.6 SOLUTION

Let u = x + 2. Then du = d x. Hence " 3 1

61.

(x + 2)3 d x =

34 1 4 5 54 3 − = 136. u du = u = 4 3 4 4 3

" 5

" 1 " 6 x √ dx +33 d x 0 (x 2 +x 1) 1

SOLUTION

63.

Substitution Method

Let u = x 2 + 1. Then du = 2x d x or 12 du = x d x. Hence " 1 " 1 2 1 1 1 1 −2 2 1 3 x d x = du = − u = − 16 + 4 = 16 = 0.1875. 2 1 u3 2 2 0 (x 2 + 1)3 1

" 4 " 2 2 x x√ + 9 d x 5x + 6 d x 0 −1

SOLUTION

Let u = x 2 + 9. Then du = 2x d x or 12 du = x d x. Hence " 4

1 x2 + 9 dx =

" 25 √

2 9

0

1 u du = 2

2 3/2 25 98 1 u = 3 (125 − 27) = 3 . 3 9

" 2 " 2 (x + 1)(xx2+ +32x)3 d x dx 1 0 (x 2 + 6x + 1)3 1 2 SOLUTION Let u = x + 2x. Then du = (2x + 2)d x and so 2 du = (x + 1)d x. Hence

65.

" 2 1

67.

(x + 1)(x 2 + 2x)3 d x =

" 1 8 3 1 1 4 8 1 4015 u du = u = (84 − 34 ) = . 2 3 2 4 8 8 3

" π /2 " 17 cos 3x d x−2/3 (x − 9) dx 0 10

SOLUTION

Let u = 3x. Then du = 3 d x or 13 du = d x. Hence " π /2 0

3π /2 " 1 1 3π /2 1 1 cos 3x d x = cos u du = sin u =− −0=− . 3 0 3 3 3 0

" π /2 " π /2 3

cos x sin x d xπ dx cos 3x + 0 2 0 SOLUTION Let u = cos x. Then du = − sin x d x. Hence

69.

" π /2 0

" π /4 71. Evaluate 0 SOLUTION

"

dx √

cos3 x sin x d x = −

using u = 2 +

2 x+sec2x) tan(2 x3d x

Let u = 2 +

√

√

" 0 1

u 3 du =

" 1 0

u 3 du =

1 4 1 1 1 u = −0= . 4 0 4 4

x.

1 d x, so that x. Then du = √ 2 x

√ 2 x du = d x 2(u − 2) du = d x. From this, we get: " " "

dx u−2 u −2 − 2u −3 du = 2 −u −1 + u −2 + C √ 3 = 2 3 du = 2 (2 + x) u √ √ 1 1 −2 − x + 1 1+ x =2 − √ + + C = −2 √ 2 +C =2 √ 2 √ 2 + C. 2+ x (2 + x) (2 + x) (2 + x)

substitution " 2use In Exercises 73–74, to evaluate the integral in terms of f (x). Evaluate r 5 − 4 − r 2 dr . 0

281

282

CHAPTER 5

THE INTEGRAL

" 73.

f (x)3 f (x) d x

SOLUTION

Let u = f (x). Then du = f (x) d x. Hence " " 1 1 f (x)3 f (x) d x = u 3 du = u 4 + C = f (x)4 + C. 4 4

" π /6 " 1/2 " 1 f (x) 75. Show that f (sin θ ) d θ = f (u) du. d x 0 0 1 − u2 f (x)2 SOLUTION Let u = sin θ . Then u(π /6) = 1/2 and u(0) = 0, as required. Furthermore, du = cos θ d θ , so that dθ = If sin θ = u, then u 2 + cos2 θ = 1, so that cos θ = " π /6 0

du . cos θ

1 − u 2 . Therefore d θ = du/ 1 − u 2 . This gives

f (sin θ ) d θ =

" 1/2 0

f (u)

1 1 − u2

du.

" π /2

Further Insights and sinn xChallenges cos x d x, where n is an integer, n = −1. Evaluate

0 77. Use the substitution u = 1 + x 1/n to show that " " 1 + x 1/n d x = n u 1/2 (u − 1)n−1 du

Evaluate for n = 2, 3. SOLUTION

"

n

Let u = 1 + x 1/n . Then x = (u − 1)n and d x = n(u − 1)n−1 du. Accordingly,

" 1 + x 1/n d x =

u 1/2 (u − 1)n−1 du. For n = 2, we have " " " 1 + x 1/2 d x = 2 u 1/2 (u − 1)1 du = 2 (u 3/2 − u 1/2 ) du =2

For n = 3, we have

"

4 2 5/2 2 3/2 4 − u u + C = (1 + x 1/2 )5/2 − (1 + x 1/2 )3/2 + C. 5 3 5 3 "

=3 =

" u 1/2 (u − 1)2 du = 3

1 + x 1/3 d x = 3

(u 5/2 − 2u 3/2 + u 1/2 ) du

2 2 2 7/2 − (2) u u 5/2 + u 3/2 + C 7 5 3

12 6 (1 + x 1/3 )7/2 − (1 + x 1/3 )5/2 + 2(1 + x 1/3 )3/2 + C. 7 5

" a

" π /2 79. Show that f "(x)π /2 d x = 0 ifd θf is an odd function. dθ Evaluate−a I = . Hint: Use substitution to show that I is equal to J = 6,000 6,000 θ 1 + tan θ 1 + cot 0 0 " continuous. SOLUTION We assume that f is If f (x) is an odd function, then f (−x) = − f (x). Let u = −x. Then π /2 x = and −u then and du = −d check thatx or I +−du J == d x. Accordingly, dθ. 0 " a " 0 " a " 0 " a f (x) d x = f (x) d x + f (x) d x = − f (−u) du + f (x) d x −a

−a

=

" a 0

0

f (x) d x −

" a 0

a

0

f (u) du = 0.

81. Show that the two regions in Figure 4 have the same area. Then use the identity cos2 u = 12 (1 + cos 2u) to compute (a) Use the substitution u = x/a to prove that the hyperbola y = x −1 (Figure 3) has the following special property: the second area. " b/a " b 1 1 dx = d x. If a, b > 0, then x a x 1 (b) Show that the areas under the hyperbola over the intervals [1, 2], [2, 4], [4, 8], . . . are all equal.

Chapter Review Exercises y

283

y

1

1

y = 1 − x2

y = cos2 u

x

p 2

1 (A)

u

(B)

FIGURE 4

!1 SOLUTION The area of the region in Figure 4(A) is given by 0 1 − x 2 d x. Let x = sin u. Then d x = cos u du and 1 − x 2 = 1 − sin2 u = cos u. Hence, " 1 " π /2 " π /2 1 − x2 dx = cos u · cos u du = cos2 u du. 0

0

0

This last integral represents the area of the region in Figure 4(B). The two regions in Figure 4 therefore have the same area. ! π /2 Let’s now focus on the deﬁnite integral 0 cos2 u du. Using the trigonometric identity cos2 u = 12 (1 + cos 2u), we have π /2 " " π /2 1 π /2 1 1 π 1 π cos2 u du = 1 + cos 2u du = = · −0= . u + sin 2u 2 0 2 2 2 2 4 0 0 83. Area of an Ellipse Prove the formula A = π ab for the area of the ellipse with equation Area of a Circle The number π is deﬁned as one-half the circumference of the unit circle. Prove that the area of a circle of radius r is A = π r 2 . The case r = 1x 2follows y 2 from Exercise 81. Prove it for all r > 0 by showing that + =1 " r a2 b2 " 1 " a r2 − x2 dx = r2 1 − x 2 d x. 2 0 0 use the formula for the area of a circle (Figure 5). 1 − (x/a) d x, change variables, and Hint: Show that A = 2b −a

y b

−a

a

x

−b

FIGURE 5 Graph of 2

x2 y2 + 2 = 1. 2 a b

2

Consider the ellipse with equation x 2 + y2 = 1; here a, b > 0. The area between the part of the ellipse in a b

!a 2 f (x) d x. By symmetry, the part of the elliptical the upper half-plane, y = f (x) = b2 1 − x 2 , and the x-axis is −a SOLUTION

a

region in the lower half-plane has the same area. Accordingly, the area enclosed by the ellipse is " a " a " a x2 2 f (x) d x = 2 b2 1 − 2 d x = 2b 1 − (x/a)2 d x a −a −a −a Now, let u = x/a. Then x = au and a du = d x. Accordingly, " a " 1

x 2

π = π ab 2b 1− d x = 2ab 1 − u 2 du = 2ab a 2 −a −1 !1 Here we recognized that −1 1 − u 2 du represents the area of the upper unit semicircular disk, which by Exercise 81 is 2( π4 ) = π2 .

CHAPTER REVIEW EXERCISES In Exercises 1–4, refer to the function f (x) whose graph is shown in Figure 1.

284

CHAPTER 5

THE INTEGRAL y 3 2 1 x 1

2

3

4

FIGURE 1

1. Estimate L 4 and M4 on [0, 4]. SOLUTION

and

With n = 4 and an interval of [0, 4], x = 4−0 4 = 1. Then, 1 5 23 L 4 = x( f (0) + f (1) + f (2) + f (3)) = 1 +1+ +2 = 4 2 4 1 3 5 7 1 9 9 M4 = x f + f + f + f =1 +2+ + = 7. 2 2 2 2 2 4 4

Estimate R4 , [a, L 4 ,b]and is larger than 3. Find an interval on M which R4 3]. 4 on [1,

" b a

f (x) d x. Do the same for L 4 .

! In general, R N is larger than ab f (x) d x on any interval [a, b] over which f (x) is increasing. Given the ! graph of f (x), we may take [a, b] = [0, 2]. In order for L 4 to be larger than ab f (x) d x, f (x) must be decreasing over the interval [a, b]. We may therefore take [a, b] = [2, 3]. SOLUTION

In Exercises 5–8, let f (x) = x 2 + 4x. Justify " 2 Sketch the graph of f (x) and the corresponding rectangles 5. Calculate R6 , M6 , and L 6 for f (x) on the interval 9 3 [1, 4]. f (x) d x ≤ ≤ for each approximation. 2 4 1

SOLUTION

Let f (x) = x 2 + 4x. A uniform partition of [1, 4] with N = 6 subintervals has x =

4−1 1 = , 6 2

and x ∗j = a +

x j = a + jx = 1 +

j−

j , 2

j 1 3 x = + . 2 4 2

Now, R6 = x 1 = 2

6 #

f (x j ) =

j=1

1 2

3 5 7 f + f (2) + f + f (3) + f + f (4) 2 2 2

33 65 105 463 + 12 + + 21 + + 32 = . 4 4 4 8

The rectangles corresponding to this approximation are shown below. y 35 30 25 20 15 10 5 x 1

2

3

4

Next, M6 = x 1 = 2

6 #

j=1

f (x ∗j ) =

1 2

5 7 9 11 13 15 f + f + f + f + f + f 4 4 4 4 4 4

105 161 225 297 377 465 + + + + + 16 16 16 16 16 16

=

The rectangles corresponding to this approximation are shown below.

1630 815 = . 32 16

Chapter Review Exercises y 35 30 25 20 15 10 5 x 1

2

3

4

Finally, L 6 = x

5 #

=

f (x j ) =

j=0

1 2

f (1) + f

3 5 7 + f (2) + f + f (3) + f 2 2 2

1 33 65 105 5+ + 12 + + 21 + 2 4 4 4

=

355 . 8

The rectangles corresponding to this approximation are shown below. y 35 30 25 20 15 10 5 x 1

2

3

4

" 2 " 4 7. Find a formula for L N for f (x) on [0, 2] and compute f (x) d x by taking the limit. Find a formula for R N for f (x) on [1, 4] and compute f (x) d x by taking the limit. 0 SOLUTION

1 Let f (x) = x 2 + 4x and N be a positive integer. Then

x =

2 2−0 = N N

and x j = a + jx = 0 +

2j 2j = N N

for 0 ≤ j ≤ N . Thus, L N = x

N −1 #

f (x j ) =

j=0

=

−1 N −1 −1 # 4 j2 2 N# 8j 8 N# 2 + 16 + j j = N j=0 N 2 N N 3 j=0 N 2 j=0

32 12 4 4(N − 1)(2N − 1) 8(N − 1) + . = + + N 3 N 3N 2 3N 2

Finally, " 2 0

4 32 12 + + N N →∞ 3 3N 2

f (x) d x = lim

=

32 . 3

9. Calculate R6 , M6 , and L 6 for f (x)" =x (x 2 + 1)−1 on the interval [0, 1]. Use FTC I to evaluate A(x) = f (t) dt. SOLUTION Let f (x) = (x 2 + 1)−1 . A uniform partition of [0, 1] with N = 6 subintervals has −2 x =

1 1−0 = , 6 6

and x ∗j = a +

j−

x j = a + jx =

j , 6

1 2j − 1 x = . 2 12

Now, R6 = x

6 # j=1

f (x j ) =

1 6

1 1 1 2 5 f + f + f + f + f + f (1) 6 3 2 3 6

285

286

CHAPTER 5

THE INTEGRAL

=

1 6

36 9 4 9 36 1 + + + + + 37 10 5 13 61 2

≈ 0.742574.

Next, M6 = x =

1 6

6 #

f (x ∗j ) =

j=1

1 6

1 1 5 7 3 11 f + f + f + f + f + f 12 4 12 12 4 12

144 16 144 144 16 144 + + + + + 145 17 169 193 25 265

≈ 0.785977.

Finally, L 6 = x

5 #

=

f (x j ) =

j=0

1 6

1 1 1 2 5 + f + f + f + f 6 3 2 3 6

f (0) + f

1 36 9 4 9 36 1+ + + + + 6 37 10 5 13 61

≈ 0.825907.

11. Which approximation to the area is represented by the shaded rectangles in Figure 3? Compute R5 and L 5 . Let R N be the N th right-endpoint approximation for f (x) = x 3 on [0, 4] (Figure 2). y 64(N + 1)2 . (a) Prove that R N = 30 N2 (b) Prove that the area of the region below the right-endpoint rectangles and above the graph is equal to 18

64(2N + 1) . N2

6

x 1

2

3

4

5

FIGURE 3 SOLUTION There are ﬁve rectangles and the height of each is given by the function value at the right endpoint of the subinterval. Thus, the area represented by the shaded rectangles is R5 . From the ﬁgure, we see that x = 1. Then

R5 = 1(30 + 18 + 6 + 6 + 30) = 90

and

L 5 = 1(30 + 30 + 18 + 6 + 6) = 90.

In Exercises 13–34, evaluate the integral. Calculate any two Riemann sums for f (x) = x 2 on the interval [2, 5], but choose partitions with at least ﬁve " subintervals of unequal widths and intermediate points that are neither endpoints nor midpoints. 13. (6x 3 − 9x 2 + 4x) d x " 3 (6x 3 − 9x 2 + 4x) d x = x 4 − 3x 3 + 2x 2 + C. SOLUTION 2 " " 13 − 1)2 d x 15. (2x (4x 3 − 2x 5 ) d x " " 0 4 (2x 3 − 1)2 d x = (4x 6 − 4x 3 + 1) d x = x 7 − x 4 + x + C. SOLUTION 7 " 4 x" 4+ 1 17. dx x 2(x 5/2 − 2x −1/2 ) d x 1 " 4 " 1 x +1 SOLUTION d x = (x 2 + x −2 ) d x = x 3 − x −1 + C. 2 3 x " 4 " 42 |x − 9| d x 19. r −2 dr −1 1

SOLUTION

" 4 −1

|x 2 − 9| d x =

" 3 −1

(9 − x 2 ) d x +

= (27 − 9) − −9 + " 21.

" 32 θ d θ csc [t] dt 1

" 4 3

1 3

(x 2 − 9) d x =

+

9x −

4 1 3 3 1 3 x + x − 9x 3 3 −1 3

64 − 36 − (9 − 27) = 30. 3

Chapter Review Exercises

" csc2 θ d θ = − cot θ + C.

SOLUTION

"

" π2/4 (9t − 4) dt sec sec t tan t dt 0 SOLUTION Let u = 9t − 4. Then du = 9dt and " " 1 1 1 sec2 u du = tan u + C = tan(9t − 4) + C. sec2 (9t − 4) dt = 9 9 9

23.

"

" π−/34)11 dt (9t sin 4θ d θ 0 SOLUTION Let u = 9t − 4. Then du = 9dt and " " 1 1 12 1 u 11 du = (9t − 4)11 dt = u +C = (9t − 4)12 + C. 9 108 108

25.

"

" 22 (3 sin θ ) cos(3θ ) d θ 4y + 1 d y 6 SOLUTION Let u = sin(3θ ). Then du = 3 cos(3θ )d θ and " " 1 1 1 u 2 du = u 3 + C = sin3 (3θ ) + C. sin2 (3θ ) cos(3θ ) d θ = 3 9 9

27.

" 29.

(2x " π3/2+ 3x) d x 2 )5 θ ) sin θ d θ (3x 4 +sec 9x2 (cos 0

SOLUTION

Let u = 3x 4 + 9x 2 . Then du = (12x 3 + 18x) d x = 6(2x 3 + 3x) d x and "

" 31.

1 (2x 3 + 3x) d x = 6 (3x 4 + 9x 2 )5

"

u −5 du = −

1 −4 1 u + C = − (3x 4 + 9x 2 )−4 + C. 24 24

√ " −2 sin θ 412x − cos d xθ dθ −4 (x 2 + 2)3

SOLUTION

" 33.

Let u = 4 − cos θ . Then du = sin θ d θ and " " √ 2 2 sin θ 4 − cos θ d θ = u 1/2 du = u 3/2 + C = (4 − cos θ )3/2 + C. 3 3

/3 + sin y" π2y 3 dθ y 0

SOLUTION

dθ cos2/3 θ Let u = 2y + 3. Then du = 2d y, y = 12 (u − 3) and " " " √ 1 1 (u − 3) u du = (u 3/2 − 3u 1/2 ) du y 2y + 3 d y = 4 4 1 1 2 5/2 1 − 2u 3/2 + C = = u (2y + 3)5/2 − (2y + 3)3/2 + C. 4 5 10 2

35. Combine to write as a single integral " 8 √ " 8 t 2 t + 8 dt 1

0 SOLUTION

f (x) d x +

" 0 −2

f (x) d x +

" 6 8

f (x) d x

First, rewrite " 8 0

f (x) d x =

" 6 0

f (x) d x +

" 8 6

f (x) d x

and observe that " 6 8

f (x) d x = −

" 8 6

f (x) d x.

287

288

CHAPTER 5

THE INTEGRAL

Thus, " 8 0

f (x) d x +

" 6 8

f (x) d x =

" 6 0

f (x) d x.

Finally, " 8 0

f (x) d x +

" 0 −2

f (x) d x +

" 6 8

f (x) d x =

" 6 0

f (x) d x +

" 0 −2

f (x) d x =

" 6 −2

f (x) d x.

" x t dt " x . 37. Find inﬂection points of A(x) = Let A(x) = f (x) d x, where + is 1 the function shown in Figure 4. Indicate on the graph of f where the 3 ft 2(x) SOLUTION Let local minima,

0

maxima, and points of inﬂection of A(x) occur and identify the intervals where A(x) is increasing, decreasing, concave up, or concave down. " x t dt . A(x) = 3 t2 + 1

Then x A (x) = 2 x +1 and A (x) =

(x 2 + 1)(1) − x(2x) 1 − x2 = 2 . 2 2 (x + 1) (x + 1)2

Thus, A(x) is concave down for |x| > 1 and concave up for |x| < 1. A(x) therefore has inﬂection points at x = ±1. 2 (in thousands of gallons per hour), 39. On A a typical city watert at ratemoves of r (t) = velocity 100 + 72t − as 3t shown particleday, startsa at theconsumes origin at time = the 0 and with v(t) in Figure 5. where t is the number of hours past midnight. What is the daily water consumption? How much water is consumed (a) How many times does the particle return to the origin in the ﬁrst 12 s? between 6 PM and midnight? (b) Where is the particle located at time t = 12? SOLUTION With a consumption rate of r (t) = 100 + 72t − 3t 2 thousand gallons per hour, the daily consumption of (c) At which time t is the particle’s distance to the origin at a maximum? water is " 24 24 (100 + 72t − 3t 2 ) dt = 100t + 36t 2 − t 3 = 100(24) + 36(24)2 − (24)3 = 9312, 0

0

or 9.312 million gallons. From 6 PM to midnight, the water consumption is " 24 18

24

(100 + 72t − 3t 2 ) dt = 100t + 36t 2 − t 3 18

= 100(24) + 36(24)2 − (24)3 − 100(18) + 36(18)2 − (18)3 = 9312 − 7632 = 1680,

or 1.68 million gallons. 41. Cost engineers at NASA have the task of projecting the cost P of major space projects. It has been found that the per bicycle), which The learning curve for producing bicycles certain = 12x, −1/5 cost C of developing a projection increases with Pinata the rate factory dC/d Pis≈L(x) 21P −0.65 where(in C hours is in thousands of dollars means that it takes a bike mechanic L(n) hours to assemble the nth bicycle. If 24 bicycles are produced, and P in millions of dollars. What is the cost of developing a projection for a project whose cost turns out to be how P =long $35 does it take to produce the second batch of 12? million? SOLUTION Assuming it costs nothing to develop a projection for a project with a cost of $0, the cost of developing a projection for a project whose cost turns out to be $35 million is

" 35 0

35 21P −0.65 d P = 60P 0.35 = 60(35)0.35 ≈ 208.245, 0

or $208,245. 43. Let fof(x) a positive increasing continuous function b], where 0 ≤ a

Chapter Review Exercises

289

y f (b)

y = f (x) f (a) x a

b

FIGURE 7 SOLUTION We can construct the shaded region in Figure 7 by taking a rectangle of length b and height f (b) and removing a rectangle of length a and height f (a) as well as the region between the graph of y = f (x) and the x-axis over the interval [a, b]. The area of the resulting region is then the area of the large rectangle minus the area of the small rectangle and minus the area under the curve y = f (x); that is, " b f (x) d x. I = b f (b) − a f (a) −

a

In Exercises 45–49, express the limit as an integral (or multiple of an integral) and evaluate. How can we interpret the quantity I in Eq. (1) if a < b ≤ 0? Explain with a graph. N 2j 2 # 45. lim sin N N →∞ N j=1 Let f (x) = sin x and N be a positive integer. A uniform partition of the interval [0, 2] with N subintervals

SOLUTION

has x =

2 N

and

xj =

2j N

for 0 ≤ j ≤ N . Then N N # 2 # 2j sin f (x j ) = R N ; = x N j=1 N j=1 consequently, 2 " 2 N 2j 2 # sin sin x d x = − cos x = 1 − cos 2. = N N →∞ N 0 0 j=1 lim

−1 π N# π j N π # +4k 47. lim 4 sin limN j=0 32+ N N →∞ N N →∞ N k=1

SOLUTION Let f (x) = sin x and N be a positive integer. A uniform partition of the interval [π /2, 3π /2] with N subintervals has

x =

π N

and

xj =

π πj + 2 N

for 0 ≤ j ≤ N . Then −1 N −1 # π N# π πj sin f (x j ) = L N ; + = x N j=0 2 N j=0 consequently, 3π /2 " 3π /2 −1 π N# π πj sin sin x d x = − cos x = 0. + = 2 N N →∞ N π /2 π /2 j=0 lim

1k + 2kN+ · · · N k (k > 0) 4 # 1 N →∞ N k+1 lim 4k 2 N N →∞ + ) k=1 (3 SOLUTION Observe that N

49.

lim

1 1k + 2k + 3k + · · · + N k = N N k+1

)

k k k * N k 2 3 N 1 # j 1 k + + + ··· . = N N N N N j=1 N

290

CHAPTER 5

THE INTEGRAL

Now, let f (x) = x k and N be a positive integer. A uniform partition of the interval [0, 1] with N subintervals has x =

1 N

xj =

and

j N

for 0 ≤ j ≤ N . Then N k N # 1 # j = x f (x j ) = R N ; N j=1 N j=1

consequently, 1 " 1 N k 1 1 1 # j = xk dx = x k+1 = . N k + 1 k + 1 N →∞ N 0 0 j=1 lim

" 1 " π /4 9 f (x) dxx, dassuming that f (x) is an even continuous function such that 51. Evaluate x , using the properties of odd functions. Evaluate 0 2 cos x −π /4 " 2 " 1 f (x) d x = 5, f (x) d x = 8 −2

1

Using the given information

SOLUTION

" 2 −2

f (x) d x =

" 1 −2

f (x) d x +

" 2 1

f (x) d x = 13.

Because f (x) is an even function, it follows that " 0 −2

f (x) d x =

" 2 0

f (x) d x,

so " 2 0

f (x) d x =

13 . 2

Finally, " 1 0

f (x) d x =

" 2 0

f (x) d x −

" 2 1

f (x) d x =

13 3 −5= . 2 2

53. Show that Plot the graph of f (x) = sin mx sin " nx on [0, π ] for the pairs (m, n) = (2, 4), (3, 5) and in each case guess " π f (x) d x =more x F(x) − G(x) f (x) d x. Experiment xwith a few values (including two cases with m = n) and formulate the value of I = 0

" a conjecture for when I is zero. where F (x) = f (x) and G (x) = F(x). Use this to evaluate x cos x d x. Suppose F (x) = f (x) and G (x) = F(x). Then

SOLUTION

d (x F(x) − G(x)) = x F (x) + F(x) − G (x) = x f (x) + F(x) − F(x) = x f (x). dx Therefore, x F(x) − G(x) is an antiderivative of x f (x) and " x f (x) d x = x F(x) − G(x) + C. To evaluate

55.

!

x cos x d x, note that f (x) = cos x. Thus, we may take F(x) = sin x and G(x) = − cos x. Finally, " x cos x d x = x sin x + cos x + C.

" 2 Prove Plot the graph of f (x) = x −2 sin x and show that 0.2 ≤ f (x) d x ≤ 0.9. 1 " " 2 2 1 1 2x d x ≤ 4 and 3−x d x ≤ . 2≤ ≤ 9 3 1 1

Chapter Review Exercises SOLUTION

291

Let f (x) = x −2 sin x. From the ﬁgure below, we see that 0.2 ≤ f (x) ≤ 0.9

for 1 ≤ x ≤ 2. Therefore, 0.2 =

" 1 0

0.2 d x ≤

" 1 0

f (x) d x ≤

" 1 0

0.9 d x = 0.9.

y 1 0.8 0.6

x−2sin x

0.4 0.2 x 0.5

1

1.5

2

In Exercises 57–62, ﬁnd the derivative. " 1 " xbounds for Find upper and lower f (x) d x, where f (x) has the graph shown in Figure 8. 57. A (x), where A(x) = sin(t 3 ) dt 0 3

SOLUTION

Let A(x) =

" x 3

sin(t 3 ) dt. Then A (x) = sin(x 3 ).

" y d " x 3x d x cos t (π ), d y A−2 where A(x) = dt 2 1+t " y d SOLUTION 3x d x = 3 y . d y −2 " x3 "√ sin x 61. G (2), where G(x) = t + 13 dt t dt G (x), where G(x) = 0 −2 " x3 √ SOLUTION Let G(x) = t + 1 dt. Then 59.

0

G (x) =

x3 + 1

d 3 x = 3x 2 x 3 + 1 dx

√ and G (2) = 3(2)2 8 + 1 = 36. " 9 If f (x) is increasing and concave up on [a, b], then L N is more accurate than R N . Explain with a graph: 63. 1 (1), where H (x) = dt Which isHmore accurate if f (x) is increasing and concave down? 4x 2 t SOLUTION Consider the ﬁgure below, which displays a portion of the graph of an increasing, concave up function. y

x

The shaded rectangles represent the differences between the right-endpoint approximation R N and the left-endpoint approximation L N . In particular, the portion of each rectangle that lies below the graph of y = f (x) is the amount by which L N underestimates the area under the graph, whereas the portion of each rectangle that lies above the graph of y = f (x) is the amount by which R N overestimates the area. Because the graph of y = f (x) is increasing and concave up, the lower portion of each shaded rectangle is smaller than the upper portion. Therefore, L N is more accurate (introduces less error) than R N . By similar reasoning, if f (x) is increasing and concave down, then R N is more accurate than L N . Explain with a graph: If f (x) is linear on [a, b], then the

" b a

f (x) d x =

1 (R N + L N ) for all N . 2

APPLICATIONS OF 6 THE INTEGRAL 6.1 Area Between Two Curves Preliminary Questions 1. What is the area interpretation of

" b a

f (x) − g(x) d x if f (x) ≥ g(x)?

!b

SOLUTION Because f (x) ≥ g(x), a ( f (x) − g(x)) d x represents the area of the region bounded between the graphs of y = f (x) and y = g(x), bounded on the left by the vertical line x = a and on the right by the vertical line x = b. " b 2. Is f (x) − g(x) d x still equal to the area between the graphs of f and g if f (x) ≥ 0 but g(x) ≤ 0?

a

SOLUTION

Yes. Since f (x) ≥ 0 and g(x) ≤ 0, it follows that f (x) − g(x) ≥ 0.

3. Suppose that f (x) ≥ g(x) on [0, 3] and g(x) ≥ f (x) on [3, 5]. Express the area between the graphs over [0, 5] as a sum of integrals. SOLUTION Remember that to calculate an area between two curves, one must subtract the equation for the lower curve from the equation for the upper curve. Over the interval [0, 3], y = f (x) is the upper curve. On the other hand, over the interval [3, 5], y = g(x) is the upper curve. The area between the graphs over the interval [0, 5] is therefore given by " 3 " 5 ( f (x) − g(x)) d x + (g(x) − f (x)) d x.

0

3

4. Suppose that the graph of x = f (y) lies to the left of the y-axis. Is

" b a

f (y) d y positive or negative?

If the graph of x = f (y) lies to the left of the y-axis, then for each value of y, the corresponding value of x ! is less than zero. Hence, the value of ab f (y) d y is negative. SOLUTION

Exercises 1. Find the area of the region between y = 3x 2 + 12 and y = 4x + 4 over [−3, 3] (Figure 8). y y = 3x 2 + 12

50

25 y = 4x + 4

−2 −3

−1

x 1

2

3

FIGURE 8

As the graph of y = 3x 2 + 12 lies above the graph of y = 4x + 4 over the interval [−3, 3], the area between the graphs is " 3 " 3

3 (3x 2 + 12) − (4x + 4) d x = (3x 2 − 4x + 8) d x = x 3 − 2x 2 + 8x = 102. SOLUTION

−3

−3

−3

3. Let f (x) = x and g(x) = 2 − x 2 [Figure 9(B)]. Compute the area of the region in Figure 9(A), which lies between y = 2 − x 2 and y = −2 over [−2, 2]. (a) Find the points of intersection of the graphs. (b) Find the area enclosed by the graphs of f and g. SOLUTION

(a) Setting f (x) = g(x) yields 2 − x 2 = x, which simpliﬁes to 0 = x 2 + x − 2 = (x + 2)(x − 1). Thus, the graphs of y = f (x) and y = g(x) intersect at x = −2 and x = 1.

S E C T I O N 6.1

Area Between Two Curves

293

(b) As the graph of y = x lies below the graph of y = 2 − x 2 over the interval [−2, 1], the area between the graphs is 1 " 1

1 1 9 (2 − x 2 ) − x d x = 2x − x 3 − x 2 = . 3 2 2 −2 −2 In Exercises 5–7, ﬁnd the area between y = sin x and y = cos x over the interval. Sketch the curves if necessary. Let f (x) = 8x − 10 and g(x) = x 2 − 4x + 10. π 5. (a) 0, Find the points of intersection of the graphs. (b) 4Compute the area of the region below the graph of f and above the graph of g. π SOLUTION Over the interval [0, 4 ], the graph of y = sin x lies below that of y = cos x. Hence, the area between the two curves is √ √ " π /4 π /4 √ 2 2 = + − (0 + 1) = 2 − 1. (cos x − sin x) d x = (sin x + cos x) 2 2 0 0 7. [0, π ] π π , π π SOLUTION 4 2Over the interval [0, 4 ], the graph of y = sin x lies below that of y = cos x, while over the interval [ 4 , π ], the orientation of the graphs is reversed. The area between the graphs over [0, π ] is then " π /4 0

(cos x − sin x) d x +

" π π /4

(sin x − cos x) d x

π π /4 + (− cos x − sin x) = (sin x + cos x) 0

√

π /4

√ √ 2 2 2 2 = + − (0 + 1) + (1 − 0) − − − = 2 2. 2 2 2 2 √

√

In Exercises 8–10, let f (x) = 20 + x − x 2 and g(x) = x 2 − 5x. 9. Find the area of the region enclosed by the two graphs. Find the area between the graphs of f and g over [1, 3]. SOLUTION Setting f (x) = g(x) gives 20 + x − x 2 = x 2 − 5x, which simpliﬁes to 0 = 2x 2 − 6x − 20 = 2(x − 5)(x + 2). Thus, the curves intersect at x = −2 and x = 5. With y = 20 + x − x 2 being the upper curve, the area between the two curves is 5 " 5

" 5

2 343 (20 + x − x 2 ) − (x 2 − 5x) d x = 20 + 6x − 2x 2 d x = 20x + 3x 2 − x 3 = . 3 3 −2 −2 −2 In Exercises 11–14, of the between shaded region ingraphs the ﬁgure. Compute theﬁnd areathe of area the region the two over [4, 8] as a sum of two integrals. 11.

y

y = x 3 − 2x 2 + 10 −2

x y=

3x 2

2 + 4x − 10

FIGURE 10 SOLUTION

As the graph of y = x 3 − 2x 2 + 10 lies above the graph of y = 3x 2 + 4x − 10, the area of the shaded

region is " 2

" 2

(x 3 − 2x 2 + 10) − (3x 2 + 4x − 10) d x = x 3 − 5x 2 − 4x + 20 d x −2

−2

=

2 1 4 5 3 160 x − x − 2x 2 + 20x = . 4 3 3 −2

294

CHAPTER 6

A P P L I C AT I O N S O F T H E I N T E G R A L

13.

y

1 y=− x 2

−1

x

1 y = x 1 − x2

FIGURE 12 √ √ 3 3 1 1 SOLUTION Setting 2 x = x 1 − x 2 yields x = 0 or 2 = 1 − x 2 , so that x = ± 2 . Over the interval [− 2 , 0], √ y = 12 x is the upper curve but over the interval [0, 23 ], y = x 1 − x 2 is the upper curve. The area of the shaded region is then " 0 " √3/2 1 2 dx + 2 − 1 x dx 1 − x 1 − x x − x x √ 2 − 3/2 2 0

=

0 √ 5 5 1 1 2 3/2 5 1 2 1 2 3/2 2 3/2 x + (1 − x ) + = . + − (1 − x ) − x = √ 4 3 3 4 48 48 24 − 3/2 0

15. Find the area of the region enclosed by the curves y = x 3 − 6x and y = 8 − 3x 2 . Setting x 3 − 6x = 8 − 3x 2 yields (x + 1)(x + 4)(x − 2) = 0, so the two curves intersect at x = −4, x = −1 and x = 2. Over the interval [−4, −1], y = x 3 − 6x is the upper curve, while y = 8 − 3x 2 is the upper curve over the interval [−1, 2]. The area of the region enclosed by the two curves is then SOLUTION

" 2

" −1

(x 3 − 6x) − (8 − 3x 2 ) d x + (8 − 3x 2 ) − (x 3 − 6x) d x −4

=

−1

−1 2 1 4 81 1 81 81 x − 3x 2 − 8x + x 3 + 8x − x 3 − x 4 + 3x 2 = + = . 4 4 4 4 2 −4 −1

In Exercises 17–18, ﬁnd the area between the graphs of x = sin y and x = 21 − cos y over the given interval (Figure 14). Find the area of the region enclosed by the semicubical parabola y = x 3 and the line x = 1. y x = sin y x = 1 − cos y 2

x −

2

FIGURE 14

π 17. 0 ≤ y ≤ 2 SOLUTION As shown in the ﬁgure, the graph on the right is x = sin y and the graph on the left is x = 1 − cos y. Therefore, the area between the two curves is given by

" π /2 0

π /2 π π = − + 1 − (−1) = 2 − . (sin y − (1 − cos y)) d y = (− cos y − y + sin y) 2 2 0

2 19. Find the π area ofπthe region lying to the right of x = y + 4y − 22 and the left of x = 3y + 8. − ≤y≤ 2 Setting2 y 2 + 4y − 22 = 3y + 8 yields SOLUTION

0 = y 2 + y − 30 = (y + 6)(y − 5), so the two curves intersect at y = −6 and y = 5. The area in question is then given by " 5

−6

(3y + 8) − (y 2 + 4y − 22)

5 " 5

y3 1331 y2 2 −y − y + 30 d y = − dy = − + 30y = . 3 2 6 −6 −6

y-axis and as an integral 21. Calculate the area enclosed by x = 9 − y 2 and x = 5 in 2two ways: as an integral along the Find the area of the region lying to the right of x = y − 5 and the left of x = 3 − y 2 . along the x-axis.

S E C T I O N 6.1 SOLUTION

Area Between Two Curves

295

Along the y-axis, we have points of intersection at y = ±2. Therefore, the area enclosed by the two curves

is 2 " 2

" 2

32 1 . 9 − y2 − 5 d y = 4 − y 2 d y = 4y − y 3 = 3 3 −2 −2 −2 Along the x-axis, we have integration limits of x = 5 and x = 9. Therefore, the area enclosed by the two curves is 9 " 9 √ 4 32 32 2 9 − x d x = − (9 − x)3/2 = 0 − − = . 3 3 3 5 5 In Exercises 23–24, ﬁnd the area of the region using the method (integration along either the x- or y-axis) that requires Figure 15 shows the graphs of x = y 3 − 26y + 10 and x = 40 − 6y 2 − y 3 . Match the equations with the curve you to evaluate just one integral. and compute the area of the shaded region. 23. Region between y 2 = x + 5 and y 2 = 3 − x From the ﬁgure below, we see that integration along the x-axis would require two integrals, but integration along the y-axis requires only one integral. Setting y 2 − 5 = 3 − y 2 yields points of intersection at y = ±2. Thus, the area is given by SOLUTION

2 " 2

" 2

64 2 . (3 − y 2 ) − (y 2 + 5) d y = 8 − 2y 2 d y = 8y − y 3 = 3 3 −2 −2 −2 y 2

x = 3 − y2

1 −4

x

−2

−1

x = y2 − 5

2

−2

In Exercises 25–41, sketch thex region enclosed by the and compute its area as an integral along the x- or y-axis. Region between y= and x + y = 8 over [2,curves 3] 25. y = 4 − x 2 ,

y = x2 − 4

Setting 4 − x 2 = x 2 − 4 yields 2x 2 = 8 or x 2 = 4. Thus, the curves y = 4 − x 2 and y = x 2 − 4 intersect at x = ±2. From the ﬁgure below, we see that y = 4 − x 2 lies above y = x 2 − 4 over the interval [−2, 2]; hence, the area of the region enclosed by the curves is SOLUTION

2 " 2 2 64 (8 − 2x 2 ) d x = 8x − x 3 = . (4 − x 2 ) − (x 2 − 4) d x = 3 3 −2 −2 −2

" 2

y y = 4 − x2

4 2 −2

−1

x −2 −4

2 27. x =y sin = xy,2 −x6,= πy y= 6 − x 3 , SOLUTION

1

2

y = x2 − 4

y-axis

Here, integration along the y-axis will require less work than integration along the x-axis. The curves

π intersect when 2y π = sin y or when y = 0, ± 2 . From the graph below, we see that both curves are symmetric with respect to the origin. It follows that the portion of the region enclosed by the curves in the ﬁrst quadrant is identical to the region enclosed in the third quadrant. We can therefore determine the total area enclosed by the two curves by doubling the area enclosed in the ﬁrst quadrant. In the ﬁrst quadrant, x = sin y lies to the right of x = 2y π , so the total area enclosed by the two curves is

2

π /2 " π /2

1 π π 2 =2 0− − (−1 − 0) = 2 − . sin y − y d y = 2 − cos y − y 2 π π 4 2 0 0

296

CHAPTER 6

A P P L I C AT I O N S O F T H E I N T E G R A L y x=

2

y

1 x = sin y x

−1

1 −1

−3 , y = 4 − x, y = x 29. y =x 3x +y= 4, x − y = 0, y 3+ 3x = 4

The curves y = 3x −3 and y = 4 − x intersect at x = 1, the curves y = 3x −3 and y = x/3 intersect at x = 3 and the curves y = 4 − x and y = x/3 intersect at x = 3. From the graph below, we see that the top of the −3 region enclosed √ by the three curves is√always bounded by y = 4 − x. The bottom of the region is bounded by y = 3x for 1 ≤ x ≤ 3 and by y = x/3 for 3 ≤ x ≤ 3. The total area of the region is then SOLUTION

√

√ " √3

" 3 2 2 3 1 1 2 3 −2 3 −3 (4 − x) − 3x d x + √ (4 − x) − x d x = 4x − x + x + 4x − x √ = 2. 3 2 2 3 1 3 3 1 y 3 y= 3 x3

2

1 y=

y=4−x

x 3 x 1

0

2

3

√ √ 2, y = −x√ x − 2, x = 4 31. y = x x − √ y = 2 − x, y = x, √ x =0 √ SOLUTION Note that y = x x − 2 and y = −x x − 2 are the upper and lower branches, respectively, of the curve y 2 = x 2 (x − 2). The area enclosed by this curve and the vertical line x = 4 is " 4 " 4

√ √ √ x x − 2 − (−x x − 2) d x = 2x x − 2 d x. 2

2

Substitute u = x − 2. Then du = d x, x = u + 2 and √ " 2

4 5/2 8 3/2 2 128 2 3/2 1/2 u 2u du = 2x x − 2 d x = 2(u + 2) u du = + 4u + u = 15 . 5 3 2 0 0 0

" 4

" 2

√

√

y 4

y 2 = x 2 (x − 2)

2 x −2

1

2

3

4

−4

33. x = |y|, x = 6 − y22 y = |x|, y = x − 6 SOLUTION From the graph below, we see that integration along the y-axis will require less work than integration along the x-axis. Moreover, the region is symmetric with respect to the x-axis, so the total area can be determined by doubling the area of the upper portion of the region. For y > 0, setting y = 6 − y 2 yields 0 = y 2 + y − 6 = (y − 2)(y + 3), so the curves intersect at y = 2. Because x = 6 − y 2 lies to the right of x = |y| = y in the ﬁrst quadrant, we ﬁnd the total area of the region is 2

2 " 2 1 1 8 44 6 − y 2 − y d y = 2 6y − y 3 − y 2 = 2 12 − − 2 = . 3 2 3 3 0 0

S E C T I O N 6.1

Area Between Two Curves

297

y 2 x = |y| 1 x 1

−1

2

3

4

5

6

x = 6 − y2

−2

35. x = 12 − y, x = y, x = 2y x = |y|, x = 1 − |y| SOLUTION From the graph below, we see that the bottom of the region enclosed by the three curves is always bounded by y = x2 . On the other hand, the top of the region is bounded by y = x for 0 ≤ x ≤ 6 and by y = 12 − x for 6 ≤ x ≤ 8. The area of the region is then 8 " 6

" 8

x x 3 1 6 x− (12 − x) − dx + d x = x 2 + 12x − x 2 = 9 + (96 − 48) − (72 − 27) = 12. 2 2 4 0 4 0 6 6 y 6

y = 12 − x

4

y=x

2

y=

x 2 x

0

2

4

6

8

37. x = 2y, 3x + 1 = (y − 1)2 x = y − 18y, y + 2x = 0 SOLUTION Setting 2y = (y − 1)2 − 1 yields 0 = y 2 − 4y = y(y − 4), so the two curves intersect at y = 0 and at y = 4. From the graph below, we see that x = 2y lies to the right of x + 1 = (y − 1)2 over the interval [0, 4] along the y-axis. Thus, the area of the region enclosed by the two curves is 4 " 4

" 4

1 32 . 2y − ((y − 1)2 − 1) d y = 4y − y 2 d y = 2y 2 − y 3 = 3 3 0 0 0 y 4

x + 1 = ( y − 1) 2

3 x = 2y

2 1

x 2

4

6

8

+ x 2 (in1/2 the region x > 0) 39. y = 6, y = x −2 1/2 x + y = 1, x +y =1 −2 SOLUTION Setting 6 = x + x 2 yields 0 = x 4 − 6x 2 + 1, which is a quadratic equation in the variable x 2 . By the quadratic formula, √ √ 6 ± 36 − 4 = 3 ± 2 2. x2 = 2 Now, √ √ √ √ and 3 − 2 2 = ( 2 − 1)2 , 3 + 2 2 = ( 2 + 1)2 √ √ so the two curves intersect at x = 2 − 1 and x = 2 + 1. Note there are also two points of intersection with x < 0, but as the problem speciﬁes the region is for x > 0, we neglect these other two values. From the graph below, we see that y = 6 lies above y = x −2 + x 2 , so the area of the region enclosed by the two curves for x > 0 is √ " √2+1

1 3 2+1 16 −2 2 −1 6 − (x + x ) d x = 6x + x − x √ = . √ 3 3 2−1 2−1

298

CHAPTER 6

A P P L I C AT I O N S O F T H E I N T E G R A L y 6

4 y = x −2 + x 2 2

x 0

1

2

3

π 3π 41. y = sin x, y = csc2 x, x = , x = 2π y = cos x, y = cos(2x), 4x = 0, x 4= 3 π 3π SOLUTION Over the interval [ 4 , 4 ], y = csc2 x lies above y = sin x. The area of the region enclosed by the two curves is then √ √ " 3π /4 √ 2 3π /4 2 2 csc x − sin x d x = − cot x + cos x = 1− − −1 + = 2 − 2. 2 2 π /4 π /4 y 2 1.5

y = csc 2 x

1

y = sin x

0.5 x 0

0.5

1

1.5

2

43. Sketch a region whose x area is represented2by Plot y = the same set of axes. Use a computer algebra system to ﬁnd the points and y = (x − 1) on " √2/2 x2 + 1 2 −curves. of intersection numerically and compute the √ area between 1 − x the |x| d x − 2/2

and evaluate using geometry.

1 − x 2 − |x| with the yTOP − yBOT template for calculating area, we see that the region in question is bounded along the top by the curve y = 1 − x 2 (the upper half of the unit circle) and is bounded along the bottom by the curve y = |x|. Hence, the region is 14 of the unit circle (see the ﬁgure below). The area of the region must then be SOLUTION

Matching the integrand

1 π π (1)2 = . 4 4 y

0.8

0.4 x

−0.4

0.4

45. Express the area (not signed) of the shaded region in Figure 16 as a sum of three integrals involving the functions f Two athletes run in the same direction along a straight track with velocities v1 (t) and v2 (t) (in ft/s). Assume and g. that y " 5 " 20 f(x)(v1 (t) − v2 (t)) dt = 5 (v1 (t) − v2 (t)) dt = 2, 0

" 35 30

0

x 3

5

9

(v1 (t) − v2 (t)) dt = −2 g(x)

" t2 16v2 (t) dt. v1 (t) − (a) Give a verbal interpretation of the integral FIGURE t1

(b) Is enough information given to determine the distance between the two runners at time t = 5 s? (c) If the runners begin at the same time and place, how far ahead is runner 1 at time t = 20 s? (d) Suppose that runner 1 is 8 ft ahead at t = 30 s. How far is she ahead or behind at t = 35 s?

S E C T I O N 6.1

Area Between Two Curves

299

SOLUTION Because either the curve bounding the top of the region or the curve bounding the bottom of the region or both change at x = 3 and at x = 5, the area is calculated using three integrals. Speciﬁcally, the area is " 3 " 5 " 9 ( f (x) − g(x)) d x + ( f (x) − 0) d x + (0 − f (x)) d x

0

=

" 3 0

3

( f (x) − g(x)) d x +

" 5 3

5

f (x) d x −

" 9 5

f (x) d x.

47. Set up (but do not evaluate) an integral that expresses the 2area between the circles x 2 + y 2 = 2 and x 2 + 2 (y − 1)2Find = 1.the area enclosed by the curves y = c − x and y = x − c as a function of c. Find the value of c for which this area is equal to 1. SOLUTION Setting 2 − y 2 = 1 − (y − 1)2 yields y = 1. The two circles therefore intersect at the points (1, 1) and (−1, 1). From the graph below, we see that over the interval [−1, 1], the upper half of the circle x 2 + y 2 = 2 lies above the lower half of the circle x 2 + (y − 1)2 = 1. The area enclosed by the two circles is therefore given by the integral " 1 2 − x 2 − (1 − 1 − x 2 ) d x. −1

y x2 + y2 = 2 1 x 2 + (y − 1)2 = 1 x 1

−1

49. Find a numerical approximation to the area above y = 1 − (x/π ) and below y = sin x (ﬁnd the points of Set up (but do not evaluate) an integral that expresses the area between the graphs of y = (1 + x 2 )−1 and y = x 2 . intersection numerically). SOLUTION The region in question is shown in the ﬁgure below. Using a computer algebra system, we ﬁnd that y = 1 − x/π and y = sin x intersect on the left at x = 0.8278585215. Analytically, we determine the two curves intersect on the right at x = π . The area above y = 1 − x/π and below y = sin x is then " π

x sin x − 1 − d x = 0.8244398727, π 0.8278585215 where the deﬁnite integral was evaluated using a computer algebra system. y 1 y = sin x

y=1−

x x

1

0

2

3

51. Use a computer algebra system to ﬁnd a numerical approximation to the number c (besides zero) in [0, π2 ], Find a numerical approximation to the area above y = |x| and below y = cos x. where the curves y = sin x and y = tan2 x intersect. Then ﬁnd the area enclosed by the graphs over [0, c]. SOLUTION The region in question is shown in the ﬁgure below. Using a computer algebra system, we ﬁnd that y = sin x and y = tan2 x intersect at x = 0.6662394325. The area of the region enclosed by the two curves is then " 0.6662394325

sin x − tan2 x d x = 0.09393667698, 0

where the deﬁnite integral was evaluated using a computer algebra system. y

0.8 0.6 0.4

y = sin x y = tan 2 x

0.2 x 0

0.2

0.4

0.6

The back of Jon’s guitar (Figure 17) has a length 19 in. He measured the widths at 1-in. intervals, beginning and ending 12 in. from the ends, obtaining the results

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Further Insights and Challenges 53. Find the line y = mx that divides the area under the curve y = x(1 − x) over [0, 1] into two regions of equal area. SOLUTION

First note that " 1 0

x(1 − x) d x =

" 1

0

1 2 1 3 1 1 x − x2 dx = x − x = . 2 3 6 0

Now, the line y = mx and the curve y = x(1 − x) intersect when mx = x(1 − x), or at x = 0 and at x = 1 − m. The area of the region enclosed by the two curves is then " 1−m 0

1−m " 1−m

x2 1 1 3 2 (1 − m)x − x d x = (1 − m) = (1 − m)3 . − x (x(1 − x) − mx) d x = 2 3 6 0 0

To have 16 (1 − m)3 = 12 · 16 requires m =1−

55.

1/3 1 ≈ 0.206299. 2

calculation) following for any n >by 0: the line y = cx (Figure LetExplain c be thegeometrically number such (without that the area under y why = sinthe x over [0, π ] holds is divided in half 18). Find an equation for c and solve this"equation numerically using a computer algebra system. " 1 1 xn dx + x 1/n d x = 1 0

SOLUTION

0

Let A1 denote the area of region 1 in the ﬁgure below. Deﬁne A2 and A3 similarly. It is clear from the

ﬁgure that A1 + A2 + A3 = 1. Now, note that x n and x 1/n are inverses of each other. Therefore, the graphs of y = x n and y = x 1/n are symmetric about the line y = x, so regions 1 and 3 are also symmetric about y = x. This guarantees that A1 = A3 . Finally, " 1 0

xn dx +

" 1 0

x 1/n d x = A3 + ( A2 + A3 ) = A1 + A2 + A3 = 1. y 1 1

2 3 x 0

1

6.2 Setting Up Integrals: Volume, Density, Average Value Preliminary Questions 1. What is the average value of f (x) on [1, 4] if the area between the graph of f (x) and the x-axis is equal to 9? Assuming that f (x) ≥ 0 over the interval [1, 4], the fact that the area between the graph of f and the x-axis ! is equal to 9 indicates that 14 f (x) d x = 9. The average value of f over the interval [1, 4] is then SOLUTION

!4

1 f (x) d x = 9 = 3.

4−1

3

2. Find the volume of a solid extending from y = 2 to y = 5 if the cross section at y has area A(y) = 5 for all y. SOLUTION Because the cross-sectional area of the solid is constant, the volume is simply the cross-sectional area times the length, or 5 × 3 = 15.

3. Describe the horizontal cross sections of an ice cream cone and the vertical cross sections of a football (when it is held horizontally).

Setting Up Integrals: Volume, Density, Average Value

S E C T I O N 6.2

301

SOLUTION The horizontal cross sections of an ice cream cone, as well as the vertical cross sections of a football (when held horizontally), are circles.

4. What is the formula for the total population within a circle of radius R around a city center if the population has a radial function? SOLUTION

Because the population density is a radial function, the total population within a circle of radius R is 2π

" R 0

r ρ(r ) dr,

where ρ(r ) is the radial population density function. 5. What is the deﬁnition of ﬂow rate? SOLUTION

The ﬂow rate of a ﬂuid is the volume of ﬂuid that passes through a cross-sectional area at a given point per

unit time. 6. Which assumption about ﬂuid velocity did we use to compute the ﬂow rate as an integral? SOLUTION To express ﬂow rate as an integral, we assumed that the ﬂuid velocity depended only on the radial distance from the center of the tube.

Exercises 1. Let V be the volume of a pyramid of height 20 whose base is a square of side 8. (a) Use similar triangles as in Example 1 to ﬁnd the area of the horizontal cross section at a height y. (b) Calculate V by integrating the cross-sectional area. SOLUTION

(a) We can use similar triangles to determine the side length, s, of the square cross section at height y. Using the diagram below, we ﬁnd 8 s = 20 20 − y

s=

or

2 (20 − y). 5

4 (20 − y)2 . The area of the cross section at height y is then given by 25

20 − y 20

s

8

(b) The volume of the pyramid is 20 " 20 4 1280 4 (20 − y)2 d y = − (20 − y)3 = . 75 3 0 25 0 3. Use the method of Exercise 2 to ﬁnd the formula for the volume of a right circular cone of height h whose base is a Let V be the volume of a right circular cone of height 10 whose base is a circle of radius 4 (Figure 16). circle of radius r (Figure 16). SOLUTION

(a) From similar triangles (see Figure 16), (a) Use similar triangles to ﬁnd the area of a horizontal cross section at a height y. r h (b) Calculate V by integrating the cross-sectional area. = , h−y r0 where r0 is the radius of the cone at a height of y. Thus, r0 = r − rhy . (b) The volume of the cone is h " h

r y 3 r y 2 −h π r − h hπ r 3 πr 2h r− π dy = = = . h r 3 r 3 3 0 0

Calculate the volume of the ramp in Figure 17 in three ways by integrating the area of the cross sections: (a) Perpendicular to the x-axis (rectangles) (b) Perpendicular to the y-axis (triangles)

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5. Find the volume of liquid needed to ﬁll a sphere of radius R to height h (Figure 18). y

R h

FIGURE 18 Sphere ﬁlled with liquid to height h. SOLUTION

The radius r at any height y is given by r =

R 2 − (R − y)2 . Thus, the volume of the ﬁlled portion of

the sphere is

π

" h 0

r2 dy =

π

" h

0

R 2 − (R − y)2

dy = π

" h 0

(2Ry − y 2 ) d y =

π

h h3 2 . = π Rh − 3 3

y3 Ry 2 −

0

7. Derive a formula for the volume of the wedge in Figure 19(B) in terms of the constants a, b, and c. Find the volume of the wedge in Figure 19(A) by integrating the area of vertical cross sections. The line from c to a is given by the equation (z/c) + (x/a) = 1 and the line from b to a is given by (y/b) + (x/a) = 1. The cross sections perpendicular to the x-axis are right triangles with height c(1 − x/a) and base b(1 − x/a). Thus we have " a

1 1 x 3 a 1 2 bc (1 − x/a) d x = − abc 1 − = 6 abc. 2 6 a 0 0 SOLUTION

In Exercises 9–14, ﬁnd the volume of the solid with given2base 2and cross sections. Let B be the solid whose base is the unit circle x + y = 1 and whose vertical cross √ sections perpendicular to thatthe thecross vertical crossperpendicular sections haveto area =are3(1 − x 2 ) whose and compute 9. the Thex-axis base isare theequilateral unit circletriangles. x 2 + y 2 Show = 1 and sections theA(x) x-axis triangles height the volume of B. and base are equal. SOLUTION At each location x, the side of the triangular cross section that lies in the base of the solid extends from the 2 2 top half of the unit circle (with y = 1 − x ) to the bottom half (with y = − 1 − x ). The triangle therefore has base 2 and height equal to 2 1 − x 2 and area 2(1 − x ). The volume of the solid is then 1 " 1 1 8 2(1 − x 2 ) d x = 2 x − x 3 = . 3 3 −1 −1

2 where −3 ≤ x ≤ 3. The cross sections perpendicular to the x-axis are 9− 11. TheThe basebase is the semicircle y = is the triangle enclosed byx x, + y = 1, the x-axis, and the y-axis. The cross sections perpendicular to squares. the y-axis are semicircles. SOLUTION For each x, the base of the square cross section extends from the semicircle y = 9 − x 2 to the x-axis.

2 9 − x 2 = 9 − x 2 . The volume of the solid is The square therefore has a base with length 9 − x 2 and an area of then 3 " 3

1 9 − x 2 d x = 9x − x 3 = 36. 3 −3 −3 y =interval 3. The cross to the y-axis are squares. 13. TheThe basebase is the enclosed y =sides x 2 and is aregion square, one of by whose is the [0, ] sections along theperpendicular √x-axis. The cross sections perpendicular 2. SOLUTION At any location y,of theheight distance to = thexparabola from the y-axis is y. Thus the base of the square will have to the x-axis are rectangles f (x) √ length 2 y. Therefore the volume is " 3 " 3 3 √ √ 4y d y = 2y 2 = 18. 2 y 2 y dy = 0

0

0

15. Find the volume of the solid whose base is the region |x| + |y| ≤ 1 and whose vertical cross sections perpendicular The base is the region(with enclosed by y = x 2 and y = 3. The cross sections perpendicular to the y-axis are rectangles to the y-axis are semicircles diameter along the base). of height y 3 . SOLUTION The region R in question is a diamond shape connecting the points (1, 0), (0, −1), (−1, 0), and (0, 1). Thus, in the lower half of the x y-plane, the radius of the circles is y + 1 and in the upper half, the radius is 1 − y. Therefore, the volume is " " π 0 π 1 π 1 1 π + = . (y + 1)2 d y + (1 − y)2 d y = 2 −1 2 0 2 3 3 3

Setting Up Integrals: Volume, Density, Average Value

S E C T I O N 6.2

17. Find the volume V of a regular tetrahedron whose face is an equilateral triangle of side s (Figure 20). Show that the volume of a pyramid of height h whose base is an equilateral triangle of side s is equal to

303

√

3 2 hs . 12

s

s

FIGURE 20 Regular tetrahedron. SOLUTION Our ﬁrst task is to determine the relationship between the height of the tetrahedron, h, and the side length of the equilateral triangles, s. Let B be the orthocenter of the tetrahedron (the point directly below the apex), and let b denote the distance from B to each corner of the base triangle. By the Law of Cosines, we have

s 2 = b2 + b2 − 2b2 cos 120◦ = 3b2 , so b2 = 13 s 2 . Thus 2 h 2 = s 2 − b2 = s 2 3

or

h=s

2 . 3

Therefore, using similar triangles, the side length of the equilateral triangle at height z above the base is h−z z s =s−√ . h 2/3 The volume of the tetrahedron is then given by √ √ √ 2 3 s 2/3 " s √2/3 √ s3 2 z z 3 2 dz = − = s−√ s−√ . 4 12 12 2/3 2/3 0 0

19. A frustum of a pyramid is a pyramid with its top cut off [Figure 22(A)]. Let V be the volume of a frustum of height The area of an ellipse is π ab, where a and b are the lengths of the semimajor and semiminor axes (Figure 21). h whose base is a square of side a and top is a square of side b with a > b ≥ 0. Compute the volume of a cone of height 12 whose base is an ellipse with semimajor ha axis a = 6 and semiminor axis (a) Show [Figure 22(B)]. b = 4.that if the frustum were continued to a full pyramid, it would have height a−b (b) Show that the cross section at height x is a square of side (1/ h)(a(h − x) + bx). (c) Show that V = 13 h(a 2 + ab + b2 ). A papyrus dating to the year 1850 BCE indicates that Egyptian mathematicians had discovered this formula almost 4,000 years ago.

b h

a (A)

(B)

FIGURE 22 SOLUTION

(a) Let H be the height of the full pyramid. Using similar triangles, we have the proportion H H −h = a b which gives H=

ha . a−b

(b) Let w denote the side length of the square cross section at height x. By similar triangles, we have a w = . H H −x Substituting the value for H from part (a) gives w=

a(h − x) + bx . h

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(c) The volume of the frustrum is " h 1 0

h

(a(h − x) + bx)

2

" h

1 a 2 (h − x)2 + 2ab(h − x)x + b2 x 2 d x dx = 2 h 0 h 1 a2 2 1 h 2 a + ab + b2 . = 2 − (h − x)3 + abhx 2 − abx 3 + b2 x 3 = 3 3 3 3 h 0

21. Figure 24 shows the solid S obtained by intersecting two cylinders of radius r whose axes are perpendicular. A plane inclined at an angle of 45◦ passes through a diameter of the base of a cylinder of radius r . Find the (a) The horizontal cross section of each cylinder at distance y from the central axis is a rectangular strip. Find the strip’s volume of the region within the cylinder and below the plane (Figure 23). width. (b) Find the area of the horizontal cross section of S at distance y. (c) Find the volume of S as a function of r .

S y

FIGURE 24 Intersection of two cylinders intersecting at right angles. SOLUTION

(a) The horizontal cross section at distance y from the central axis (for −r ≤ y ≤ r ) is a square of width w = 2 r 2 − y2. (b) The area of the horizontal cross section of S at distance y from the central axis is w2 = 4(r 2 − y 2 ). (c) The volume of the solid S is then r " r

1 16 3 r . r 2 − y 2 d y = 4 r 2 y − y 3 = 4 3 3 −r −r 23. Calculate the volume of a cylinder inclined at an angle θ = 30◦ whose height is 10 and whose base is a circle of Let S be the solid obtained by intersecting two cylinders of radius r whose axes intersect at an angle θ . Find the radius 4 (Figure 25). volume of S as a function of r and θ . 4

10 30°

FIGURE 25 Cylinder inclined at an angle θ = 30◦ . SOLUTION

The area of each circular cross section is π (4)2 = 16π , hence the volume of the cylinder is " 10 0

10 16π d x = (16π x) = 160π 0

25. Find the total mass of a 2-m rod whose linear density function is ρ(x) = 1 + 0.5 sin(π x) kg/m for 0 ≤ x ≤ 2. Find the total mass of a 1-m rod whose linear density function is ρ(x) = 10(x + 1)−2 kg/m for 0 ≤ x ≤ 1. SOLUTION The total mass of the rod is cos π x 2 ρ(x) d x = (1 + .5 sin π x) d x = x − .5 = 2 kg, π 0 0 0

" 2

" 2

27. Calculate the population within a 10-mile radius of the city center if the radial population density is ρ(r ) = 4(1 + A mineral deposit along a strip of length 6 cm has density s(x) = 0.01x(6 − x) g/cm for 0 ≤ x ≤ 6. Calculate r 2 )1/3 (in thousands per square mile). the total mass of the deposit.

Setting Up Integrals: Volume, Density, Average Value

S E C T I O N 6.2 SOLUTION

305

The total population is 2π

" 10 0

r · ρ(r ) dr = 2π

" 10 0

10

4r (1 + r 2 )1/3 dr = 3π (1 + r 2 )4/3

0

≈ 4423.59 thousand ≈ 4.4 million. 29. Table 1 lists the population density (in people per squared kilometer) as a function of distance r (in kilometers) from Odzala National Park in the Congo has a high density of gorillas. Suppose that the radial population density is the center of a rural town. Estimate the total population within a 2-km radius of the center by taking the average of the ρ(r ) = 52(1 + r 2 )−2 gorillas per square kilometer, where r is the distance from a large grassy clearing with a source left- and right-endpoint approximations. of food and water. Calculate the number of gorillas within a 5-km radius of the clearing. TABLE 1

SOLUTION

Population Density

r

ρ(r )

r

ρ(r )

0.0

125.0

1.2

37.6

0.2

102.3

1.4

30.8

0.4

83.8

1.6

25.2

0.6

68.6

1.8

20.7

0.8

56.2

2.0

16.9

1.0

46.0

The total population is given by 2π

" 2 0

r · ρ(r ) dr.

With r = 0.2, the left- and right-endpoint approximations to the required deﬁnite integral are L 10 = .2(2π )[0(125) + (.2)(102.3) + (.4)(83.8) + (.6)(68.6) + (.8)(56.2) + (1)(46) + (1.2)(37.6) + (1.4)(30.8) + (1.6)(25.2) + (1.8)(20.7)] = 442.24; R10 = .2(2π )[(.2)(102.3) + (.4)(83.8) + (.6)(68.6) + (0.8)(56.2) + (1)(46) + (1.2)(37.6) + (1.4)(30.8) + (1.6)(25.2) + (1.8)(20.7) + (2)(16.9)] = 484.71. This gives an average of 463.475. Thus, there are roughly 463 people within a 2-km radius of the town center. 2 + 2)−2 deer per km2 , where r is the distance (in ρ(rcm ) =whose 150(r mass 31. TheFind density deer in aofforest is the plate radialoffunction the of total mass a circular radius 20 density is the radial function ρ(r ) = 0.03 + kilometers) to a small meadow. Calculate the number of deer in the region 2 ≤ r ≤ 5 km. 0.01 cos(π r 2 ) g/cm2 . SOLUTION The number of deer in the region 2 ≤ r ≤ 5 km is

2π

5

−2 1 = −150π 1 − 1 ≈ 61 deer. r (150) r 2 + 2 dr = −150π 27 6 r 2 + 2 2 2

" 5

33. Find the ﬂow rate through a tube of radius 4 cm, assuming that the velocity of ﬂuid particles at a distance r cm from 4 the center is v(r ) =a 16 − r 2 cm/s. g/cm has ﬁnite total mass, even Show that circular plate of radius 2 cm with radial mass density ρ(r ) = r though theThe density SOLUTION ﬂow becomes rate is inﬁnite at the origin. 2π

" R 0

r v(r ) dr = 2π

4 " 4

1 cm3 r 16 − r 2 dr = 2π 8r 2 − r 4 = 128π . 4 s 0 0

35. A solid rod of radius 1 cm is placed in a pipe of radius 3 cm so that their axes are aligned. Water ﬂows through −5 Use function Poiseuille’s ) be the of the blood in rate an arterial capillaryofofthe radius 4 × 10 the pipeLet andv(raround thevelocity rod. Find ﬂow if the velocity waterR is=given by them.radial v(r Law ) = 6 (m-s)−1 to determine the velocity at the center of the capillary and the ﬂow rate (use (Example 6) with k = 10 0.5(r − 1)(3 − r ) cm/s. correct units). SOLUTION The ﬂow rate is 2π

" 3 1

r (.5)(r − 1)(3 − r ) dr = π

" 3

1

3 4 3 8π cm3 1 −r 3 + 4r 2 − 3r dr = π − r 4 + r 3 − r 2 = . 4 3 2 3 s 1

In Exercises 37–44,the calculate over the given interval. Bureau of Fisheries created the depth contour map in To estimate volumethe V average of Lake Nogebow, the Minnesota Figure 26 and determined the area of the cross section of the lake at the depths recorded in the table below. Estimate V by taking the average of the right- and left-endpoint approximations to the integral of cross-sectional area.

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37. f (x) = x 3 , SOLUTION

[0, 1]

The average is " 1 " 1 1 1 1 1 x3 dx = x 3 d x = x 4 = . 1−0 0 4 0 4 0

39. f (x) = cos x, [0, π2 ] f (x) = x 3 , [−1, 1] SOLUTION The average is " π /2 π /2 2 1 2

sin t 0 = . cos x d x = π /2 − 0 0 π π 41. f (s) = s −2 , 2[2, 5] f (x) = sec x, [0, π4 ] SOLUTION The average is " 5 1 −1 5 1 1 −2 s ds = − s = . 5−2 2 3 10 2 2 , [−1, 3] − 3x 43. f (x) = 2x 3sin( π /x) , [1, 2] f (x) = SOLUTION The average is x2

3 " 3

1 1 4 1 2x 3 − 3x 2 d x = x − x 3 = 3. 3 − (−1) −1 4 2 −1 45. Let M be thenaverage value of f (x) = x 3 on [0, A], where A > 0. Which theorem guarantees that f (c) = M has a f (x) = x , [0, 1] solution c in [0, A]? Find c. SOLUTION

The Mean Value Theorem for Integrals guarantees that f (c) = M has a solution c in [0, A]. With f (x) =

x 3 on [0, A], M=

" A 1 1 4 A A3 1 x3 dx = x = . A−0 0 A 4 0 4

3

Solving f (c) = c3 = A4 for c yields A c= √ . 3 4 47. Which of f (x) = x sin2 x and g(x) = x 2 sin2 x has a larger average value over [0, 1]? Over [1, 2]? Let f (x) = 2 sin x − x. Use a computer algebra system to plot f (x) and estimate: 2 SOLUTION The functions f and f (x).g differ only in the power of x multiplying sin x. It is also important to note that (a) The positive root α of 2 2 x ≥ The 0 foraverage all x. Now, x ∈ on (0,[0, 1),αx].> x so sin (b) valuefor M each of f (x) (c) A value c ∈ [0, α ] such that f (c)f = (x)M. = x sin2 x > x 2 sin2 x = g(x). Thus, over [0, 1], f (x) will have a larger average value than g(x). On the other hand, for each x ∈ (1, 2), x 2 > x, so g(x) = x 2 sin2 x > x sin2 x = f (x). Thus, over [1, 2], g(x) will have the larger average value. 49. Sketch the graph of a function f (x)sin such x that fπ(x) ≥ 0 on [0, 1] and f (x) ≤ 0 on [1, 2], whose average on that the average value of f (x) = over [ 2 , π ] is less than 0.41. Sketch the graph if necessary. [0, 2] isShow negative. x SOLUTION

Many solutions will exist. One could be y 1 x 1 −1 −2

2

S E C T I O N 6.2

Setting Up Integrals: Volume, Density, Average Value

307

π 51. TheFind temperature T (t)ofatf time (in + hours) in the an art museum varies T (t)M=are 70arbitrary + 5 cos constants. t . Find the the average (x) =t ax b over interval [−M, M], according where a, b,toand 12 average over the time periods [0, 24] and [2, 6]. SOLUTION

• The average temperature over the 24-hour period is

" 24

π

π 24 1 1 60 70 + 5 cos sin t dt = 70t + t = 70◦ F. 24 − 0 0 12 24 π 12 0 • The average temperature over the 4-hour period is

" 6

π

π 6 1 1 60 70 + 5 cos sin t dt = 70t + t = 72.4◦ F. 6−2 2 12 4 π 12 2 53. What is the average area of the circles whose radii vary from 0 to 1? A ball is thrown in the air vertically from ground level with initial velocity 64 ft/s. Find the average height of the SOLUTION Thetime average area extending is ball over the interval from the time of the ball’s release to its return to ground level. Recall that the height at time t is h(t) = 64t − 16t 2 . " 1 π 1 π 1 π r 2 dr = r 3 = . 1−0 0 3 0 3 55. The acceleration of a particle is a(t) = t − t 3 m/s2 for 0 ≤ t ≤ 1. Compute2the average acceleration and average . Find its average velocity during the object withinterval zero initial velocity accelerates a constant ratevelocity of 10 m/s velocityAn over the time [0, 1], assuming that theatparticle’s initial is zero. ﬁrst 15 s. SOLUTION The average acceleration is " 1

1 1 2 1 4 1 1 t − t = m/s2 . t − t 3 dt = 1−0 0 2 4 4 0 An acceleration a(t) = t − t 3 with zero initial velocity gives v(t) =

1 2 1 4 t − t . 2 4

Thus the average velocity is given by 1 " 1 1 7 1 2 1 4 1 3 1 t − t t − t 5 = m/s. dt = 1−0 0 2 4 6 20 60 0 √ 57. Let f (x) = x. Find a value of c in [4, 9] such that f (c) is equal to the average of f on [4, 9]. Let M be the average value of f (x) = x 4 on [0, 3]. Find a value of c in [0, 3] such that f (c) = M. SOLUTION The average value is " 9 " √ 1 9√ 2 3/2 9 38 1 x dx = x dx = x = . 9−4 4 5 4 15 15 4 Then f (c) =

√

c = 38 15 implies c=

38 2 1444 = ≈ 6.417778. 15 225

Give an example of a function (necessarily discontinuous) that does not satisfy the conclusion of the MVT for Further Insights and Challenges Integrals.

59. An object is tossed in the air vertically from ground level with initial velocity v0 ft/s at time t = 0. Find the average speed of the object over the time interval [0, T ], where T is the time the object returns to earth. The height is given by h(t) = v0 t − 16t 2 . The ball is at ground level at time t = 0 and T = v0 /16. The velocity is given by v(t) = v0 − 32t and thus the speed is given by s(t) = |v0 − 32t|. The average speed is SOLUTION

" v0 /16 " " 1 16 v0 /32 16 v0 /16 |v0 − 32t| dt = (v0 − 32t) dt + (32t − v0 ) dt v0 /16 − 0 0 v0 0 v0 v0 /32 v0 /32 16

v0 /16 16

v0 t − 16t 2 16t 2 − v0 t + = v0 /2. = 0 v0 /32 v0 v0 Review the MVT stated in Section 4.3 (Theorem 1, p. 192) and show how it can be used, together with the Fundamental Theorem of Calculus, to prove the MVT for integrals.

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6.3 Volumes of Revolution Preliminary Questions 1. Which of the following is a solid of revolution? (a) Sphere (b) Pyramid

(c) Cylinder

(d) Cube

SOLUTION The sphere and the cylinder have circular cross sections; hence, these are solids of revolution. The pyramid and cube do not have circular cross sections, so these are not solids of revolution.

2. True or false? When a solid is formed by rotating the region under a graph about the x-axis, the cross sections perpendicular to the x-axis are circular disks. SOLUTION

True. The cross sections will be disks with radius equal to the value of the function.

3. True or false? When a solid is formed by rotating the region between two graphs about the x-axis, the cross sections perpendicular to the x-axis are circular disks. SOLUTION

False. The cross sections may be washers.

4. Which of the following integrals expresses the volume of the solid obtained by rotating the area between y = f (x) and y = g(x) over [a, b] around the x-axis [assume f (x) ≥ g(x) ≥ 0]? " b 2 (a) π f (x) − g(x) d x a " b (b) π f (x)2 − g(x)2 d x a

SOLUTION The correct answer is (b). Cross sections of the solid will be washers with outer radius f (x) and inner radius g(x). The area of the washer is then π f (x)2 − π g(x)2 = π ( f (x)2 − g(x)2 ).

Exercises In Exercises 1–4, (a) sketch the solid obtained by revolving the region under the graph of f (x) about the x-axis over the given interval, (b) describe the cross section perpendicular to the x-axis located at x, and (c) calculate the volume of the solid. 1. f (x) = x + 1,

[0, 3]

SOLUTION

(a) A sketch of the solid of revolution is shown below: y 2 x 1

2

3

−2

(b) Each cross section is a disk with radius x + 1. (c) The volume of the solid of revolution is

π

" 3 0

(x + 1)2 d x = π

" 3 0

(x 2 + 2x + 1) d x = π

3 1 3 x + x 2 + x = 21π . 3 0

√ 3. f (x) = x + 1, [1, 4] f (x) = x 2 , [1, 3] SOLUTION

(a) A sketch of the solid of revolution is shown below: y 2 1 x −1 −2

(b) Each cross section is a disk with radius

√

x + 1.

1

2

3

4

S E C T I O N 6.3

Volumes of Revolution

309

(c) The volume of the solid of revolution is 4 " 4 " 4 √ 21π 1 2 ( x + 1)2 d x = π (x + 1) d x = π x + x = . 2 2 1 1 1

π

In Exercises 5–12,−1ﬁnd the volume of the solid obtained by rotating the region under the graph of the function about the f (x) = x , [1, 2] x-axis over the given interval. 5. f (x) = x 2 − 3x, SOLUTION

[0, 3]

The volume of the solid of revolution is

π

" 3 0

(x 2 − 3x)2 d x =

π

" 3 0

(x 4 − 6x 3 + 9x 2 ) d x =

π

3 81π 1 5 3 4 3 x − x + 3x = . 5 2 10 0

7. f (x) = x 5/31, [1, 8] f (x) = 2 , [1, 4] SOLUTION The x volume of the solid of revolution is " 8 " 8 3π 13/3 8 3π 13 24573π π (x 5/3 )2 d x = π x 10/3 d x = x = 13 (2 − 1) = 13 . 13 1 1 1 2 9. f (x)f (x) = = 4 −, x 2[1, , 3] [0, 2] x +1 SOLUTION

The volume of the solid of revolution is

π

" 3 1

3 2 " 3 2 d x = 4π (x + 1)−2 d x = −4π (x + 1)−1 = π . x +1 1 1

√ x + 1, [0, π ] 11. f (x) = cos f (x) = x 4 + 1, [1, 3] SOLUTION The volume of the solid of revolution is

π

π " π " π √ ( cos x + 1)2 d x = π (cos x + 1) d x = π (sin x + x) = π 2 . 0

0

0

√ (a) sketch the region enclosed by the curves, (b) describe the cross section perpendicular to the In Exercises 13–18, f (x) = cos x sin x, [0, π /2] x-axis located at x, and (c) ﬁnd the volume of the solid obtained by rotating the region about the x-axis. 13. y = x 2 + 2,

y = 10 − x 2

SOLUTION

(a) Setting x 2 + 2 = 10 − x 2 yields 2x 2 = 8, or x 2 = 4. The two curves therefore intersect at x = ±2. The region enclosed by the two curves is shown in the ﬁgure below. y y = 10 − x 2 8 4 y = x2 + 2 −2

x

−1

1

2

(b) When the region is rotated about the x-axis, each cross section is a washer with outer radius R = 10 − x 2 and inner radius r = x 2 + 2. (c) The volume of the solid of revolution is " 2

" 2

2 (10 − x 2 )2 − (x 2 + 2)2 d x = π π (96 − 24x 2 ) d x = π 96x − 8x 3 = 256π . −2

15. y = 16 − x, y = 3x + 12, y = x 2 , y = 2x + 3

−2

−2

x = −1

SOLUTION

(a) Setting 16 − x = 3x + 12, we ﬁnd that the two lines intersect at x = 1. The region enclosed by the two curves is shown in the ﬁgure below.

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A P P L I C AT I O N S O F T H E I N T E G R A L y

y = 3x + 12

10

−1

y = 16 − x

x

−0.5

0.5

1

(b) When the region is rotated about the x-axis, each cross section is a washer with outer radius R = 16 − x and inner radius r = 3x + 12. (c) The volume of the solid of revolution is

π

1 " 1

" 1 8 656π (16 − x)2 − (3x + 12)2 d x = π (112 − 104x − 8x 2 ) d x = π 112x − 52x 2 − x 3 = . 3 3 −1 −1 −1

π 17. y = sec x, 1 y = 0,5 x = − 4 , y = , y = −x x 2

x=

π 4

SOLUTION

(a) The region in question is shown in the ﬁgure below. y

y = sec x

1.2 0.8 0.4 x

−0.4

0.4

(b) When the region is rotated about the x-axis, each cross section is a circular disk with radius R = sec x. (c) The volume of the solid of revolution is π /4 " π /4 2 π (sec x) d x = π (tan x) = 2π . −π /4

−π /4

In Exercises 19–22, ﬁnd the volume of the solid obtained by rotating the region enclosed by the graphs about the y-axis π = secinterval. x, y = csc x, y = 0, x = 0, and x = . over theygiven 2 √ 19. x = y, x = 0; 1 ≤ y ≤ 4 SOLUTION When the region in question (shown in the ﬁgure below) is rotated about the y-axis, each cross section is a √ disk with radius y. The volume of the solid of revolution is

" 4 √ 2 15π π y 2 4 π y dy = = 2 . 2 1 1 y 4 3 x=

y

2 1 x 0

0.5

1

1.5

2

√ 21. x = y 2 , √ x = y; 0 ≤ y ≤ 1 x = sin y, x = 0; 0 ≤ y ≤ π SOLUTION When the region in question (shown in the ﬁgure below) is rotated about the y-axis, each cross section is a √ washer with outer radius R = y and inner radius r = y 2 . The volume of the solid of revolution is 1 " 1

y2 3π y 5 √ 2 2 2 ( y) − (y ) d y = π π − . = 2 5 10 0 0

S E C T I O N 6.3

Volumes of Revolution

311

y 1 x = y2 x=

y

x 0

1

In Exercises 23–28, ﬁnd the volume of the solid obtained by rotating region A in Figure 10 about the given axis. x = 4 − y, x = 16 − y 2 ; −3 ≤ y ≤ 4 y

y = x2 + 2

6 A 2

B x 1

2

FIGURE 10

23. x-axis Rotating region A about the x-axis produces a solid whose cross sections are washers with outer radius R = 6 and inner radius r = x 2 + 2. The volume of the solid of revolution is 2 " 2

" 2 1 704π 4 (6)2 − (x 2 + 2)2 d x = π π (32 − 4x 2 − x 4 ) d x = π 32x − x 3 − x 5 = . 3 5 15 0 0 0

SOLUTION

25. y = 2 y = −2 SOLUTION Rotating the region A about y = 2 produces a solid whose cross sections are washers with outer radius R = 6 − 2 = 4 and inner radius r = x 2 + 2 − 2 = x 2 . The volume of the solid of revolution is 2 " 2

128π 1 π . 42 − (x 2 )2 d x = π 16x − x 5 = 5 5 0 0 27. x = −3 y-axis

Rotating region A about x = −3 produces a solid whose cross sections are washers with outer radius √ y − 2 − (−3) = y − 2 + 3 and inner radius r = 0 − (−3) = 3. The volume of the solid of revolution is

SOLUTION

R=

√

π

" 6

2

(3 +

6 " 6 1 y − 2)2 − (3)2 d y = π (6 y − 2 + y − 2) d y = π 4(y − 2)3/2 + y 2 − 2y = 40π . 2 2 2

In Exercises x = 229–34, ﬁnd the volume of the solid obtained by rotating region B in Figure 10 about the given axis. 29. x-axis Rotating region B about the x-axis produces a solid whose cross sections are disks with radius R = x 2 + 2. The volume of the solid of revolution is 2 " 2 " 2 376π 1 5 4 3 π (x 2 + 2)2 d x = π (x 4 + 4x 2 + 4) d x = π x + x + 4x = . 5 3 15 0 0 0 SOLUTION

31. y = 6 y = −2 SOLUTION Rotating region B about y = 6 produces a solid whose cross sections are washers with outer radius R = 6 − 0 = 6 and inner radius r = 6 − (x 2 + 2) = 4 − x 2 . The volume of the solid of revolution is " 2

" 2

824π 8 3 1 5 2 2 2 2 2 4 6 − (4 − x ) d y = π 20 + 8x − x d y = π 20x + x − x = π . 3 5 15 0 0 0 33. x = 2 y-axis Hint for Exercise 32: Express the volume as a sum of two integrals along the y-axis, or use Exercise 26.

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CHAPTER 6

A P P L I C AT I O N S O F T H E I N T E G R A L SOLUTION Rotating region B about x = 2 produces a solid with two different cross sections. For each y ∈ [0, 2], the √ cross section is a disk with radius R = 2; for each y ∈ [2, 6], the cross section is a disk with radius R = 2 − y − 2. The volume of the solid of revolution is " 2 " 6 " 2 " 6 π (2)2 d y + π (2 − y − 2)2 d y = π 4 dy + π (2 + y − 4 y − 2) d y

0

2

0

2

2 6 1 2 8 3/2 = 32π . = π (4y) + π 2y + y − (y − 2) 2 3 3 0 2

In Exercises 35–46, ﬁnd the volume of the solid obtained by rotating the region enclosed by the graphs about the given x = −3 axis. 35. y = x 2 ,

y = 12 − x,

x = 0,

about y = −2

Rotating the region enclosed by y = x 2 , y = 12 − x and the y-axis (shown in the ﬁgure below) about y = −2 produces a solid whose cross sections are washers with outer radius R = 12 − x − (−2) = 14 − x and inner radius r = x 2 − (−2) = x 2 + 2. The volume of the solid of revolution is " 3

" 3 (14 − x)2 − (x 2 + 2)2 d x = π π (192 − 28x − 3x 2 − x 4 ) d x SOLUTION

0

0

3 1 1872π = π 192x − 14x 2 − x 3 − x 5 = . 5 5 0 y y = 12 − x

12

8

4

y = x2 x

0

1

2

3

37. y = 16 − x, y = 3x + 12, x = 0, about y-axis y = x 2 , y = 12 − x, x = 0, about y = 15 SOLUTION Rotating the region enclosed by y = 16 − x, y = 3x + 12 and the y-axis (shown in the ﬁgure below) about the y-axis produces a solid with two different cross sections. For each y ∈ [12, 15], the cross section is a disk with radius R = 13 (y − 12); for each y ∈ [15, 16], the cross section is a disk with radius R = 16 − y. The volume of the solid of revolution is 2 " 15 " 16 1 π (16 − y)2 d y (y − 12) d y + π 3 12 15 " 15 " 16 1 2 (y 2 − 32y + 256) d y (y − 24y + 144) d y + π =π 12 9 15 15 16 4 π 1 3 1 3 2 2 = y − 12y + 144y + π y − 16y + 256y = π . 9 3 3 3 12 15 y 16 14 12 10 8 6 4 2

y = 16 − x y = 3x + 12

x 0

9 y= , +about 39. y =y =2 ,16 − x, 10y−=x 23x 12, x-axis x = 0, x

0.2 0.4 0.6 0.8

1

about x = 2

SOLUTION The region enclosed by the two curves is shown in the ﬁgure below. Note that the region consists of two pieces that are symmetric with respect to the y-axis. We may therefore compute the volume of the solid of revolution by

S E C T I O N 6.3

Volumes of Revolution

313

considering one of the pieces and doubling the result. Rotating the portion of the region in the ﬁrst quadrant about the x-axis produces a solid whose cross sections are washers with outer radius R = 10 − x 2 and inner radius r = 9x −2 . The volume of the solid of revolution is " 3

" 3

(10 − x 2 )2 − (9x −2 )2 d x = 2π 100 − 20x 2 + x 4 − 81x −4 d x 2π 1

1

= 2π

3 20 3 1 5 −3 = 1472π . x + x + 27x 100x − 3 5 15 1

y

y = 10 − x 2

8 6 4 y = 92 x

2 −3 −2 −1

1

x 2

3

1 5 41. y = , 9y = − x, about y-axis y x= 2 , y2 = 10 − x 2 , about y = 12 x 5 SOLUTION Rotating the region enclosed by y = x −1 and y = 2 − x (shown in the ﬁgure below) about the y-axis produces a solid whose cross sections are washers with outer radius R = 52 − y and inner radius r = y −1 . The volume of the solid of revolution is 2 " 2 " 2 25 5 −1 2 dy = π π − y − (y ) − 5y + y 2 − y −2 d y 2 4 1/2 1/2 = π

2 5 1 9π 25 y − y 2 + y 3 + y −1 . = 4 2 3 8 1/2

y 2 1.5

y = 2.5 − x

1 0.5

y=

1 x x

0

0.5

1

1.5

2

43. y = x 3 , y = x 1/3 , about y-axis x = 2, x = 3, y = 16 − x 4 , y = 0, about y-axis SOLUTION Rotating the region enclosed by y = x 3 and y = x 1/3 (shown in the ﬁgure below) about the y-axis produces a solid whose cross sections are washers with outer radius R = y 1/3 and inner radius r = y 3 . The volume of the solid of revolution is " 1

" 1 32π 3 5/3 1 7 1 (y 1/3 )2 − (y 3 )2 d y = π π (y 2/3 − y 6 ) d y = π − y = y . 5 7 35 −1 −1 −1 y 1

y = x 1/3 y = x3 x

−1

1

−1

about x-axis 45. y 2 = 4x, 3 y = x, 1/3 y = x , y = x , about x = −2 SOLUTION Rotating the region enclosed by y 2 = 4x and y = x (shown in the ﬁgure below) about the x-axis produces √ a solid whose cross sections are washers with outer radius R = 2 x and inner radius r = x. The volume of the solid of

314

CHAPTER 6

A P P L I C AT I O N S O F T H E I N T E G R A L

revolution is " 4

√ 2 1 3 4 32π 2 2 (2 x) − x d x = π 2x − x = π . 3 3 0 0 y 4 3

y 2 = 4x

2

y=x

1 x 0

1

2

3

4

47. Sketch the hypocycloid x 2/3 + y 2/3 = 1 and ﬁnd the volume of the solid obtained by revolving it about the 2= y 4x, y = x, about y = 8 x-axis. SOLUTION

A sketch of the hypocycloid is shown below. y 1

x

−1

1

−1

3/2 For the hypocycloid, y = ± 1 − x 2/3 . Rotating this region about the x-axis will produce a solid whose cross 3/2

sections are disks with radius R = 1 − x 2/3 . Thus the volume of the solid of revolution will be 1 " 1

2 −x 3 32π 9 7/3 9 5/3 2/3 3/2 (1 − x ) π dx = π − x +x = + x . 3 7 5 105 −1 −1

49. A “bead” is formed by removing a cylinder of radius r from the center of a sphere of radius R (Figure 12). Find the 2 2 solid generated rotating volume The of the bead with r =by 1 and R =the 2. region between the branches of the hyperbola y − x = 1 about the x-axis is called a hyperboloid (Figure 11). Find the volume of the hyperboloid for −a ≤ x ≤ a. y

y

h r

R

x

x

FIGURE 12 A bead is a sphere with a cylinder removed.

The equation of the outer circle is x 2 + y 2 = 22 , and the inner cylinder intersects the sphere when √ y = ± 3. Each cross section of the bead is a washer with outer radius 4 − y 2 and inner radius 1, so the volume is given by 2 " √3 " √3

√ 2 2 π √ 4− y −1 d y = π √ 3 − y 2 d y = 4π 3. SOLUTION

− 3

Further Insights and Challenges

− 3

x 2 y 2 + = 1 πaround the x-axis is 51. The solid generated by rotating the region inside the ellipse with equation 3 Find the volume V of the bead (Figure 12) in terms of r and R. Thena show that b V = h , where h is the 6 4 2 is theVvolume the ellipseinisterms rotated called an ellipsoid. ShowThis thatformula the ellipsoid volumeconsequence: height of the bead. has a has surprising can beifexpressed of around h alone,the it 3 π ab . WhatSince y-axis? follows that two beads of height 2 in., one formed from a sphere the size of an orange and the other the size of the earth, would have the same volume! Can you explain intuitively how this is possible?

S E C T I O N 6.3

Volumes of Revolution

315

SOLUTION

• Rotating the ellipse about the x-axis produces an ellipsoid whose cross sections are disks with radius R = b 1 − (x/a)2 . The volume of the ellipsoid is then

π

2 a " a " a 4 1 1 d x = b2 π b 1 − (x/a)2 1 − 2 x 2 d x = b2 π x − 2 x 3 = π ab2 . 3 a 3a −a −a −a

• Rotating the ellipse about the y-axis produces an ellipsoid whose cross sections are disks with radius R = a 1 − (y/b)2 . The volume of the ellipsoid is then

2 b " b " b 4 1 1 d y = a2 π a 1 − (y/b)2 1 − 2 y 2 d y = a 2 π y − 2 y 3 = π a 2 b. 3 b 3b −b −b −b 53. The curve y = f (x) in Figure 14, called a tractrix, has the following property: the tangent line at each point (x, y) A doughnut-shaped solid is called a torus (Figure 13). Use the washer method to calculate the volume of the on the curve has slope torus obtained by rotating the region inside the circle with equation (x − a)2 + y 2 = b2 around the y-axis (assume that a > b). Hint: Evaluate the integral by interpreting it as −ythe area of a circle. dy = . dx 1 − y2 y 1

c

R

y = f (x) x a

2

FIGURE 14 The tractrix.

Let R be the shaded region under the graph of 0 ≤ x ≤ a in Figure 14. Compute the volume V of the solid obtained by revolving R around the x-axis in terms of the constant c = f (a). Hint: Use the disk method and the substitution u = f (x) to show that V =π

" 1 u 1 − u 2 du c

SOLUTION Let y = f (x) be the tractrix depicted in Figure 14. Rotating the region R about the x-axis produces a solid whose cross sections are disks with radius f (x). The volume of the resulting solid is then " a V =π [ f (x)]2 d x.

0

Now, let u = f (x). Then − f (x) −u dx = d x; du = f (x) d x = 2 1 − [ f (x)] 1 − u2 hence, 1 − u2 du, dx = − u and V =π

" c

u2

1

1 − u2 − du u

=π

" 1 u 1 − u 2 du. c

Carrying out the integration, we ﬁnd 1 π π V = − (1 − u 2 )3/2 = (1 − c2 )3/2 . 3 3 c Verify the formula " x2 x1

1 (x − x 1 )(x − x 2 ) d x = (x 1 − x2 )3 6

316

CHAPTER 6

A P P L I C AT I O N S O F T H E I N T E G R A L

55. Let R be the region in the unit circle lying above the cut with the line y = mx + b (Figure 16). Assume the points where the line intersects the circle lie above the x-axis. Use the method of Exercise 54 to show that the solid obtained by π rotating R about the x-axis has volume V = hd 2 , with h and d as in the ﬁgure. 6 y = mx + b

y

R

d h

x2 + y2 = 1 x

FIGURE 16

Let x 1 and x 2 denote the x-coordinates of the points of intersection between the circle x 2 + y 2 = 1 and the line y = mx + b with x 1 < x 2 . Rotating the regionenclosed by the two curves about the x-axis produces a solid whose cross sections are washers with outer radius R = 1 − x 2 and inner radius r = mx + b. The volume of the resulting solid is then " x2

V =π (1 − x 2 ) − (mx + b)2 d x SOLUTION

x1

Because x1 and x 2 are roots of the equation (1 − x 2 ) − (mx + b)2 = 0 and (1 − x 2 ) − (mx + b)2 is a quadratic polynomial in x with leading coefﬁcient −(1 + m 2 ), it follows that (1 − x 2 ) − (mx + b)2 = −(1 + m 2 )(x − x 1 )(x − x 2 ). Therefore, " x2 π (x − x 1 )(x − x 2 ) d x = (1 + m 2 )(x 2 − x 1 )3 . V = −π (1 + m 2 ) 6 x1 From the diagram, we see that h = x 2 − x 1 . Moreover, by the Pythagorean theorem, d 2 = h 2 + (mh)2 = (1 + m 2 )h 2 . Thus, π π π V = (1 + m 2 )h 3 = h (1 + m 2 )h 2 = hd 2 . 6 6 6

6.4 The Method of Cylindrical Shells Preliminary Questions 1. Consider the region R under the graph of the constant function f (x) = h over the interval [0, r ]. What are the height and radius of the cylinder generated when R is rotated about: (a) the x-axis (b) the y-axis SOLUTION

(a) When the region is rotated about the x-axis, each shell will have radius h and height r. (b) When the region is rotated about the y-axis, each shell will have radius r and height h. 2. Let V be the volume of a solid of revolution about the y-axis. (a) Does the Shell Method for computing V lead to an integral with respect to x or y? (b) Does the Disk or Washer Method for computing V lead to an integral with respect to x or y? SOLUTION

(a) The Shell method requires slicing the solid parallel to the axis of rotation. In this case, that will mean slicing the solid in the vertical direction, so integration will be with respect to x. (b) The Disk or Washer method requires slicing the solid perpendicular to the axis of rotation. In this case, that means slicing the solid in the horizontal direction, so integration will be with respect to y.

Exercises In Exercises 1–10, sketch the solid obtained by rotating the region underneath the graph of the function over the given interval about the y-axis and ﬁnd its volume. 1. f (x) = x 3 ,

[0, 1]

The Method of Cylindrical Shells

S E C T I O N 6.4 SOLUTION

317

A sketch of the solid is shown below. Each shell has radius x and height x 3 , so the volume of the solid is 2π

" 1 0

x · x 3 d x = 2π

" 1 0

x 4 d x = 2π

2 1 5 1 x = π. 5 5 0

y 1

x

−1

1

3. f (x) = 3x + √ 2, [2, 4] f (x) = x, [0, 4] SOLUTION A sketch of the solid is shown below. Each shell has radius x and height 3x + 2, so the volume of the solid is " 4 " 4

4 2π x(3x + 2) d x = 2π (3x 2 + 2x) d x = 2π x 3 + x 2 = 136π . 2

2

2

y 14 12 10 8 6 4 2 x

−4

4

5. f (x) = 4 − x 2 , 2[0, 2] f (x) = 1 + x , [1, 3] SOLUTION A sketch of the solid is shown below. Each shell has radius x and height 4 − x 2 , so the volume of the solid is 2 " 2 " 2 1 2π x(4 − x 2 ) d x = 2π (4x − x 3 ) d x = 2π 2x 2 − x 4 = 8π . 4 0 0 0 y

2

x

−2

2

√ 2 ), [0, π ] 7. f (x) = sin(x f (x) = x 2 + 9, [0, 3] SOLUTION A sketch of the solid is shown below. Each shell has radius x and height sin x 2 , so the volume of the solid is " √π 2π x sin(x 2 ) d x. 0

Let u = x 2 . Then du = 2x d x and π " √π " π 2π x sin(x 2 ) d x = π sin u du = −π (cos u) = 2π . 0

0

0

y 1 0.5

−2

−1

x 1

2

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CHAPTER 6

A P P L I C AT I O N S O F T H E I N T E G R A L

9. f (x) = x + 1−1− 2x 2 , [0, 1] f (x) = x , [1, 3] SOLUTION A sketch of the solid is shown below. Each shell has radius x and height x + 1 − 2x 2 , so the volume of the solid is " 1 " 1 1 3 1 2 1 4 1 2 2π x(x + 1 − 2x 2 ) d x = 2π (x 2 + x − 2x 3 ) d x = 2π x + x − x = π. 3 2 2 3 0 0 0 y 1 0.5 x

−1

1

In Exercises 11–14, xuse the Shell Method to compute the volume of the solids obtained by rotating the region enclosed f (x) =of , [1, 4] the y-axis. by the graphs the functions about 1 + x3 11. y = x 2 ,

y = 8 − x 2,

x =0

The region enclosed by y = x 2 , y = 8 − x 2 and the y-axis is shown below. When rotating this region about the y-axis, each shell has radius x and height 8 − x 2 − x 2 = 8 − 2x 2 . The volume of the resulting solid is SOLUTION

2π

" 2 0

x(8 − 2x 2 ) d x = 2π

2 1 (8x − 2x 3 ) d x = 2π 4x 2 − x 4 = 16π . 2 0 0

" 2

y 8 y = 8 − x2 6 4 2

y = x2 x

0

0.5

1

1.5

2

√ x2 13. y = x, y = y = 8 − x 3 , y = 8 − 4x √ SOLUTION The region enclosed by y = x and y = x 2 is shown below. When rotating this region about the y-axis, √ each shell has radius x and height x − x 2 . The volume of the resulting solid is 2π

" 1 0

√ x( x − x 2 ) d x = 2π

" 1 0

(x 3/2 − x 3 ) d x = 2π

3 2 5/2 1 4 1 − x = π. x 5 4 10 0

y 1 y=

x y = x2

x 0

1

In Exercises y= 1 − |x −15–16, 1|, yuse = 0the Shell Method to compute the volume of rotation of the region enclosed by the curves about the y-axis. Use a computer algebra system or graphing utility to ﬁnd the points of intersection numerically. 15. y = 12 x 2 ,

y = sin(x 2 )

The region enclosed by y = 12 x 2 and y = sin x 2 is shown below. When rotating this region about the y-axis, each shell has radius x and height sin x 2 − 12 x 2 . Using a computer algebra system, we ﬁnd that the x-coordinate of the point of intersection on the right is x = 1.376769504. Thus, the volume of the resulting solid of revolution is " 1.376769504 1 x sin x 2 − x 2 d x = 1.321975576. 2π 2 0 SOLUTION

S E C T I O N 6.4

The Method of Cylindrical Shells

319

y 1 y = sin x 2 y=

x2 2 x

0

1

In Exercises 17–22, sketch the solid obtained by rotating the region underneath the graph of the function over the interval y = 1 − x 4 , y = x, x = 0 about the given axis and calculate its volume using the Shell Method. 17. f (x) = x 3 , [0, 1], SOLUTION

x =2

A sketch of the solid is shown below. Each shell has radius 2 − x and height x 3 , so the volume of the solid

is 2π

" 1 0

(2 − x) x 3

d x = 2π

" 1 0

(2x 3 − x 4 ) d x = 2π

x4 x5 − 2 5

1 3π . = 5 0

y 1

x 0

4

19. f (x) = x −4 ,3 [−3, −1], x = 4 f (x) = x , [0, 1], x = −2 SOLUTION A sketch of the solid is shown below. Each shell has radius 4 − x and height x −4 , so the volume of the solid is " −1 " −1

280π 1 −2 4 −3 −1 −4 −4 −3 d x = 2π 2π (4x − x ) d x = 2π x − x (4 − x) x = 81 . 2 3 −3 −3 −3 y

0.8

0.4 x

−2

10

21. f (x) = a − bx, [0, a/b], x = −1, a, b > 0 1 2], isxshown = 0 below. Each shell has radius x − (−1) = x + 1 and height a − bx, so the f (x) = SOLUTION A sketch of,the[0, solid 2 x +1 volume of the solid is " a/b " a/b

a + (a − b)x − bx 2 d x (x + 1) (a − bx) d x = 2π 2π 0

0

a − b 2 b 3 a/b x − x 2 3 0 2 2 3 a a 2 (a + 3b) a a (a − b) = = 2π − π. + b 2b2 3b2 3b2 = 2π ax +

y a

−2 − a /b

x −2 −1

a/b

320

CHAPTER 6

A P P L I C AT I O N S O F T H E I N T E G R A L

In Exercises 23–28, use the Shell Method to calculate the volume of rotation about the x-axis for the region underneath f (x) = 1 − x 2 , [−1, 1], x = c (with c > 1) the graph. 23. y = x,

0≤x ≤1

SOLUTION When the region shown below is rotated about the x-axis, each shell has radius y and height 1 − y. The volume of the resulting solid is

2π

" 1 0

y(1 − y) d y = 2π

" 1 0

(y − y 2 ) d y = 2π

π 1 2 1 3 1 y − y = . 2 3 3 0

y 1 0.8 0.6

y=x

0.4 0.2 x 0

0.2 0.4 0.6 0.8

1

25. y = x 1/3 − 2,2 8 ≤ x ≤ 27 y =4−x , 0≤ x ≤2 SOLUTION When the region shown below is rotated about the x-axis, each shell has radius y and height 27 − (y + 2)3 . The volume of the resulting solid is 2π

" 1 0

" 1

19y − 12y 2 − 6y 3 − y 4 d y y · 27 − (y + 2)3 d y = 2π 0

= 2π

1 3 1 38π 19 2 y − 4y 3 − y 4 − y 5 = . 2 2 5 5 0

y 1 0.8 0.6

y=

3

5

10 15 20 25 30

x−2

0.4 0.2 x 0

27. y = x −2 ,−12 ≤ x ≤ 4 y = x , 1 ≤ x ≤ 4. Sketch the region and express the volume as a sum of two integrals. SOLUTION When the region shown below is rotated about the x-axis, two different shells are generated. For each 1 ], the shell has radius y and height 4 − 2 = 2; for each y ∈ [ 1 , 1 ], the shell has radius y and height √1 − 2. y ∈ [0, 16 16 4 y The volume of the resulting solid is 2π

" 1/16 0

2y d y + 2π

" 1/4 1/16

y(y −1/2 − 2) d y = 2π

" 1/16 0

2y d y + 2π

" 1/4 1/16

(y 1/2 − 2y) d y

1/4

1/16 2 3/2 = 2π y 2 + 2π − y 2 y 0 3 1/16 =

11π 7π π + = . 128 384 192

y 0.25 0.2 y = 12 x

0.15 0.1 0.05

x 0

y=

√

x, 1 ≤ x ≤ 4

1

2

3

4

The Method of Cylindrical Shells

S E C T I O N 6.4

321

29. Use both the Shell and Disk Methods to calculate the volume of the solid obtained by rotating the region under the graph of f (x) = 8 − x 3 for 0 ≤ x ≤ 2 about: (a) the x-axis (b) the y-axis SOLUTION

(a) x-axis: Using the disk method, the cross sections are disks with radius R = 8 − x 3 ; hence the volume of the solid is

π

" 2 0

(8 − x 3 )2 d x = π

64x − 4x 4 +

1 7 2 576π x = . 7 7 0

With the shell method, each shell has radius y and height (8 − y)1/3 . The volume of the solid is 2π

" 8 0

y (8 − y)1/3 d y

Let u = 8 − y. Then d y = −du, y = 8 − u and 2π

" 8 0

y (8 − y)1/3 d y = 2π

" 8 0

= 2π

(8 − u) · u 1/3 du = 2π

" 8 0

(8u 1/3 − u 4/3 ) du

8 3 576π . 6u 4/3 − u 7/3 = 7 7 0

(b) y-axis: With the shell method, each shell has radius x and height 8 − x 3 . The volume of the solid is 2π

" 2 0

2 1 96π . 4x 2 − x 5 = 5 5 0

x(8 − x 3 ) d x = 2π

Using the disk method, the cross sections are disks with radius R = (8 − y)1/3 . The volume is then given by

π

8 3π 96π 2/3 5/3 (8 − y) d y = − (8 − y) = . 5 5 0 0

" 8

31. Assume that the graph in Figure 9(A) can be described by both y = f (x) and x = h(y). Let V be the volume of the Sketch the solid of rotation about the y-axis for the region under the graph of the constant function f (x) = c solid obtained by rotating the region under the curve about the y-axis. (where c > 0) for 0 ≤ x ≤ r . (a) Describe the ﬁgures generated by rotating segments AB and C B about the y-axis. (a) Find the volume without using integration. (b) Set up integrals that compute V by the Shell and Disk Methods. (b) Use the Shell Method to compute the volume. y

y y = f(x) x = h(y)

1.3

y = g(x)

B

A

A'

B'

x C

x

2

C'

(A)

2 (B)

FIGURE 9 SOLUTION

(a) When rotated about the y-axis, the segment AB generates a disk with radius R = h(y) and the segment C B generates a shell with radius x and height f (x). (b) Based on Figure 9(A) and the information from part (a), when using the Shell Method, V = 2π

" 2 0

x f (x) d x;

when using the Disk Method, V =π

" 1.3 0

(h(y))2 d y.

In Exercises 33–38, use the Shell Method to ﬁnd the volume of the solid obtained by rotating region A in Figure 10 about Let W be the volume of the solid obtained by rotating the region under the curve in Figure 9(B) about the the given axis. y-axis. (a) Describe the ﬁgures generated by rotating segments A B and A C about the y-axis. (b) Set up an integral that computes W by the Shell Method. (c) Explain the difﬁculty in computing W by the Washer Method.

322

CHAPTER 6

A P P L I C AT I O N S O F T H E I N T E G R A L y

y = x2 + 2

6 A 2

B x 1

2

FIGURE 10

33. y-axis When rotating region A about the y-axis, each shell has radius x and height 6 − (x 2 + 2) = 4 − x 2 . The volume of the resulting solid is SOLUTION

2π

" 2 0

x(4 − x 2 ) d x = 2π

2 1 (4x − x 3 ) d x = 2π 2x 2 − x 4 = 8π . 4 0 0

" 2

35. x = 2 x = −3 SOLUTION When rotating region A about x = 2, each shell has radius 2 − x and height 6 − (x 2 + 2) = 4 − x 2 . The volume of the resulting solid is " 2

1 4 2 40π 2 3 2 2 3 2 . 8 − 2x − 4x + x d x = 2π 8x − x − 2x + x = 2π (2 − x) 4 − x d x = 2π 3 4 3 0 0 0 " 2

37. y = −2 x-axis

When rotating region A about y = −2, each shell has radius y − (−2) = y + 2 and height volume of the resulting solid is SOLUTION

2π

" 6 2

√

y − 2. The

(y + 2) y − 2 d y

Let u = y − 2. Then du = d y, y + 2 = u + 4 and 2π

" 4 √ 1024π 2 5/2 8 3/2 4 (y + 2) y − 2 d y = 2π (u + 4) u du = 2π + u = u . 5 3 15 2 0 0

" 6

In Exercises y = 639–44, use the Shell Method to ﬁnd the volumes of the solids obtained by rotating region B in Figure 10 about the given axis. 39. y-axis When rotating region B about the y-axis, each shell has radius x and height x 2 + 2. The volume of the resulting solid is SOLUTION

2π

" 2 0

x(x 2 + 2) d x = 2π

" 2 0

(x 3 + 2x) d x = 2π

2 1 4 x + x 2 = 16π . 4 0

41. x = 2 x = −3 SOLUTION When rotating region B about x = 2, each shell has radius 2 − x and height x 2 + 2. The volume of the resulting solid is 2π

" 2 0

(2 − x) x 2 + 2

2 " 2

32π 2 3 1 4 2 3 2 d x = 2π x − x + 4x − x = . 2x − x + 4 − 2x d x = 2π 3 4 3 0 0

43. y = −2 x-axis

SOLUTION When rotating region B about y = −2, two different shells are generated. For each y ∈ [0, 2], the resulting shell has radius √ y − (−2) = y + 2 and height 2; for each y ∈ [2, 6], the resulting shell has radius y − (−2) = y + 2 and height 2 − y − 2. The volume of the solid is then

2π

" 2 0

2(y + 2) d y + 2π

" 6 2

(y + 2)(2 −

y − 2) d y = 2π

" 6 0

2(y + 2) d y − 2π

= 120π − 2π

" 6 2

" 6 2

(y + 2) y − 2 d y

(y + 2) y − 2 d y.

S E C T I O N 6.4

The Method of Cylindrical Shells

323

In the remaining integral, let u = y − 2, so du = d y and y + 2 = u + 4. Then 2π

" 4 √ 2 5/2 8 3/2 4 1024π (y + 2) y − 2 d y = 2π (u + 4) u du = 2π + u u = 15 . 5 3 2 0 0

" 6

Finally, the volume of the solid is 1024π 776π = . 15 15

120π −

45. Use the Shell Method to compute the volume of a sphere of radius r . y=8 SOLUTION A sphere of radius r can be generated by rotating the region under the semicircle y = r 2 − x 2 around the x-axis. Each shell has radius y and height

2 2 2 2 r − y − − r − y = 2 r 2 − y2. Thus, the volume of the sphere is 2π

" r 0

2y r 2 − y 2 d y.

Let u = r 2 − y 2 . Then du = −2y d y and 2π

2 " r2

√ 2 3/2 r 4 3 u 2y r 2 − y 2 d y = 2π u du = 2π = 3 πr . 3 0 0 0

" r

2+ 47. UseUse the the Shell Method to compute the volume of the by rotating the interior of the circle (x −ra)from Shell Method to calculate the volume V torus of theobtained “bead” formed by removing a cylinder of radius y 2 =the r 2center about of thea y-axis, where a > r . Hint: Evaluate the integral by interpreting part of it as the area of a circle. sphere of radius R (compare with Exercise 49 in Section 6.3). SOLUTION When rotating the region enclosed by the circle (x − a)2 + y 2 = r 2 about the y-axis each shell has radius x and height

r 2 − (x − a)2 − − r 2 − (x − a)2 = 2 r 2 − (x − a)2 .

The volume of the resulting torus is then 2π

" a+r a−r

2x r 2 − (x − a)2 d x.

Let u = x − a. Then du = d x, x = u + a and " a+r " r 2 2 2π 2x r − (x − a) d x = 2π 2(u + a) r 2 − u 2 du −r

a−r

= 4π

" r

−r

" r u r 2 − u 2 du + 4a π r 2 − u 2 du. −r

Now, " r −r

u r 2 − u 2 du = 0

because the integrand is an odd function and the integration interval is symmetric with respect to zero. Moreover, the other integral is one-half the area of a circle of radius r ; thus, " r 1 r 2 − u 2 du = π r 2 . 2 −r Finally, the volume of the torus is 4π (0) + 4a π

1 2 πr 2

= 2π 2 ar 2 .

49. Use the most convenient method to compute the volume of the solid obtained by rotating the region in Figure 12 Use the Shell or Disk Method (whichever is easier) to compute the volume of the solid obtained by rotating the about the axis: region in Figure 11 about: (a) x = 4 (b) y = −2 (a) the x-axis (b) the y-axis

324

CHAPTER 6

A P P L I C AT I O N S O F T H E I N T E G R A L y y = x3 + 2 y = 4 − x2 x 1

2

FIGURE 12

Examine Figure 12. If the indicated region is sliced vertically, then the top of the slice lies along the curve y = x 3 + 2 and the bottom lies along the curve y = 4 − x 2 . On the other hand, the left end of a horizontal slice switches from y = 4 − x 2 to y = x 3 + 2 at y = 3. Here, vertical slices will be more convenient. (a) Now, suppose the region in Figure 12 is rotated about x = 4. Because a vertical slice is parallel to x = 4, we will calculate the volume of the resulting solid using the shell method. Each shell has radius 4 − x and height x 3 + 2 − (4 − x 2 ) = x 3 + x 2 − 2, so the volume is SOLUTION

2π

" 2 1

(4 − x)(x 3 + x 2 − 2) d x = 2π

2 3 4 563π 1 . − x 5 + x 4 + x 3 + x 2 − 8x = 5 4 3 30 1

(b) Now suppose the region is rotated about y = −2. Because a vertical slice is perpendicular to y = −2, we will calculate the volume of the resulting solid using the disk method. Each cross section is a washer with outer radius R = x 3 + 2 − (−2) = x 3 + 4 and inner radius r = 4 − x 2 − (−2) = 6 − x 2 , so the volume is

π

2 " 2

1748π 1 7 1 5 (x 3 + 4)2 − (6 − x 2 )2 d x = π x − x + 2x 4 + 4x 3 − 20x = . 7 5 35 1 1

Further Insights and Challenges

x 2 y 2 = 1 to (Figure Show that obtained rotating 51. Let R The be the region bounded by theofellipse surface area of a sphere radius r ais 4π+r 2 . bUse this derive13). the formula forthe thesolid volume V of abysphere of 4 2 radius R in a new way. R about the y-axis (called an ellipsoid) has volume 3 π a b. (a) Show that the volume of a thin spherical shell of inner radius r and thickness x is approximately 4π r 2 x. y (b) Approximate V by decomposing the sphere of radius R into N thin spherical shells of thickness x = R/N . b (c) Show that the approximation is a Riemann sum which converges to an integral. Evaluate the integral. R a

FIGURE 13 The ellipse

x 2 a

+

x

y 2 b

= 1.

SOLUTION Let’s slice the portion of the ellipse in the ﬁrst and fourth quadrants horizontally and rotate the slices about the y-axis. The resulting ellipsoid has cross sections that are disks with radius a2 y2 R = a2 − 2 . b

Thus, the volume of the ellipsoid is b " b a2 y2 a 2 y 3 2 2 a − 2 dy = π a y − π b 3b2 −b

−b

) =π

a2 b a2b − 3

−

a2 b −a 2 b + 3

* =

4 2 π a b. 3

The bell-shaped curve in Figure 14 is the graph of a certain function y = f (x) with the following property: The tangent line at a point (x, y) on the graph has slope d y/d x = −x y. Let R be the shaded region under the graph for 6.50 Work Energy ≤ x ≤ and a in Figure 14. Use the Shell Method and the substitution u = f (x) to show that the solid obtained by revolving R around the y-axis has volume V = 2π (1 − c), where c = f (a). Observe that as c → 0, the region R becomes inﬁnite but the volume V approaches 2π . Preliminary Questions 1. Why is integration needed to compute the work performed in stretching a spring? SOLUTION Recall that the force needed to extend or compress a spring depends on the amount by which the spring has already been extended or compressed from its equilibrium position. In other words, the force needed to move a spring is variable. Whenever the force is variable, work needs to be computed with an integral.

2. Why is integration needed to compute the work performed in pumping water out of a tank but not to compute the work performed in lifting up the tank?

S E C T I O N 6.5

Work and Energy

325

SOLUTION To lift a tank through a vertical distance d, the force needed to move the tank remains constant; hence, no integral is needed to calculate the work done in lifting the tank. On the other hand, pumping water from a tank requires that different layers of the water be moved through different distances, and, depending on the shape of the tank, may require different forces. Thus, pumping water from a tank requires that an integral be evaluated.

3. Which of the following represents the work required to stretch a spring (with spring constant k) a distance x beyond its equilibrium position: kx, −kx, 12 mk 2 , 12 kx 2 , or 12 mx 2 ? SOLUTION

The work required to stretch a spring with spring constant k a distance x beyond its equilibrium position is x " x 1 1 ky d y = ky 2 = kx 2 . 2 2 0 0

Exercises 1. How much work is done raising a 4-kg mass to a height of 16 m above ground? SOLUTION

The force needed to lift a 4-kg object is a constant (4 kg)(9.8 m/s2 ) = 39.2 N.

The work done in lifting the object to a height of 16 m is then (39.2 N)(16 m) = 627.2 J. In Exercises work (in joules) required to stretch or ftcompress a spring as indicated, assuming that the How 3–6, muchcompute work is the done raising a 4-lb mass to a height of 16 above ground? spring constant is k = 150 kg/s2 . 3. Stretching from equilibrium to 12 cm past equilibrium SOLUTION

The work required to stretch the spring 12 cm past equilibrium is " .12

.12 150x d x = 75x 2 = 1.08 J. 0

0

5. Stretching from 5 to 15 cm past equilibrium Compressing from equilibrium to 4 cm past equilibrium SOLUTION The work required to stretch the spring from 5 cm to 15 cm past equilibrium is " .15 .05

.15 150x d x = 75x 2 = 1.5 J. .05

7. If 5 J of work are needed to stretch a spring 10 cm beyond equilibrium, how much work is required to stretch it Compressing the spring 4 more cm when it is already compressed 5 cm 15 cm beyond equilibrium? SOLUTION

First, we determine the value of the spring constant as follows: " 0.1 0

kx d x =

1 2 0.1 kx = 0.005k = 5 J. 2 0

Thus, k = 1000 kg/s2 . Next, we calculate the work required to stretch the spring 15 cm beyond equilibrium: " 0.15 0

0.15 1000x d x = 500x 2 = 11.25 J. 0

9. If 10 ft-lb of work are needed to stretch a spring 1 ft beyond equilibrium, how far will the spring stretch if a 10-lb If 5 J of work are needed to stretch a spring 10 cm beyond equilibrium, how much work is required to compress weight is attached to its end? it 5 cm beyond equilibrium? SOLUTION First, we determine the value of the spring constant as follows: 1 2 1 1 kx d x = kx = k = 10 ft-lb. 2 2 0 0

" 1

Thus k = 20 lb/ft. Balancing the forces acting on the weight, we have 10 lb = kd = 20d, which implies d = 0.5 ft. A 10-lb weight will therefore stretch the spring 6 inches. In Exercises 11–14, calculate the work against gravity required to build the structure out of brick using the method of Show that the work required to stretch a spring from position a to position b is 12 k(b2 − a 2 ), where k is the Examples 2 and 3. Assume that brick has density 80 lb/ft3 . spring constant. How do you interpret the negative work obtained when |b| < |a|?

326

CHAPTER 6

A P P L I C AT I O N S O F T H E I N T E G R A L

11. A tower of height 20 ft and square base of side 10 ft SOLUTION The volume of one layer is 100y ft3 and so the weight of one layer is 8000y lb. Thus, the work done against gravity to build the tower is

W =

" 20 0

20 8000y d y = 4000y 2 = 1.6 × 106 ft-lb. 0

13. A 20-ft-high tower in the shape of a right circular cone with base of radius 4 ft A cylindrical tower of height 20 ft and radius 10 ft y 2 2 SOLUTION From similar triangles, the area of one layer is π 4 − 5 ft , so the volume of each small layer is y 2 y 2 3 π 4 − 5 y ft . The weight of one layer is then 80π 4 − 5 y lb. Finally, the total work done against gravity to build the tower is " 20

y 2 128,000π ft-lb. 80π 4 − y dy = 5 3 0 15. Built around 2600 BCE, the Great Pyramid of Giza in Egypt is 485 ft high (due to erosion, its current height is slightly A structure in the shape of a hemisphere of radius 4 ft less) and has a square base of side 755.5 ft (Figure 6). Find the work needed to build the pyramid if the density of the stone is estimated at 125 lb/ft3 .

FIGURE 6 The Great Pyramid in Giza, Egypt. SOLUTION

From similar triangles, the area of one layer is 755.5 −

755.5 2 2 ft , y 485

so the volume of each small layer is 755.5 2 755.5 − y y ft3 . 485 The weight of one layer is then 755.5 2 125 755.5 − y y lb. 485 Finally, the total work needed to build the pyramid was " 485 0

755.5 2 125 755.5 − y y d y = 1.399 × 1012 ft-lb. 485

In Exercises 16–20, calculate the work (in joules) required to pump all of the water out of the tank. Assume that the tank is full, distances are measured in meters, and the density of water is 1,000 kg/m3 . 17. The hemisphere in Figure 8; water exits from the spout as shown. The box in Figure 7; water exits from a small hole at the top. 10

2

FIGURE 8

S E C T I O N 6.5

Work and Energy

327

SOLUTION Place the origin at the center of the hemisphere, and let the positive y-axis point downward. The radius of a layer of water at depth y is 100 − y 2 m, so the volume of the layer is π (100 − y 2 )y m3 , and the force needed to lift the layer is 9800π (100 − y 2 )y N. The layer must be lifted y + 2 meters, so the total work needed to empty the tank is

" 10 0

9800π (100 − y 2 )(y + 2) d y =

112700000π J ≈ 1.18 × 108 J. 3

19. The horizontal cylinder in Figure 10; water exits from a small hole at the top. Hint: Evaluate the integral by interThe conical tank in Figure 9; water exits through the spout as shown. preting part of it as the area of a circle. Water exits here

r

FIGURE 10

Place the origin along the axis of the cylinder. At location y, the layer of water is a rectangular slab of length , width 2 r 2 −y 2 and thickness y. Thus, the volume of the layer is 2 r 2 − y 2 y, and the force needed to lift the layer is 19600 r 2 − y 2 y. The layer must be lifted a distance r − y, so the total work needed to empty the tank is given by " r " r " r

19600 r 2 − y 2 (r − y) d y = 19600r r 2 − y 2 d y − 19600 y r 2 − y 2 d y. SOLUTION

−r

−r

−r

Now, " r −r

y r 2 − y 2 du = 0

because the integrand is an odd function and the integration interval is symmetric with respect to zero. Moreover, the other integral is one-half the area of a circle of radius r ; thus, " r 1 r 2 − y2 d y = πr 2. 2 −r Finally, the total work needed to empty the tank is 1 2 π r − 19600(0) = 9800π r 3 J. 19600r 2 21. Find the work W required to empty the tank in Figure 7 if it is half full of water. The trough in Figure 11; water exits by pouring over the sides. SOLUTION Place the origin on the top of the box, and let the positive y-axis point downward. Note that with this coordinate system, the bottom half of the box corresponds to y values from 2.5 to 5. The volume of one layer of water is 32y m3 , so the force needed to lift each layer is (9.8)(1000)32y = 313600y N. Each layer must be lifted y meters, so the total work needed to empty the tank is " 5 2.5

5 313600y d y = 156800y 2 = 2.94 × 106 J. 2.5

23. Find the work required to empty the tank in Figure 9 if it is half full of water. Assume the tank in Figure 7 is full of water and let W be the work required to pump out half of the water. Do SOLUTION origin the vertex of theininverted and let the y-axis point you expectPlace W tothe equal theatwork computed Exercisecone, 21? Explain andpositive then compute W . upward. Consider a layer of water at a height of y meters. From similar triangles, the area of the layer is

π

y 2 2

m2 ,

so the volume is

π

y 2 2

y m3 .

CHAPTER 6

A P P L I C AT I O N S O F T H E I N T E G R A L

Thus the weight of one layer is 9800π

y 2 2

y N.

The layer must be lifted 12 − y meters, so the total work needed to empty the half-full tank is " 5 0

9800π

y 2 2

(12 − y) d y =

1684375π J ≈ 2.65 × 106 J. 2

25. Assume that the tank in Figure 9 is full. Assume the tank in Figure 9 is full of water and ﬁnd the work required to pump out half of the water. (a) Calculate the work F(y) required to pump out water until the water level has reached level y. Plot F(y). (b) (c)

What is the signiﬁcance of F (y) as a rate of change?

(d)

If your goal is to pump out all of the water, at which water level y0 will half of the work be done?

SOLUTION

(a) Place the origin at the vertex of the inverted cone, and let the positive y-axis point upward. Consider a layer of water at a height of y meters. From similar triangles, the area of the layer is

π

y 2 2

m2 ,

so the volume is

π

y 2 2

y m3 .

Thus the weight of one layer is 9800π

y 2 2

y N.

The layer must be lifted 12 − y meters, so the total work needed to pump out water until the water level has reached level y is " 10 y

9800π

y 2 2

(12 − y) d y = 3675000π − 9800π y 3 +

1225π 4 y J. 2

(b) A plot of F(y) is shown below. y 1 × 10 −7 Work, J

328

8 × 10 −6 6 × 10 −6 4 × 10 −6 2 × 10 −6 x 0

2

4 6 8 Depth, m

10

(c) First, note that F (y) < 0; as y increases, less water is being pumped from the tank, so F(y) decreases. Therefore, when the water level in the tank has reached level y, we can interpret −F (y) as the amount of work per meter needed to remove the next layer of water from the tank. In other words, −F (y) is a “marginal work” function. (d) The amount of work needed to empty the tank is 3675000π J. Half of this work will be done when the water level reaches height y0 satisfying 3675000π − 9800π y03 +

1225π 4 y0 = 1837500π . 2

Using a computer algebra system, we ﬁnd y0 = 6.91 m. 27. How much work is done lifting a 3-m chain over the side of a building if the chain has mass density 4 kg/m? How much work is done lifting a 25-ft chain over the side of a building (Figure 12)? Assume that the chain has SOLUTION segment of the of length ysegments, located a estimate distance the y j meters from the top the building. a density Consider of 4 lb/ft.a Hint: Break up chain the chain into N work performed on of a segment, and The compute work needed to liftasthis segment chain to the top of the building is approximately the limit N→ ∞ as of anthe integral. W j ≈ (4y)(9.8)y j J.

S E C T I O N 6.5

Work and Energy

329

Summing over all segments of the chain and passing to the limit as y → 0, it follows that the total work is " 3 0

3 4 · 9.8y d y = 19.6y 2 = 176.4 J. 0

29. A 20-foot chain with mass density 3 lb/ft is initially coiled on the ground. How much work is performed in lifting An 8-ft chain weighs 16 lb. Find the work required to lift the chain over the side of a building. the chain so that it is fully extended (and one end touches the ground)? SOLUTION Consider a segment of the chain of length y that must be lifted y j feet off the ground. The work needed to lift this segment of the chain is approximately

W j ≈ (3y)y j ft-lb. Summing over all segments of the chain and passing to the limit as y → 0, it follows that the total work is " 20 0

3y d y =

3 2 20 y = 600 ft-lb. 2 0

31. A 1,000-lb wrecking ball hangs from a 30-ft cable of density 10 lb/ft attached to a crane. Calculate the work done if How much work is done lifting a 20-ft chain with mass density 3 lb/ft (initially coiled on the ground) so that its the crane lifts the ball from ground level to 30 ft in the air by drawing in the cable. top end is 30 ft above the ground? SOLUTION We will treat the cable and the wrecking ball separately. Consider a segment of the cable of length y that must be lifted y j feet. The work needed to lift the cable segment is approximately W j ≈ (10y)y j ft-lb. Summing over all of the segments of the cable and passing to the limit as y → 0, it follows that lifting the cable requires " 30 0

30 10y d y = 5y 2 = 4500 ft-lb. 0

Lifting the 1000 lb wrecking ball 30 feet requires an additional 30,000 ft-lb. Thus, the total work is 34,500 ft-lb. In Exercises 32–34, use Newton’s Universal Law of Gravity, according to which the gravitational force between two objects of mass m and M separated by a distance r has magnitude G Mm/r 2 , where G = 6.67 × 10−11 m3 kg−1 s−1 . Although the Universal Law refers to point masses, Newton proved that it also holds for uniform spherical objects, where r is the distance between their centers. 33. Use the result of Exercise 32 to calculate the work required to place a 2,000-kg satellite in an orbit 1,200 km above Two spheres of mass M and m are separated by a distance r1 . Show that 24 the work required to increase6 the the surface of the earth. Assume that the earth is a sphere−1 of mass −1 Me = 5.98 × 10 kg and radius re = 6.37 × 10 m. to aasdistance r2 is equal to W = G Mm(r1 − r2 ). Treatseparation the satellite a point mass. The satellite will move from a distance r1 = re to a distance r2 = re + 1200000. Thus, from Exercise 32, 1 1 ≈ 1.99 × 1010 J. − W = (6.67 × 10−11 )(5.98 × 1024 )(2000) 6.37 × 106 6.37 × 106 + 1200000

SOLUTION

35. Assume that the pressure P and volume V of the gas in a 30-in. cylinder of radius 3 in. with a movable piston are Use the result of Exercise 32 to compute the work required to move a 1,500-kg satellite from an orbit 1,000 to related by P V 1.4 = k, where k is a constant (Figure 13). When the cylinder is full, the gas pressure is 200 lb/in.2 . 1,500 km above the surface of the earth. (a) Calculate k. (b) Calculate the force on the piston as a function of the length x of the column of gas (the force is P A, where A is the piston’s area). (c) Calculate the work required to compress the gas column from 30 to 20 in.

3 x

FIGURE 13 Gas in a cylinder with a piston. SOLUTION

(a) We have P = 200 and V = 270π . Thus k = 200(270π )1.4 = 2.517382 × 106 lb-in2.2 .

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(b) The area of the piston is A = 9π and the volume of the cylinder as a function of x is V = 9π x, which gives P = k/V 1.4 = k/(9π x)1.4 . Thus F = PA =

k 9π = k(9π )−0.4 x −1.4 . (9π x)1.4

(c) Since the force is pushing against the piston, in order to calculate work, we must calculate the integral of the opposite force, i.e., we have " 20 1 −0.4 20 x −1.4 d x = −k(9π )−0.4 x W = −k(9π )−0.4 = 74677.8 in-lb. −0.4 30 30

Further Insights and Challenges 37. Work-Kinetic Energy Theorem The kinetic energy of an object of mass m moving with velocity v is KE = 1 mv 2 . A 20-foot chain with linear mass density 2 (x) = 0.02x(20 − x) lb/ft the time interval [t1 , t2 ] due to a net force F(x) acting along the (a) Suppose that the object moves from x1 to x 2ρduring interval [x 1 , x2 ]. Let x(t) be the position of the object at time t. Use the Change of Variables formula to show that the on the ground. worklies performed is equal to (a) How much work is done lifting the chain extended (and one end touches the ground)? " x2 so that it is "fully t2 (b) How much work is done liftingWthe so that has a height d xits=top end F(x(t))v(t) dt of 30 ft? = chain F(x) x1

t1

(b) By Newton’s Second Law, F(x(t)) = ma(t), where a(t) is the acceleration at time t. Show that d 1 mv(t)2 = F(x(t))v(t) dt 2 (c) Use the FTC to show that the change in kinetic energy during the time interval [t1 , t2 ] is equal to " t2 F(x(t))v(t) dt t1

(d) Prove the Work-Kinetic Energy Theorem: The change in KE is equal to the work W performed. SOLUTION

(a) Let x 1 = x(t1 ) and x2 = x(t2 ), then x = x(t) gives d x = v(t) dt. By substitution we have " x2 " t2 W = F(x) d x = F(x(t))v(t) dt. x1

t1

(b) Knowing F(x(t)) = m · a(t), we have d 1 2 m · v(t) = m · v(t) v (t) dt 2

(Chain Rule)

= m · v(t) a(t) = v(t) · F(x(t))

(Newton’s 2nd law)

(c) From the FTC, 1 m · v(t)2 = 2

" F(x(t)) v(t) dt.

Since K E = 12 m v 2 , K E = K E(t2 ) − K E(t1 ) =

(d)

W =

" x2 x1

F(x) d x =

" t2 1 1 F(x(t)) v(t) dt. m v(t2 )2 − m v(t1 )2 = 2 2 t1 " t2 t1

F(x(t)) v(t) dt

(Part (a))

= K E(t2 ) − K E(t1 )

(Part (c))

= K E

(as required)

39. With what initial velocity v0 must we ﬁre a rocket so it attains a maximum height r above the earth? Hint: Use the A model train of mass 0.5 kg is placed at one end of a straight 3-m electric track. Assume that a force F(x) = results of Exercises 32 and 37. As the rocket reaches its maximum height, its KE decreases from 12 mv02 to zero. (Exercise 37) to 3x − x 2 N acts on the train at distance x along the track. Use the Work-Kinetic Energy Theorem determine the velocity of the train when it reaches the end of the track.

Chapter Review Exercises SOLUTION

331

The work required to move the rocket a distance r from the surface of the earth is 1 1 W (r ) = G Me m − . re r + re

As the rocket climbs to a height r , its kinetic energy is reduced by the amount W (r ). The rocket reaches its maximum height when its kinetic energy is reduced to zero, that is, when 1 2 1 1 − mv = G Me m . 2 0 re r + re Therefore, its initial velocity must be v0 =

2G Me

1 1 − . re r + re

41. Calculate escape velocity, the minimum initial velocity of an object to ensure that it will continue traveling into With what initial velocity must we ﬁre a rocket so it attains a maximum height of r = 20 km above the surface space and never fall back to earth (assuming that no force is applied after takeoff). Hint: Take the limit as r → ∞ in of the earth? Exercise 39. SOLUTION The result of the previous exercise leads to an interesting conclusion. The initial velocity v0 required to reach a height r does not increase beyond all bounds as r tends to inﬁnity; rather, it approaches a ﬁnite limit, called the escape velocity: 1 2G Me 1 − vesc = lim 2G Me = r →∞ re r + re re

In other words, vesc is large enough to insure that the rocket reaches a height r for every value of r ! Therefore, a rocket ﬁred with initial velocity vesc never returns to earth. It continues traveling indeﬁnitely into outer space. Now, let’s see how large escape velocity actually is: vesc =

2 · 6.67 × 10−11 · 5.989 × 1024 6.37 × 106

1/2 ≈ 11,190 m/sec.

Since one meter per second is equal to 2.236 miles per hour, escape velocity is approximately 11,190(2.236) = 25,020 miles per hour.

CHAPTER REVIEW EXERCISES In Exercises 1–4, ﬁnd the area of the region bounded by the graphs of the functions. 1. y = sin x,

y = cos x,

0≤x ≤

5π 4

The region bounded by the graphs of y = sin x and y = cos x over the interval [0, 54π ] is shown below. For π x ∈ [0, 4 ], the graph of y = cos x lies above the graph of y = sin x, whereas, for x ∈ [ π4 , 54π ], the graph of y = sin x SOLUTION

lies above the graph of y = cos x. The area of the region is therefore given by " 5π /4 " π /4 (cos x − sin x) d x + (sin x − cos x) d x π /4

0

5π /4 π /4 + (− cos x − sin x) = (sin x + cos x) π /4

0

√ √ √ √ √ 2 2 2 2 2 2 = + − (0 + 1) + + − − − = 3 2 − 1. 2 2 2 2 2 2 √

√

y 1 0.5

y = sin x x 1

−0.5 −1

2

y = cos x

3

4

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= x 2 − 1, h(x) = x 2 + x − 2 3. f (x) = x 2 +32x, g(x) f (x) = x − 2x 2 + x, g(x) = x 2 − x SOLUTION The region bounded by the graphs of y = x 2 + 2x, y = x 2 − 1 and y = x 2 + x − 2 is shown below. For each x ∈ [−2, − 12 ], the graph of y = x 2 + 2x lies above the graph of y = x 2 + x − 2, whereas, for each x ∈ [− 12 , 1], the graph of y = x 2 − 1 lies above the graph of y = x 2 + x − 2. The area of the region is therefore given by " −1/2

" 1

(x 2 + 2x) − (x 2 + x − 2) d x + (x 2 − 1) − (x 2 + x − 2) d x −2

−1/2

−1/2 1 1 2 1 = + − x 2 + x x + 2x 2 2 −2 −1/2 1 1 1 1 9 = − 1 − (2 − 4) + − + 1 − − − = . 8 2 8 2 4 y = x 2 + 2x −2

y

y = x2 − 1

−1

1

x

−2 y = x2 + x − 2

In Exercises 5–8, sketch the region bounded π by the graphs of the functions and ﬁnd its area. f (x) = sin x, g(x) = sin 2x, ≤ x ≤ π 3 5. f (x) = x 3 − x 2 − x + 1, g(x) = 1 − x 2 , 0 ≤ x ≤ 1 Hint: Use geometry to evaluate the integral. SOLUTION The region bounded by the graphs of y = x 3 − x 2 − x + 1 and y = 1 − x 2 is shown below. As the 3 2 2 graph of y = 1 − x lies above the graph of y = x − x − x + 1, the area of the region is given by " 1 1 − x 2 − (x 3 − x 2 − x + 1) d x. 0

Now, the region below the graph of y = circle; thus,

1 − x 2 but above the x-axis over the interval [0, 1] is one-quarter of the unit " 1 0

1 − x2 dx =

1 π. 4

Moreover, " 1 0

(x 3 − x 2 − x + 1) d x =

1 5 1 4 1 3 1 2 1 1 1 x − x − x + x = − − + 1 = . 4 3 2 4 3 2 12 0

Finally, the area of the region shown below is 1 5 π− . 4 12 y 1 0.8

y=

1 − x2

0.6 0.4 0.2

y = x3 − x2 − x + 1 x

0

0.2 0.4 0.6 0.8

1

7. y = 4 −1x 2 , y = 3x, y=4 x = 1 − y 2 , by0the ≤ ygraphs ≤ 1 of y = 4 − x 2 , y = 3x and y = 4 is shown below. For x ∈ [0, 1], the y, x = y SOLUTION 2The region bounded graph of y = 4 lies above the graph of y = 4 − x 2 , whereas, for x ∈ [1, 43 ], the graph of y = 4 lies above the graph of y = 3x. The area of the region is therefore given by 4/3 " 1 " 4/3 1 1 3 1 16 8 3 1 (4 − (4 − x 2 )) d x + (4 − 3x) d x = x 3 + 4x − x 2 = + − − 4− = . 3 0 2 3 3 3 2 2 0 1 1

Chapter Review Exercises

333

y y=4

4

y = 3x

y = 4 − x2

3 2 1

x 0

0.2 0.4 0.6 0.8 1 1.2

Use graphing utility to locate the points of intersection of y = e−x and y = 1 − x 2 and ﬁnd the area 9. 3 −a 2y 2 = ytwo y, x = y 2 − y betweenx the curves+(approximately). The region bounded by the graphs of y = e−x and y = 1 − x 2 is shown below. One point of intersection clearly occurs at x = 0. Using a computer algebra system, we ﬁnd that the other point of intersection occurs at x = 0.7145563847. As the graph of y = 1 − x 2 lies above the graph of y = e−x , the area of the region is given by " 0.7145563847

1 − x 2 − e−x d x = 0.08235024596 SOLUTION

0

y 1 y = 1 − x2

0.8 y = e −x

0.6 0.4 0.2

x 0

0.2

0.4

0.6

0.8

11. Find the total weight of a 3-ft metal rod of linear density Figure 1 shows a solid whose horizontal cross section at height y is a circle of radius (1 + y)−2 for 0 ≤ y ≤ H . 2 Find the volume of the solid. ρ(x) = 1 + 2x + x 3 lb/ft. 9 SOLUTION

The total weight of the rod is " 3 0

ρ(x) d x =

x + x2 +

1 4 3 9 33 x =3+9+ = lb. 18 2 2 0

In Exercises the(in average valueunits) of the through functionaover Find 13–16, the ﬂowﬁnd rate the correct pipethe ofinterval. diameter 6 cm if the velocity of ﬂuid particles at a distance r from the center of the pipe is v(r ) = (3 − r ) cm/s. 3 13. f (x) = x − 2x + 2, [−1, 2] SOLUTION

The average value is

2 " 2

1 1 4 1 1 9 1 x 3 − 2x + 2 d x = x − x 2 + 2x = (4 − 4 + 4) − −1−2 = . 2 − (−1) −1 3 4 3 4 4 −1 15. f (x) = |x|, [−4, 4] f (x) = 9 − x 2 , [0, 3] Hint: Use geometry to evaluate the integral. SOLUTION The average value is " " 4 " 4 0 1 1 1 1 2 4 1 1 2 0 |x| d x = (−x) d x + x dx = − x + x = [(0 + 8) + (8 − 0)] = 2. 4 − (−4) −4 8 8 2 2 0 8 −4 0 −4 (x) = x[x], [0,g(t) 3] on [2, 5] is 9. Find 17. Thefaverage value of SOLUTION

" 5 g(t) dt. 2

The average value of the function g(t) on [2, 5] is given by " 5 " 1 5 1 g(t) dt = g(t) dt. 5−2 2 3 2

Therefore, " 5 2

g(t) dt = 3(average value) = 3(9) = 27.

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A P P L I C AT I O N S O F T H E I N T E G R A L

19. UseFor theall Shell to ﬁnd the volume of the obtained the region between y = x 2 and y = mx x ≥Method 0, the average value of R(x) oversolid [0, x] is equalby torevolving x. Find R(x). about the x-axis (Figure 2). y y = x2 y = mx

x

FIGURE 2 SOLUTION Setting x 2 = mx yields x(x − m) = 0, so the two curves intersect at (0, 0) and (m, m 2 ). To use the shell method, we must slice the solid parallel to the axis of rotation; as we are revolving about the x-axis, this implies a √ horizontal slice and integration in y. For each y ∈ [0, m 2 ], the shell has radius y and height y − my . The volume of the solid is therefore given by

2π

" m2 y

√

0

y y− d y = 2π m

2 5/2 y3 − y 5 3m

m 2 2m 5 m5 2π 5 − = m . = 2π 5 3 15 0

21. Let R be the intersection of the circles of radius 1 centered at (1, 0) and (0, 1). Express as an integral (but do not Use(a)the to ﬁnd the volume of the solid byx-axis. revolving the region between y = x 2 and evaluate): thewasher area ofmethod R and (b) the volume of revolution of Robtained about the y = mx about the y-axis (Figure 2). SOLUTION The region R is shown below. y 1

(x − 1)2 + y2 = 1

0.8 0.6 0.4 0.2

x2 + (y − 1)2 = 1 x

0

0.2 0.4 0.6 0.8

1

(a) A vertical slice of R has its top along the upper left arc of the circle (x − 1)2 + y 2 = 1 and its bottom along the lower right arc of the circle x 2 + (y − 1)2 = 1. The area of R is therefore given by " 1 1 − (x − 1)2 − (1 − 1 − x 2 ) d x. 0

(b) and use the washer method, each cross section is a washer with outer radius If we revolve R about the x-axis 1 − (x − 1)2 and inner radius 1 − 1 − x 2 . The volume of the solid is therefore given by

π

" 1 0

(1 − (x − 1)2 ) − (1 −

1 − x 2 )2 d x.

In Exercises 23–31, the volume of the solid obtained theexpressing region enclosed by theofcurves about the given Use the Shellﬁnd Method to set up an integral (but doby notrotating evaluate) the volume the solid obtained by axis.rotating the region under y = cos x over [0, π /2] about the line x = π . 23. y = 2x,

y = 0,

x = 8;

x-axis

SOLUTION The region bounded by the graphs of y = 2x, y = 0 and x = 8 is shown below. Let’s choose to slice the region vertically. Because a vertical slice is perpendicular to the axis of rotation, we will use the washer method to calculate the volume of the solid of revolution. For each x ∈ [0, 8], the cross section is a circular disk with radius R = 2x. The volume of the solid is therefore given by

π

" 8 0

(2x)2 d x =

4π 3 8 2048π x = . 3 3 0

Chapter Review Exercises

335

y 16 12 y = 2x

8 4

x 0

2

4

6

8

1, 8;axis x= −2−3 25. y =y x=2 − 2x,1, yy==0,2x − x= axis x= SOLUTION The region bounded by the graphs of y = x 2 − 1 and y = 2x − 1 is shown below. Let’s choose to slice the region vertically. Because a vertical slice is parallel to the axis of rotation, we will use the shell method to calculate the volume of the solid of revolution. For each x ∈ [0, 2], the shell has radius x − (−2) = x + 2 and height (2x − 1) − (x 2 − 1) = 2x − x 2 . The volume of the solid is therefore given by 2π

2 1 (x + 2)(2x − x 2 ) d x = 2π 2x 2 − x 4 = 2π (8 − 4) = 8π . 4 0 0

" 2

y 3 2 y = 2x − 1

1

x 1 −1

2

y = x2 − 1

27. y 2 = x 3 , 2 y = x, x = 8; axis x = −1 y = x − 1, y = 2x − 1, axis y = 4 SOLUTION The region bounded by the graphs of y 2 = x 3 , y = x and x = 8 is composed of two components, shown below. Let’s choose to slice the region vertically. Because a vertical slice is parallel to the axis of rotation, we will use the shell method to calculate the volume of the solid of revolution. For each x ∈ [0, 1], the shell as radius x − (−1) = x + 1 and height x − x 3/2 ; for each x ∈ [1, 8], the shell also has radius x + 1, but the height is x 3/2 − x. The volume of the solid is therefore given by 2π

" 1 0

(x + 1)(x − x 3/2 ) d x + 2π

" 8 1

(x + 1)(x 3/2 − x) d x =

y

√ 2π (12032 2 − 7083). 35

y

1

20

0.8 15 0.6 0.4

y=x

y2 = x3

10

y2 = x3

5

0.2

y=x x

x 0

0.2 0.4 0.6 0.8

1

0

2

4

6

8

29. y = 2−x 2 +−14x − 3, y = 0; axis y = −1 y = x , x = 1, x = 3; axis y = −3 SOLUTION The region bounded by the graph of y = −x 2 + 4x − 3 and the x-axis is shown below. Let’s choose to slice the region vertically. Because a vertical slice is perpendicular to the axis of rotation, we will use the washer method to calculate the volume of the solid of revolution. For each x ∈ [1, 3], the cross section is a washer with outer radius R = −x 2 + 4x − 3 − (−1) = −x 2 + 4x − 2 and inner radius r = 0 − (−1) = 1. The volume of the solid is therefore given by

π

3 " 3

20 3 1 5 (−x 2 + 4x − 2)2 − 1 d x = π x − 2x 4 + x − 8x 2 + 3x 5 3 1 1 1 20 56π 243 =π − 162 + 180 − 72 + 9 − −2+ −8+3 = . 5 5 3 15

336

CHAPTER 6

A P P L I C AT I O N S O F T H E I N T E G R A L y y = −x 2 + 4x − 3

1 0.8 0.6 0.4 0.2

x 0

0.5 1 1.5 2 2.5 3

31. y 2 = x −1 , x 3= 1, x = 3; axis x = −3 x = 4y − y , y = 0, y = 2; y-axis SOLUTION The region bounded by the graphs of y 2 = x −1 , x = 1 and x = 3 is shown below. Let’s choose to slice the region vertically. Because a vertical slice is parallel to the axis of rotation, we will use the shell method to calculate the volume of the solid of revolution. For each x ∈ [1, 3], the shell has radius x − (−3) = x + 3 and height 1 1 2 = √ . √ − −√ x x x The volume of the solid is therefore given by 2π

" 3

2 (x + 3) √ d x = 2π x 1

3 √ 40 4 3/2 + 12x 1/2 = 2π 16 3 − x . 3 3 1

y 1

x 1

2

3

1 x

y2 =

−1

In Exercises 32–34, the regions refer to the graph of the hyperbola y 2 − x 2 = 1 in Figure 3. Calculate the volume of revolution about both the x- and y-axes. y 3 y=x

2 1 −c

−1 −2

x

c

y2 − x 2 = 1

−3

FIGURE 3

33. The region between the upper branch of the hyperbola and the line y = x for 0 ≤ x ≤ c. The shaded region between the upper branch of the hyperbola and the x-axis for −c ≤ x ≤ c. SOLUTION

• x-axis: Let’s choose to slice the region vertically. Because a vertical slice is perpendicular to the axis of rotation,

we will use the washer method to calculate the volume of the solid of revolution. For each x ∈ [0, c], cross sections are washers with outer radius R = 1 + x 2 and inner radius r = x. The volume of the solid is therefore given by c " c

2 2 (1 + x ) − x d x = π x = cπ . π 0

0

• y-axis: Let’s choose to slice the region vertically. Because a vertical slice is parallel to the axis of rotation, we will

use the shell method to calculate the volume of the solid of revolution. For each x ∈ [0, c], the shell has radius x and height 1 + x 2 − x. The volume of the solid is therefore given by " c c 2π

2π

(1 + x 2 )3/2 − x 3 = (1 + c2 )3/2 − c3 − 1 . 2π x 1 + x2 − x dx = 3 3 0 0 2 35. LetThe a >region 0. Show that when the branch region between y = a and x −y ax between the upper of the hyperbola = 2.and the x-axis is rotated about the x-axis, the resulting volume is independent of the constant a.

Chapter Review Exercises

337

Setting a x − ax 2 = 0 yields x = 0 and x = 1/a. Using the washer method, cross sections are circular disks with radius R = a x − ax 2 . The volume of the solid is therefore given by SOLUTION

π

" 1/a 0

a 2 (x − ax 2 ) d x =

π

π 1 2 2 1 3 3 1/a 1 1 =π a x − a x − = , 2 3 2 3 6 0

which is independent of the constant a. In Exercises 37–38, water is pumpedlength into aisspherical tank aofforce radius a source locatedto2 20 ft below a hole the A spring whose equilibrium 15 cm exerts of 550ftNfrom when it is stretched cm. Find theat work 3. bottom (Figure 4). The density of water is 64.2 lb/ft required to stretch the spring from 22 to 24 cm.

5

2 Water source

FIGURE 4

37. Calculate the work required to ﬁll the tank. SOLUTION Place the origin at the base of the sphere with the positive y-axis pointing upward. The equation for the great is then x 2 + (y − 5)2 = 25. At location y, the horizontal cross section is a circle of radius circle of the sphere 25 − (y − 5)2 = 10y − y 2 ; the volume of the layer is then π (10y − y 2 )y ft3 , and the force needed to lift the layer is 64.2π (10y − y 2 )y lb. The layer of water must be lifted y + 2 feet, so the work required to ﬁll the tank is given by " 10 " 10 (y + 2)(10y − y 2 ) d y = 64.2π (8y 2 + 20y − y 3 ) d y 64.2π

0

0

= 64.2π

10 1 8 3 y + 10y 2 − y 4 3 4 0

= 74900π ≈ 235,305 ft-lb. 3 of water. The container is raised vertically at a constant speed of 39. A container lb required is ﬁlled with Calculateweighing the work 50 F(h) to ﬁll20 thefttank to height h ft from the bottom of the sphere. 2 ft/s for 1 min, during which time it leaks water at a rate of 13 ft3 /s. Calculate the total work performed in raising the container. The density of water is 64.2 lb/ft3 . SOLUTION Let t denote the elapsed time of the ascent of the container, and let y denote the height of the container. Given that the speed of ascent is 2 ft/s, y = 2t; moreover, the volume of water in the container is

1 1 20 − t = 20 − y ft3 . 3 6 The force needed to lift the container and its contents is then 1 50 + 64.2 20 − y = 1334 − 10.7y lb, 6 and the work required to lift the container and its contents is " 120 0

120 (1334 − 10.7y) d y = (1334y − 5.35y 2 ) = 83040 ft-lb. 0

Let W be the work (against the sun’s gravitational force) required to transport an 80-kg person from Earth to Mars when the two planets are aligned with the sun at their minimal distance of 55.7 × 106 km. Use Newton’s Universal Law of Gravity (see Exercises 32–34 in Section 6.5) to express W as an integral and evaluate it. The sun has mass Ms = 1.99 × 1030 kg, and the distance from the sun to the earth is 149.6 × 106 km.

THE EXPONENTIAL 7 FUNCTION 7.1 Derivative of bx and the Number e Preliminary Questions 1. Which of the following equations is incorrect? (a) 32 · 35 = 37

√ (b) ( 5)4/3 = 52/3

(c) 32 · 23 = 1

(d) (2−2 )−2 = 16

SOLUTION

(a) (b) (c) (d)

This equation is correct: 32 · 35 = 32+5 = 37 . √ This equation is correct: ( 5)4/3 = (51/2 )4/3 = 5(1/2)·(4/3) = 52/3 . This equation is incorrect: 32 · 23 = 9 · 8 = 72 = 1. this equation is correct: (2−2 )−2 = 2(−2)·(−2) = 24 = 16.

2. Which of the following functions can be differentiated using the Power Rule? (b) 2e (c) x e (a) x 2

(d) e x

SOLUTION The Power Rule applies when the function has a variable base and a constant exponent. Therefore, the Power Rule applies to (a) x 2 and (c) x e .

3. For which values of b does b x have a negative derivative? SOLUTION

The function b x has a negative derivative when 0 < b < 1.

4. For which values of b is the graph of y = b x concave up? SOLUTION

The graph of y = b x is concave up for all b > 0 except b = 1.

5. Which point lies on the graph of y = b x for all b? SOLUTION

The point (0, 1) lies on the graph of y = b x for all b.

6. Which of the following statements is not true? (a) (e x ) = e x eh − 1 (b) lim =1 h h→0 (c) The tangent line to y = e x at x = 0 has slope e. (d) The tangent line to y = e x at x = 0 has slope 1. SOLUTION

(a) This statement is true: (e x ) = e x . (b) This statement is true: d x eh − 1 = e = e0 = 1. lim h d x x=0 h→0 (c) This statement is false: the tangent line to y = e x at x = 0 has slope e0 = 1. (d) This statement is true: the tangent line to y = e x at x = 0 has slope e0 = 1.

Exercises 1. Rewrite as a whole number (without using a calculator): (a) 70 (c)

(43 )5

(45 )3 (e) 8−1/3 · 85/3

SOLUTION

(a) 70 = 1.

(b) 102 (2−2 + 5−2 ) (d) 274/3 (f) 3 · 41/4 − 12 · 2−3/2

S E C T I O N 7.1

(b) (c) (d) (e) (f)

Derivative of bx and the Number e

339

102 (2−2 + 5−2 ) = 100(1/4 + 1/25) = 25 + 4 = 29. (43 )5 /(45 )3 = 415 /415 = 1. (27)4/3 = (271/3 )4 = 34 = 81. 8−1/3 · 85/3 = (81/3 )5 /81/3 = 25 /2 = 24 = 16. 3 · 41/4 − 12 · 2−3/2 = 3 · 21/2 − 3 · 22 · 2−3/2 = 0.

In Exercises 2–10, solve for the unknown variable. 3. e2x = e x+1 92x = 98 SOLUTION If e2x = e x+1 then 2x = x + 1, and x = 1. x+1 5. 3x =t 2 13 4t−3 e =e 1 SOLUTION Rewrite ( 3 ) x+1 as (3−1 ) x+1 = 3−x−1 . Then 3 x = 3−x−1 , which requires x = −x − 1. Thus, x = −1/2. 7. 4−x √ = 2x+1 ( 5)x = 125 SOLUTION Rewrite 4−x as (22 )−x = 2−2x . Then 2−2x = 2 x+1 , which requires −2x = x + 1. Solving for x gives x = −1/3. 9. k 3/2 4= 27 12 b = 10 SOLUTION Raise both sides of the equation to the two-thirds power. This gives k = (27)2/3 = (271/3 )2 = 32 = 9. In Exercises 11–14, determine the limit. (b2 )x+1 = b−6 11. lim 4x x→∞

SOLUTION

lim 4x = ∞.

x→∞

1 −x 13. limlim 4−x x→∞ 4 x→∞ SOLUTION

lim

x→∞

1 −x 4

= lim 4x = ∞. x→∞

In Exercises 15–18, 2 ﬁnd the equation of the tangent line at the point indicated. lim e x−x x→∞ 15. y = 4e x , x 0 = 0 Let f (x) = 4e x . Then f (x) = 4e x and f (0) = 4. At x0 = 0, f (0) = 4, so the equation of the tangent line is y = 4(x − 0) + 4 = 4x + 4. SOLUTION

17. y = e x+24x , x 0 = −1 y = e , x0 = 0 SOLUTION Let f (x) = e x+2 . Then f (x) = e x+2 and f (−1) = e1 . At x 0 = −1, f (−1) = e, so the equation of the tangent line is y = e(x + 1) + e = ex + 2e. In Exercises 19–41, ﬁnd the derivative. 2 y = e x , x0 = 1 19. f (x) = 7e2x + 3e4x SOLUTION

d (7e2x + 3e4x ) = 14e2x + 12e4x . dx

21. f (x) = eπ x f (x) = e−5x d πx SOLUTION e = π eπ x . dx 23. f (x) = 4e−x−4x+9 + 7e−2x f (x) = e d SOLUTION (4e−x + 7e−2x ) = −4e−x − 14e−2x . dx 25. f (x) = x 2 e2x 2 ex f (x) =d 2 2x 2x 2 2x SOLUTION x e = 2xe + 2x e . dx −2x )4 27. f (x) = (2e3x + 2e f (x) = (1 + e x )4 d SOLUTION (2e3x + 2e−2x )4 = 4(2e3x + 2e−2x )3 (6e3x − 4e−2x ). dx

29. f (x) = e1/x 2 f (x) = e x +2x−3

340

CHAPTER 7

THE EXPONENTIAL FUNCTION

SOLUTION

−e1/x d 1/x = . e dx x2

31. f (x) = esin x3 f (x) = e d sin x SOLUTION = cos xesin x . e dx 33. f (x) = sin(e x )2 2 f (x) =de(x +2x+3) x SOLUTION sin(e ) = e x cos(e x ). dx 1√ 35. f (t)f (t) = = e −3t t 1−e d 1 d SOLUTION = (1 − e−3t )−1 = −(1 − e−3t )−2 (3e−3t ). dt 1 − e−3t dt −2t ) 37. f (t) = cos(te f (t) = et+1/2t−1 d SOLUTION cos(te−2t ) = − sin(te−2t )(t (−2e−2t ) + e−2t ) = (2te−2t − e−2t ) sin(te−2t ). dt

) 39. f (x) = tan(e5−6x ex f (x) =d 3xtan(e + 15−6x ) = −6e5−6x sec2 (e5−6x ). SOLUTION dx x

41. f (x) = ee x+1 e +x f (x) =d exx x 2e − SOLUTION e = 1e x ee . dx In Exercises 42–47, ﬁnd the critical points and determine whether they are local minima, maxima, or neither. 43. f (x) = x + ex −x f (x) = e − x SOLUTION Setting f (x) = 1 − e−x equal to zero and solving for x gives e−x = 1 which is true if and only if x = 0. f (x) = e−x , so f (0) = e0 = 1 > 0. Therefore, x = 0 corresponds to a local minimum. 45. f (x) = x 2 e x x e for x > 0 f (x) = SOLUTION Setting f (x) = (x 2 + 2x)e x equal to zero and solving for x gives (x 2 + 2x)e x = 0 which is true if and x only if x = 0 or x = −2. Now, f (x) = (x 2 + 4x + 2)e x . Because f (0) = 2 > 0, x = 0 corresponds to a local minimum. On the other hand, f (−2) = (4 − 8 + 2)e−2 = −2/e2 < 0, so x = −2 corresponds to a local maximum. t 47. g(t) = (t 3 − 2t)e et g(t) = 2 SOLUTION t +1

g (t) = (t 3 − 2t)(et ) + et (3t 2 − 2) = et (t 3 + 3t 2 − 2t − 2). √ The critical points occur when t 3 + 3t 2 − 2t − 2 = 0. This occurs when t = 1, −2 ± 2. g (t) = et (3t 2 + 6t − 2) + et (t 3 + 3t 2 − 2t − 2) = et (t 3 + 6t 2 + 4t − 4). Thus, • g (1) = 7e > 0, so t = 1 corresponds to a local minimum; √ √ • g (−2 + 2) ≈ −2.497 < 0, so t = −2 + 2 corresponds to a local maximum; and √ √ • g (−2 − 2) ≈ .4108 > 0, so t = −2 − 2 corresponds to a local minimum.

In Exercises 48–53, ﬁnd the critical points and points of inﬂection. Then sketch the graph. 49. y = e−x +−x ex y = xe SOLUTION Let f (x) = e−x + e x . Then f (x) = −e−x + e x =

e2x − 1 . ex

Thus, x = 0 is a critical point, and f (x) is increasing for x > 0 and decreasing for x < 0. Observe that f (x) = e−x + e x > 0 for all x, so f (x) is concave up for all x and there are no points of inﬂection. A graph of y = f (x) is shown below.

S E C T I O N 7.1

Derivative of bx and the Number e

341

y

6 4

−2

x

−1

1

2

−x 51. y =y e= e−x cos x on [−π /2, π /2] 2

Let f (x) = e−x . Then f (x) = −2xe−x and x = 0 is the only critical point. Further, f (x) is increasing for x < 0 and decreasing for x > 0. Now, 2

SOLUTION

2

f (x) = 4x 2 e−x − 2e−x = 2e−x (2x 2 − 1), 2

2

√

2

√

√

so f (x) is concave up for |x| > 22 , is concave down for |x| < 22 and has points of inﬂection at x = ± 22 . A graph of y = f (x) is shown below. y 1

0.4 0.2 −2

x

−1

1

2

on [0, 10] 53. y = x 2 e−x y = ex − x SOLUTION Let f (x) = x 2 e−x . Then f (x) = −x 2 e−x + 2xe−x = x(2 − x)e−x , so x = 0 and x = 2 are critical points. Also, f (x) is increasing for 0 < x < 2 and decreasing for 2 < x < 10. Since f (x) = −(2x − x 2 )e−x + (2 − 2x)e−x = (x 2 − 4x + 2)e−x , √ √ √ √ we ﬁnd f (x) is concave up for 0 < x√< 2 − 2 and for 2 + 2 < x < 10, is concave down for 2 − 2 < x < 2 + 2, and has inﬂection points at x = 2 ± 2. A graph of y = f (x) is shown below. y 0.6 0.4 0.2 x 0

2

4

6

8

10

In Exercises 55–68, evaluate integral. Use Newton’s Methodthe to indeﬁnite ﬁnd the two solutions of e x = 5x to three decimal places (Figure 7). " 55. (e x + 2) d x " SOLUTION

" 57.

" 2 dy ye y 4x e dx

SOLUTION

" 59.

(e x + 2) d x = e x + 2x + C.

Use the substitution u = y 2 , du = 2y d y. Then " " 2 1 1 1 2 eu du = eu + C = e y + C. ye y d y = 2 2 2

" dt e−9t 4x (e + 1) d x

342

CHAPTER 7

THE EXPONENTIAL FUNCTION SOLUTION

" 61.

" dt et ext + 1−x (e + e ) d x

SOLUTION

" 63.

Use the substitution u = −9t, du = −9 dt. Then " " 1 1 1 eu du = − eu + C = − e−9t + C. e−9t dt = − 9 9 9

Use the substitution u = et + 1, du = et dt. Then " " √ 2 2 et et + 1 dt = u du = u 3/2 + C = (et + 1)3/2 + C. 3 3

" 10x ) d x (7 − e−x (e − 4x) d x

SOLUTION

First, observe that " " " " (7 − e10x ) d x = 7 d x − e10x d x = 7x − e10x d x.

In the remaining integral, use the substitution u = 10x, du = 10 d x. Then " " 1 1 u 1 10x eu du = e10x d x = + C. e +C = e 10 10 10 Finally, " (7 − e10x ) d x = 7x − " 65.

2 " −4x d xe4x xe e2x −

SOLUTION

" 67.

1 10x + C. e 10

dx ex Use the substitution u = −4x 2 , du = −8x d x. Then " " 2 2 1 1 1 eu du = − eu + C = − e−4x + C. xe−4x d x = − 8 8 8

" ex d xx √ x eex +cos(e 1 ) dx

SOLUTION

Use the substitution u = e x + 1, du = e x d x. Then " √

ex ex + 1

" dx =

√ √ du √ = 2 u + C = 2 e x + 1 + C. u

69. Find" an approximation to m 4 using the limit deﬁnition and estimate the slope of the tangent line to y = 4x at x = 0 and x = 2.e x (e2x + 1)3 d x SOLUTION

Recall

4h − 1 . h h→0

m 4 = lim Using a table of values, we ﬁnd h

4h − 1 h

.01 .001 .0001 .00001

1.39595 1.38726 1.38639 1.38630

Thus m 4 ≈ 1.386. Knowing that y (x) = m 4 · 4x , it follows that y (0) ≈ 1.386 and y (2) ≈ 1.386 · 16 = 22.176. 71. Find the area between y = e x and y = e−x over [0, 2]. Find the area between y = e x and y = e2x over [0, 1].

S E C T I O N 7.1 SOLUTION

Derivative of bx and the Number e

343

Over [0, 2], the graph of y = e x lies above the graph of y = e−x . Hence, the area between the graphs is 2 1 1 (e x − e−x ) d x = (e x + e−x ) = e2 + 2 − (1 + 1) = e2 − 2 + 2 . e e 0 0

" 2

73. Find the volume obtained by revolving y = e x about the x-axis for 0 ≤ x ≤ 1. Find the area bounded by y = e2 , y = e x , and x = 0. SOLUTION Each cross section of the solid is a disk with radius R = e x . The volume is then

π

" 1 0

e2x d x =

π 2x 1 π e = (e2 − 1). 2 2 0

x f (x). Show that if g(x) is another function satisfying g (x) = g(x), then g(x) = Ce forx some constant C. Hint: Compute the derivative of g(x)e−x . 75. Prove that f (x) = e is not a polynomial function. Hint: Differentiation lowers the degree of a polynomial by 1.

TheInsights function fand (x) =Challenges e satisﬁes f (x) = Further x

Assume f (x) = e x is a polynomial function of degree n. Then f (n+1) (x) = 0. But we know that any derivative of e x is e x and e x = 0. Hence, e x cannot be a polynomial function. SOLUTION

77. Generalize Exercise 76; that is, use induction (if you are familiar with this method of proof) to prove that for all Recall the following property of integrals: If f (t) ≥ g(t) for all t ≥ 0, then for all x ≥ 0, n ≥ 0, " x " x 1 2 f (t)1 dt3 ≥ 1 x e ≥ 1 + x + x + x + · · · +g(t) xdtn (x ≥ 0) 0 20 6 n! t

x ≥ 1 + x for x ≥ 0. Then prove, by e76. > Assume 1. Use (5) prove thatis etrue thetostatement for n = k. We need to prove the successive the1.following inequalities (for x ≥ 0): statement is trueintegration, for n = k + By the Induction Hypothesis, x ≥ 1for The inequality because SOLUTION For n e=≥1,1eholds + tx ≥by0 Exercise

x 2 k /k!. 1 1 2 1 3 x ≥ · · ·1+ x + x +xx + e x ≥ 1e +≥x 1++ xx 2+, x /2e+ 2 2 6 Integrating both sides of this inequality yields " x et dt = e x − 1 ≥ x + x 2 /2 + · · · + x k+1 /(k + 1)! 0

or e x ≥ 1 + x + x 2 /2 + · · · + x k+1 /(k + 1)! as required. 79. Calculate the ﬁrst three derivatives xof f (x) = xe x . Then guess the formula for f (n) (x) (use induction to prove it if x ex e you are Use familiar with this of proof). ≥ = ∞. Then use Exercise 77 to prove more and conclude that lim Exercise 76 tomethod show that x→∞ x 2 6 x2 x x x x x x x x SOLUTION f (x) = e e + xe , f (x) = e + e + xe = 2e + xe , f (x) = 2e x + e x + xe x = 3e x + xe x . So all n. generally that lim(n) n = ∞ for x→∞ one would guess that f x(x) = ne x + xe x . Assuming this is true for f (n) (x), we verify that f (n+1) (x) = ( f (n) (x)) = x x x x x ne + e + xe = (n + 1)e + xe . 81. Prove in two ways that the numbers m a satisfy Consider the equation e x = λ x, where λ is a constant. m a + mdraw intuition, a graph of y = e x and the line y = λ x. (a) For which λ does it have a unique solution?mFor ab = b (b) For which λ does it have at least one solution? (a) First method: Use the limit deﬁnition of m b and ah − 1 (ab)h − 1 bh − 1 h =b + h h h (b) Second method: Apply the Product Rule to a x b x = (ab)x . SOLUTION

(ab)h − 1 bh (a h − 1) bh − 1 ah − 1 bh − 1 = lim + = lim bh lim + lim h h h h h h→0 h→0 h→0 h→0 h→0

(a) m ab = lim

ah − 1 bh − 1 + lim = ma + mb. h h h→0 h→0

= lim

So, m ab = m a + m b . (b) m ab (ab)x = ((ab)x ) = (a x b x ) = (a x ) b x + (b x ) a x = m a a x b x + m b a x b x = a x b x (m a + m b ) = (ab)x (m a + m b ). Therefore, we have m ab (ab)x = (ab)x (m a + m b ). Dividing both sides by (ab)x , we see that m ab = m a + m b .

344

CHAPTER 7

THE EXPONENTIAL FUNCTION

7.2 Inverse Functions Preliminary Questions 1. (a) (c) (e)

Which of the following satisfy f −1 (x) = f (x)? f (x) = x f (x) = 1 f (x) = |x|

SOLUTION

(b) f (x) = 1 − x √ (d) f (x) = x (f) f (x) = x −1

The functions (a) f (x) = x, (b) f (x) = 1 − x and (f) f (x) = x −1 satisfy f −1 (x) = f (x).

2. The graph of a function looks like the track of a roller coaster. Is the function one-to-one? Because the graph looks like the track of a roller coaster, there will be several locations at which the graph has the same height. The graph will therefore fail the horizontal line test, meaning that the function is not one-to-one. SOLUTION

3. Consider the function f that maps teenagers in the United States to their last names. Explain why the inverse of f does not exist. SOLUTION Many different teenagers will have the same last name, so this function will not be one-to-one. Consequently, the function does not have an inverse.

4. View the following fragment of a train schedule for the New Jersey Transit System as deﬁning a function f from towns to times. Is f one-to-one? What is f −1 (6:27)?

SOLUTION

Trenton

6:21

Hamilton Township

6:27

Princeton Junction

6:34

New Brunswick

6:38

This function is one-to-one, and f −1 (6:27) = Hamilton Township.

5. A homework problem asks for a sketch of the graph of the inverse of f (x) = x + cos x. Frank, after trying but failing to ﬁnd a formula for f −1 (x), says it’s impossible to graph the inverse. Bianca hands in an accurate sketch without solving for f −1 . How did Bianca complete the problem? SOLUTION

The graph of the inverse function is the reﬂection of the graph of y = f (x) through the line y = x.

Exercises 1. Show that f (x) = 7x − 4 is invertible by ﬁnding its inverse. y+4 x +4 SOLUTION Solving y = 7x − 4 for x yields x = . Thus, f −1 (x) = . 7 7 3. What is the largest interval containing zero on which f (x) = sin x is one-to-one? Is f (x) = x 2 + 2 one-to-one? If not, describe a domain on which it is one-to-one. SOLUTION Looking at the graph of sin x, the function is one-to-one on the interval [−π /2, π /2]. 5. Verify that f (x) = x 3 x+− 3 2and g(x) = (x − 3)1/3 are inverses by showing that f (g(x)) = x and g( f (x)) = x. Show that f (x) = is invertible by ﬁnding its inverse. SOLUTION x +3

(a) What is the domain of3 f (x)? The range of f −1 (x)? • f (g(x)) = (x − 3)1/3 +−13 = x − 3 + 3 = x. (b) What is the

domain of f (x)?

The range of f (x)? • g( f (x)) =

x3 + 3 − 3

1/3

= x3

1/3

= x.

t + 1R is v(R) = t + 1 M and radius 7. TheRepeat escapeExercise velocity 5from and g(t) = . for af planet (t) = of mass t − 1 constant. Find the inverse of v(R) as a function of R. t − 1 SOLUTION

To ﬁnd the inverse, we solve y=

2G M R

R=

2G M . y2

for R. This yields

Therefore, v −1 (R) =

2G M . R2

2G M , where G is the universal gravitational R

S E C T I O N 7.2

Inverse Functions

345

In Exercises ﬁnd avertically domain in onthe which f islands one-to-one and alater. formula forofthe of f functions restrictedare to one-to-one? this domain. A ball9–16, is tossed air and T seconds Which theinverse following Sketch the graphs of f and f −1 . (a) The ball’s height s(t) for 0 ≤ t ≤ T ball’s 9. (b) f (x)The = 3x − 2velocity v(t) for 0 ≤ t ≤ T (c) The ball’s maximum height as a function of initial velocity SOLUTION The linear function f (x) = 3x − 2 is one-to-one for all real numbers. Solving y = 3x − 2 for x gives x = (y + 2)/3. Thus,

f −1 (x) =

x +2 . 3 y

f −1(x) =

x+2 3

−2

1 x

−1

1 −1 −2

f(x) = 3x − 2

1 11. f (x)f (x) = =4−x x +1 The graph of f (x) = 1/(x + 1) given below shows that f passes the horizontal line test, and is therefore 1 1 1 one-to-one, on its entire domain {x : x = −1}. Solving y = for x gives x = − 1. Thus, f −1 (x) = − 1. x +1 y x SOLUTION

y

y

4

4 y = f −1(x) 2

y = f(x) x

−4

2

−4

4

x

−2

−4

1 13. f (s) = 2 f (x)s=

1 7x − 3 SOLUTION To make f (s) = s −2 one-to-one, we must restrict the domain to either {s : s > 0} or {s : s < 0}. If we 1 1 1 choose the domain {s : s > 0}, then solving y = 2 for s yields s = √ . Hence, f −1 (s) = √ . Had we chosen the y s s 1 −1 domain {s : s < 0}, the inverse would have been f (s) = − √ . s y

y 8 6

4

4 y = f(s)

2 −2

−1

y = f −1(s)

2 s

1

2

s 1

2

3

4

15. f (z) = z 3 1 f (x) = 3 3 SOLUTION The x function 2 + 1 f (z) = z is one-to-one over its entire domain (see the graph below). Solving y = z for z 1/3 −1 1/3 yields y = z. Thus, f (z) = z .

346

CHAPTER 7

THE EXPONENTIAL FUNCTION y y = f(z) 3 2 y = f −1(z)

1

z

−3 −2 −1 −1

1

2

3

−2 −3

17. For each function shown in Figure 15, sketch the graph of the inverse (restrict the function’s domain if necessary). f (x) = x 3 + 9 y

y

y

x

x

x (A)

(B)

(C)

y

y

y

x x

x (E)

(D)

(F)

FIGURE 15

Here, we apply the rule that the graph of f −1 is obtained by reﬂecting the graph of f across the line y = x. For (C) and (D), we must restrict the domain of f to make f one-to-one. y (b) y (c) y (a) SOLUTION

x

x (A)

(d)

x

(B)

(e)

y

(C)

(f)

y

y

x x x (D)

(E)

(F)

19. Let n be a non-zero integer. Find a domain on which f (x) = (1 − x n )1/n coincides with its inverse. Hint: The Which ofon thewhether graphs ninisFigure 16odd. is the graph of a function satisfying f −1 = f ? answer depends even or SOLUTION

First note

1/n n 1/n = 1 − (1 − x n ) = (x n )1/n = x, f ( f (x)) = 1 − (1 − x n )1/n

so f (x) coincides with its inverse. For the domain and range of f , let’s ﬁrst consider the case when n > 0. If n is even, then f (x) is deﬁned only when 1 − x n ≥ 0. Hence, the domain is −1 ≤ x ≤ 1. The range is 0 ≤ y ≤ 1. If n is odd, then f (x) is deﬁned for all real numbers, and the range is also all real numbers. Now, suppose n < 0. Then −n > 0, and 1/−n 1 −1/−n x −n f (x) = 1 − −n = . x x −n − 1 If n is even, then f (x) is deﬁned only when x −n − 1 > 0. Hence, the domain is |x| > 1. The range is y > 1. If n is odd, then f (x) is deﬁned for all real numbers except x = 1. The range is all real numbers except y = 1. 21. Show that the inverse of f (x) = e−x exists (without ﬁnding it explicitly). What is the domain of f −1 ? Let f (x) = x 7 + x + 1. SOLUTION (a) Show that f −1 exists (but do not attempt to ﬁnd it). Hint: Show that f is strictly increasing. (b) What is the domain of f −1 ? (c) Find f −1 (3).

S E C T I O N 7.2

Inverse Functions

347

y

6 4 2

−2

y = e−x x

−1

1

2

Notice that the graph of f (x) = e−x (shown above) passes the horizontal line test. That is, f (x) = e−x is one-to-one. By Theorem 3, f −1 (x) exists and the domain of f −1 (x) is the range of f (x), namely (0, ∞) 23. Let f (x) = x 2 − 2x. Determine a domain on which f −1 exists and ﬁnd a formula for f −1 on this domain. Show that f (x) = (x 2 + 1)−1 is one-to-one on (−∞, 0] and ﬁnd a formula for f −1 for this domain. SOLUTION From the graph of y = x 2 − 2x shown below, we see that if the domain of f is restricted to either x ≤ 1 or x ≥ 1, then f is one-to-one and f −1 exists. To ﬁnd a formula for f −1 , we solve y = x 2 − 2x for x as follows: y + 1 = x 2 − 2x + 1 = (x − 1)2 x −1=± y+1 x =1± y+1 −1 If the √ domain of f is restricted to x ≤ 1, then we choose the negative sign in front of the radical and f −1(x) = 1 − √ x + 1. If the domain of f is restricted to x ≥ 1, we choose the positive sign in front of the radical and f (x) = 1 + x + 1. y y = x 2 − 2x 6 4 2 x

−1

2

3

two ways: using Theorem 2 and 25. Find the inverse g(x) of f (x) −1 = x 2 + 9 with domain x ≥ 0 and calculate g (x) in −1 Show that f (x) = x + x is one-to-one on [1, ∞) and ﬁnd a formula for f on this domain. What is the by direct calculation. −1 domain of f ? SOLUTION To ﬁnd a formula for g(x) = f −1 (x), solve y = x 2 + 9 for x. This yields x = ± y 2 − 9. Because the domain of f was restricted to x ≥ 0, we must choose the positive sign in front of the radical. Thus g(x) = f −1 (x) = x 2 − 9. Because x 2 + 9 ≥ 9 for all x, it follows that f (x) ≥ 3 for all x. Thus, the domain of g(x) = f −1 (x) is x ≥ 3. The range of g is the restricted domain of f : y ≥ 0. By Theorem 2, g (x) =

1 . f (g(x))

With f (x) = it follows that

x x2 + 9

x2 − 9 = f (g(x)) = ( x 2 − 9)2 + 9

,

x2 − 9 = √ x2

since the domain of g is x ≥ 3. Thus, g (x) =

1 f (g(x))

=

x x2 − 9

.

This agrees with the answer we obtain by differentiating directly: x 2x = . g (x) = 2 2 2 x −9 x −9

x2 − 9 x

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In Exercises 27–32, use Theorem 2 to calculate3 g (x), where g(x) is the inverse of f (x). Let g(x) be the inverse of f (x) = x + 1. Find a formula for g(x) and calculate g (x) in two ways: using Theorem 27. f (x) = 7x2 and + 6 then by direct calculation. Let f (x) = 7x + 6 then f (x) = 7. Solving y = 7x + 6 for x and switching variables, we obtain the inverse g(x) = (x − 6)/7. Thus, SOLUTION

1 1 = . f (g(x)) 7

g (x) =

29. f (x) = x −5√ f (x) = 3 − x SOLUTION Let f (x) = x −5 , then f (x) = −5x −6 . Solving y = x −5 for x and switching variables, we obtain the inverse g(x) = x −1/5 . Thus, g (x) =

1 1 = − x −6/5 . 5 −5(x −1/5 )−6

x 31. f (x) = f (x) x=+4x13 − 1 SOLUTION

x , then Let f (x) = x+1

(x + 1) − x 1 = . (x + 1)2 (x + 1)2

f (x) =

x for x and switching variables, we obtain the inverse g(x) = x . Thus Solving y = x+1 1−x

g (x) = 1

1 1 = . (x/(1 − x) + 1)2 (1 − x)2

33. Let g(x) be the inverse of f (x) = x 3 + 2x + 4. Calculate g(7) [without ﬁnding a formula for g(x)] and then −1 f (x) = 2 + x calculate g (7). SOLUTION

Let g(x) be the inverse of f (x) = x 3 + 2x + 4. Because f (1) = 13 + 2(1) + 4 = 7,

it follows that g(7) = 1. Moreover, f (x) = 3x 2 + 2, and g (7) =

1 1 1 = = . f (g(7)) f (1) 5

In Exercises 35–40, calculate g(b) and g (b), where g is the inverse of f (in the given domain, if indicated). x3 Find g (− 12 ), where g(x) is the inverse of f (x) = 2 . x +1 35. f (x) = x + cos x, b = 1 f (0) = 1, so g(1) = 0. f (x) = 1 − sin x so f (g(1)) = f (0) = 1 − sin 0 = 1. Thus, g (1) = 1/1 = 1. 37. f (x) = x 2 +3 6x for x ≥ 0, b = 4 f (x) = 4x − 2x, b = −2 SOLUTION To determine g(4), we solve f (x) = x 2 + 6x = 4 for x. This yields: SOLUTION

x 2 + 6x = 16 x 2 + 6x − 16 = 0 (x + 8)(x − 2) = 0 or x = −8, 2. Because the domain of f has been restricted to x ≥ 0, we have g(4) = 2. With f (x) =

x +3 x 2 + 6x

,

it then follows that g (4) = 1 1 = for x ≤ −6, 39. f (x)f (x) = = x,2 +b 6x x +1 4

b=4

1

1

4

= = . f (g(4)) f (2) 5

S E C T I O N 7.3 SOLUTION

f (3) = 1/4, so g(1/4) = 3. f (x) =

g (1/4) = −16.

Logarithms and Their Derivatives

349

−1 so f (g(1/4)) = f (3) = −1 2 = −1/16. Thus, (x+1)2 (3+1)

41. Let f (x) = xxn and g(x) = x 1/n . Compute g (x) using Theorem 2 and check your answer using the Power Rule. f (x) = e , b = e SOLUTION Note that g(x) = f −1 (x) (see problem 55 from 6.1). Therefore, g (x) =

1 f (g(x))

=

x 1/n−1 1 1 1 1 = = = = (x 1/n−1 ) n−1 1/n n−1 1−1/n n n n(g(x)) n(x ) n(x )

which agrees with the Power Rule. graphical reasoning to determine if the following statements are true or false. If false, modify the statement Further Use Insights and Challenges

to make it correct. 43. Show that if f (x) is odd and f −1 (x) exists, then f −1 (x) is odd. Show, on the other hand, that an even function does (a) If (x) is strictly increasing, then f −1 (x) is strictly decreasing. not have anfinverse. (b) If f (x) is strictly decreasing, then−1f −1 (x) is strictly decreasing. SOLUTION Suppose f (x) is odd and f (x) exists. Because f (x) is odd, f (−x) = − f (x). Let y = f −1 (x), then (c) If f (x) is concave up, then f −1 (x) is concave up. f (y) = x. Since f (x) is odd, f (−y) = − f (y) = −x. Thus f −1 (−x) = −y = − f −1 (x). Hence, f −1 is odd. is concave (d)the If other f (x) hand, is concave down, thenthen f −1 (x) On if f (x) is even, f (−x) = f (x).up. Hence, f is not one-to-one and f −1 does not exist. (e) Linear functions f (x) = ax + b (a = 0) are always one-to-one. (x). thatone-to-one. g (x) x −1 . We will this[a, in the next 45. Let gQuadratic bethe theIntermediate inverse of a function f satisfying f (x) + show bx + c (a= are always (f) Use polynomials f (x) = ax 2xto Value Theorem that if = ff0) (x) isShow continuous and= one-to-one on anapply interval b], then section to show that the inverse of f (x) = e (the natural logarithm) is an antiderivative of x −1 . f(g) (x)sin is xanisincreasing or a decreasing function. not one-to-one. SOLUTION

g (x) =

1 1 1 1 = −1 = = . f (g(x)) x f ( f (x)) f ( f −1 (x))

7.3 Logarithms and Their Derivatives Preliminary Questions 1. Compute logb2 (b4 ). SOLUTION

Because b4 = (b2 )2 , logb2 (b4 ) = 2.

2. When is ln x negative? SOLUTION

ln x is negative for 0 < x < 1.

3. What is ln(−3)? Explain. SOLUTION

ln(−3) is not deﬁned.

4. Explain the phrase “the logarithm converts multiplication into addition.” SOLUTION

This phrase is a verbal description of the general property of logarithms that states log(ab) = log a + log b.

5. What are the domain and range of ln x? SOLUTION

The domain of ln x is x > 0 and the range is all real numbers.

6. Does x −1 have an antiderivative for x < 0? If so, describe one. SOLUTION

Yes, ln(−x) is an antiderivative of x −1 for x < 0.

Exercises In Exercises 1–12, calculate directly (without using a calculator). 1. log3 27 SOLUTION

log3 27 = log3 33 = 3 log3 3 = 3.

3. log2 (25/31) log5 25 SOLUTION

log2 25/3 =

5. log64 4 log2 (85/3 )

5 5 log2 2 = . 3 3

350

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SOLUTION

log64 4 = log64 641/3 =

1 1 log64 64 = . 3 3

7. log8 2 + log4 2 log7 (492 ) 1/3 + log 41/2 = 1 + 1 = 5 . SOLUTION log8 2 + log4 2 = log8 8 4 3 2 6 9. log4 48 − log4 12 log25 30 + log25 65 48 SOLUTION log4 48 − log4 12 = log4 = log4 4 = 1. 12 11. ln(e3 )√ + ln(e4 ) ln( e · e7/5 ) SOLUTION ln(e3 ) + ln(e4 ) = 3 + 4 = 7. 13. Write as the natural log of a single expression: 4 (a) 2 ln 5log +23 3ln+ 4 log2 24

(b) 5 ln(x 1/2 ) + ln(9x)

SOLUTION

(a) 2 ln 5 + 3 ln 4 = ln 52 + ln 43 = ln 25 + ln 64 = ln(25 · 64) = ln 1600. (b) 5 ln x 1/2 + ln 9x = ln x 5/2 + ln 9x = ln(x 5/2 · 9x) = ln(9x 7/2 ). In Exercises 15–20, solve 2for the unknown. Solve for x: ln(x + 1) − 3 ln x = ln(2). 15. 7e5t = 100 SOLUTION

Divide the equation by 7 and then take the natural logarithm of both sides. This gives 1 100 100 or t = ln . 5t = ln 7 5 7

2 17. 2x −2x =8 6e−4t = 2 SOLUTION Since 8 = 23 , we have x 2 − 2x − 3 = 0 or (x − 3)(x + 1) = 0. Thus, x = −1 or x = 3.

19. ln(x 42t+1 ) − ln(x 2 1−t )=2 e = 9e SOLUTION

ln(x 4 ) − ln(x 2 ) = ln

x4 x2

= ln(x 2 ) = 2 ln x. Thus, 2 ln x = 2 or ln x = 1. Hence, x = e.

21. The population of a city millions) at time t (years) is P(t) = 2.4e0.06t , where t = 0 is the year 2000. When 2 ) =(in log y + 3 log (y 14 3 3 will the population double from its size at t = 0? SOLUTION

Population doubles when 4.8 = 2.4e.06t . Thus, 0.06t = ln 2 or t =

ln 2 ≈ 11.55 years. 0.06

In Exercises 23–40, ﬁnd theGutenberg–Richter derivative. According to the law, the number N of earthquakes worldwide of Richter magnitude M approximately satisﬁes a relation ln N = 16.17 − bM for some constant b. Find b, assuming that there are 800 23. y = x ln x earthquakes of magnitude M = 5 per year. How many earthquakes of magnitude M = 7 occur per year? d x SOLUTION x ln x = ln x + = ln x + 1. dx x 2 25. y =y (ln = tx)ln t − t 1 2 d SOLUTION (ln x)2 = (2 ln x) = ln x. dx x x

27. y = ln(x 3 +23x + 1) y = ln(x ) d 3x 2 + 3 . SOLUTION ln(x 3 + 3x + 1) = 3 dx x + 3x + 1 29. y = ln(sin t s+ 1) y = ln(2 ) cos t d ln(sin t + 1) = . SOLUTION dt sin t + 1 ln x 31. y =y = x 2 ln x x SOLUTION

1 (x) − ln x 1 − ln x d ln x = . = x dx x x2 x2

33. y = ln(ln x) 2 y = e(ln x)

S E C T I O N 7.3

SOLUTION

Logarithms and Their Derivatives

351

d 1 ln(ln x) = . dx x ln x

35. y = ln((ln x)3 ) 3 y = (ln(ln x)) 3(ln x)2 3 d ln((ln x)3 ) = . SOLUTION = dx x ln x x(ln x)3 Alternately, because ln((ln x)3 ) = 3 ln(ln x), d 1 d ln((ln x)3 ) = 3 ln(ln x) = 3 · . dx dx x ln x 37. y = ln(tan x) y = ln((x + 1)(2x + 9)) d 1 1 SOLUTION ln(tan x) = · sec2 x = . dx tan x sin x cos x 39. y = 5x x +1 y = lnd x3 + ln 1 5 · 5x . SOLUTION 5x = dx In Exercises 41–44, compute the derivative using Eq. (1). 2 y = 5x −x 41. f (x), f (x) = log2 x SOLUTION

f (x) = log2 x =

ln x 1 1 . Thus, f (x) = · . ln 2 x ln 2

43. f (3), f (x) = log5 x d 2 log10 (x 3 + ln xx ) 1 1 d SOLUTIONx f (x) = , so f (x) = . Thus, f (3) = . ln 5 x ln 5 3 ln 5 In Exercises d 45–56, ﬁnd an equation of the tangent line at the point indicated. log (sin t) dt= 4x3, x = 3 45. f (x) SOLUTION Let f (x) = 4 x . Then f (3) = 43 = 64. f (x) = ln 4 · 4 x , so f (3) = ln 4 · 64. Therefore, the equation of the tangent line is y = 64 ln 4(x − 3) + 64.

√ 47. s(t) = 37t , √t =x2 f (x) = ( 2) , x = 2 SOLUTION Let s(t) = 37t . Then s(2) = 314 . s (t) = 7 ln 3 · 37t , so s (2) = 7 ln 3 · 314 . Therefore the equation of the tangent line is y = 7 ln 3 · 314 (t − 2) + 314 . 49. f (x) = 5x −2x+9 , x =1 f (x) = π 3x+9 , x = 1 2 2 SOLUTION Let f (x) = 5 x −2x+9 . Then f (1) = 58 . f (x) = ln 5 · 5 x −2x+9 (2x − 2), so f (1) = ln 5(0) = 0. Therefore, the equation of the tangent line is y = 58 . 2

51. s(t) = ln(8 − 4t), t = 1 s(t) = ln t, t = 5 −4 SOLUTION Let s(t) = ln(8 − 4t). Then s(1) = ln(8 − 4) = ln 4. s (t) = 8−4t , so s (1) = −4/4 = −1. Therefore the equation of the tangent line is y = −1(t − 1) + ln 4. 53. f (x) = 4 ln(9x + 2), x = 2 f (x) = ln(x 2 ), x = 4 36 SOLUTION Let f (x) = 4 ln(9x + 2). Then f (2) = 4 ln 20. f (x) = 9x+2 , so f (2) = 9/5. Therefore the equation of the tangent line is y = (9/5)(x − 2) + 4 ln 20. 55. f (x) = log5 x, x = 2 π f (x) = ln(sin x), x = 2 1 1 4 SOLUTION Let f (x) = log5 x. Then f (2) = log5 2 = ln ln 5 . f (x) = x ln 5 , so f (2) = 2 ln 5 . Therefore the equation of the tangent line is y=

1 ln 2 . (x − 2) + 2 ln 5 ln 5

In Exercises 57–60, ﬁnd the−1 local extreme values in the domain {x : x > 0} and use the Second Derivative Test to f (x) = log2 (x + x ), x = 1 determine whether these values are local minima or maxima. 57. g(x) =

ln x x

352

CHAPTER 7

THE EXPONENTIAL FUNCTION SOLUTION

Let g(x) = lnxx . Then g (x) =

x(1/x) − ln x 1 − ln x = . 2 x x2

We know g (x) = 0 when 1 − ln x = 0, or when x = e. g (x) = so g (e) =

x 2 (−1/x) − (1 − ln x)(2x) −3 + 2 ln x = x4 x3

−1 < 0. Thus, g(e) is a local maximum. e3

59. g(x) = x − ln x ln x g(x) Let = g(x) 2 SOLUTION x 2 = x − ln x. Then g (x) = 1 − 1/x, and g (x) = 0 when x = 1. g (x) = 1/x , so g (1) = 1 > 0. Thus g(1) is a local minimum. In Exercises g(x)61–66, = x lnﬁnd x the derivative using the methods of Example 9. 61. f (x) = x 2x SOLUTION

Method 1: x 2x = e2x ln x , so d 2x x = e2x ln x (2 + 2 ln x) = x 2x (2 + 2 ln x). dx

Method 2: Let y = x 2x . Then, ln y = 2x ln x. By logarithmic differentiation 1 y = 2x · + 2 ln x, y x so y = y(2 + 2 ln x) = x 2x (2 + 2 ln x) . x

e 63. f (x)f (x) = x= x cos x x x SOLUTION Method 1: x e = ee ln x , so x d ex x = ee ln x dx

x x x e e + e x ln x = x e + e x ln x . x x

x Method 2: Let y = x e . Then ln y = e x ln x. By logarithmic differentiation

y 1 = e x · + e x ln x, y x so y = y

x x x e e + e x ln x = x e + e x ln x . x x

x

65. f (x) = x 2 x 2 f (x) = x x x SOLUTION Method 1: x 2 = e2 ln x , so x x x ln x x 2 2 d 2x 2 x 2 x x =e + (ln x)(ln 2)2 = x + (ln x)(ln 2)2 . dx x x x Method 2: Let y = x 2 . Then ln y = 2x ln x. By logarithmic differentiation

1 y = 2x + (ln x)(ln 2)2x , y x so x y = x 2

x 2 x + (ln x)(ln 2)2 . x

In Exercises 67–74,x xevaluate the derivative using logarithmic differentiation as in Example 8. f (x) = e

Logarithms and Their Derivatives

S E C T I O N 7.3

353

67. y = (x + 2)(x + 4) SOLUTION Let y = (x + 2)(x + 4). Then ln y = ln((x + 2)(x + 4)) = ln(x + 2) + ln(x + 4). By logarithmic differentiation

y 1 1 = + y x +2 x +4 or y = (x + 2)(x + 4)

1 1 + x +2 x +4

= (x + 4) + (x + 2) = 2x + 6.

x(x + 1)3 + 2)(x + 4) 69. y =y = (x + 1)(x (3x − 1)2 SOLUTION

3 Let y = x(x+1)2 . Then ln y = ln x + 3 ln(x + 1) − 2 ln(3x − 1). By logarithmic differentiation

(3x−1)

y 1 3 6 = + − , y x x + 1 3x − 1 so y =

(x + 1)3 3x(x + 1)2 6x(x + 1)3 + − . (3x − 1)2 (3x − 1)2 (3x − 1)3

√ 2) x − 9 71. y = (2x + 1)(4x 2 x(x + 1) y = Let √ y = (2x + 1)(4x 2 )√x − 9. Then SOLUTION x +1 ln y = ln(2x + 1) + ln 4x 2 + ln(x − 9)1/2 = ln(2x + 1) + ln 4 + 2 ln x +

1 ln(x − 9). 2

By logarithmic differentiation 2 2 1 y = + + , y 2x + 1 x 2(x − 9) so

√ y = (2x + 1)(4x 2 ) x − 9

2 2 1 + + . 2x + 1 x 2(x − 9)

73. y = (x 2 + 1)(x 2 + 2)(x 2 + 3)2 x(x + 2) y = Let y = (x 2 + 1)(x 2 + 2)(x 2 + 3)2 . Then ln y = ln(x 2 + 1) + ln(x 2 + 2) + 2 ln(x 2 + 3). By logarithmic SOLUTION (2x + 1)(2x + 2) differentiation y 2x 2x 4x = 2 + 2 + 2 , y x +1 x +2 x +3 so y = (x 2 + 1)(x 2 + 2)(x 2 + 3)2

2x 2x 4x + + . x2 + 1 x2 + 2 x2 + 3

In Exercises 75–78, x cosﬁnd x the local extrema and points of inﬂection, and sketch the graph of y = f (x) over the given interval. y = (x + 1) sin x 75. y = x 2 − ln x, SOLUTION

[ 12 , 2]

Let y = x 2 − ln x. Then y = 2x −

1 2x 2 − 1 = x x

√

√

√

and y = 2 + 12 . Thus, the function is decreasing on ( 12 , 22 ), is increasing on ( 22 , 2), has a local minimum at x = 22 , x and is concave up over the entire interval ( 12 , 2). A graph of y = x 2 − ln x is shown below.

354

CHAPTER 7

THE EXPONENTIAL FUNCTION y 3 2 1 x 0

0.5

1

1.5

2

2 + 4), [0, ∞) 77. y = ln(xln t y = 3 , [1, ∞)2 SOLUTION Let t y = ln(x + 4). Then

2x y = 2 x +4

y =

and

(x 2 + 4)(2) − 2x(2x) 8 − 2x 2 = 2 . 2 2 (x + 4) (x + 4)2

Thus, the function is increasing on (0, ∞), is concave up on (0, 2), is concave down on (2, ∞) and has a point of inﬂection at x = 2. A graph of y = ln(x 2 + 4) is shown below. y 4 3 2 1 x 0

2

4

6

8

In Exercises 79–96, evaluate the indeﬁnite integral, using substitution if necessary. y = 2−1/t , (0, 4) " dx 79. 2x + 4 SOLUTION

81.

Let u = 2x + 4. Then du = 2 d x, and " " 1 1 1 dx = du = ln |2x + 4| + C. 2x + 4 2 u 2

" "2 x dtxdt x3 + t 2 2+ 4

SOLUTION

Let u = x 3 + 2. Then du = 3x 2 d x, and "

x2 dx 1 = 3 3 x +2

"

du 1 = ln |x 3 + 2| + C. u 3

"

" tan(4x (3x+−1)1)d dx x 9 − 2x + 3x 2 ! ! sin(4x+1) SOLUTION First we rewrite tan(4x + 1) d x as cos(4x+1) d x. Let u = cos(4x + 1). Then du = −4 sin(4x + 1) d x, and " " sin(4x + 1) 1 du 1 dx = − = − ln | cos(4x + 1)| + C. cos(4x + 1) 4 u 4 83.

" 85.

" cos x dx 2 sin cotx x+d3x

SOLUTION

Let u = 2 sin x + 3. Then du = 2 cos x d x, and " " cos x 1 du 1 dx = = ln(2 sin x + 3) + C, 2 sin x + 3 2 u 2

where we have used the fact that 2 sin x + 3 ≥ 1 to drop the absolute value. " 4" ln x + 5 87. ln x d x x dx x

S E C T I O N 7.3 SOLUTION

Logarithms and Their Derivatives

355

Let u = 4 ln x + 5. Then du = (4/x)d x, and " " 1 1 1 4 ln x + 5 u du = u 2 + C = (4 ln x + 5)2 + C. dx = x 4 8 8

"

"d x 2 x ln(ln x x) d x x SOLUTION Let u = ln x. Then du = (1/x)d x, and " " dx 1 = du = ln |u| + C = ln | ln x| + C. x ln x u 89.

"

ln(ln x) " dx dx x ln x (4x − 1) ln(8x − 2) 1 1 · d x and SOLUTION Let u = ln(ln x). Then du = ln x x " " ln(ln x) u2 (ln(ln x))2 d x = u du = +C = + C. x ln x 2 2 91.

"

" 3x d x cot x ln(sin x) d x " 3x 3x d x = SOLUTION + C. ln 3 " " x dx 95. cos x 3xsin 2 x3 d x 93.

SOLUTION

Let u = sin x. Then du = cos x d x, and "

" cos x 3sin x d x =

3u du =

3u 3sin x +C = + C. ln 3 ln 3

In Exercises evaluate the deﬁnite integral. " 97–102, 1 3x+2 dx " 2 1 2 97. dx 1 x 2 " 2 1 SOLUTION d x = ln |x| = ln 2 − ln 1 = ln 2. x 1 1 " e 1 " 12 99. d x1 1 x dx e 4 "x e 1 SOLUTION d x = ln |x| = ln e − ln 1 = 1. x 1

" −e " 41 dtdt 101. −e2 t 4 2 3t "+ −e SOLUTION

1

−e e 1 = ln | − e| − ln | − e2 | = ln 2 = ln(1/e) = −1. dt = ln |t| 2 e −e2 t −e

103. " e2 Find a good numerical approximation to the coordinates of the point on the graph of y = ln x − x closest 1 10). to the origin (Figure dt e t ln t y 0.5 x 1

2

3

4

5

FIGURE 10 Graph of y = ln x − x.

356

CHAPTER 7

THE EXPONENTIAL FUNCTION

The distance from the origin to the point (x, ln x − x) on the graph of y = ln x − x is d = As usual, we will minimize d 2 . Let d 2 = f (x) = x 2 + (ln x − x)2 . Then 1 f (x) = 2x + 2(ln x − x) −1 . x SOLUTION

x 2 + (ln x − x)2 .

To determine x, we need to solve 4x +

2 ln x − 2 ln x − 2 = 0. x

This yields x ≈ .632784. Thus, the point on the graph of y = ln x − x that is closest to the origin is approximately (0.632784, −1.090410). x 105. the formula (ln of f (x)) forf x(x) > to 0. show that ln x and ln(2x) have the same derivative. Is there a Find Use the minimum value f (x)== fx (x)/ simpler explanation of this result? SOLUTION

Observe (ln x) =

1 x

and (ln 2x) =

2 1 = . 2x x

As an alternative explanation, note that ln(2x) = ln 2 + ln x. Hence, ln x and ln(2x) differ by a constant, which implies the two functions have the same derivative. 107. What is b if the slope of the tangent line to the curve y = b x at x = 0 is 4? Why is the area under the hyperbola y = 1/x between 1 and 3 equal to the area under the same hyperbola = ln b · b x so y (0) = ln b. Setting this equal to 4 and solving for b yields b = e4 . SOLUTION between y5 and 15? For which value of b is it also equal to the area between 10 and b? 109. The energy E (in joules) radiated as seismic waves from an earthquake of Richter magnitude M is given by the Let f E (x)==4.8 ln + x. 1.5M. Use the Linear Approximation to estimate f = ln(e2 + 0.1) − ln(e2 ). formula log 10 (a) Express E as a function of M. (b) Show that when M increases by 1, the energy increases by a factor of approximately 31. (c) Calculate d E/d M. SOLUTION

(a) Solving log10 E = 4.8 + 1.5M for E yields E = 104.8+1.5M . (b) Using the formula from part (a), we ﬁnd 106.3+1.5M 104.8+1.5(M+1) E(M + 1) = = 101.5 ≈ 31.6228. = E(M) 104.8+1.5M 104.8+1.5M (c) Again using the formula from part (a), dE = 1.5(ln 10)104.8+1.5M . dM

Further Insights and Challenges 111. Show that loga b logb a = 1. (a) Show that if f and g are differentiable, then ln b ln a ln b ln a and logb a = . Thus loga b · logb a = · = 1. SOLUTION loga b = (x)b f (x)ln a g ln ln a ln bd ln( f (x)g(x)) = + f (x) g(x) 113. Use Exercise 112 to verify the formula d x loga x Verify the formula logb x = for a, b > 0. d 1 ( f (x)g(x)) log b logb x =that the left-hand side of Eq. (5) is equal to . (b) Give a new proof of the Producta Rule by observing dx (ln b)x f (x)g(x) SOLUTION

d ln x 1 d logb x = = . dx d x ln b (ln b)x

115. Show that if f (x) is strictly increasing and satisﬁes f (x y) = f (x) + f (y), then its inverse g(x) satisﬁes g(x + Deﬁning ln x as an Integral Deﬁne a function ϕ (x) in the domain x > 0: y) = g(x)g(y). " x SOLUTION Let x = f (w) and y = f (z). Then 1 ϕ (x) = dt t g(x + y) = g( f (w) + f (z)) = g( f 1(wz)) = wz = g(x) · g(y). This exercise proceeds as if we didn’t know that ϕ (x) = ln x and shows directly that ϕ (x) has all the basic properties Exercises provide a the mathematically elegant approach to the exponential and logarithm functions, which avoids of the114–116 logarithm. Prove following statements: x for irrational " bofisdeﬁning " eab This a continuation of the previous exercises. g(x) be the inverse of ϕ (x). Show that the problem x and two of proving thatLet e x is differentiable. 1 1 (a) g(x)g(y) dt = dt for all a, b > 0. Hint: Use the substitution u = t/a. = g(x + y). t 1 t a r number. (b) g(r ) ==e ϕfor (ab) (a)any + ϕrational (b). Hint: Break up the integral from 1 to ab into two integrals and use (a). (b) ϕ

S E C T I O N 7.4

Exponential Growth and Decay

357

7.4 Exponential Growth and Decay Preliminary Questions 1. Two quantities increase exponentially with growth constants k = 1.2 and k = 3.4, respectively. Which quantity doubles more rapidly? SOLUTION Doubling time is inversely proportional to the growth constant. Consequently, the quantity with k = 3.4 doubles more rapidly.

2. If you are given both the doubling time and the growth constant of a quantity that increases exponentially, can you determine the initial amount? SOLUTION

No. To determine the initial amount, we need to know the amount at one instant in time.

3. A cell population grows exponentially beginning with one cell. Does it take less time for the population to increase from one to two cells than from 10 million to 20 million cells? SOLUTION Because growth from one cell to two cells and growth from 10 million to 20 million cells both involve a doubling of the population, both increases take exactly the same amount of time.

4. Referring to his popular book A Brief History of Time, the renowned physicist Stephen Hawking said, “Someone told me that each equation I included in the book would halve its sales.” If this is so, write a differential equation satisﬁed by the sales function S(n), where n is the number of equations in the book. SOLUTION

Let S(0) denote the sales with no equations in the book. Translating Hawking’s observation into an equation

yields S(n) =

S(0) . 2n

Differentiating with respect to n then yields dS d = S(0) 2−n = − ln 2S(0)2−n = − ln 2S(n). dn dn 5. Carbon dating is based on the assumption that the ratio R of C14 to C12 in the atmosphere has been constant over the past 50,000 years. If R were actually smaller in the past than it is today, would the age estimates produced by carbon dating be too ancient or too recent? SOLUTION If R were actually smaller in the past than it is today, then we would be overestimating the amount of decay and therefore overestimating the age. Our estimates would be too ancient.

Exercises 1. A certain bacteria population P obeys the exponential growth law P(t) = 2,000e1.3t (t in hours). (a) How many bacteria are present initially? (b) At what time will there be 10,000 bacteria? SOLUTION

(a) P(0) = 2000e0 = 2000 bacteria initially. (b) We solve 2000e1.3t = 10, 000 for t. Thus, e1.3t = 5 or t=

1 ln 5 ≈ 1.24 hours. 1.3

3. A certain RNA molecule has a doubling time of 3 minutes. Find the growth constant k and the differential equation A quantity P of obeys the exponential P(t) = e5tStarting (t in years). for the number N (t) molecules present atgrowth time t law (in minutes). with one molecule, how many will be present At what time t is P = 10? after(a) 10 min? ln 2 (b) At what time t is P = 20? ln 2 SOLUTION The doubling time is so k = . Thus, the differential equation is N (t) = k N (t) = doubling time (c) What is the doubling time forkP? ln 2 N (t). With one molecule initially, 3 N (t) = e(ln 2/3)t = 2t/3 . Thus, after ten minutes, there are N (10) = 210/3 ≈ 10.079, or 10 molecules present. A quantity P obeys the exponential growth law P(t) = Cekt (t in years). Find the formula for P(t), assuming that the doubling time is 7 years and P(0) = 100.

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5. The decay constant of Cobalt-60 is 0.13 years−1 . What is its half-life? SOLUTION

Half-life =

ln 2 ≈ 5.33 years. 0.13

7. Find all solutions to the differential equation y = −5y. Which solution satisﬁes the initial condition y(0) = 3.4? Find the decay constant of Radium-226, given that its half-life is 1,622 years. SOLUTION y = −5y, so y(t) = Ce−5t for some constant C. The initial condition y(0) = 3.4 determines C = 3.4. Therefore, y(t) = 3.4e−5t . y(2) = 4. 9. Find the solution to y = 3y satisfying √ Find the solution to y = 2y satisfying y(0) = 20. 4 SOLUTION y = 3y, so y(t) = Ce3t for some constant C. The initial condition y(2) = 4 determines C = . e6 4 3t Therefore, y(t) = 6 e = 4e3(t−2) . e 11. (a) (b) (c)

The population of a city is P(t) = 2 · e0.06t (in millions), where t is measured in years. Find the function y = f (t) that satisﬁes the differential equation y = −0.7y and initial condition y(0) = 10. Calculate the doubling time of the population. How long does it take for the population to triple in size? How long does it take for the population to quadruple in size?

SOLUTION

(a) Since k = 0.06, the doubling time is ln 2 ≈ 11.55 years. k (b) The tripling time is calculated in the same way as the doubling time. Solve for in the equation P(t + ) = 3P(t) 2 · e0.06(t+) = 3(2e0.06t ) 2 · e0.06t e0.06 = 3(2e0.06t ) e0.06 = 3 0.06 = ln 3, or = ln 3/0.06 ≈ 18.31 years. (c) Since the population doubles every 11.55 years, it quadruples after 2 × 11.55 = 23.10 years. 13. Assuming that population growth is approximately exponential, which of the two sets of data is most likely to The population of Washington state increased from 4.86 million in 1990 to 5.89 million in 2000. Assuming represent the population (in millions) of a city over a 5-year period? exponential growth, (a) What will the population be in 2010? Year 2000 2001 2002 2003 2004 (b) What is the doubling time? Data I 3.14 3.36 3.60 3.85 4.11 Data II 3.14 3.24 3.54 4.04 4.74 SOLUTION If the population growth is approximately exponential, then the ratio between successive years’ data needs to be approximately the same.

Year

2000

2001

2002

2003

2004

Data I Ratios

3.14 3.36 3.60 3.85 4.11 1.07006 1.07143 1.06944 1.06753

Data II Ratios

3.14 3.24 3.54 4.04 4.74 1.03185 1.09259 1.14124 1.17327

As you can see, the ratio of successive years in the data from “Data I” is very close to 1.07. Therefore, we would expect exponential growth of about P(t) ≈ (3.14)(1.07t ). 15. The Beer–Lambert Law is used in spectroscopy to determine the molar absorptivity α or the concentration c of Lightdissolved intensityin The intensity of light passing through an 10). absorbing medium exponentially withasthe a compound a solution at low concentrations (Figure The law states decreases that the intensity I of light it distance traveled. Suppose the decay constant for a certain plastic block is k = 2 when the distance is measured in passes through the solution satisﬁes ln(I /I0 ) = α cx, where I0 is the initial intensity and x is the distance traveled by the How must the block be toequation reduce the byI afor factor one-third? light.feet. Show thatthick I satisﬁes a differential d I intensity /d x = −k someofconstant k.

S E C T I O N 7.4

Exponential Growth and Decay

359

Intensity I

Distance

x

Solution

I0 0

x

FIGURE 10 Light of intensity passing through a solution.

SOLUTION

ln

I I0

= α cx so

I = eα cx or I = I0 eα cx . Therefore, I0 dI = I0 eα cx (α c) = I (α c) = −k I, dx

where k = −α c is a constant. 17. A 10-kg quantity of a radioactive isotope decays to 3 kg after 17 years. Find the decay constant of the isotope. An insect population triples in size after 5 months. Assuming exponential growth, when will it quadruple in size? ln(3/10) SOLUTION P(t) = 10e−kt . Thus P(17) = 3 = 10e−17k , so k = ≈ 0.071 years−1 . −17 19. Chauvet Caves In 1994, rock climbers in southern France stumbled on a cave containing prehistoric cave paint14 to C12 ratio showedout that samplearcheologist of sheepskinHelene parchment discovered bythat archaeologists had byaFrench Valladas showed the paintings area C between 29,700 ings. A Measurements C14 -analysis carried equal to 40% of that found in the atmosphere. Approximately how old is the parchment? and 32,400 years old, much older than any previously known human art. Given that the C14 to C12 ratio of the atmosphere is R = 10−12 , what range of C14 to C12 ratios did Valladas ﬁnd in the charcoal specimens? SOLUTION

The C14 -C12 ratio found in the specimens ranged from 10−12 e−0.000121(32400) ≈ 1.98 × 10−14

to 10−12 e−0.000121(29700) ≈ 2.75 × 10−14 . 21. Atmospheric Pressure The atmospheric pressure P(h) (in pounds per square inch) at a height h (in miles) above A paleontologist has discovered the remains of animals that appear to have died at the onset of the Holocene ice sea level on earth satisﬁes a differential equation P = −k P for some positive constant k. age. She applies carbon dating to test her theory that the Holocene age started between 10,000 and 12,000 years ago. (a) Measurements with a barometer show that P(0) =to14.7 P(10) = 2.13. What is the decay constant k? 14 to C12 ratio would she expect ﬁnd and in the animal remains? What range of C (b) Determine the atmospheric pressure 15 miles above sea level. SOLUTION

(a) Because P = −k P for some positive constant k, P(h) = Ce−kh where C = P(0) = 14.7. Therefore, P(h) = 14.7e−kh . We know that P(10) = 14.7e−10k = 2.13. Solving for k yields 2.13 1 k = − ln ≈ 0.193 miles−1 . 10 14.7 (b) P(15) = 14.7e−0.193(15) ≈ 0.813 pounds per square inch. 23. A quantity P increases exponentially with doubling time 6 hours. After how many hours has P increased by 50%? Inversion of Sugar When cane sugar is dissolved in water, it converts to invert sugar over a period of several ln 2 −1 . P willexponentially. −0.2 f. hours. TheThe percentage (t) ofisunconverted at time t decreases Suppose thatwhen f =1.5P SOLUTION doublingf time have increased by 50% = 6 socane k ≈sugar 0.1155 hours 0 = k What percentage of cane sugar remains after 5 hours? After 10 hours? ln 1.5 P0 e0.1155t , or when t = ≈ 3.5 hours. 0.1155 25. Moore’s Law In 1965, Gordon Moore predicted that the number N of transistors on a microchip would increase Two bacteria colonies are cultivated in a laboratory. The ﬁrst colony has a doubling time of 2 hours and the second exponentially. a doubling time of 3 hours. Initially, the ﬁrst colony contains 1,000 bacteria and the second colony 3,000 bacteria. (a) Does thetime tablet of data below conﬁrm Moore’s prediction for the period from 1971 to 2000? If so, estimate the growth At what will sizes of the colonies be equal? constant k. Plot the data in the table. (b) (c) Let N (t) be the number of transistors t years after 1971. Find an approximate formula N (t) ≈ Cekt , where t is the number of years after 1971. (d) Estimate the doubling time in Moore’s Law for the period from 1971 to 2000. (e) If Moore’s Law continues to hold until the end of the decade, how many transistors will a chip contain in 2010?

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(f) Can Moore have expected his prediction to hold indeﬁnitely? Transistors

Year

No. Transistors

4004 8008 8080 8086 286 386 processor 486 DX processor Pentium processor Pentium II processor Pentium III processor Pentium 4 processor

1971 1972 1974 1978 1982 1985 1989 1993 1997 1999 2000

2,250 2,500 5,000 29,000 120,000 275,000 1,180,000 3,100,000 7,500,000 24,000,000 42,000,000

SOLUTION

(a) Yes, the graph looks like an exponential graph especially towards the latter years. We estimate the growth constant by setting 1971 as our starting point, so P0 = 2250. Therefore, P(t) = 2250ekt . In 2000, t = 29. Therefore, P(29) = 2250e29k = 42000000, so k = ln 18666.67 ≈ 0.339. Note: A better estimate can be found by calculating k for each time 29 period and then averaging the k values. (b) y 4×10 7 3×10 7 2×10 7 1×10 7 x 1980 1985 1990 1995 2000

(c) (d) (e) (f)

N (t) = 2250e0.339t The doubling time is ln 2/0.339 ≈ 2.04 years. In 2010, t = 39 years. Therefore, N (39) = 2250e0.339(39) ≈ 1,241,623,327. No, you can’t make a microchip smaller than an atom.

In Exercises 27–28, the Gompertz differential equation: Assume that we in aconsider certain country, the rate at which jobs are created is proportional to the number of people who million jobs 3 months later, how many jobs will

y15.1 al