Stewart J. Calculus

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P R O B L E M S P L U S

202

11. The ﬁgure shows a rotating wheel with radius 40 cm and a connecting rod with length

1.2 m. The pin slides back and forth along the -axis as the wheel rotates counterclockwise

at a rate of 360 revolutions per minute.

(a) Find the angular velocity of the connecting rod, , in radians per second,

when .

(b) Express the distance in terms of .

12. Tangent lines and are drawn at two points and on the parabola and they

intersect at a point . Another tangent line is drawn at a point between and ; it

intersects at and at . Show that

13. Let and be the tangent and normal lines to the ellipse at any point on

the ellipse in the ﬁrst quadrant. Let and be the - and -intercepts of and and be

the intercepts of . As moves along the ellipse in the ﬁrst quadrant (but not on the axes),

what values can , , , and take on? First try to guess the answers just by looking at the

ﬁgure. Then use calculus to solve the problem and see how good your intuition is.

14. Evaluate .

15. (a) Use the identity for (see Equation 14b in Appendix D) to show that if two

lines and intersect at an angle , then

where and are the slopes of and , respectively.

(b) The angle between the curves and at a point of intersection is deﬁned to be the

angle between the tangent lines to and at (if these tangent lines exist). Use part (a)

to ﬁnd, correct to the nearest degree, the angle between each pair of curves at each point

of intersection.

(i) and

(ii) and

16. Let be a point on the parabola with focus . Let be the angle

between the parabola and the line segment , and let be the angle between the horizontal

line and the parabola as in the ﬁgure. Prove that . (Thus, by a principle of geo-

metrical optics, light from a source placed at will be reﬂected along a line parallel to the F

y ! y

F!p, 0"y

! 4pxP!x

, y

# 4x $ y

$ 3 ! 0x

# y

! 3

y ! !x # 2"

y ! x

tan

# m

1 $ m

tan !x # y"

lim

x l 0

sin!3 $ x"

# sin 9

Tyxy

#9 $ y

#4 ! 1NT

! 1

y ! x

x !

#dt

(x,0)

F I G U R E F O R P RO B LE M 1 1

P R O B L E M S P L U S

203

-axis. This explains why paraboloids, the surfaces obtained by rotating parabolas about their

axes, are used as the shape of some automobile headlights and mirrors for telescopes.)

17. Suppose that we replace the parabolic mirror of Problem 16 by a spherical mirror. Although

the mirror has no focus, we can show the existence of an approximate focus. In the ﬁgure,

is a semicircle with center . A ray of light coming in toward the mirror parallel to the axis

along the line will be reﬂected to the point on the axis so that (the

angle of incidence is equal to the angle of reﬂection). What happens to the point as is

taken closer and closer to the axis?

18. If and are differentiable functions with and , show that

19. Evaluate .

20. Given an ellipse , where , ﬁnd the equation of the set of all points

from which there are two tangents to the curve whose slopes are (a) reciprocals and (b) nega-

tive reciprocals.

21. Find the two points on the curve that have a common tangent line.

22. Suppose that three points on the parabola have the property that their normal lines

intersect at a common point. Show that the sum of their -coordinates is 0.

23. A lattice point in the plane is a point with integer coordinates. Suppose that circles with radius

are drawn using all lattice points as centers. Find the smallest value of such that any line

with slope intersects some of these circles.

24. A cone of radius centimeters and height centimeters is lowered point ﬁrst at a rate of

1 cm#s into a tall cylinder of radius centimeters that is partially ﬁlled with water. How fast is

the water level rising at the instant the cone is completely submerged?

25. A container in the shape of an inverted cone has height 16 cm and radius 5 cm at the top. It is

partially ﬁlled with a liquid that oozes through the sides at a rate proportional to the area of

the container that is in contact with the liquid. (The surface area of a cone is , where is

the radius and is the slant height.) If we pour the liquid into the container at a rate of

, then the height of the liquid decreases at a rate of 0.3 cm#min when the height is

10 cm. If our goal is to keep the liquid at a constant height of 10 cm, at what rate should we

pour the liquid into the container?

26. (a) The cubic function has three distinct zeros: 0, 2, and 6. Graph

and its tangent lines at the average of each pair of zeros. What do you notice?

(b) Suppose the cubic function has three distinct zeros:

, , and . Prove, with the help of a computer algebra system, that a tangent line drawn at

the average of the zeros intersects the graph of at the third zero.fa and b

cba

!x" ! !x # a"!x # b"!x # c"

!x" ! x!x # 2"!x # 6"

CAS

2 cm

#min

y ! x

# 2x

# x

a " bx

$ y

! 1

lim

x l 0

sin!a $ 2x" # 2 sin!a $ x" $ sin a

lim

f !x"

t!x"

f '!0"

t'!0"

t'!0" " 0f !0" ! t!0" ! 0tf

!PQO ! !OQRRPQ

F(p,0)

P(⁄,›)

¥=4px

y=›

∫

F I G U R E F O R P R O B L E M 1 7

204

We have already investigated some of the applications of derivatives, but now that we

know the differentiation rules we are in a better position to pursue the applications of

differentiation in greater depth. Here we learn how derivatives affect the shape of a

graph of a function and, in particular, how they help us locate maximum and minimum

values of functions. Many practical problems require us to minimize a cost or maximize

an area or somehow ﬁnd the best possible outcome of a situation. In particular, we will

be able to investigate the optimal shape of a can and to explain the location of rainbows

in the sky.

Calculus reveals all the important aspects of graphs of functions.

Members of the family of functions are illustrated.f !x" ! cx ! sin x

APPLICATIONS OF

DIFFERENTIATION

MAXIMUM AND MINIMUM VALUES

Some of the most important applications of differential calculus are optimization prob-

lems, in which we are required to ﬁnd the optimal (best) way of doing something. Here are

examples of such problems that we will solve in this chapter:

■

What is the shape of a can that minimizes manufacturing costs?

■

What is the maximum acceleration of a space shuttle? (This is an important

question to the astronauts who have to withstand the effects of acceleration.)

■

What is the radius of a contracted windpipe that expels air most rapidly during

a cough?

■

At what angle should blood vessels branch so as to minimize the energy expended

by the heart in pumping blood?

These problems can be reduced to ﬁnding the maximum or minimum values of a function.

Let’s ﬁrst explain exactly what we mean by maximum and minimum values.

DEFINITION A function has an absolute maximum (or global maximum)

at if for all in , where is the domain of . The number is

called the maximum value of on . Similarly, has an absolute minimum at

if for all in and the number is called the minimum value of

on . The maximum and minimum values of are called the extreme values of .

Figure 1 shows the graph of a function with absolute maximum at and absolute

minimum at . Note that is the highest point on the graph and is the low-

est point. If we consider only values of near [for instance, if we restrict our attention

to the interval ], then is the largest of those values of and is called a local

maximum value of . Likewise, is called a local minimum value of because

for near [in the interval , for instance]. The function also has a local

minimum at . In general, we have the following deﬁnition.

DEFINITION A function has a local maximum (or relative maximum) at

if when x is near c. [This means that for all in some

open interval containing c.] Similarly, has a local minimum at if

when is near c.

EXAMPLE 1 The function takes on its (local and absolute) maximum value

of 1 inﬁnitely many times, since for any integer and for

all . Likewise, is its minimum value, where is any integer. M

EXAMPLE 2 If , then because for all . Therefore

is the absolute (and local) minimum value of . This corresponds to the fact that the

origin is the lowest point on the parabola . (See Figure 2.) However, there is no

highest point on the parabola and so this function has no maximum value. M

EXAMPLE 3 From the graph of the function , shown in Figure 3, we see that

this function has neither an absolute maximum value nor an absolute minimum value. In

fact, it has no local extreme values either. M

f !x" ! x

y ! x

f !0" ! 0xx

" 0f !x" " f !0"f !x" ! x

ncos!2n ! 1"

! $1x

$1 % cos x % 1ncos 2n

! 1

f !x" ! cos x

f !c" % f !x"cf

xf !c" " f !x"f !c" " f !x"

f!b, d"cxf !c" % f !x"

ff !c"f

f !x"f !b"!a, c"

!a, f !a""!d, f !d""a

ffD

ff !c"Dxf !c" % f !x"

cfDf

f !c"fDDxf !c" " f !x"c

4.1

205

f(a)

f(d)

d ea c

F I G U R E 1

Minimum value f(a),

maximum value f(d)

F I G U R E 2

Minimum value 0, no maximum

y=≈

F I G U R E 3

No minimum, no maximum

y=˛

EXAMPLE 4 The graph of the function

is shown in Figure 4. You can see that is a local maximum, whereas the

absolute maximum is . (This absolute maximum is not a local maximum

because it occurs at an endpoint.) Also, is a local minimum and

is both a local and an absolute minimum. Note that has neither a local nor an absolute

maximum at .

We have seen that some functions have extreme values, whereas others do not. The

following theorem gives conditions under which a function is guaranteed to possess

extreme values.

THE EXTREME VALUE THEOREM If is continuous on a closed interval ,

then attains an absolute maximum value and an absolute minimum value

at some numbers and in .

The Extreme Value Theorem is illustrated in Figure 5. Note that an extreme value can

be taken on more than once. Although the Extreme Value Theorem is intuitively very plau-

sible, it is difﬁcult to prove and so we omit the proof.

Figures 6 and 7 show that a function need not possess extreme values if either hypoth-

esis (continuity or closed interval) is omitted from the Extreme Value Theorem.

The function f whose graph is shown in Figure 6 is deﬁned on the closed interval [0, 2]

but has no maximum value. (Notice that the range of f is [0, 3). The function takes on val-

ues arbitrarily close to 3, but never actually attains the value 3.) This does not contradict

the Extreme Value Theorem because f is not continuous. [Nonetheless, a discontinuous

function could have maximum and minimum values. See Exercise 13(b).]

F I G U R E 7

This continuous function g has

no maximum or minimum.

F I G U R E 6

This function has minimum value

f(2)=0, but no maximum value.

F I G U R E 5

ba c d

ba c¡ d c™

d=ba c

#a, b$dcf !d "

f !c"f

#a, b$f

x ! 4

f !3" ! $27f !0" ! 0

f !$1" ! 37

f !1" ! 5

$1 % x % 4f !x" ! 3x

$ 16x

! 18x

206

|| ||

CHAPTER 4 APPLICATIONS OF DIFFERENTIATION

F I G U R E 4

(_1,37)

1 2 3 4 5

(3,_27)

(1,5)

y=3x$-16˛+18≈

The function t shown in Figure 7 is continuous on the open interval (0, 2) but has nei-

ther a maximum nor a minimum value. [The range of t is . The function takes on

arbitrarily large values.] This does not contradict the Extreme Value Theorem because the

interval (0, 2) is not closed.

The Extreme Value Theorem says that a continuous function on a closed interval has a

maximum value and a minimum value, but it does not tell us how to ﬁnd these extreme

values. We start by looking for local extreme values.

Figure 8 shows the graph of a function with a local maximum at and a local minimum

at . It appears that at the maximum and minimum points the tangent lines are horizontal

and therefore each has slope 0. We know that the derivative is the slope of the tangent line,

so it appears that and . The following theorem says that this is always

true for differentiable functions.

FERMAT’S THEOREM If has a local maximum or minimum at , and if

exists, then .

PROOF Suppose, for the sake of deﬁniteness, that has a local maximum at c. Then,

according to Deﬁnition 2, if is sufﬁciently close to . This implies that if

is sufﬁciently close to 0, with being positive or negative, then

and therefore

We can divide both sides of an inequality by a positive number. Thus, if and is

sufﬁciently small, we have

Taking the right-hand limit of both sides of this inequality (using Theorem 2.3.2), we get

But since exists, we have

and so we have shown that .

If , then the direction of the inequality (5) is reversed when we divide by :

So, taking the left-hand limit, we have

f &!c" ! lim

f !c ! h" $ f !c"

! lim

f !c ! h" $ f !c"

" 0

f !c ! h" $ f !c"

" 0

f &!c" % 0

f &!c" ! lim

f !c ! h" $ f !c"

! lim

f !c ! h" $ f !c"

f &!c"

lim

f !c ! h" $ f !c"

% lim

0 ! 0

f !c ! h" $ f !c"

% 0

hh ( 0

f !c ! h" $ f !c" % 0

f !c" " f !c ! h"

cxf !c" " f !x"

f &!c" ! 0

f &!c"cf

f &!d" ! 0f &!c" ! 0

!1, )"

SECTION 4.1 MAXIMUM AND MINIMUM VALUES

|| ||

207

N Fermat’s Theorem is named after Pierre

Fermat (1601–1665), a French lawyer who took

up mathematics as a hobby. Despite his amateur

status, Fermat was one of the two inventors of

analytic geometry (Descartes was the other). His

methods for ﬁnding tangents to curves and maxi-

mum and minimum values (before the invention

of limits and derivatives) made him a forerunner

of Newton in the creation of differential calculus.

{

c, f(c)

}

{

d, f(d)

}

F I G U R E 8

We have shown that and also that . Since both of these inequalities

must be true, the only possibility is that .

We have proved Fermat’s Theorem for the case of a local maximum. The case of a

local minimum can be proved in a similar manner, or we could use Exercise 70 to

deduce it from the case we have just proved (see Exercise 71). M

The following examples caution us against reading too much into Fermat’s Theorem.

We can’t expect to locate extreme values simply by setting and solving for .

EXAMPLE 5 If , then , so . But has no maximum or

minimum at 0, as you can see from its graph in Figure 9. (Or observe that for

but for .) The fact that simply means that the curve

has a horizontal tangent at . Instead of having a maximum or minimum at ,

the curve crosses its horizontal tangent there.

EXAMPLE 6 The function has its (local and absolute) minimum value at 0,

but that value can’t be found by setting because, as was shown in Example 5

in Section 3.2, does not exist. (See Figure 10.) M

WARNING

Examples 5 and 6 show that we must be careful when using Fermat’s

Theorem. Example 5 demonstrates that even when there need not be a maximum

or minimum at . (In other words, the converse of Fermat’s Theorem is false in general.)

Furthermore, there may be an extreme value even when does not exist (as in

Example 6).

Fermat’s Theorem does suggest that we should at least start looking for extreme values

of at the numbers where or where does not exist. Such numbers are

given a special name.

DEFINITION A critical number of a function is a number in the domain of

such that either or does not exist.

EXAMPLE 7 Find the critical numbers of .

SOLUTION The Product Rule gives

[The same result could be obtained by ﬁrst writing .] Therefore

if , that is, , and does not exist when . Thus the

critical numbers are and .

In terms of critical numbers, Fermat’s Theorem can be rephrased as follows (compare

Deﬁnition 6 with Theorem 4):

If f has a local maximum or minimum at c, then c is a critical number of f.

x ! 0f &!x"x !

12 $ 8x ! 0f &!x" ! 0

f !x" ! 4x

3%5

$ x

8%5

$5x ! 3!4 $ x"

2%5

12 $ 8x

2%5

f &!x" ! x

3%5

!$1" ! !4 $ x"

(

$2%5

)

! $x

3%5

3!4 $ x"

2%5

f !x" ! x

3%5

!4 $ x"

f &!c"f &!c" ! 0f

f &!c"f &!c" ! 0cf

f &!c"

f &!c" ! 0

f &!0"

f &!x" ! 0

f !x" !

!0, 0"!0, 0"

y ! x

f &!0" ! 0x

0x ( 0

( 0

ff &!0" ! 0f &!x" ! 3x

f !x" ! x

xf &!x" ! 0

f &!c" ! 0

f &!c" % 0f &!c" " 0

208

|| ||

CHAPTER 4 APPLICATIONS OF DIFFERENTIATION

If ƒ=˛, then fª(0)=0 but ƒ

has no maximum or minimum.

y=˛

F I G U R E 1 0

If ƒ=|x|, then f(0)=0 is a

minimum value, but fª(0) does not exist.

y=|x|

F I G U R E 1 1

3.5

_0.5 5

N Figure 11 shows a graph of the function

in Example 7. It supports our answer because

there is a horizontal tangent when and

a vertical tangent when .x ! 0

x ! 1.5

To ﬁnd an absolute maximum or minimum of a continuous function on a closed interval,

we note that either it is local [in which case it occurs at a critical number by (7)] or it occurs

at an endpoint of the interval. Thus the following three-step procedure always works.

THE CLOSED INTERVAL METHOD To ﬁnd the absolute maximum and minimum

values of a continuous function on a closed interval :

1. Find the values of at the critical numbers of in .

2. Find the values of at the endpoints of the interval.

3. The largest of the values from Steps 1 and 2 is the absolute maximum value;

the smallest of these values is the absolute minimum value.

EXAMPLE 8 Find the absolute maximum and minimum values of the function

SOLUTION Since is continuous on , we can use the Closed Interval Method:

Since exists for all , the only critical numbers of occur when , that is,

or . Notice that each of these critical numbers lies in the interval .

The values of at these critical numbers are

The values of at the endpoints of the interval are

Comparing these four numbers, we see that the absolute maximum value is

and the absolute minimum value is .

Note that in this example the absolute maximum occurs at an endpoint, whereas the

absolute minimum occurs at a critical number. The graph of is sketched in Figure 12. M

If you have a graphing calculator or a computer with graphing software, it is possible

to estimate maximum and minimum values very easily. But, as the next example shows,

calculus is needed to ﬁnd the exact values.

EXAMPLE 9

(a) Use a graphing device to estimate the absolute minimum and maximum values of

the function .

(b) Use calculus to ﬁnd the exact minimum and maximum values.

SOLUTION

(a) Figure 13 shows a graph of in the viewing rectangle by . By mov-

ing the cursor close to the maximum point, we see that the -coordinates don’t change

very much in the vicinity of the maximum. The absolute maximum value is about 6.97

and it occurs when . Similarly, by moving the cursor close to the minimum point,

we see that the absolute minimum value is about and it occurs when . It is x ' 1.0$0.68

x ' 5.2

#$1, 8$#0, 2

f !x" ! x $ 2 sin x, 0 % x % 2

f !2" ! $3

f !4" ! 17

f !4" ! 17f

(

)

f !2" ! $3f !0" ! 1

(

, 4

)

x ! 2x ! 0

f &!x" ! 0fxf &!x"

f &!x" ! 3x

$ 6x ! 3x!x $ 2"

f !x" ! x

$ 3x

! 1

[

, 4

]

% x % 4f !x" ! x

$ 3x

! 1

!a, b"ff

#a, b$f

SECTION 4.1 MAXIMUM AND MINIMUM VALUES

|| ||

209

F I G U R E 1 2

1 2

3 4

(4,17)

(2,_3)

y=˛-3≈+1

F I G U R E 1 3

2π

possible to get more accurate estimates by zooming in toward the maximum and mini-

mum points, but instead let’s use calculus.

(b) The function is continuous on . Since ,

we have when and this occurs when or . The values

of at these critical points are

and

The values of at the endpoints are

Comparing these four numbers and using the Closed Interval Method, we see that the

absolute minimum value is and the absolute maximum value is

. The values from part (a) serve as a check on our work.

EXAMPLE 10 The Hubble Space Telescope was deployed on April 24, 1990, by the space

shuttle Discovery. A model for the velocity of the shuttle during this mission, from liftoff

at until the solid rocket boosters were jettisoned at , is given by

(in feet per second). Using this model, estimate the absolute maximum and minimum

values of the acceleration of the shuttle between liftoff and the jettisoning of the boosters.

SOLUTION We are asked for the extreme values not of the given velocity function, but

rather of the acceleration function. So we ﬁrst need to differentiate to ﬁnd the acceleration:

We now apply the Closed Interval Method to the continuous function a on the interval

. Its derivative is

The only critical number occurs when :

Evaluating at the critical number and at the endpoints, we have

So the maximum acceleration is about and the minimum acceleration is

about .

21.52 ft%s

62.87 ft%s

a!126" ' 62.87a!t

" ' 21.52a!0" ! 23.61

a!t"

0.18058

0.007812

' 23.12

a&!t" ! 0

a&!t" ! 0.007812t $ 0.18058

0 % t % 126

! 0.003906t

$ 0.18058t ! 23.61

a!t" !

v&!t" !

!0.001302t

$ 0.09029t

! 23.61t $ 3.083"

v!t" ! 0.001302t

$ 0.09029t

! 23.61t $ 3.083

t ! 126 st ! 0

f !5

%3" ! 5

%3 !

f !

%3" !

%3 $

f !2

" ! 2

' 6.28andf !0" ! 0

f !5

%3" !

$ 2 sin

' 6.968039

f !

%3" !

$ 2 sin

' $0.684853

%3x !

%3cos x !

f &!x" ! 0

f &!x" ! 1 $ 2 cos x#0, 2

$f !x" ! x $ 2 sin x

210

|| ||

CHAPTER 4 APPLICATIONS OF DIFFERENTIATION

NASA

SECTION 4.1 MAXIMUM AND MINIMUM VALUES

|| ||

211

at 2 and is not continuous at 2.

12. (a) Sketch the graph of a function on [$1, 2] that has an

absolute maximum but no local maximum.

(b) Sketch the graph of a function on [$1, 2] that has a local

maximum but no absolute maximum.

(a) Sketch the graph of a function on [$1, 2] that has an

absolute maximum but no absolute minimum.

(b) Sketch the graph of a function on [$1, 2] that is discontin-

uous but has both an absolute maximum and an absolute

minimum.

14. (a) Sketch the graph of a function that has two local maxima,

one local minimum, and no absolute minimum.

(b) Sketch the graph of a function that has three local minima,

two local maxima, and seven critical numbers.

15–28 Sketch the graph of by hand and use your sketch to

ﬁnd the absolute and local maximum and minimum values of .

(Use the graphs and transformations of Sections 1.2 and 1.3.)

15. ,

16. ,

17. ,

18. ,

19. ,

20. ,

21. ,

22. ,

23. ,

24. ,

26.

27.

28.

29– 42 Find the critical numbers of the function.

29. 30.

31. 32.

33. 34.

35. 36.

h!p" !

p $ 1

! 4

t!y" !

y $ 1

$ y ! 1

t!t" !

3t $ 4

s!t" ! 3t

! 4t

$ 6t

f !x" ! x

! x

! xf !x" ! x

! 3x

$ 24x

f !x" ! x

! x

$ xf !x" ! 5x

! 4x

f !x" !

(

4 $ x

2x $ 1

if $2 % x

if 0 % x % 2

f !x" !

(

1 $ x

2x $ 4

if 0 % x

if 2 % x % 3

f !x" ! 1 $ x

f !x" ! 1 $

25.

%2 % t % 3

%2f !t" ! cos t

f !t" ! 1%t

$2 % x

f !x" ! 1 ! !x ! 1"

$3 % x % 2f !x" ! x

0 % x % 2f !x" ! x

0 % x

f !x" ! x

x % 2f !x" ! x

f !x" ! x

x % 5f !x" ! 3 $ 2x

x " 1f !x" ! 8 $ 3x

13.

1. Explain the difference between an absolute minimum and a

local minimum.

2. Suppose is a continuous function deﬁned on a closed

interval .

(a) What theorem guarantees the existence of an absolute max-

imum value and an absolute minimum value for ?

(b) What steps would you take to ﬁnd those maximum and

minimum values?

3– 4 For each of the numbers a, b, c, d, r, and s, state whether the

function whose graph is shown has an absolute maximum or min-

imum, a local maximum or minimum, or neither a maximum

nor a minimum.

3. 4.

5–6 Use the graph to state the absolute and local maximum and

minimum values of the function.

5. 6.

7–10 Sketch the graph of a function that is continuous on

[1, 5] and has the given properties.

7. Absolute minimum at 2, absolute maximum at 3,

local minimum at 4

8. Absolute minimum at 1, absolute maximum at 5,

local maximum at 2, local minimum at 4

Absolute maximum at 5, absolute minimum at 2,

local maximum at 3, local minima at 2 and 4

10. has no local maximum or minimum, but 2 and 4 are critical

numbers

(a) Sketch the graph of a function that has a local maximum

at 2 and is differentiable at 2.

(b) Sketch the graph of a function that has a local maximum

at 2 and is continuous but not differentiable at 2.

11.

0 x

y=©

0 x

y=ƒ

a b c d r s

#a, b$

E X E R C I S E S

4.1