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11: Applications of Taylor Polynomials

11: Applications of Taylor Polynomials

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97817_11_ch11_p792-801.qk_97817_11_ch11_p792-801 11/3/10 5:33 PM Page 793



APPLICATIONS OF TAYLOR POLYNOMIALS



SECTION 11.11



x ෇ 0.2



x ෇ 3.0



T2͑x͒

T4͑x͒

T6͑x͒

T8͑x͒

T10͑x͒



1.220000

1.221400

1.221403

1.221403

1.221403



8.500000

16.375000

19.412500

20.009152

20.079665



ex



1.221403



20.085537



793



The values in the table give a numerical demonstration of the convergence of the Taylor

polynomials Tn͑x͒ to the function y ෇ e x. We see that when x ෇ 0.2 the convergence is very

rapid, but when x ෇ 3 it is somewhat slower. In fact, the farther x is from 0, the more slowly

Tn͑x͒ converges to e x.

When using a Taylor polynomial Tn to approximate a function f, we have to ask the questions: How good an approximation is it? How large should we take n to be in order

to achieve a desired accuracy? To answer these questions we need to look at the absolute

value of the remainder:



Խ R ͑x͒ Խ ෇ Խ f ͑x͒ Ϫ T ͑x͒ Խ

n



n



There are three possible methods for estimating the size of the error:



Խ



Խ



1. If a graphing device is available, we can use it to graph Rn͑x͒ and thereby esti-



mate the error.



2. If the series happens to be an alternating series, we can use the Alternating Series



Estimation Theorem.

3. In all cases we can use Taylor’s Inequality (Theorem 11.10.9), which says that if



Խf



͑nϩ1͒



Խ



͑x͒ ഛ M , then

M



Խ R ͑x͒ Խ ഛ ͑n ϩ 1͒! Խ x Ϫ a Խ



nϩ1



n



v



EXAMPLE 1



3

(a) Approximate the function f ͑x͒ ෇ s

x by a Taylor polynomial of degree 2 at a ෇ 8.

(b) How accurate is this approximation when 7 ഛ x ഛ 9?



SOLUTION



(a)



3

f ͑x͒ ෇ s

x ෇ x 1͞3



f ͑8͒ ෇ 2



f Ј͑x͒ ෇ 13 xϪ2͞3



f Ј͑8͒ ෇ 121



f Љ͑x͒ ෇ Ϫ 29 xϪ5͞3



1

f Љ͑8͒ ෇ Ϫ 144



Ϫ8͞3

f ٞ͑x͒ ෇ 10

27 x



Thus the second-degree Taylor polynomial is

T2͑x͒ ෇ f ͑8͒ ϩ



f Ј͑8͒

f Љ͑8͒

͑x Ϫ 8͒ ϩ

͑x Ϫ 8͒2

1!

2!



1

෇ 2 ϩ 121 ͑x Ϫ 8͒ Ϫ 288

͑x Ϫ 8͒2



The desired approximation is

1

3

x Ϸ T2͑x͒ ෇ 2 ϩ 121 ͑x Ϫ 8͒ Ϫ 288

͑x Ϫ 8͒2

s



(b) The Taylor series is not alternating when x Ͻ 8, so we can’t use the Alternating

Series Estimation Theorem in this example. But we can use Taylor’s Inequality with

n ෇ 2 and a ෇ 8:

M

R2͑x͒ ഛ

xϪ8 3

3!



Խ



Խ



Խ



Խ



Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).

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97817_11_ch11_p792-801.qk_97817_11_ch11_p792-801 11/3/10 5:33 PM Page 794



794



CHAPTER 11



INFINITE SEQUENCES AND SERIES



Խ



Խ



where f ٞ͑x͒ ഛ M. Because x ജ 7, we have x 8͞3 ജ 7 8͞3 and so

10

1

10

1

ؒ



ؒ

Ͻ 0.0021

27 x 8͞3

27 7 8͞3



f ٞ͑x͒ ෇

2.5



Therefore we can take M ෇ 0.0021. Also 7 ഛ x ഛ 9, so Ϫ1 ഛ x Ϫ 8 ഛ 1 and

x Ϫ 8 ഛ 1. Then Taylor’s Inequality gives



Խ



T™



Խ



Խ R ͑x͒ Խ ഛ

2



#x



y= œ

15



0



FIGURE 2



0.0021

0.0021

ؒ 13 ෇

Ͻ 0.0004

3!

6



Thus, if 7 ഛ x ഛ 9, the approximation in part (a) is accurate to within 0.0004.

Let’s use a graphing device to check the calculation in Example 1. Figure 2 shows that

3

the graphs of y ෇ s

x and y ෇ T2͑x͒ are very close to each other when x is near 8. Figure 3 shows the graph of R2͑x͒ computed from the expression



Խ



0.0003



Խ

Խ R ͑x͒ Խ ෇ Խ sx Ϫ T ͑x͒ Խ

3



2



y=|R™(x)|



2



We see from the graph that



Խ R ͑x͒ Խ Ͻ 0.0003

2



7



9

0



FIGURE 3



when 7 ഛ x ഛ 9. Thus the error estimate from graphical methods is slightly better than the

error estimate from Taylor’s Inequality in this case.



v



EXAMPLE 2



(a) What is the maximum error possible in using the approximation

sin x Ϸ x Ϫ



x5

x3

ϩ

3!

5!



when Ϫ0.3 ഛ x ഛ 0.3? Use this approximation to find sin 12Њ correct to six decimal

places.

(b) For what values of x is this approximation accurate to within 0.00005?

SOLUTION



(a) Notice that the Maclaurin series

sin x ෇ x Ϫ



x3

x5

x7

ϩ

Ϫ

ϩ иии

3!

5!

7!



is alternating for all nonzero values of x, and the successive terms decrease in size

because x Ͻ 1, so we can use the Alternating Series Estimation Theorem. The error

in approximating sin x by the first three terms of its Maclaurin series is at most



Խ Խ



Ϳ Ϳ



Խ Խ



x7

x 7



7!

5040



Խ Խ



If Ϫ0.3 ഛ x ഛ 0.3, then x ഛ 0.3, so the error is smaller than

͑0.3͒7

Ϸ 4.3 ϫ 10Ϫ8

5040



Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).

Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.



97817_11_ch11_p792-801.qk_97817_11_ch11_p792-801 11/3/10 5:33 PM Page 795



APPLICATIONS OF TAYLOR POLYNOMIALS



SECTION 11.11



795



To find sin 12Њ we first convert to radian measure:

sin 12Њ ෇ sin

Ϸ



ͩ ͪ ͩ ͪ

ͩ ͪ ͩ ͪ

12␲

180





Ϫ

15





15



෇ sin





15



3



1

ϩ

3!





15



5



1

Ϸ 0.20791169

5!



Thus, correct to six decimal places, sin 12Њ Ϸ 0.207912.

(b) The error will be smaller than 0.00005 if



ԽxԽ



7



5040



Ͻ 0.00005



Solving this inequality for x, we get



ԽxԽ



7



Ͻ 0.252



Խ x Խ Ͻ ͑0.252͒



1͞7



or



Ϸ 0.821



Խ Խ



So the given approximation is accurate to within 0.00005 when x Ͻ 0.82.

TEC Module 11.10/11.11 graphically



Խ



shows the remainders in Taylor polynomial

approximations.

4.3 ϫ 10–*



What if we use Taylor’s Inequality to solve Example 2? Since f ͑7͒͑x͒ ෇ Ϫcos x, we have

f ͑x͒ ഛ 1 and so

1

R6͑x͒ ഛ

x 7

7!

͑7͒



Խ



Խ



Խ



Խ Խ



So we get the same estimates as with the Alternating Series Estimation Theorem.

What about graphical methods? Figure 4 shows the graph of



y=| Rß(x)|



Խ R ͑x͒ Խ ෇ Խ sin x Ϫ ( x Ϫ x ϩ x ) Խ

and we see from it that Խ R ͑x͒ Խ Ͻ 4.3 ϫ 10 when Խ x Խ ഛ 0.3. This is the same estimate

that we obtained in Example 2. For part (b) we want Խ R ͑x͒ Խ Ͻ 0.00005, so we graph both

y ෇ Խ R ͑x͒ Խ and y ෇ 0.00005 in Figure 5. By placing the cursor on the right intersection

point we find that the inequality is satisfied when Խ x Խ Ͻ 0.82. Again this is the same esti1

6



6



0.3

0



6



6



FIGURE 4



mate that we obtained in the solution to Example 2.

If we had been asked to approximate sin 72Њ instead of sin 12Њ in Example 2, it would

have been wise to use the Taylor polynomials at a ෇ ␲͞3 (instead of a ෇ 0) because they

are better approximations to sin x for values of x close to ␲͞3. Notice that 72Њ is close to

60Њ (or ␲͞3 radians) and the derivatives of sin x are easy to compute at ␲͞3.

Figure 6 shows the graphs of the Maclaurin polynomial approximations



0.00006

y=0.00005



y=| Rß(x)|



_1



5



Ϫ8



6



_0.3



1

120



3



1



T1͑x͒ ෇ x



0



T5͑x͒ ෇ x Ϫ



FIGURE 5



x3

x5

ϩ

3!

5!



T3͑x͒ ෇ x Ϫ



x3

3!



T7͑x͒ ෇ x Ϫ



x3

x5

x7

ϩ

Ϫ

3!

5!

7!



to the sine curve. You can see that as n increases, Tn͑x͒ is a good approximation to sin x on

a larger and larger interval.

y







T∞



x



0



y=sin x

FIGURE 6











Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).

Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.



97817_11_ch11_p792-801.qk_97817_11_ch11_p792-801 11/3/10 5:33 PM Page 796



796



CHAPTER 11



INFINITE SEQUENCES AND SERIES



One use of the type of calculation done in Examples 1 and 2 occurs in calculators and

computers. For instance, when you press the sin or e x key on your calculator, or when a

computer programmer uses a subroutine for a trigonometric or exponential or Bessel function, in many machines a polynomial approximation is calculated. The polynomial is often

a Taylor polynomial that has been modified so that the error is spread more evenly throughout an interval.



Applications to Physics

Taylor polynomials are also used frequently in physics. In order to gain insight into an equation, a physicist often simplifies a function by considering only the first two or three terms

in its Taylor series. In other words, the physicist uses a Taylor polynomial as an approximation to the function. Taylor’s Inequality can then be used to gauge the accuracy of the

approximation. The following example shows one way in which this idea is used in special

relativity.



v EXAMPLE 3 In Einstein’s theory of special relativity the mass of an object moving

with velocity v is

m0

m෇

Ϫ

s1 v 2͞c 2

where m 0 is the mass of the object when at rest and c is the speed of light. The kinetic

energy of the object is the difference between its total energy and its energy at rest:

K ෇ mc 2 Ϫ m 0 c 2

(a) Show that when v is very small compared with c, this expression for K agrees with

1

classical Newtonian physics: K ෇ 2 m 0 v 2.

(b) Use Taylor’s Inequality to estimate the difference in these expressions for K when

v ഛ 100 m͞s.



Խ Խ



SOLUTION



(a) Using the expressions given for K and m, we get

The upper curve in Figure 7 is the graph of

the expression for the kinetic energy K of an

object with velocity v in special relativity. The

lower curve shows the function used for K in

classical Newtonian physics. When v is much

smaller than the speed of light, the curves are

practically identical.



K ෇ mc 2 Ϫ m 0 c 2 ෇



ͫͩ ͪ



m0c2

Ϫ m0c2 ෇ m0 c2

s1 Ϫ v 2͞c 2



Ϫ1͞2



c2



ͬ



Ϫ1



Խ Խ



͑1 ϩ x͒Ϫ1͞2 ෇ 1 Ϫ 12 x ϩ



(Ϫ 12 )(Ϫ 32 ) x 2 ϩ (Ϫ 12 )(Ϫ 32 )(Ϫ 52) x 3 ϩ и и и

2!



3!



෇ 1 Ϫ 12 x ϩ 38 x 2 Ϫ 165 x 3 ϩ и и и



K=mc@-m¸c@



and

K = 21 m ¸ √ @



FIGURE 7



v2



With x ෇ Ϫv 2͞c 2, the Maclaurin series for ͑1 ϩ x͒Ϫ1͞2 is most easily computed as a

binomial series with k ෇ Ϫ12 . (Notice that x Ͻ 1 because v Ͻ c.) Therefore we have



K



0







c







ͫͩ

ͩ



K ෇ m0 c2

෇ m0 c2







ͪ ͬ



1 v2

3 v4

5 v6

ϩ

ϩ

ϩ иии Ϫ 1

2

4

2 c

8 c

16 c 6



ͪ



1 v2

3 v4

5 v6

ϩ

ϩ

ϩ иии

2

4

2 c

8 c

16 c 6



If v is much smaller than c, then all terms after the first are very small when compared



Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).

Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.



97817_11_ch11_p792-801.qk_97817_11_ch11_p792-801 11/3/10 5:33 PM Page 797



APPLICATIONS OF TAYLOR POLYNOMIALS



SECTION 11.11



797



with the first term. If we omit them, we get



ͩ ͪ

1 v2

2 c2



K Ϸ m0 c2



෇ 12 m 0 v 2



(b) If x ෇ Ϫv 2͞c 2, f ͑x͒ ෇ m 0 c 2 ͓͑1 ϩ x͒Ϫ1͞2 Ϫ 1͔, and M is a number such that

f Љ͑x͒ ഛ M , then we can use Taylor’s Inequality to write



Խ



Խ



M



Խ R ͑x͒ Խ ഛ 2! x

1



2



Խ Խ



We have f Љ͑x͒ ෇ 34 m 0 c 2͑1 ϩ x͒Ϫ5͞2 and we are given that v ഛ 100 m͞s, so

3m 0 c 2



Խ f Љ͑x͒ Խ ෇ 4͑1 Ϫ v ͞c ͒

2



2 5͞2







3m 0 c 2

4͑1 Ϫ 100 2͞c 2 ͒5͞2



͑෇ M͒



Thus, with c ෇ 3 ϫ 10 8 m͞s,



Խ



Խ



R1͑x͒ ഛ



1

3m 0 c 2

100 4

ؒ

ؒ

Ͻ ͑4.17 ϫ 10Ϫ10 ͒m 0

2 4͑1 Ϫ 100 2͞c 2 ͒5͞2

c4



Խ Խ



So when v ഛ 100 m͞s, the magnitude of the error in using the Newtonian expression

for kinetic energy is at most ͑4.2 ϫ 10Ϫ10 ͒m 0.

Another application to physics occurs in optics. Figure 8 is adapted from Optics,

4th ed., by Eugene Hecht (San Francisco, 2002), page 153. It depicts a wave from the point

source S meeting a spherical interface of radius R centered at C. The ray SA is refracted

toward P.

ăr

A



Lo



h

R



V







ăt Li



S



C



so

FIGURE 8



si





n



Refraction at a spherical interface



P



Courtesy of Eugene Hecht



ăi



Using Fermats principle that light travels so as to minimize the time taken, Hecht derives

the equation

1



n1

n2

1

ϩ



ᐉo

ᐉi

R



ͩ



n2 si

n1 so

Ϫ

ᐉi

ᐉo



ͪ



where n1 and n2 are indexes of refraction and ᐉo , ᐉi , so , and si are the distances indicated in

Figure 8. By the Law of Cosines, applied to triangles ACS and ACP, we have

Here we use the identity

cos͑␲ Ϫ ␾͒ ෇ Ϫcos ␾



2



ᐉo ෇ sR 2 ϩ ͑so ϩ R͒2 Ϫ 2R͑so ϩ R͒ cos ␾

ᐉi ෇ sR 2 ϩ ͑si Ϫ R͒2 ϩ 2R͑si Ϫ R͒ cos ␾



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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.



97817_11_ch11_p792-801.qk_97817_11_ch11_p792-801 11/3/10 5:33 PM Page 798



798



INFINITE SEQUENCES AND SERIES



CHAPTER 11



Because Equation 1 is cumbersome to work with, Gauss, in 1841, simplified it by using the

linear approximation cos ␾ Ϸ 1 for small values of ␾. (This amounts to using the Taylor

polynomial of degree 1.) Then Equation 1 becomes the following simpler equation [as you

are asked to show in Exercise 34(a)]:

n1

n2

n2 Ϫ n1

ϩ



so

si

R



3



The resulting optical theory is known as Gaussian optics, or first-order optics, and has

become the basic theoretical tool used to design lenses.

A more accurate theory is obtained by approximating cos ␾ by its Taylor polynomial of

degree 3 (which is the same as the Taylor polynomial of degree 2). This takes into account

rays for which ␾ is not so small, that is, rays that strike the surface at greater distances h

above the axis. In Exercise 34(b) you are asked to use this approximation to derive the more

accurate equation

4



ͫ ͩ



n2

n2 Ϫ n1

n1

n1

ϩ



ϩ h2

so

si

R

2so



1

1

ϩ

so

R



ͪ



2



ϩ



n2

2si



ͩ



1

1

Ϫ

R

si



ͪͬ

2



The resulting optical theory is known as third-order optics.

Other applications of Taylor polynomials to physics and engineering are explored in

Exercises 32, 33, 35, 36, 37, and 38, and in the Applied Project on page 801.



11.11 Exercises

8. f ͑x͒ ෇ x cos x,



; 1. (a) Find the Taylor polynomials up to degree 6 for

f ͑x͒ ෇ cos x centered at a ෇ 0. Graph f and these

polynomials on a common screen.

(b) Evaluate f and these polynomials at x ෇ ␲͞4, ␲͞2,

and ␲.

(c) Comment on how the Taylor polynomials converge

to f ͑x͒.



; 2. (a) Find the Taylor polynomials up to degree 3 for

f ͑x͒ ෇ 1͞x centered at a ෇ 1. Graph f and these

polynomials on a common screen.

(b) Evaluate f and these polynomials at x ෇ 0.9 and 1.3.

(c) Comment on how the Taylor polynomials converge

to f ͑x͒.



; 3–10 Find the Taylor polynomial T3͑x͒ for the function f

centered at the number a. Graph f and T3 on the same screen.

3. f ͑x͒ ෇ 1͞x,



a෇2



4. f ͑x͒ ෇ x ϩ e Ϫx,

5. f ͑x͒ ෇ cos x,

6. f ͑x͒ ෇ e



Ϫx



;



a ෇ ␲͞2



sin x,



7. f ͑x͒ ෇ ln x,



a෇0



a෇0



a෇1



Graphing calculator or computer required



9. f ͑x͒ ෇ xe Ϫ2x,



a෇0



10. f ͑x͒ ෇ tanϪ1 x,



CAS



a෇0



a෇1



11–12 Use a computer algebra system to find the Taylor polynomials Tn centered at a for n ෇ 2, 3, 4, 5. Then graph these

polynomials and f on the same screen.

11. f ͑x͒ ෇ cot x ,



a ෇ ␲͞4



3

12. f ͑x͒ ෇ s

1 ϩ x2 ,



a෇0



13–22



(a) Approximate f by a Taylor polynomial with degree n at the

number a.

(b) Use Taylor’s Inequality to estimate the accuracy of the

approximation f ͑x͒ Ϸ Tn͑x͒ when x lies in the given

interval.

; (c) Check your result in part (b) by graphing Rn ͑x͒ .



Խ



13. f ͑x͒ ෇ sx ,

Ϫ2



14. f ͑x͒ ෇ x ,



CAS Computer algebra system required



a ෇ 4,

a ෇ 1,



n ෇ 2,

n ෇ 2,



Խ



4 ഛ x ഛ 4.2

0.9 ഛ x ഛ 1.1



1. Homework Hints available at stewartcalculus.com



Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).

Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.



97817_11_ch11_p792-801.qk_97817_11_ch11_p792-801 11/3/10 5:33 PM Page 799



CHAPTER 11.11



15. f ͑x͒ ෇ x 2͞3,



a ෇ 1,



n ෇ 3,



0.8 ഛ x ഛ 1.2



16. f ͑x͒ ෇ sin x,



a ෇ ␲͞6, n ෇ 4, 0 ഛ x ഛ ␲͞3



17. f ͑x͒ ෇ sec x,



a ෇ 0,



18. f ͑x͒ ෇ ln͑1 ϩ 2x͒,

19. f ͑x͒ ෇ e x ,

2



a ෇ 1,



a ෇ 0,



20. f ͑x͒ ෇ x ln x,



n ෇ 2,



n ෇ 3,



a ෇ 1,



conductivity and is measured in units of ohm-meters (⍀ -m).

The resistivity of a given metal depends on the temperature

according to the equation



0.5 ഛ x ഛ 1.5



␳ ͑t͒ ෇ ␳ 20 e ␣ ͑tϪ20͒



0 ഛ x ഛ 0.1



n ෇ 3,



0.5 ഛ x ഛ 1.5



21. f ͑ x͒ ෇ x sin x,



a ෇ 0,



n ෇ 4,



Ϫ1 ഛ x ഛ 1



22. f ͑x͒ ෇ sinh 2x,



a ෇ 0,



n ෇ 5,



Ϫ1 ഛ x ഛ 1



23. Use the information from Exercise 5 to estimate cos 80Њ cor-



rect to five decimal places.



;



24. Use the information from Exercise 16 to estimate sin 38Њ



correct to five decimal places.

25. Use Taylor’s Inequality to determine the number of terms of



the Maclaurin series for e x that should be used to estimate

e 0.1 to within 0.00001.

26. How many terms of the Maclaurin series for ln͑1 ϩ x͒ do



you need to use to estimate ln 1.4 to within 0.001?



799



32. The resistivity ␳ of a conducting wire is the reciprocal of the



Ϫ0.2 ഛ x ഛ 0.2



n ෇ 3,



APPLICATIONS OF TAYLOR POLYNOMIALS



;



where t is the temperature in ЊC. There are tables that list the

values of ␣ (called the temperature coefficient) and ␳ 20 (the

resistivity at 20ЊC) for various metals. Except at very low

temperatures, the resistivity varies almost linearly with temperature and so it is common to approximate the expression

for ␳ ͑t͒ by its first- or second-degree Taylor polynomial

at t ෇ 20.

(a) Find expressions for these linear and quadratic

approximations.

(b) For copper, the tables give ␣ ෇ 0.0039͞ЊC and

␳ 20 ෇ 1.7 ϫ 10 Ϫ8 ⍀ -m. Graph the resistivity of copper

and the linear and quadratic approximations for

Ϫ250ЊC ഛ t ഛ 1000ЊC.

(c) For what values of t does the linear approximation agree

with the exponential expression to within one percent?



33. An electric dipole consists of two electric charges of equal



magnitude and opposite sign. If the charges are q and Ϫq and

are located at a distance d from each other, then the electric

field E at the point P in the figure is



; 27–29 Use the Alternating Series Estimation Theorem or

Taylor’s Inequality to estimate the range of values of x for which

the given approximation is accurate to within the stated error.

Check your answer graphically.

27. sin x Ϸ x Ϫ



x3

6



28. cos x Ϸ 1 Ϫ



x4

x2

ϩ

2

24



( Խ error Խ Ͻ 0.01)

( Խ error Խ Ͻ 0.005)



E෇



q

q

Ϫ

D2

͑D ϩ d͒2



By expanding this expression for E as a series in powers of

d͞D, show that E is approximately proportional to 1͞D 3

when P is far away from the dipole.

q

D



x3

x5

29. arctan x Ϸ x Ϫ

ϩ

3

5



_q



P

d



( Խ error Խ Ͻ 0.05)

34. (a) Derive Equation 3 for Gaussian optics from Equation 1



30. Suppose you know that



f ͑n͒͑4͒ ෇



͑Ϫ1͒ n n!

3 n ͑n ϩ 1͒



and the Taylor series of f centered at 4 converges to f ͑x͒

for all x in the interval of convergence. Show that the fifthdegree Taylor polynomial approximates f ͑5͒ with error less

than 0.0002.

31. A car is moving with speed 20 m͞s and acceleration 2 m͞s2



at a given instant. Using a second-degree Taylor polynomial,

estimate how far the car moves in the next second. Would it

be reasonable to use this polynomial to estimate the distance

traveled during the next minute?



by approximating cos ␾ in Equation 2 by its first-degree

Taylor polynomial.

(b) Show that if cos ␾ is replaced by its third-degree Taylor

polynomial in Equation 2, then Equation 1 becomes

Equation 4 for third-order optics. [Hint: Use the first two

terms in the binomial series for ᐉoϪ1 and ᐉiϪ1. Also, use

␾ Ϸ sin ␾.]

35. If a water wave with length L moves with velocity v across a



body of water with depth d, as in the figure on page 800, then

v2 ෇



tL

2␲ d

tanh

2␲

L



(a) If the water is deep, show that v Ϸ stL͑͞2␲͒ .

(b) If the water is shallow, use the Maclaurin series for tanh

to show that v Ϸ std . (Thus in shallow water the veloc-



Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).

Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.



97817_11_ch11_p792-801.qk_97817_11_ch11_p792-801 11/3/10 5:33 PM Page 800



800



INFINITE SEQUENCES AND SERIES



CHAPTER 11



ity of a wave tends to be independent of the length of the

wave.)

(c) Use the Alternating Series Estimation Theorem to show that

if L Ͼ 10d, then the estimate v 2 Ϸ td is accurate to within

0.014tL.



L



36. A uniformly charged disk has radius R and surface charge den-



sity ␴ as in the figure. The electric potential V at a point P at a

distance d along the perpendicular central axis of the disk is

V ෇ 2␲ ke␴ (sd 2 ϩ R 2 Ϫ d)

where ke is a constant (called Coulomb’s constant). Show that



␲ ke R 2␴

d



mum angle ␪ 0 with the vertical is

T෇4



ͱ ͫ



T ෇ 2␲



L

t



R

P



37. If a surveyor measures differences in elevation when making



plans for a highway across a desert, corrections must be made

for the curvature of the earth.

(a) If R is the radius of the earth and L is the length of the

highway, show that the correction is

C ෇ R sec͑L͞R͒ Ϫ R

(b) Use a Taylor polynomial to show that

L2

5L 4



ϩ

2R

24R 3

(c) Compare the corrections given by the formulas in parts (a)

and (b) for a highway that is 100 km long. (Take the radius

of the earth to be 6370 km.)

C



R

R



y



␲͞2



dx

s1 Ϫ k 2 sin 2x



0







ͬ



12 2

12 3 2 4

12 3 25 2 6

k

ϩ

k

ϩ

k ϩ иии

22

2 242

2 2426 2



ͱ



T Ϸ 2␲



L

(1 ϩ 14 k 2 )

t



(b) Notice that all the terms in the series after the first one have

coefficients that are at most 14. Use this fact to compare this

series with a geometric series and show that



ͱ



2␲



L



L

t



If ␪ 0 is not too large, the approximation T Ϸ 2␲ sL͞t ,

obtained by using only the first term in the series, is often

used. A better approximation is obtained by using two

terms:



for large d



d



ͱ



1

where k ෇ sin ( 2 ␪ 0 ) and t is the acceleration due to gravity. (In

Exercise 42 in Section 7.7 we approximated this integral using

Simpson’s Rule.)

(a) Expand the integrand as a binomial series and use the result

of Exercise 50 in Section 7.1 to show that



d







38. The period of a pendulum with length L that makes a maxi-



L

t



(1 ϩ 14 k 2 ) ഛ T ഛ 2␲



ͱ



L 4 Ϫ 3k 2

t 4 Ϫ 4k 2



(c) Use the inequalities in part (b) to estimate the period of a

pendulum with L ෇ 1 meter and ␪ 0 ෇ 10Њ. How does it

compare with the estimate T Ϸ 2␲ sL͞t ? What if

␪ 0 ෇ 42Њ ?

39. In Section 3.8 we considered Newton’s method for approxi-



mating a root r of the equation f ͑x͒ ෇ 0, and from an initial

approximation x 1 we obtained successive approximations

x 2 , x 3 , . . . , where

x nϩ1 ෇ x n Ϫ



f ͑x n͒

f Ј͑x n͒



Use Taylor’s Inequality with n ෇ 1, a ෇ x n , and x ෇ r to show

that if f Љ͑x͒ exists on an interval I containing r, x n , and x nϩ1,

and f Љ͑x͒ ഛ M, f Ј͑x͒ ജ K for all x ʦ I , then



Խ



Խ



Խ



Խx



nϩ1



Խ



Խ



Ϫr ഛ



M

xn Ϫ r

2K



Խ



Խ



2



[This means that if x n is accurate to d decimal places, then x nϩ1

is accurate to about 2d decimal places. More precisely, if the

error at stage n is at most 10Ϫm, then the error at stage n ϩ 1 is

at most ͑M͞2K ͒10Ϫ2m.]



Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).

Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.



97817_11_ch11_p792-801.qk_97817_11_ch11_p792-801 11/3/10 5:33 PM Page 801



APPLIED PROJECT



© Luke Dodd / Photo Researchers, Inc.



APPLIED PROJECT



RADIATION FROM THE STARS



801



RADIATION FROM THE STARS

Any object emits radiation when heated. A blackbody is a system that absorbs all the radiation that

falls on it. For instance, a matte black surface or a large cavity with a small hole in its wall (like a

blastfurnace) is a blackbody and emits blackbody radiation. Even the radiation from the sun is

close to being blackbody radiation.

Proposed in the late 19th century, the Rayleigh-Jeans Law expresses the energy density of

blackbody radiation of wavelength ␭ as

f ͑␭͒ ෇



8␲ kT

␭4



where ␭ is measured in meters, T is the temperature in kelvins (K), and k is Boltzmann’s constant. The Rayleigh-Jeans Law agrees with experimental measurements for long wavelengths

but disagrees drastically for short wavelengths. [The law predicts that f ͑␭͒ l ϱ as ␭ l 0 ϩ but

experiments have shown that f ͑␭͒ l 0.] This fact is known as the ultraviolet catastrophe.

In 1900 Max Planck found a better model (known now as Planck’s Law) for blackbody

radiation:

f ͑␭͒ ෇



8␲ hc␭Ϫ5

e hc͑͞␭ kT ͒ Ϫ 1



where ␭ is measured in meters, T is the temperature (in kelvins), and

h ෇ Planck’s constant ෇ 6.6262 ϫ 10Ϫ34 Jиs

c ෇ speed of light ෇ 2.997925 ϫ 10 8 m͞s

k ෇ Boltzmann’s constant ෇ 1.3807 ϫ 10Ϫ23 J͞K

1. Use l’Hospital’s Rule to show that



lim f ͑␭͒ ෇ 0



␭ l 0ϩ



and



lim f ͑␭͒ ෇ 0



␭lϱ



for Planck’s Law. So this law models blackbody radiation better than the Rayleigh-Jeans

Law for short wavelengths.

2. Use a Taylor polynomial to show that, for large wavelengths, Planck’s Law gives approxi-



mately the same values as the Rayleigh-Jeans Law.



; 3. Graph f as given by both laws on the same screen and comment on the similarities and

differences. Use T ෇ 5700 K (the temperature of the sun). (You may want to change from

meters to the more convenient unit of micrometers: 1 ␮m ෇ 10Ϫ6 m.)

4. Use your graph in Problem 3 to estimate the value of ␭ for which f ͑␭͒ is a maximum



under Planck’s Law.



; 5. Investigate how the graph of f changes as T varies. (Use Planck’s Law.) In particular,

graph f for the stars Betelgeuse (T ෇ 3400 K), Procyon (T ෇ 6400 K), and Sirius

(T ෇ 9200 K), as well as the sun. How does the total radiation emitted (the area under the

curve) vary with T ? Use the graph to comment on why Sirius is known as a blue star and

Betelgeuse as a red star.



;



Graphing calculator or computer required



Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).

Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.



97817_11_ch11_p802-808.qk_97817_11_ch11_p802-808 11/3/10 5:34 PM Page 802



802



CHAPTER 11



11



INFINITE SEQUENCES AND SERIES



Review



Concept Check

1. (a) What is a convergent sequence?



(c) If a series is convergent by the Alternating Series Test, how

do you estimate its sum?



(b) What is a convergent series?

(c) What does lim n l ϱ an ෇ 3 mean?

(d) What does ͸ϱn෇1 an ෇ 3 mean?



8. (a) Write the general form of a power series.



(b) What is the radius of convergence of a power series?

(c) What is the interval of convergence of a power series?



2. (a) What is a bounded sequence?



(b) What is a monotonic sequence?

(c) What can you say about a bounded monotonic sequence?

3. (a) What is a geometric series? Under what circumstances is



9. Suppose f ͑x͒ is the sum of a power series with radius of



convergence R.

(a) How do you differentiate f ? What is the radius of convergence of the series for f Ј?

(b) How do you integrate f ? What is the radius of convergence

of the series for x f ͑x͒ dx ?



it convergent? What is its sum?

(b) What is a p-series? Under what circumstances is it

convergent?

4. Suppose ͸ a n ෇ 3 and s n is the nth partial sum of the series.



What is lim n l ϱ a n ? What is lim n l ϱ sn?

10. (a) Write an expression for the nth-degree Taylor polynomial



5. State the following.



(a)

(b)

(c)

(d)

(e)

(f )

(g)



of f centered at a.

(b) Write an expression for the Taylor series of f centered at a.

(c) Write an expression for the Maclaurin series of f .

(d) How do you show that f ͑x͒ is equal to the sum of its

Taylor series?

(e) State Taylor’s Inequality.



The Test for Divergence

The Integral Test

The Comparison Test

The Limit Comparison Test

The Alternating Series Test

The Ratio Test

The Root Test



11. Write the Maclaurin series and the interval of convergence for



6. (a) What is an absolutely convergent series?



each of the following functions.

(a) 1͑͞1 Ϫ x͒

(b) e x

(c) sin x

(d) cos x

(e) tanϪ1x

(f ) ln͑1 ϩ x͒



(b) What can you say about such a series?

(c) What is a conditionally convergent series?

7. (a) If a series is convergent by the Integral Test, how do you



estimate its sum?

(b) If a series is convergent by the Comparison Test, how do

you estimate its sum?



12. Write the binomial series expansion of ͑1 ϩ x͒ k. What is the



radius of convergence of this series?



True-False Quiz

Determine whether the statement is true or false. If it is true, explain why.

If it is false, explain why or give an example that disproves the statement.

1. If lim n l ϱ a n ෇ 0, then ͸ a n is convergent.



ϱ



10.



͚



n෇0



͑Ϫ1͒ n

1



n!

e



11. If Ϫ1 Ͻ ␣ Ͻ 1, then lim n l ϱ ␣ n ෇ 0.



2. The series ͸ϱn෇1 n Ϫsin 1 is convergent.

3. If lim n l ϱ a n ෇ L, then lim n l ϱ a 2nϩ1 ෇ L.



12. If ͸ a n is divergent, then ͸ a n is divergent.



4. If ͸ cn 6 is convergent, then ͸ cn͑Ϫ2͒ is convergent.



13. If f ͑x͒ ෇ 2x Ϫ x ϩ x Ϫ и и и converges for all x,



n



Խ Խ



2



n



then f ٞ͑0͒ ෇ 2.



5. If ͸ cn 6 is convergent, then ͸ cn͑Ϫ6͒ is convergent.

n



n



1

3



3



6. If ͸ cn x n diverges when x ෇ 6, then it diverges when x ෇ 10.



14. If ͕a n ͖ and ͕bn ͖ are divergent, then ͕a n ϩ bn ͖ is divergent.



7. The Ratio Test can be used to determine whether ͸ 1͞n



15. If ͕a n ͖ and ͕bn ͖ are divergent, then ͕a n bn ͖ is divergent.



3



converges.

8. The Ratio Test can be used to determine whether ͸ 1͞n!



converges.

9. If 0 ഛ a n ഛ bn and ͸ bn diverges, then ͸ a n diverges.



16. If ͕a n ͖ is decreasing and a n Ͼ 0 for all n, then ͕a n ͖ is



convergent.

17. If a n Ͼ 0 and ͸ a n converges, then ͸ ͑Ϫ1͒ n a n converges.



Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).

Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.



97817_11_ch11_p802-808.qk_97817_11_ch11_p802-808 11/3/10 5:34 PM Page 803



CHAPTER 11



18. If a n Ͼ 0 and lim n l ϱ ͑a nϩ1͞a n ͒ Ͻ 1, then lim n l ϱ a n ෇ 0.



803



21. If a finite number of terms are added to a convergent series,



then the new series is still convergent.



19. 0.99999 . . . ෇ 1



ϱ



20. If lim a n ෇ 2, then lim ͑a nϩ3 Ϫ a n͒ ෇ 0.

nlϱ



REVIEW



22. If



nlϱ



͚a



n



෇ A and



n෇1



ϱ



͚b



n



ϱ



͚a



෇ B, then



n෇1



n



bn ෇ AB.



n෇1



Exercises

1–8 Determine whether the sequence is convergent or divergent.

If it is convergent, find its limit.



2 ϩ n3

1. a n ෇

1 ϩ 2n 3



9 nϩ1

2. a n ෇

10 n



n3

3. a n ෇

1 ϩ n2



4. a n ෇ cos͑n␲͞2͒



5. a n ෇



ϱ



27.



n sin n

n2 ϩ 1



6. a n ෇



29.



sn



෇ 0 and use a graph to find the

smallest value of N that corresponds to ␧ ෇ 0.1 in the precise definition of a limit.



; 10. Show that lim n l ϱ n e



11–22 Determine whether the series is convergent or divergent.



ϱ



13.



͚



n෇1



15.



14.



n෇2



nsln n



͚



n෇1

ϱ



19.



n3

5n

1



ϱ



17.



12.



ϱ



͚



͚



n෇1



ϱ



n2 ϩ 1

n3 ϩ 1

͑Ϫ1͒ n



n෇1



sn ϩ 1



ͩ



n

16. ͚ ln

3n ϩ 1

n෇1

ϱ



cos 3n

1 ϩ ͑1.2͒ n



18.



1 ؒ 3 ؒ 5 ؒ и и и ؒ ͑2n Ϫ 1͒

5 n n!



20.



͚ ͑Ϫ1͒



nϪ1



n෇1



sn

nϩ1



͚



n෇1

ϱ



͚



n෇1

ϱ



22.



͚



n෇1



n෇2



͑Ϫ1͒nsn

ln n



ϱ



͑Ϫ3͒ nϪ1

2 3n



͚ ͓tan



28.



͚



1

n͑n ϩ 3͒



ϱ



͑Ϫ1͒ n ␲ n

3 2n ͑2n͒!



n෇1



Ϫ1



͑n ϩ 1͒ Ϫ tanϪ1n͔



30.



͚



n෇0



32. Express the repeating decimal 4.17326326326 . . . as a



fraction.

33. Show that cosh x ജ 1 ϩ 2 x 2 for all x.

1



34. For what values of x does the series ͸ϱn෇1 ͑ln x͒ n converge?

ϱ



͚



35. Find the sum of the series



n෇1



mal places.



͑Ϫ1͒ nϩ1

correct to four decin5



36. (a) Find the partial sum s5 of the series ͸ϱn෇1 1͞n 6 and



estimate the error in using it as an approximation to the

sum of the series.

(b) Find the sum of this series correct to five decimal places.



ϱ



͚

ϱ



ϱ



21.



͚



n෇1



͚



e3

e4

e2

Ϫ

ϩ

Ϫ иии

31. 1 Ϫ e ϩ

2!

3!

4!



8. ͕͑Ϫ10͒ n͞n!͖



n

n3 ϩ 1



26.



n෇1



4 Ϫn



͚



͚

ϱ



ln n



a nϩ1 ෇ 13 ͑a n ϩ 4͒. Show that ͕a n ͖ is increasing and a n Ͻ 2

for all n. Deduce that ͕a n ͖ is convergent and find its limit.



n෇1



n෇1



ϱ



͑Ϫ1͒n͑n ϩ 1͒3 n

2 2nϩ1



27–31 Find the sum of the series.



9. A sequence is defined recursively by the equations a 1 ෇ 1,



11.



͚



n෇1



7. ͕͑1 ϩ 3͞n͒4n ͖



ϱ



ϱ



25.



ͪ



n 2n

͑1 ϩ 2n 2 ͒n

͑Ϫ5͒ 2n

n 2 9n



37. Use the sum of the first eight terms to approximate the sum



of the series ͸ϱn෇1 ͑2 ϩ 5 n ͒Ϫ1. Estimate the error involved in

this approximation.

ϱ



38. (a) Show that the series



͚



n෇1



nn

is convergent.

͑2n͒!



nn

෇ 0.

(b) Deduce that lim

n l ϱ ͑2n͒!

39. Prove that if the series ͸ϱn෇1 an is absolutely convergent, then



the series



sn ϩ 1 Ϫ sn Ϫ 1

n



ϱ



͚



n෇1



ͩ ͪ



nϩ1

an

n



is also absolutely convergent.

23–26 Determine whether the series is conditionally convergent, absolutely convergent, or divergent.

ϱ



23.



͚ ͑Ϫ1͒



n Ϫ1͞3



nϪ1



ϱ



24.



n෇1



;



͚ ͑Ϫ1͒



n Ϫ3



nϪ1



n෇1



40– 43 Find the radius of convergence and interval of convergence of the series.

ϱ



40.



͚ ͑Ϫ1͒



n෇1



n



xn

n2 5n



ϱ



41.



͚



n෇1



͑x ϩ 2͒ n

n 4n



Graphing calculator or computer required



Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).

Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.



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