1. Trang chủ >
2. Giáo án - Bài giảng >
3. Toán học >

# 9: Representations of Functions as Power Series

Bạn đang xem bản rút gọn của tài liệu. Xem và tải ngay bản đầy đủ của tài liệu tại đây (25.43 MB, 608 trang )

97817_11_ch11_p762-771.qk_97817_11_ch11_p762-771 11/3/10 5:31 PM Page 771

SECTION 11.9

REPRESENTATIONS OF FUNCTIONS AS POWER SERIES

771

tions by polynomials. (Scientists do this to simplify the expressions they deal with; computer

scientists do this to represent functions on calculators and computers.)

1

A geometric illustration of Equation 1 is shown

in Figure 1. Because the sum of a series is the

limit of the sequence of partial sums, we have

where

1

෇ lim sn͑x͒

nlϱ

1Ϫx

ϱ

1

෇ 1 ϩ x ϩ x2 ϩ x3 ϩ и и и ෇ ͚ xn

1Ϫx

n෇0

ԽxԽ Ͻ 1

We first encountered this equation in Example 6 in Section 11.2, where we obtained it by

observing that the series is a geometric series with a ෇ 1 and r ෇ x. But here our point of

view is different. We now regard Equation 1 as expressing the function f ͑x͒ ෇ 1͑͞1 Ϫ x͒

as a sum of a power series.

s¡¡

y

s∞

sn͑x͒ ෇ 1 ϩ x ϩ x 2 ϩ и и и ϩ x n

f

is the nth partial sum. Notice that as n

increases, sn͑x͒ becomes a better approximation to f ͑x͒ for Ϫ1 Ͻ x Ͻ 1.

s™

FIGURE 1

0

_1

1

ƒ=

and some partial sums

1-x

v

x

1

EXAMPLE 1 Express 1͑͞1 ϩ x 2 ͒ as the sum of a power series and find the interval of

convergence.

SOLUTION Replacing x by Ϫx 2 in Equation 1, we have

ϱ

1

1

෇ ͚ ͑Ϫx 2 ͒n

2

2

1ϩx

1 Ϫ ͑Ϫx ͒

n෇0

ϱ

͚ ͑Ϫ1͒ x

n 2n

෇ 1 Ϫ x2 ϩ x4 Ϫ x6 ϩ x8 Ϫ и и и

n෇0

Խ

Խ

Because this is a geometric series, it converges when Ϫx 2 Ͻ 1, that is, x 2 Ͻ 1, or

x Ͻ 1. Therefore the interval of convergence is ͑Ϫ1, 1͒. (Of course, we could have

determined the radius of convergence by applying the Ratio Test, but that much work is

unnecessary here.)

Խ Խ

EXAMPLE 2 Find a power series representation for 1͑͞x ϩ 2͒.

SOLUTION In order to put this function in the form of the left side of Equation 1, we first

factor a 2 from the denominator:

1

2ϩx

Խ

1

1

ͩ ͪ ͫ ͩ ͪͬ

͚ͩ ͪ ͚

x

2 1ϩ

2

1

2

ϱ

n෇0

Խ

Ϫ

2 1Ϫ Ϫ

x

2

n

ϱ

n෇0

x

2

͑Ϫ1͒n n

x

2 nϩ1

Խ Խ

This series converges when Ϫx͞2 Ͻ 1, that is, x Ͻ 2. So the interval of convergence is ͑Ϫ2, 2͒.

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_p772-781.qk_97817_11_ch11_p772-781 11/3/10 5:31 PM Page 772

772

CHAPTER 11

INFINITE SEQUENCES AND SERIES

EXAMPLE 3 Find a power series representation of x 3͑͞x ϩ 2͒.

SOLUTION Since this function is just x 3 times the function in Example 2, all we have to

do is to multiply that series by x 3:

It’s legitimate to move x 3 across the

sigma sign because it doesn’t depend on n.

[Use Theorem 11.2.8(i) with c ෇ x 3.]

ϱ

ϱ

x3

1

͑Ϫ1͒ n

͑Ϫ1͒ n

෇ x3 ؒ

෇ x 3 ͚ nϩ1 x n ෇ ͚ nϩ1 x nϩ3

xϩ2

xϩ2

n෇0 2

n෇0 2

෇ 12 x 3 Ϫ 14 x 4 ϩ 18 x 5 Ϫ 161 x 6 ϩ и и и

Another way of writing this series is as follows:

ϱ

x3

͑Ϫ1͒ nϪ1 n

෇ ͚

x

xϩ2

2 nϪ2

n෇3

As in Example 2, the interval of convergence is ͑Ϫ2, 2͒.

Differentiation and Integration of Power Series

The sum of a power series is a function f ͑x͒ ෇ ͸ϱn෇0 cn͑x Ϫ a͒ n whose domain is the interval of convergence of the series. We would like to be able to differentiate and integrate such

functions, and the following theorem (which we won’t prove) says that we can do so by differentiating or integrating each individual term in the series, just as we would for a polynomial. This is called term-by-term differentiation and integration.

2

Theorem If the power series

͸ cn͑x Ϫ a͒ n has radius of convergence R Ͼ 0,

then the function f defined by

f ͑x͒ ෇ c0 ϩ c1͑x Ϫ a͒ ϩ c2͑x Ϫ a͒2 ϩ и и и ෇

ϱ

͚ c ͑x Ϫ a͒

n

n

n෇0

is differentiable (and therefore continuous) on the interval ͑a Ϫ R, a ϩ R͒ and

(i) f Ј͑x͒ ෇ c1 ϩ 2c2͑x Ϫ a͒ ϩ 3c3͑x Ϫ a͒2 ϩ и и и ෇

ϱ

͚ nc ͑x Ϫ a͒

n

nϪ1

n෇1

In part (ii), x c0 dx ෇ c0 x ϩ C1 is written as

c0͑x Ϫ a͒ ϩ C, where C ෇ C1 ϩ ac0 , so all

the terms of the series have the same form.

(ii)

y f ͑x͒ dx ෇ C ϩ c ͑x Ϫ a͒ ϩ c

0

෇Cϩ

1

ϱ

͚c

n

n෇0

͑x Ϫ a͒2

͑x Ϫ a͒3

ϩ c2

ϩ иии

2

3

͑x Ϫ a͒ nϩ1

nϩ1

The radii of convergence of the power series in Equations (i) and (ii) are both R.

NOTE 1 Equations (i) and (ii) in Theorem 2 can be rewritten in the form

(iii)

͚ͫ

y ͚ͫ

d

dx

ϱ

n෇0

ϱ

(iv)

n෇0

ͬ

ͬ

ϱ

cn͑x Ϫ a͒ n ෇

͚

n෇0

cn͑x Ϫ a͒ n dx ෇

d

͓cn͑x Ϫ a͒ n ͔

dx

ϱ

͚ y c ͑x Ϫ a͒

n

n

dx

n෇0

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_p772-781.qk_97817_11_ch11_p772-781 11/3/10 5:32 PM Page 773

REPRESENTATIONS OF FUNCTIONS AS POWER SERIES

SECTION 11.9

773

We know that, for finite sums, the derivative of a sum is the sum of the derivatives and the

integral of a sum is the sum of the integrals. Equations (iii) and (iv) assert that the same is

true for infinite sums, provided we are dealing with power series. (For other types of series

of functions the situation is not as simple; see Exercise 38.)

NOTE 2 Although Theorem 2 says that the radius of convergence remains the same

when a power series is differentiated or integrated, this does not mean that the interval of

convergence remains the same. It may happen that the original series converges at an endpoint, whereas the differentiated series diverges there. (See Exercise 39.)

NOTE 3 The idea of differentiating a power series term by term is the basis for a powerful method for solving differential equations. We will discuss this method in Chapter 17.

EXAMPLE 4 In Example 3 in Section 11.8 we saw that the Bessel function

J0͑x͒ ෇

ϱ

͚

n෇0

͑Ϫ1͒ n x 2n

2 2n͑n!͒2

is defined for all x. Thus, by Theorem 2, J0 is differentiable for all x and its derivative is

found by term-by-term differentiation as follows:

J0Ј͑x͒ ෇

ϱ

͚

n෇0

v

ϱ

d ͑Ϫ1͒ nx 2n

͑Ϫ1͒ n 2nx 2nϪ1

͚ 2 2n͑n!͒2

dx 2 2n͑n!͒2

n෇1

EXAMPLE 5 Express 1͑͞1 Ϫ x͒2 as a power series by differentiating Equation 1. What

SOLUTION Differentiating each side of the equation

ϱ

1

෇ 1 ϩ x ϩ x2 ϩ x3 ϩ и и и ෇ ͚ xn

1Ϫx

n෇0

we get

ϱ

1

෇ 1 ϩ 2x ϩ 3x 2 ϩ и и и ෇ ͚ nx nϪ1

2

͑1 Ϫ x͒

n෇1

If we wish, we can replace n by n ϩ 1 and write the answer as

ϱ

1

෇ ͚ ͑n ϩ 1͒x n

2

͑1 Ϫ x͒

n෇0

According to Theorem 2, the radius of convergence of the differentiated series is the

same as the radius of convergence of the original series, namely, R ෇ 1.

EXAMPLE 6 Find a power series representation for ln͑1 ϩ x͒ and its radius of

convergence.

SOLUTION We notice that the derivative of this function is 1͑͞1 ϩ x͒. From Equation 1

we have

1

1

෇ 1 Ϫ x ϩ x2 Ϫ x3 ϩ и и и

1ϩx

1 Ϫ ͑Ϫx͒

ԽxԽ Ͻ 1

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_p772-781.qk_97817_11_ch11_p772-781 11/3/10 5:32 PM Page 774

774

CHAPTER 11

INFINITE SEQUENCES AND SERIES

Integrating both sides of this equation, we get

1

dx ෇ y ͑1 Ϫ x ϩ x 2 Ϫ x 3 ϩ и и и͒ dx

1ϩx

ln͑1 ϩ x͒ ෇ y

෇xϪ

x2

x3

x4

ϩ

Ϫ

ϩ иии ϩ C

2

3

4

ϱ

͚ ͑Ϫ1͒

n෇1

nϪ1

xn

ϩC

n

ԽxԽ Ͻ 1

To determine the value of C we put x ෇ 0 in this equation and obtain ln͑1 ϩ 0͒ ෇ C.

Thus C ෇ 0 and

ln͑1 ϩ x͒ ෇ x Ϫ

ϱ

x2

x3

x4

xn

ϩ

Ϫ

ϩ и и и ෇ ͚ ͑Ϫ1͒ nϪ1

2

3

4

n

n෇1

ԽxԽ Ͻ 1

The radius of convergence is the same as for the original series: R ෇ 1.

v

EXAMPLE 7 Find a power series representation for f ͑x͒ ෇ tanϪ1x.

SOLUTION We observe that f Ј͑x͒ ෇ 1͑͞1 ϩ x 2 ͒ and find the required series by integrating

the power series for 1͑͞1 ϩ x 2 ͒ found in Example 1.

tanϪ1x ෇ y

The power series for tanϪ1x obtained

in Example 7 is called Gregory’s series after

the Scottish mathematician James Gregory

(1638–1675), who had anticipated some of

Newton’s discoveries. We have shown that

Gregory’s series is valid when Ϫ1 Ͻ x Ͻ 1,

but it turns out (although it isn’t easy to prove)

that it is also valid when x ෇ Ϯ1. Notice that

when x ෇ 1 the series becomes

1

1

1

෇1Ϫ ϩ Ϫ ϩиии

4

3

5

7

This beautiful result is known as the Leibniz

formula for ␲.

1

dx ෇ y ͑1 Ϫ x 2 ϩ x 4 Ϫ x 6 ϩ и и и͒ dx

1 ϩ x2

෇CϩxϪ

x3

x5

x7

ϩ

Ϫ

ϩ иии

3

5

7

To find C we put x ෇ 0 and obtain C ෇ tanϪ1 0 ෇ 0. Therefore

tanϪ1x ෇ x Ϫ

ϱ

x3

x5

x7

x 2nϩ1

ϩ

Ϫ

ϩ и и и ෇ ͚ ͑Ϫ1͒ n

3

5

7

2n ϩ 1

n෇0

Since the radius of convergence of the series for 1͑͞1 ϩ x 2 ͒ is 1, the radius of convergence of this series for tanϪ1x is also 1.

EXAMPLE 8

(a) Evaluate x ͓1͑͞1 ϩ x 7 ͔͒ dx as a power series.

(b) Use part (a) to approximate x00.5 ͓1͑͞1 ϩ x 7 ͔͒ dx correct to within 10Ϫ7.

SOLUTION

(a) The first step is to express the integrand, 1͑͞1 ϩ x 7 ͒, as the sum of a power series.

As in Example 1, we start with Equation 1 and replace x by Ϫx 7:

ϱ

1

1

͚ ͑Ϫx 7 ͒ n

1 ϩ x7

1 Ϫ ͑Ϫx 7 ͒

n෇0

ϱ

͚ ͑Ϫ1͒ x

n 7n

෇ 1 Ϫ x 7 ϩ x 14 Ϫ и и и

n෇0

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_p772-781.qk_97817_11_ch11_p772-781 11/3/10 5:32 PM Page 775

SECTION 11.9

This example demonstrates one way in

which power series representations are useful.

Integrating 1͑͞1 ϩ x 7 ͒ by hand is incredibly

difficult. Different computer algebra systems

return different forms of the answer, but they

are all extremely complicated. (If you have a

CAS, try it yourself.) The infinite series answer

that we obtain in Example 8(a) is actually much

easier to deal with than the finite answer

provided by a CAS.

REPRESENTATIONS OF FUNCTIONS AS POWER SERIES

775

Now we integrate term by term:

y

ϱ

ϱ

1

x 7nϩ1

n 7n

n

dx

͑Ϫ1͒

x

dx

C

ϩ

͑Ϫ1͒

y n෇0

͚

͚

1 ϩ x7

7n ϩ 1

n෇0

෇CϩxϪ

Խ

x8

x 15

x 22

ϩ

Ϫ

ϩ иии

8

15

22

Խ

Խ Խ

This series converges for Ϫx 7 Ͻ 1, that is, for x Ͻ 1.

(b) In applying the Fundamental Theorem of Calculus, it doesn’t matter which antiderivative we use, so let’s use the antiderivative from part (a) with C ෇ 0:

y

0.5

0

ͫ

ͬ

1

x8

x 15

x 22

dx ෇ x Ϫ

ϩ

Ϫ

ϩ иии

7

1ϩx

8

15

22

1͞2

0

1

1

1

1

͑Ϫ1͒ n

Ϫ

ϩ

Ϫ

ϩ

и

и

и

ϩ

ϩ иии

2

8 и 28

15 и 2 15

22 и 2 22

͑7n ϩ 1͒2 7nϩ1

This infinite series is the exact value of the definite integral, but since it is an alternating

series, we can approximate the sum using the Alternating Series Estimation Theorem. If

we stop adding after the term with n ෇ 3, the error is smaller than the term with n ෇ 4:

1

Ϸ 6.4 ϫ 10Ϫ11

29 и 2 29

So we have

y

0.5

0

11.9

1

1

1

1

1

dx Ϸ Ϫ

ϩ

Ϫ

Ϸ 0.49951374

7

8

15

1ϩx

2

8ؒ2

15 ؒ 2

22 ؒ 2 22

Exercises

1. If the radius of convergence of the power series ͸ϱn෇0 cn x n

is 10, what is the radius of convergence of the series

͸ϱn෇1 ncn x nϪ1 ? Why?

2. Suppose you know that the series ͸ϱn෇0 bn x n converges for

7. f ͑x͒ ෇

x

9 ϩ x2

8. f ͑x͒ ෇

x

2x 2 ϩ 1

9. f ͑x͒ ෇

1ϩx

1Ϫx

10. f ͑x͒ ෇

x2

a3 Ϫ x3

Խ x Խ Ͻ 2. What can you say about the following series? Why?

ϱ

͚

n෇0

11–12 Express the function as the sum of a power series by first

using partial fractions. Find the interval of convergence.

bn

x nϩ1

n ϩ1

11. f ͑x͒ ෇

3–10 Find a power series representation for the function and determine the interval of convergence.

3. f ͑x͒ ෇

5. f ͑x͒ ෇

;

1

1ϩx

4. f ͑x͒ ෇

2

3Ϫx

6. f ͑x͒ ෇

Graphing calculator or computer required

5

1 Ϫ 4x 2

1

x ϩ 10

3

x2 Ϫ x Ϫ 2

12. f ͑x͒ ෇

xϩ2

2x 2 Ϫ x Ϫ 1

13. (a) Use differentiation to find a power series representation for

f ͑x͒ ෇

1

͑1 ϩ x͒2

What is the radius of convergence?

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_p772-781.qk_97817_11_ch11_p772-781 11/3/10 5:32 PM Page 776

776

CHAPTER 11

INFINITE SEQUENCES AND SERIES

33. Use the result of Example 7 to compute arctan 0.2 correct to

(b) Use part (a) to find a power series for

five decimal places.

1

f ͑x͒ ෇

͑1 ϩ x͒3

34. Show that the function

(c) Use part (b) to find a power series for

ϱ

f ͑x͒ ෇

͚

n෇0

x2

f ͑x͒ ෇

͑1 ϩ x͒3

is a solution of the differential equation

f Љ͑x͒ ϩ f ͑x͒ ෇ 0

14. (a) Use Equation 1 to find a power series representation for

f ͑x͒ ෇ ln͑1 Ϫ x͒. What is the radius of convergence?

(b) Use part (a) to find a power series for f ͑x͒ ෇ x ln͑1 Ϫ x͒.

(c) By putting x ෇ 12 in your result from part (a), express ln 2

as the sum of an infinite series.

15–20 Find a power series representation for the function and

15. f ͑x͒ ෇ ln͑5 Ϫ x͒

17. f ͑x͒ ෇

x

͑1 ϩ 4x͒ 2

19. f ͑x͒ ෇

1ϩx

͑1 Ϫ x͒ 2

͑Ϫ1͒ n x 2n

͑2n͒!

35. (a) Show that J0 (the Bessel function of order 0 given in

Example 4) satisfies the differential equation

x 2 J0Љ͑x͒ ϩ x J0Ј͑x͒ ϩ x 2 J0 ͑x͒ ෇ 0

(b) Evaluate x01 J0 ͑x͒ dx correct to three decimal places.

36. The Bessel function of order 1 is defined by

16. f ͑x͒ ෇ x 2 tanϪ1 ͑x 3 ͒

18. f ͑x͒ ෇

ͩ ͪ

20. f ͑x͒ ෇

x ϩx

͑1 Ϫ x͒ 3

x

2Ϫx

J1͑x͒ ෇

3

ϱ

͚

n෇0

͑Ϫ1͒ n x 2nϩ1

n! ͑n ϩ 1͒! 2 2nϩ1

(a) Show that J1 satisfies the differential equation

2

x 2J1Љ͑x͒ ϩ x J1Ј͑x͒ ϩ ͑x 2 Ϫ 1͒J1͑x͒ ෇ 0

(b) Show that J0Ј͑x͒ ෇ ϪJ1͑x͒.

; 21–24 Find a power series representation for f, and graph f and

37. (a) Show that the function

several partial sums sn͑x͒ on the same screen. What happens as n

increases?

f ͑x͒ ෇

ϱ

͚

n෇0

x

21. f ͑x͒ ෇ 2

x ϩ 16

ͩ ͪ

1ϩx

1Ϫx

23. f ͑x͒ ෇ ln

22. f ͑x͒ ෇ ln͑x ϩ 4͒

2

24. f ͑x͒ ෇ tanϪ1͑2x͒

xn

n!

is a solution of the differential equation

f Ј͑x͒ ෇ f ͑x͒

(b) Show that f ͑x͒ ෇ e x.

38. Let fn ͑x͒ ෇ ͑sin nx͒͞n 2. Show that the series ͸ fn͑x͒

25–28 Evaluate the indefinite integral as a power series. What is

25.

27.

t

y 1Ϫt

8

dt

x 2 ln͑1 ϩ x͒ dx

y

26.

28.

t

y 1ϩt

3

dt

converges for all values of x but the series of derivatives

͸ fnЈ͑x͒ diverges when x ෇ 2n␲, n an integer. For what values

of x does the series ͸ f nЉ͑x͒ converge?

39. Let

tanϪ1 x

dx

x

y

f ͑x͒ ෇

ϱ

͚

n෇1

xn

n2

Find the intervals of convergence for f , f Ј, and f Љ.

29–32 Use a power series to approximate the definite integral to

six decimal places.

29.

31.

y

0.2

0

y

0.1

0

1

dx

1 ϩ x5

x arctan͑3x͒ dx

40. (a) Starting with the geometric series ͸ϱn෇0 x n, find the sum of

the series

ϱ

30.

32.

y

0.4

0

y

0.3

0

ln͑1 ϩ x ͒ dx

4

x2

dx

1 ϩ x4

͚ nx

n෇1

nϪ1

ԽxԽ Ͻ 1

(b) Find the sum of each of the following series.

ϱ

ϱ

n

(i) ͚ n x n, x Ͻ 1

(ii) ͚ n

n෇1

n෇1 2

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).

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_p772-781.qk_97817_11_ch11_p772-781 11/3/10 5:32 PM Page 777

SECTION 11.10

(c) Find the sum of each of the following series.

͚ n͑n Ϫ 1͒x , Խ x Խ Ͻ 1

n

y

n෇2

ϱ

(ii)

͚

n෇2

(iii)

͚

n෇1

n2

2n

41. Use the power series for tan Ϫ1 x to prove the following

expression for ␲ as the sum of an infinite series:

␲ ෇ 2s3

ϱ

͚

n෇0

1͞2

0

ϱ

n2 Ϫ n

2n

͑Ϫ1͒ n

͑2n ϩ 1͒ 3 n

777

42. (a) By completing the square, show that

ϱ

(i)

TAYLOR AND MACLAURIN SERIES

dx

x2 Ϫ x ϩ 1

3s3

(b) By factoring x 3 ϩ 1 as a sum of cubes, rewrite the integral in part (a). Then express 1͑͞x 3 ϩ 1͒ as the sum of a

power series and use it to prove the following formula

for ␲ :

␲෇

3s3

4

ϱ

͚

n෇0

͑Ϫ1͒ n

8n

ͩ

2

1

ϩ

3n ϩ 1

3n ϩ 2

ͪ

11.10 Taylor and Maclaurin Series

In the preceding section we were able to find power series representations for a certain

restricted class of functions. Here we investigate more general problems: Which functions

have power series representations? How can we find such representations?

We start by supposing that f is any function that can be represented by a power series

1

Խx Ϫ aԽ Ͻ R

f ͑x͒ ෇ c0 ϩ c1͑x Ϫ a͒ ϩ c2͑x Ϫ a͒2 ϩ c3͑x Ϫ a͒3 ϩ c4͑x Ϫ a͒4 ϩ и и и

Let’s try to determine what the coefficients cn must be in terms of f . To begin, notice that if

we put x ෇ a in Equation 1, then all terms after the first one are 0 and we get

f ͑a͒ ෇ c0

By Theorem 11.9.2, we can differentiate the series in Equation 1 term by term:

2

f Ј͑x͒ ෇ c1 ϩ 2c2͑x Ϫ a͒ ϩ 3c3͑x Ϫ a͒2 ϩ 4c4͑x Ϫ a͒3 ϩ и и и

Խx Ϫ aԽ Ͻ R

and substitution of x ෇ a in Equation 2 gives

f Ј͑a͒ ෇ c1

Now we differentiate both sides of Equation 2 and obtain

3

f Љ͑x͒ ෇ 2c2 ϩ 2 ؒ 3c3͑x Ϫ a͒ ϩ 3 ؒ 4c4͑x Ϫ a͒2 ϩ и и и

Խx Ϫ aԽ Ͻ R

Again we put x ෇ a in Equation 3. The result is

f Љ͑a͒ ෇ 2c2

Let’s apply the procedure one more time. Differentiation of the series in Equation 3 gives

4

f ٞ͑x͒ ෇ 2 ؒ 3c3 ϩ 2 ؒ 3 ؒ 4c4͑x Ϫ a͒ ϩ 3 ؒ 4 ؒ 5c5͑x Ϫ a͒2 ϩ и и и

Խx Ϫ aԽ Ͻ R

and substitution of x ෇ a in Equation 4 gives

f ٞ͑a͒ ෇ 2 ؒ 3c3 ෇ 3!c3

By now you can see the pattern. If we continue to differentiate and substitute x ෇ a, we

obtain

f ͑n͒͑a͒ ෇ 2 ؒ 3 ؒ 4 ؒ и и и ؒ ncn ෇ n!cn

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_p772-781.qk_97817_11_ch11_p772-781 11/3/10 5:32 PM Page 778

778

CHAPTER 11

INFINITE SEQUENCES AND SERIES

Solving this equation for the nth coefficient cn , we get

cn ෇

f ͑n͒͑a͒

n!

This formula remains valid even for n ෇ 0 if we adopt the conventions that 0! ෇ 1 and

f ͑0͒ ෇ f . Thus we have proved the following theorem.

5

Theorem If f has a power series representation (expansion) at a, that is, if

f ͑x͒ ෇

ϱ

͚ c ͑x Ϫ a͒

n

n

n෇0

Խx Ϫ aԽ Ͻ R

then its coefficients are given by the formula

cn ෇

f ͑n͒͑a͒

n!

Substituting this formula for cn back into the series, we see that if f has a power series

expansion at a, then it must be of the following form.

6

f ͑x͒ ෇

ϱ

͚

n෇0

f ͑n͒͑a͒

͑x Ϫ a͒n

n!

෇ f ͑a͒ ϩ

Taylor and Maclaurin

The Taylor series is named after the English

mathematician Brook Taylor (1685–1731)

and the Maclaurin series is named in honor

of the Scottish mathematician Colin Maclaurin

(1698–1746) despite the fact that the Maclaurin

series is really just a special case of the Taylor

series. But the idea of representing particular

functions as sums of power series goes back

to Newton, and the general Taylor series

was known to the Scottish mathematician

James Gregory in 1668 and to the Swiss

mathematician John Bernoulli in the 1690s.

Taylor was apparently unaware of the work of

Gregory and Bernoulli when he published his

discoveries on series in 1715 in his book

Methodus incrementorum directa et inversa.

Maclaurin series are named after Colin Maclaurin because he popularized them in his calculus

textbook Treatise of Fluxions published in 1742.

f Ј͑a͒

f Љ͑a͒

f ٞ͑a͒

͑x Ϫ a͒ ϩ

͑x Ϫ a͒2 ϩ

͑x Ϫ a͒3 ϩ и и и

1!

2!

3!

The series in Equation 6 is called the Taylor series of the function f at a (or about a or

centered at a). For the special case a ෇ 0 the Taylor series becomes

7

f ͑x͒ ෇

ϱ

͚

n෇0

f ͑n͒͑0͒ n

f Ј͑0͒

f Љ͑0͒ 2

x ෇ f ͑0͒ ϩ

x ϩ иии

n!

1!

2!

This case arises frequently enough that it is given the special name Maclaurin series.

NOTE We have shown that if f can be represented as a power series about a, then f is

equal to the sum of its Taylor series. But there exist functions that are not equal to the sum

of their Taylor series. An example of such a function is given in Exercise 74.

v

EXAMPLE 1 Find the Maclaurin series of the function f ͑x͒ ෇ e x and its radius of

convergence.

SOLUTION If f ͑x͒ ෇ e x, then f ͑n͒͑x͒ ෇ e x, so f ͑n͒͑0͒ ෇ e 0 ෇ 1 for all n. Therefore the

Taylor series for f at 0 (that is, the Maclaurin series) is

ϱ

͚

n෇0

ϱ

f ͑n͒͑0͒ n

xn

x

x2

x3

x ෇ ͚

෇1ϩ

ϩ

ϩ

ϩ иии

n!

1!

2!

3!

n෇0 n!

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.

Xem Thêm
Tải bản đầy đủ (.pdf) (608 trang) ### Tài liệu bạn tìm kiếm đã sẵn sàng tải về

Tải bản đầy đủ ngay
×