Skip to ContentGo to accessibility pageKeyboard shortcuts menu
OpenStax Logo
Calculus Volume 2

6.2 Properties of Power Series

Calculus Volume 26.2 Properties of Power Series

Learning Objectives

  • 6.2.1 Combine power series by addition or subtraction.
  • 6.2.2 Create a new power series by multiplication by a power of the variable or a constant, or by substitution.
  • 6.2.3 Multiply two power series together.
  • 6.2.4 Differentiate and integrate power series term-by-term.

In the preceding section on power series and functions we showed how to represent certain functions using power series. In this section we discuss how power series can be combined, differentiated, or integrated to create new power series. This capability is particularly useful for a couple of reasons. First, it allows us to find power series representations for certain elementary functions, by writing those functions in terms of functions with known power series. For example, given the power series representation for f(x)=11x,f(x)=11x, we can find a power series representation for f(x)=1(1x)2.f(x)=1(1x)2. Second, being able to create power series allows us to define new functions that cannot be written in terms of elementary functions. This capability is particularly useful for solving differential equations for which there is no solution in terms of elementary functions.

Combining Power Series

If we have two power series with the same interval of convergence, we can add or subtract the two series to create a new power series, also with the same interval of convergence. Similarly, we can multiply a power series by a power of x or evaluate a power series at xmxm for a positive integer m to create a new power series. Being able to do this allows us to find power series representations for certain functions by using power series representations of other functions. For example, since we know the power series representation for f(x)=11x,f(x)=11x, we can find power series representations for related functions, such as

y=3x1x2andy=1(x1)(x3).y=3x1x2andy=1(x1)(x3).

In Combining Power Series we state results regarding addition or subtraction of power series, composition of a power series, and multiplication of a power series by a power of the variable. For simplicity, we state the theorem for power series centered at x=0.x=0. Similar results hold for power series centered at x=a.x=a.

Theorem 6.2

Combining Power Series

Suppose that the two power series n=0cnxnn=0cnxn and n=0dnxnn=0dnxn converge to the functions f and g, respectively, on a common interval I.

  1. The power series n=0(cnxn±dnxn)n=0(cnxn±dnxn) converges to f±gf±g on I.
  2. For any integer m0m0 and any real number b, the power series n=0bxmcnxnn=0bxmcnxn converges to bxmf(x)bxmf(x) on I.
  3. For any integer m0m0 and any real number b, the series n=0cn(bxm)nn=0cn(bxm)n converges to f(bxm)f(bxm) for all x such that bxmbxm is in I.

Proof

We prove i. in the case of the series n=0(cnxn+dnxn).n=0(cnxn+dnxn). Suppose that n=0cnxnn=0cnxn and n=0dnxnn=0dnxn converge to the functions f and g, respectively, on the interval I. Let x be a point in I and let SN(x)SN(x) and TN(x)TN(x) denote the Nth partial sums of the series n=0cnxnn=0cnxn and n=0dnxn,n=0dnxn, respectively. Then the sequence {SN(x)}{SN(x)} converges to f(x)f(x) and the sequence {TN(x)}{TN(x)} converges to g(x).g(x). Furthermore, the Nth partial sum of n=0(cnxn+dnxn)n=0(cnxn+dnxn) is

n=0N(cnxn+dnxn)=n=0Ncnxn+n=0Ndnxn=SN(x)+TN(x).n=0N(cnxn+dnxn)=n=0Ncnxn+n=0Ndnxn=SN(x)+TN(x).

Because

limN(SN(x)+TN(x))=limNSN(x)+limNTN(x)=f(x)+g(x),limN(SN(x)+TN(x))=limNSN(x)+limNTN(x)=f(x)+g(x),

we conclude that the series n=0(cnxn+dnxn)n=0(cnxn+dnxn) converges to f(x)+g(x).f(x)+g(x).

We examine products of power series in a later theorem. First, we show several applications of Combining Power Series and how to find the interval of convergence of a power series given the interval of convergence of a related power series.

Example 6.4

Combining Power Series

Suppose that n=0anxnn=0anxn is a power series whose interval of convergence is (−1,1),(−1,1), and suppose that n=0bnxnn=0bnxn is a power series whose interval of convergence is (−2,2).(−2,2).

  1. Find the interval of convergence of the series n=0(anxn+bnxn).n=0(anxn+bnxn).
  2. Find the interval of convergence of the series n=0an3nxn.n=0an3nxn.

Checkpoint 6.4

Suppose that n=0anxnn=0anxn has an interval of convergence of (−1,1).(−1,1). Find the interval of convergence of n=0an(x2)n.n=0an(x2)n.

In the next example, we show how to use Combining Power Series and the power series for a function f to construct power series for functions related to f. Specifically, we consider functions related to the function f(x)=11xf(x)=11x and we use the fact that

11x=n=0xn=1+x+x2+x3+11x=n=0xn=1+x+x2+x3+

for |x|<1.|x|<1.

Example 6.5

Constructing Power Series from Known Power Series

Use the power series representation for f(x)=11xf(x)=11x combined with Combining Power Series to construct a power series for each of the following functions. Find the interval of convergence of the power series.

  1. f(x)=3x1+x2f(x)=3x1+x2
  2. f(x)=1(x1)(x3)f(x)=1(x1)(x3)

Checkpoint 6.5

Use the series for f(x)=11xf(x)=11x on |x|<1|x|<1 to construct a series for 1(1x)(x2).1(1x)(x2). Determine the interval of convergence.

In Example 6.5, we showed how to find power series for certain functions. In Example 6.6 we show how to do the opposite: given a power series, determine which function it represents.

Example 6.6

Finding the Function Represented by a Given Power Series

Consider the power series n=02nxn.n=02nxn. Find the function f represented by this series. Determine the interval of convergence of the series.

Checkpoint 6.6

Find the function represented by the power series n=013nxn.n=013nxn. Determine its interval of convergence.

Recall the questions posed in the chapter opener about which is the better way of receiving payouts from lottery winnings. We now revisit those questions and show how to use series to compare values of payments over time with a lump sum payment today. We will compute how much future payments are worth in terms of today’s dollars, assuming we have the ability to invest winnings and earn interest. The value of future payments in terms of today’s dollars is known as the present value of those payments.

Example 6.7

Chapter Opener: Present Value of Future Winnings

This is a picture of a stack of money. There are 100 dollar bills wrapped in groups of $10,000.
Figure 6.4 (credit: modification of work by Robert Huffstutter, Flickr)

Suppose you win the lottery and are given the following three options: (1) Receive 20 million dollars today; (2) receive 1.5 million dollars per year over the next 20 years; or (3) receive 1 million dollars per year indefinitely (being passed on to your heirs). Which is the best deal, assuming that the annual interest rate is 5%? We answer this by working through the following sequence of questions.

  1. How much is the 1.5 million dollars received annually over the course of 20 years worth in terms of today’s dollars, assuming an annual interest rate of 5%?
  2. Use the answer to part a. to find a general formula for the present value of payments of C dollars received each year over the next n years, assuming an average annual interest rate r.
  3. Find a formula for the present value if annual payments of C dollars continue indefinitely, assuming an average annual interest rate r.
  4. Use the answer to part c. to determine the present value of 1 million dollars paid annually indefinitely.
  5. Use your answers to parts a. and d. to determine which of the three options is best.

Multiplication of Power Series

We can also create new power series by multiplying power series. Being able to multiply two power series provides another way of finding power series representations for functions.

The way we multiply them is similar to how we multiply polynomials. For example, suppose we want to multiply

n=0cnxn=c0+c1x+c2x2+n=0cnxn=c0+c1x+c2x2+

and

n=0dnxn=d0+d1x+d2x2+.n=0dnxn=d0+d1x+d2x2+.

It appears that the product should satisfy

(n=0cnxn)(n=−0dnxn)=(c0+c1x+c2x2+)·(d0+d1x+d2x2+)=c0d0+(c1d0+c0d1)x+(c2d0+c1d1+c0d2)x2+.(n=0cnxn)(n=−0dnxn)=(c0+c1x+c2x2+)·(d0+d1x+d2x2+)=c0d0+(c1d0+c0d1)x+(c2d0+c1d1+c0d2)x2+.

In Multiplying Power Series, we state the main result regarding multiplying power series, showing that if n=0cnxnn=0cnxn and n=0dnxnn=0dnxn converge on a common interval I, then we can multiply the series in this way, and the resulting series also converges on the interval I.

Theorem 6.3

Multiplying Power Series

Suppose that the power series n=0cnxnn=0cnxn and n=0dnxnn=0dnxn converge to f and g, respectively, on a common interval I. Let

en=c0dn+c1dn1+c2dn2++cn1d1+cnd0=k=0nckdnk.en=c0dn+c1dn1+c2dn2++cn1d1+cnd0=k=0nckdnk.

Then

(n=0cnxn)(n=0dnxn)=n=0enxn(n=0cnxn)(n=0dnxn)=n=0enxn

and

n=0enxnconverges tof(x)·g(x)onI.n=0enxnconverges tof(x)·g(x)onI.

The series n=0enxnn=0enxn is known as the Cauchy product of the series n=0cnxnn=0cnxn and n=0dnxn.n=0dnxn.

We omit the proof of this theorem, as it is beyond the level of this text and is typically covered in a more advanced course. We now provide an example of this theorem by finding the power series representation for

f(x)=1(1x)(1x2)f(x)=1(1x)(1x2)

using the power series representations for

y=11xandy=11x2.y=11xandy=11x2.

Example 6.8

Multiplying Power Series

Multiply the power series representation

11x=n=0xn=1+x+x2+x3+11x=n=0xn=1+x+x2+x3+

for |x|<1|x|<1 with the power series representation

11x2=n=0(x2)n=1+x2+x4+x6+11x2=n=0(x2)n=1+x2+x4+x6+

for |x|<1|x|<1 to construct a power series for f(x)=1(1x)(1x2)f(x)=1(1x)(1x2) on the interval (−1,1).(−1,1).

Checkpoint 6.7

Multiply the series 11x=n=0xn11x=n=0xn by itself to construct a series for 1(1x)(1x).1(1x)(1x).

Differentiating and Integrating Power Series

Consider a power series n=0cnxn=c0+c1x+c2x2+n=0cnxn=c0+c1x+c2x2+ that converges on some interval I, and let ff be the function defined by this series. Here we address two questions about f.f.

  • Is ff differentiable, and if so, how do we determine the derivative f?f?
  • How do we evaluate the indefinite integral f(x)dx?f(x)dx?

We know that, for a polynomial with a finite number of terms, we can evaluate the derivative by differentiating each term separately. Similarly, we can evaluate the indefinite integral by integrating each term separately. Here we show that we can do the same thing for convergent power series. That is, if

f(x)=n=1cnxn=c0+c1x+c2x2+f(x)=n=1cnxn=c0+c1x+c2x2+

converges on some interval I, then

f(x)=c1+2c2x+3c3x2+f(x)=c1+2c2x+3c3x2+

and

f(x)dx=C+c0x+c1x22+c2x33+f(x)dx=C+c0x+c1x22+c2x33+

converges on I. As noted below, behavior at the endpoints of the interval must be investigated individually.

Evaluating the derivative and indefinite integral in this way is called term-by-term differentiation of a power series and term-by-term integration of a power series, respectively. The ability to differentiate and integrate power series term-by-term also allows us to use known power series representations to find power series representations for other functions. For example, given the power series for f(x)=11x,f(x)=11x, we can differentiate term-by-term to find the power series for f(x)=1(1x)2.f(x)=1(1x)2. Similarly, using the power series for g(x)=11+x,g(x)=11+x, we can integrate term-by-term to find the power series for G(x)=ln(1+x),G(x)=ln(1+x), an antiderivative of g. We show how to do this in Example 6.9 and Example 6.10. First, we state Term-by-Term Differentiation and Integration for Power Series, which provides the main result regarding differentiation and integration of power series.

Theorem 6.4

Term-by-Term Differentiation and Integration for Power Series

Suppose that the power series n=0cn(xa)nn=0cn(xa)n converges on the interval (aR,a+R)(aR,a+R) for some R>0.R>0. Let f be the function defined by the series

f(x)=n=0cn(xa)n=c0+c1(xa)+c2(xa)2+c3(xa)3+f(x)=n=0cn(xa)n=c0+c1(xa)+c2(xa)2+c3(xa)3+

for |xa|<R.|xa|<R. Then f is differentiable on the interval (aR,a+R)(aR,a+R) and we can find ff by differentiating the series term-by-term:

f(x)=n=1ncn(xa)n1=c1+2c2(xa)+3c3(xa)2+f(x)=n=1ncn(xa)n1=c1+2c2(xa)+3c3(xa)2+

for |xa|<R.|xa|<R. Also, to find f(x)dx,f(x)dx, we can integrate the series term-by-term. The resulting series converges on (aR,a+R),(aR,a+R), and we have

f(x)dx=C+n=0cn(xa)n+1n+1=C+c0(xa)+c1(xa)22+c2(xa)33+f(x)dx=C+n=0cn(xa)n+1n+1=C+c0(xa)+c1(xa)22+c2(xa)33+

for |xa|<R.|xa|<R.

The proof of this result is beyond the scope of the text and is omitted. Note that although Term-by-Term Differentiation and Integration for Power Series guarantees the same radius of convergence when a power series is differentiated or integrated term-by-term, it says nothing about what happens at the endpoints. It is possible that the differentiated and integrated power series have different behavior at the endpoints than does the original series. We see this behavior in the next examples.

Example 6.9

Differentiating Power Series

  1. Use the power series representation
    f(x)=11x=n=0xn=1+x+x2+x3+f(x)=11x=n=0xn=1+x+x2+x3+

    for |x|<1|x|<1 to find a power series representation for
    g(x)=1(1x)2g(x)=1(1x)2

    on the interval (−1,1).(−1,1). Determine whether the resulting series converges at the endpoints.
  2. Use the result of part a. to evaluate the sum of the series n=0n+14n.n=0n+14n.

Checkpoint 6.8

Differentiate the series 1(1x)2=n=0(n+1)xn1(1x)2=n=0(n+1)xn term-by-term to find a power series representation for 2(1x)32(1x)3 on the interval (−1,1).(−1,1).

Example 6.10

Integrating Power Series

For each of the following functions f, find a power series representation for f by integrating the power series for ff and find its interval of convergence.

  1. f(x)=ln(1+x)f(x)=ln(1+x)
  2. f(x)=tan−1xf(x)=tan−1x

Checkpoint 6.9

Integrate the power series ln(1+x)=n=1(−1)n+1xnnln(1+x)=n=1(−1)n+1xnn term-by-term to evaluate ln(1+x)dx.ln(1+x)dx.

Up to this point, we have shown several techniques for finding power series representations for functions. However, how do we know that these power series are unique? That is, given a function f and a power series for f at a, is it possible that there is a different power series for f at a that we could have found if we had used a different technique? The answer to this question is no. This fact should not seem surprising if we think of power series as polynomials with an infinite number of terms. Intuitively, if

c0+c1x+c2x2+=d0+d1x+d2x2+c0+c1x+c2x2+=d0+d1x+d2x2+

for all values x in some open interval I about zero, then the coefficients cn should equal dn for n0.n0. We now state this result formally in Uniqueness of Power Series.

Theorem 6.5

Uniqueness of Power Series

Let n=0cn(xa)nn=0cn(xa)n and n=0dn(xa)nn=0dn(xa)n be two convergent power series such that

n=0cn(xa)n=n=0dn(xa)nn=0cn(xa)n=n=0dn(xa)n

for all x in an open interval containing a. Then cn=dncn=dn for all n0.n0.

Proof

Let

f(x)=c0+c1(xa)+c2(xa)2+c3(xa)3+=d0+d1(xa)+d2(xa)2+d3(xa)3+.f(x)=c0+c1(xa)+c2(xa)2+c3(xa)3+=d0+d1(xa)+d2(xa)2+d3(xa)3+.

Then f(a)=c0=d0.f(a)=c0=d0. By Term-by-Term Differentiation and Integration for Power Series, we can differentiate both series term-by-term. Therefore,

f(x)=c1+2c2(xa)+3c3(xa)2+=d1+2d2(xa)+3d3(xa)2+,f(x)=c1+2c2(xa)+3c3(xa)2+=d1+2d2(xa)+3d3(xa)2+,

and thus, f(a)=c1=d1.f(a)=c1=d1. Similarly,

f(x)=2c2+3·2c3(xa)+=2d2+3·2d3(xa)+f(x)=2c2+3·2c3(xa)+=2d2+3·2d3(xa)+

implies that f(a)=2c2=2d2,f(a)=2c2=2d2, and therefore, c2=d2.c2=d2. More generally, for any integer n0,f(n)(a)=n!cn=n!dn,n0,f(n)(a)=n!cn=n!dn, and consequently, cn=dncn=dn for all n0.n0.

In this section we have shown how to find power series representations for certain functions using various algebraic operations, differentiation, or integration. At this point, however, we are still limited as to the functions for which we can find power series representations. Next, we show how to find power series representations for many more functions by introducing Taylor series.

Section 6.2 Exercises

63.

If f(x)=n=0xnn!f(x)=n=0xnn! and g(x)=n=0(−1)nxnn!,g(x)=n=0(−1)nxnn!, find the power series of 12(f(x)+g(x))12(f(x)+g(x)) and of 12(f(x)g(x)).12(f(x)g(x)).

64.

If C(x)=n=0x2n(2n)!C(x)=n=0x2n(2n)! and S(x)=n=0x2n+1(2n+1)!,S(x)=n=0x2n+1(2n+1)!, find the power series of C(x)+S(x)C(x)+S(x) and of C(x)S(x).C(x)S(x).

In the following exercises, use partial fractions to find the power series of each function.

65.

4 ( x 3 ) ( x + 1 ) 4 ( x 3 ) ( x + 1 )

66.

3 ( x + 2 ) ( x 1 ) 3 ( x + 2 ) ( x 1 )

67.

5 ( x 2 + 4 ) ( x 2 1 ) 5 ( x 2 + 4 ) ( x 2 1 )

68.

30 ( x 2 + 1 ) ( x 2 9 ) 30 ( x 2 + 1 ) ( x 2 9 )

In the following exercises, express each series as a rational function.

69.

n = 1 1 x n n = 1 1 x n

70.

n = 1 1 x 2 n n = 1 1 x 2 n

71.

n = 1 1 ( x 3 ) 2 n 1 n = 1 1 ( x 3 ) 2 n 1

72.

n = 1 ( 1 ( x 3 ) 2 n 1 1 ( x 2 ) 2 n 1 ) n = 1 ( 1 ( x 3 ) 2 n 1 1 ( x 2 ) 2 n 1 )

The following exercises explore applications of annuities.

73.

Calculate the present values P of an annuity in which $10,000 is to be paid out annually for a period of 20 years, assuming interest rates of r=0.03,r=0.05,r=0.03,r=0.05, and r=0.07.r=0.07.

74.

Calculate the present values P of annuities in which $9,000 is to be paid out annually perpetually, assuming interest rates of r=0.03,r=0.05r=0.03,r=0.05 and r=0.07.r=0.07.

75.

Calculate the annual payouts C to be given for 20 years on annuities having present value $100,000 assuming respective interest rates of r=0.03,r=0.05,r=0.03,r=0.05, and r=0.07.r=0.07.

76.

Calculate the annual payouts C to be given perpetually on annuities having present value $100,000 assuming respective interest rates of r=0.03,r=0.05,r=0.03,r=0.05, and r=0.07.r=0.07.

77.

Suppose that an annuity has a present value P=1million dollars.P=1million dollars. What interest rate r would allow for perpetual annual payouts of $50,000?

78.

Suppose that an annuity has a present value P=10million dollars.P=10million dollars. What interest rate r would allow for perpetual annual payouts of $100,000?

In the following exercises, express the sum of each power series in terms of geometric series, and then express the sum as a rational function.

79.

x+x2x3+x4+x5x6+x+x2x3+x4+x5x6+ (Hint: Group powers x3k, x3k1,x3k1, and x3k2.)x3k2.)

80.

x+x2x3x4+x5+x6x7x8+x+x2x3x4+x5+x6x7x8+ (Hint: Group powers x4k, x4k1,x4k1, etc.)

81.

xx2x3+x4x5x6+x7xx2x3+x4x5x6+x7 (Hint: Group powers x3k, x3k1,x3k1, and x3k2.)x3k2.)

82.

x2+x24x38+x416+x532x664+x2+x24x38+x416+x532x664+ (Hint: Group powers (x2)3k,(x2)3k1,(x2)3k,(x2)3k1, and (x2)3k2.)(x2)3k2.)

In the following exercises, find the power series of f(x)g(x)f(x)g(x) given f and g as defined.

83.

f ( x ) = 2 n = 0 x n , g ( x ) = n = 0 n x n f ( x ) = 2 n = 0 x n , g ( x ) = n = 0 n x n

84.

f(x)=n=1xn,g(x)=n=11nxn.f(x)=n=1xn,g(x)=n=11nxn. Express the coefficients of f(x)g(x)f(x)g(x) in terms of Hn=k=1n1k.Hn=k=1n1k.

85.

f ( x ) = g ( x ) = n = 1 ( x 2 ) n f ( x ) = g ( x ) = n = 1 ( x 2 ) n

86.

f ( x ) = g ( x ) = n = 1 n x n f ( x ) = g ( x ) = n = 1 n x n

In the following exercises, differentiate the given series expansion of f term-by-term to obtain the corresponding series expansion for the derivative of f.

87.

f ( x ) = 1 1 + x = n = 0 ( −1 ) n x n f ( x ) = 1 1 + x = n = 0 ( −1 ) n x n

88.

f ( x ) = 1 1 x 2 = n = 0 x 2 n f ( x ) = 1 1 x 2 = n = 0 x 2 n

In the following exercises, integrate the given series expansion of ff term-by-term from zero to x to obtain the corresponding series expansion for the indefinite integral of f.f.

89.

f ( x ) = 2 x ( 1 + x 2 ) 2 = n = 1 ( −1 ) n ( 2 n ) x 2 n 1 f ( x ) = 2 x ( 1 + x 2 ) 2 = n = 1 ( −1 ) n ( 2 n ) x 2 n 1

90.

f ( x ) = 2 x 1 + x 2 = 2 n = 0 ( −1 ) n x 2 n + 1 f ( x ) = 2 x 1 + x 2 = 2 n = 0 ( −1 ) n x 2 n + 1

In the following exercises, evaluate each infinite series by identifying it as the value of a derivative or integral of geometric series.

91.

Evaluate n=1n2nn=1n2n as f(12)f(12) where f(x)=n=0xn.f(x)=n=0xn.

92.

Evaluate n=1n3nn=1n3n as f(13)f(13) where f(x)=n=0xn.f(x)=n=0xn.

93.

Evaluate n=2n(n1)2nn=2n(n1)2n as f(12)f(12) where f(x)=n=0xn.f(x)=n=0xn.

94.

Evaluate n=0(−1)n2n+1n=0(−1)n2n+1 as 01f(t)dt01f(t)dt where f(x)=n=0(−1)nx2n=11+x2.f(x)=n=0(−1)nx2n=11+x2.

In the following exercises, given that 11x=n=0xn,11x=n=0xn, use term-by-term differentiation or integration to find power series for each function centered at the given point.

95.

f(x)=lnxf(x)=lnx centered at x=1x=1 (Hint: x=1(1x))x=1(1x))

96.

ln(1x)ln(1x) at x=0x=0

97.

ln(1x2)ln(1x2) at x=0x=0

98.

f(x)=2x(1x2)2f(x)=2x(1x2)2 at x=0x=0

99.

f(x)=tan−1(x2)f(x)=tan−1(x2) at x=0x=0

100.

f(x)=ln(1+x2)f(x)=ln(1+x2) at x=0x=0

101.

f(x)=0xlntdtf(x)=0xlntdt where ln(x)=n=1(−1)n1(x1)nnln(x)=n=1(−1)n1(x1)nn

102.

[T] Evaluate the power series expansion ln(1+x)=n=1(−1)n1xnnln(1+x)=n=1(−1)n1xnn at x=1x=1 to show that ln(2)ln(2) is the sum of the alternating harmonic series. Use the alternating series test to determine how many terms of the sum are needed to estimate ln(2)ln(2) accurate to within 0.001, and find such an approximation.

103.

[T] Subtract the infinite series of ln(1x)ln(1x) from ln(1+x)ln(1+x) to get a power series for ln(1+x1x).ln(1+x1x). Evaluate at x=13.x=13. What is the smallest N such that the Nth partial sum of this series approximates ln(2)ln(2) with an error less than 0.001?

In the following exercises, using a substitution if indicated, express each series in terms of elementary functions and find the radius of convergence of the sum.

104.

k = 0 ( x k x 2 k + 1 ) k = 0 ( x k x 2 k + 1 )

105.

k = 1 x 3 k 6 k k = 1 x 3 k 6 k

106.

k=1(1+x2)kk=1(1+x2)k using y=11+x2y=11+x2

107.

k=12kxk=12kx using y=2xy=2x

108.

Show that, up to powers x3 and y3, E(x)=n=0xnn!E(x)=n=0xnn! satisfies E(x+y)=E(x)E(y).E(x+y)=E(x)E(y).

109.

Differentiate the series E(x)=n=0xnn!E(x)=n=0xnn! term-by-term to show that E(x)E(x) is equal to its derivative.

110.

Show that if f(x)=n=0anxnf(x)=n=0anxn is a sum of even powers, that is, an=0an=0 if n is odd, then F=0xf(t)dtF=0xf(t)dt is a sum of odd powers, while if f is a sum of odd powers, then F is a sum of even powers.

111.

[T] Suppose that the coefficients an of the series n=0anxnn=0anxn are defined by the recurrence relation an=an1n+an2n(n1).an=an1n+an2n(n1). For a0=0a0=0 and a1=1,a1=1, compute and plot the sums SN=n=0NanxnSN=n=0Nanxn for N=2,3,4,5N=2,3,4,5 on [−1,1].[−1,1].

112.

[T] Suppose that the coefficients an of the series n=0anxnn=0anxn are defined by the recurrence relation an=an1nan2n(n1).an=an1nan2n(n1). For a0=1a0=1 and a1=0,a1=0, compute and plot the sums SN=n=0NanxnSN=n=0Nanxn for N=2,3,4,5N=2,3,4,5 on [−1,1].[−1,1].

113.

[T] Given the power series expansion ln(1+x)=n=1(−1)n1xnn,ln(1+x)=n=1(−1)n1xnn, determine how many terms N of the sum evaluated at x=−1/2x=−1/2 are needed to approximate ln(2)ln(2) accurate to within 1/1000. Evaluate the corresponding partial sum n=1N(−1)n1xnn.n=1N(−1)n1xnn.

114.

[T] Given the power series expansion tan−1(x)=k=0(−1)kx2k+12k+1,tan−1(x)=k=0(−1)kx2k+12k+1, use the alternating series test to determine how many terms N of the sum evaluated at x=1x=1 are needed to approximate tan−1(1)=π4tan−1(1)=π4 accurate to within 1/1000. Evaluate the corresponding partial sum k=0N(−1)kx2k+12k+1.k=0N(−1)kx2k+12k+1.

115.

[T] Recall that tan−1(13)=π6.tan−1(13)=π6. Assuming an exact value of (13),(13), estimate π6π6 by evaluating partial sums SN(13)SN(13) of the power series expansion tan−1(x)=k=0(−1)kx2k+12k+1tan−1(x)=k=0(−1)kx2k+12k+1 at x=13.x=13. What is the smallest number N such that 6SN(13)6SN(13) approximates π accurately to within 0.001? How many terms are needed for accuracy to within 0.00001?

Order a print copy

As an Amazon Associate we earn from qualifying purchases.

Citation/Attribution

This book may not be used in the training of large language models or otherwise be ingested into large language models or generative AI offerings without OpenStax's permission.

Want to cite, share, or modify this book? This book uses the Creative Commons Attribution-NonCommercial-ShareAlike License and you must attribute OpenStax.

Attribution information
  • If you are redistributing all or part of this book in a print format, then you must include on every physical page the following attribution:
    Access for free at https://openstax.org/books/calculus-volume-2/pages/1-introduction
  • If you are redistributing all or part of this book in a digital format, then you must include on every digital page view the following attribution:
    Access for free at https://openstax.org/books/calculus-volume-2/pages/1-introduction
Citation information

© Feb 5, 2024 OpenStax. Textbook content produced by OpenStax is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike License . The OpenStax name, OpenStax logo, OpenStax book covers, OpenStax CNX name, and OpenStax CNX logo are not subject to the Creative Commons license and may not be reproduced without the prior and express written consent of Rice University.