Find the Maclaurin series for using the definition of a Maclaurin series. [Assume that has a power series expansion.Do not show that Rn(x) tends to 0.] Also find the associated radius of convergence. f(x)=(1-x)^{-2}

tinfoQ

tinfoQ

Answered question

2020-12-17

Find the Maclaurin series for using the definition of a Maclaurin series. [Suppose it has a power series expansion. Do not show that Rn(x) tends to 0.] Find the related radius of convergence as well. f(x)=(1x)2

Answer & Explanation

dessinemoie

dessinemoie

Skilled2020-12-18Added 90 answers

Find a few derivatives and determine the values of each one at a=0. Remember that there is a (-1) that comes from the derivative of (1x)
f(x)=(1x)2
f(x)=2(1x)3(1)
=2(1x)3
f(x)=(3)2(1x)4(1)
=(3)(2)(1x)4
f(x)=(4)(3)2(1x)5(1)
=(4)(3)(2)(1x)5
f4(x)=(5)(4)(3)(2)(1x)6(1)
=(5)(4)(3)(2)(1x)5
fn(x)=(n+1)!(1x)(n+2)
Plug everything into Maclaurin general form:
f(x)=f(a)+f(a)1!x+f(a)2!x2+f(a)3!x3+...
f(x)=1+2x+3!2!x2+4!3!x3+5!4!x4+...
=1+2x+3x2+4x3+5x4+...
Find the pattern of the numbers to write in summation form
f(x)=(n=0)(n+1)xn Use the Ratio Test |an+1an|=|((n+1)+1)xn+1(n+1)xn|=|(n+2)xn+1(n+1)xn|=(n+2)|x|n+1
limn(n+2)|x|(n+1)1n1n=limn(1+2n)|x|1+1n=(1+0)|x|(1+0)=|x|
This converges when
|x|<1
The radius of convergence is half the width of this interval
R=1(1)2=1
Result
f(x)=n=0(n+1)xn,R=1

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