- #1
knowLittle
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Homework Statement
for
a.) f(x) =1/ ( (1+x)^2 )
what is the radius of convergence?
b.) Use part a.) to find a power series for
f(x)=1/ ( (1+x)^3)
c.) Use part b.) to find a power series for
f(x) =x^2 /( (1+x)^3)
Homework Equations
I want to check my work.
I used properties of functions defined by power series.
The Attempt at a Solution
a.)
## \dfrac {d} {dx}\left[ \left( -1\right) \left( 1+x\right) ^{-1}\right] =\sum _{n=0}\left( -1\right) \left( -1\right) ^{n}x^{n}##
after differentiating:
1/ ( (1+x)^2) =SUM[ -n x^(n-1) (-1)^n ]
b.)
##\dfrac {d} {dx}\left[ \left( 1+x\right) ^{-2}\right] =\dfrac {d} {dx}\left[ \sum _{n=0}-nx^{n-1}\left( -1\right) ^{n}\right]##
Differentiating:
2/( (1+x)^3)= SUM [ n(n-1) x^(n-2) (-1)^n ], and multiplying both sides by (1/2) yields the desired fuction.
c.)
Only multiply both sides by x^2 and the desired function is found.
Is this correct?
Thank you.