Power series derivative

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this is a problem that's bugging me. It's a two part question. I believe I have answered part a. I'm not sure where to start for part b, please help. I will upload the question also the answer for part a. I like the way you have answered my question earlier that's why I'm coming back to you. Thank you for all your helpcenter image description hereenter image description hereenter image description here

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Part a) is correct. The center of the power series is $2$ and its radius of convergence is $3$.

As regard part b), you should know that, for power series, in order to compute the derivative in the interval $(-1,5)$, you can take the derivative term by term (see the section Differentiation and integration here). Hence $$f'(x)=\sum_{n=1}^{\infty}\frac{n^2}{3^n}(x-2)^{n-1}=\sum_{n=0}^{\infty}\frac{(n+1)^2}{3^{n+1}}(x-2)^{n}$$ The radius of convergence of $f'$ is the same one of $f$, that is $3$ (check it!).