Calculus (3rd Edition)

Published by W. H. Freeman
ISBN 10: 1464125260
ISBN 13: 978-1-46412-526-3

Chapter 11 - Infinite Series - 11.5 The Ratio and Root Tests and Strategies for Choosing Tests - Exercises - Page 568: 22


The series converges when $|x|<1$.

Work Step by Step

Given $$\sum_{n=1}^{\infty} n^2x^{n}$$ By using the Ratio Test, we get: \begin{align*} \rho&=\lim _{n \rightarrow \infty}\left|\frac{a_{n+1}}{a_{n}}\right|\\ &=\lim _{n \rightarrow \infty}\left| \frac{(n+1)^2x^{n+1}}{ n^2x^{n}}\right|\\ &= \lim _{n \rightarrow \infty} \frac{ (n+1)^{2}}{n^2}|x|\\ &= |x| \end{align*} Thus the series converges when $|x|<1$.
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