Calculus 10th Edition

Published by Brooks Cole
ISBN 10: 1-28505-709-0
ISBN 13: 978-1-28505-709-5

Chapter 9 - Infinite Series - 9.6 Exercises - Page 634: 59

Answer

The series converges by limit comparison.

Work Step by Step

$\Sigma_{n=1}^{\infty}\frac{10n+3}{n2^{n}}$ We can compare this series to the series $\Sigma_{n=1}^{\infty}\frac{10n}{n2^{n}}=10\Sigma_{n=1}^{\infty}\frac{1}{2^{n}}=10\Sigma_{n=1}^{\infty}(\frac{1}{2})^{n}$ The series $10\Sigma_{n=1}^{\infty}(\frac{1}{2})^{n}$ converges by the geometric series test, where $|r|=\frac{1}{2}$ and $0\lt\frac{1}{2}\lt1$. If $\frac{10n+3}{n2^{n}}\leq(\frac{1}{2})^{n}$, then the original series also converges. This does not work, so instead let's use limit comparison: $\lim\limits_{n \to \infty}\frac{a_n}{b_n}=L$ If $L$ is a finite and positive number, then either both series converge or both diverge, based on the convergence or divergence of $b_n$: $\lim\limits_{n\to \infty}\frac{10n+3}{n2^{n}}\times\frac{2^n}{1}$ $\lim\limits_{n\to \infty}\frac{10n+3}{n}=\frac{\infty}{\infty}$ $\frac{\infty}{\infty}$ is an indeterminate form, so let's use L'hopital's rule. $\lim\limits_{n \to \infty}\frac{10}{1}=10$ Since $10$ is a finite and positive integer, the original series converges because $10\Sigma_{n=1}^{\infty}(\frac{1}{2})^{n}$ converges. Therefore, the original series converges by limit comparison.
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