Calculus 8th Edition

Published by Cengage
ISBN 10: 1285740629
ISBN 13: 978-1-28574-062-1

Chapter 11 - Infinite Sequences and Series - 11.4 The Comparison Tests - 11.4 Exercises - Page 771: 28

Answer

Divergent

Work Step by Step

To use the Direct Comparison Test, we need a known series $\Sigma_{n=1}^{\infty}b_{n}$ for the purpose of comparison. We will choose the harmonic series $b_{n}=\frac{1}{n}$ Now,$a_{n}=\frac{e^{1/n}}{n}$ and $b_{n}=\frac{1}{n}$ $\lim\limits_{n \to \infty}\frac{a_{n}}{b_{n}}=\lim\limits_{n \to \infty}\frac{\frac{e^{1/n}}{n}}{\frac{1}{n}}$ Since, $ e\approx 2.71 \gt 1$ $\sqrt[n] e=\sqrt[n] {2.71}\gt \sqrt[n] 1$ $\frac{e^{1/n}}{n} \gt \frac{1}{n}$ $=(1+0)^{2}\ne 1 \ne 0 \ne \infty $ for all $n$ The Comparison Test states that the p-series $\sum_{n=1}^{\infty}\frac{1}{n^{p}}$ is convergent if $p\gt 1$ and divergent if $p\leq 1$. Since,$\Sigma_{n=1}^{\infty}\frac{1}{n}$ is divergent by p- series Comparison Test with $p =1$ and so, $\Sigma \frac{e^{1/n}}{n}$ also diverges. Hence, $\Sigma_{n=1}^{\infty} \frac{e^{1/n}}{n}$ diverges by the direct comparison test.
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