Multivariable Calculus, 7th Edition

Published by Brooks Cole
ISBN 10: 0-53849-787-4
ISBN 13: 978-0-53849-787-9

Chapter 11 - Infinite Sequences and Series - 11.3 Exercises - Page 745: 23

Answer

Convergent

Work Step by Step

Let $f(x)=\frac{e^{1/x}}{x^2}$. This function is continuous and decreasing on $[1,\infty)$. Evaluate $\int_1^\infty f(x)dx$: $\int_1^\infty \frac{e^{1/x}}{x^2}dx=\int_1^\infty -e^{1/x}\cdot -\frac{1}{x^2}dx$ (Let $u=\frac{1}{x}\to du=-\frac{1}{x^2}dx$) $=\int_1^0 -e^udu$ $=\int_0^1e^udu$ $=e^u]_0^1$ $=e^1-e^0$ $=e-1$ So, the integral is convergent. Using the Integral Test for Series, $\sum_{n=1}^\infty \frac{e^{1/n}}{n^2}$ is convergent.
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