Thomas' Calculus 13th Edition

Published by Pearson
ISBN 10: 0-32187-896-5
ISBN 13: 978-0-32187-896-0

Chapter 10: Infinite Sequences and Series - Practice Exercises - Page 636: 31

Answer

Absolutely convergent

Work Step by Step

The Integral Test states that the series converges if and only if the integral $\int_a^\infty f(x) dx$ converges. Let $a_n=\int_{1}^\infty \dfrac{\ln x dx}{x^3}$ Plug $a=\ln x $ and $da=\dfrac{dx}{x}$ Then $\int_{\ln 1}^{\ln \infty} \dfrac{da}{(e^{(a)})^2}=[\dfrac{e^{-2a}}{-2}]_{0}^{\infty}$ Thus $-(\dfrac{e^{-\infty}}{2})+\dfrac{e^0}{2}=0+\dfrac{1}{2}=\dfrac{1}{2}$ Thus, the series converges by the Integral Test. Also, in the given series all the terms have positive signs, and this means that the absolute series is the same as the series. Hence, the series is absolutely convergent.
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