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 - Section 10.3 - The Integral Test - Exercises 10.3 - Page 586: 6

Answer

converges to $\dfrac{1}{\ln 2}$

Work Step by Step

Let us consider $f(x)=\dfrac{1}{x(\ln x)^2}$. Here, the function $f(x)$ is positive, continuous and decreasing for $x \geq 1$ Then $\int_2^\infty \dfrac{1}{x(\ln x)^2} dx= \lim\limits_{k \to \infty} \int_2^k \dfrac{1}{x(\ln x)^2} dx=\lim\limits_{k \to \infty} [\dfrac{-1}{\ln x}]_2^k$ and $\lim\limits_{k \to \infty} [\dfrac{-1}{\ln k} +\dfrac{1}{\ln 2}]= \dfrac{1}{\ln 2}$ Thus, the sequence $\Sigma_{n=2}^\infty \dfrac{1}{n(\ln n)^2}$ converges to $\dfrac{1}{\ln 2}$.
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