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: 19

Answer

Divergent

Work Step by Step

Let us consider $f(x)=\dfrac{\ln x}{x}$ Here, the function $f(x)$ is positive, continuous for $x \geq 2$ and $f(x)$ is decreasing for $x \gt 3$ Then $\int_3^\infty \dfrac{\ln x}{x}dx= \lim\limits_{k \to \infty} \int_3^k \dfrac{\ln x}{x}dx=\lim\limits_{k \to \infty} [(\dfrac{1}{2})(\ln^2 x)]_3^k$ and $ \lim\limits_{k \to \infty} [(\dfrac{1}{2})(\ln^2 (k)-\ln^2 (3))]= \infty$ Thus, the sequence $\Sigma_{n=2}^\infty \dfrac{\ln n}{n}$ is Divergent.
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