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: 20

Answer

Divergent

Work Step by Step

Let us consider $f(x)=\dfrac{\ln x}{\sqrt x}$ Here, the function $f(x)$ is positive, continuous for $x \geq 2$ and $f(x)$ is decreasing for $x \gt 3$. Then $\lim\limits_{k \to \infty} \int_2^k \dfrac{\ln x}{\sqrt x}dx= \lim\limits_{k \to \infty} [(2) (\ln x)e^{\ln x/2} -(4) (e^{\ln x/2})]_{\ln 2}^k$ and $\lim\limits_{k \to \infty} [(2) (\ln x) (e^{\ln x/2}) -(4) (e^{\ln x/2})]_{\ln 2}^k=\infty$ Thus, the sequence $\Sigma_{n=2}^\infty \dfrac{\ln n}{\sqrt n}$ is Divergent.
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