Calculus, 10th Edition (Anton)

Published by Wiley
ISBN 10: 0-47064-772-8
ISBN 13: 978-0-47064-772-1

Chapter 9 - Infinite Series - 9.6 Alternating Series; Absolute And Conditional Convergence - Exercises Set 9.6 - Page 646: 6

Answer

Converges

Work Step by Step

Let ${a_k} = \dfrac{{\ln k}}{k}$. Condition 1. Examine if ${a_k} \gt {a_{k + 1}}$. We plot the graph of $f\left( x \right) = \dfrac{{\ln x}}{x}$. The figure attached shows that $f$ is decreasing for $x \ge 3$. Thus, for $k \ge 3$, we have ${a_k} - {a_{k + 1}} \gt 0$. Hence, ${a_k} \gt {a_{k + 1}}$. Condition 2. Examine if $\mathop {\lim }\limits_{k \to \infty } {a_k} = 0$. $\mathop {\lim }\limits_{k \to \infty } {a_k} = \mathop {\lim }\limits_{k \to \infty } \dfrac{{\ln k}}{k}$ Applying L'Hôpital's Rule, we get $\mathop {\lim }\limits_{k \to \infty } {a_k} = \mathop {\lim }\limits_{k \to \infty } \dfrac{{\ln k}}{k} = \mathop {\lim }\limits_{k \to \infty } \dfrac{{\dfrac{1}{k}}}{1} = \mathop {\lim }\limits_{k \to \infty } \dfrac{1}{k} = 0$ Since the two conditions above are satisfied, by Theorem 9.6.1, the alternating series converges.
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