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 10.4 - Comparison Tests - Exercises 10.4 - Page 591: 28

Answer

Converges

Work Step by Step

Let $u_n=\dfrac{(\ln n)^2}{n^3}$ and $v_n=\dfrac{1}{ n^2}$ Now, $\lim\limits_{n \to \infty}\dfrac{u_n}{v_n} =\lim\limits_{n \to \infty}\dfrac{(\dfrac{\ln n}{n^3})^2}{1/ n^2}$ But $\lim\limits_{n \to \infty} \dfrac{(\ln n)^2 }{n}=\dfrac{\infty}{\infty}$ We will have to apply L-Hospital's rule. This implies that $ \lim\limits_{n \to \infty} \dfrac{2 (\ln n)(\dfrac{1}{n})}{1}=0$ Thus, the series converges by the comparison test.
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