Calculus, 10th Edition (Anton)

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

Chapter 9 - Infinite Series - 9.5 The Comparison, Ratio, And Root Tests - Exercises Set 9.5 - Page 637: 44

Answer

Converges

Work Step by Step

Apply the integral test. Let us consider that $\Sigma a_k$ is a series with positive terms such that $a_k=f(k)$ for all $k \geq a$: $\Sigma_{k=1}^{\infty}$ and $\int_a^{\infty} f(x) \ dx$ diverge or converge. We can see that $\Sigma_{k=1}^{\infty} \dfrac{\sqrt k \ln k}{k^3+1} \lt \dfrac{k \ln k}{k^3}=\dfrac{\ln k}{k^2}$ Therefore, $\int_2^{\infty} \dfrac{\ln x}{x^2} \ dx=\lim\limits_{z \to \infty}\int_2^{z} \dfrac{\ln x}{x^2} \ dx\\=\lim\limits_{b \to \infty} [\dfrac{-\ln x}{x}-\dfrac{1}{x}]_{2}^z\\=\dfrac{1}{2}[\ln (2)+1] \lt \infty$ So, we can conclude that the given series converges by the integral test.
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