Calculus 10th Edition

Published by Brooks Cole
ISBN 10: 1-28505-709-0
ISBN 13: 978-1-28505-709-5

Chapter 8 - Integration Techniques, L'Hopital's Rule, and Improper Integrals - Review Exercises - Page 580: 83

Answer

$\infty$, diverges

Work Step by Step

$\int_{1}^{\infty}x^{2}lnxdx$ $\lim\limits_{a \to \infty}\int_{1}^{a}x^{2}lnxdx$ Perform integration by parts, where the integral is equal to $uv-\int(vdu)$: $u=lnx$ $du=\frac{1}{x}dx$ $\int(v)=\int(x^{2})$ $v=\frac{1}{3}x^{3}$ $\lim\limits_{a \to \infty}[(\frac{1}{3}x^{3})(lnx)-\int(\frac{1}{3}x^{3})(\frac{1}{x})dx]_{1}^{a}$ $\lim\limits_{a \to \infty}[\frac{1}{3}x^{3}lnx-\int(\frac{1}{3}x^{2}))dx]_{1}^{a}$ $\lim\limits_{a \to \infty}[\frac{1}{3}x^{3}lnx-\frac{1}{9}x^{3}]_{1}^{a}$ $\lim\limits_{a \to \infty}[\frac{1}{3}a^{3}lna-\frac{1}{9}a^{3}-\frac{1}{3}(1)^{3}ln(1)+\frac{1}{9}(1)^{3}]$ $\lim\limits_{a \to \infty}[\frac{1}{3}a^{3}lna-\frac{1}{9}a^{3}+\frac{1}{9}]$=$\infty-\infty$ $\infty-\infty$ is indeterminate, so we must manipulate the function: $\lim\limits_{a \to \infty}[\frac{1}{3}a^{3}(lna-\frac{1}{3})+\frac{1}{9}]$=$\infty\times\infty$=$\infty$
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