Calculus: Early Transcendentals (2nd Edition)

Published by Pearson
ISBN 10: 0321947347
ISBN 13: 978-0-32194-734-5

Chapter 7 - Integration Techniques - 7.8 Improper Integrals - 7.8 Exercises - Page 579: 44

Answer

$${\text{The integral diverges}}$$

Work Step by Step

$$\eqalign{ & \int_1^\infty {\frac{{dx}}{{\root 3 \of {x - 1} }}} \cr & {\text{The integrand is not defined for }}x = 1,{\text{ then}} \cr & \int_1^\infty {\frac{{dx}}{{\root 3 \of {x - 1} }}} = \mathop {\lim }\limits_{a \to {1^ + }} \int_a^c {\frac{{dx}}{{\root 3 \of {x - 1} }}} + \mathop {\lim }\limits_{b \to \infty } \int_c^b {\frac{{{dx}}}{{{\sqrt[3]{x-1}}}} } \cr & {\text{Where }}c\,{\text{is a number in the interval }}\left( {1,\infty } \right) \cr & {\text{Integrating}} \cr & = \frac{3}{2}\mathop {\lim }\limits_{a \to {1^ + }} \left[ {{{\left( {x - 1} \right)}^{2/3}}} \right]_a^c + \frac{3}{2}\mathop {\lim }\limits_{b \to \infty } \left[ {{{\left( {x - 1} \right)}^{2/3}}} \right]_c^b \cr & = \frac{3}{2}\mathop {\lim }\limits_{a \to {1^ + }} \left[ {{{\left( {c - 1} \right)}^{2/3}} - {{\left( {a - 1} \right)}^{2/3}}} \right] + \frac{3}{2}\mathop {\lim }\limits_{b \to \infty } \left[ {{{\left( {b - 1} \right)}^{2/3}} - {{\left( {c - 1} \right)}^{2/3}}} \right] \cr & = \frac{3}{2}\mathop {\lim }\limits_{a \to {1^ + }} \left[ {{{\left( {c - 1} \right)}^{2/3}} - {{\left( {a - 1} \right)}^{2/3}}} \right] + \frac{3}{2}\mathop {\lim }\limits_{b \to \infty } \left[ {{{\left( {b - 1} \right)}^{2/3}} - 1} \right] \cr & {\text{Evaluating the limits}} \cr & = \frac{3}{2}\left[ {{{\left( {c - 1} \right)}^{2/3}} - {{\left( {1 - 1} \right)}^{2/3}}} \right] + \frac{3}{2}\left[ \infty \right] \cr & = \frac{3}{2}\left[ {{{\left( {c - 1} \right)}^{2/3}} - {{\left( {1 - 1} \right)}^{2/3}}} \right] + \frac{3}{2}\left[ \infty \right] \cr & = \frac{3}{2}{\left( {c - 1} \right)^{2/3}} + \infty \cr & = \infty \cr & {\text{The integral diverges}} \cr} $$
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