Calculus: Early Transcendentals 8th Edition

Published by Cengage Learning
ISBN 10: 1285741552
ISBN 13: 978-1-28574-155-0

Chapter 10 - Section 10.2 - Calculus with Parametric Curves - 10.2 Exercises - Page 656: 61

Answer

$$ x= t^{3}, \quad y= t^{2} , \quad 0 \leq t \leq 1 $$ The exact area of the surface obtained by rotating the given curve about the x-axis is: $$ S=\int 2 \pi y d s=\int_{0}^{1} 2 \pi t^{2} \sqrt{9 t^{4}+4 t^{2} } d t=\frac{2 \pi}{1215}(247 \sqrt{13}+64). $$

Work Step by Step

$$ x= t^{3}, \quad y= t^{2} , \quad 0 \leq t \leq 1 $$ then $$ d x / d t=3 t^{2} , \quad d y / d t =2 t $$ so $$ \begin{aligned}\left(\frac{d x}{d t}\right)^{2}+\left(\frac{d y}{d t}\right)^{2} &=\left(3 t^{2} \right)^{2}+\left(2 t\right)^{2} \\ &=9 t^{4}+4 t^{2}, \end{aligned} $$ thus the surface area of the curve, which is obtained by rotating the given curve about the x-axis, is $$ \begin{aligned} S&=\int 2 \pi y d s=\int_{0}^{1} 2 \pi t^{2} \sqrt{9 t^{4}+4 t^{2} } d t \\ &=2 \pi\int_{0}^{1} t^{2} \sqrt{t^{2}(9 t^{2}+4 )} d t \\ &\quad\quad \quad\left[\begin{array}{l}{ \text {Let }u=9 t^{2}+4, \quad \text { then } t^{2}=(u-4) / 9} \\ {d u=18 t d t, \quad \text {and so }\quad td t=\frac{1}{18} d u}\end{array}\right]\\ &=2 \pi \int_{4}^{13}\left(\frac{u-4}{9}\right) \sqrt{u}\left(\frac{1}{18} d u\right) \\ &=\frac{2 \pi}{9 \cdot 18} \int_{4}^{13}\left(u^{3 / 2}-4 u^{1 / 2}\right) d u \\ & =\frac{\pi}{81}\left[\frac{2}{5} u^{5 / 2}-\frac{8}{3} u^{3 / 2}\right]_{4}^{13} \\ &=\frac{\pi}{81} \cdot \frac{2}{15}\left[3 u^{5 / 2}-20 u^{3 / 2}\right]_{4}^{13}\\ &=\frac{2 \pi}{1215}\left[\left(3 \cdot 13^{2} \sqrt{13}-20 \cdot 13 \sqrt{13}\right)-(3 \cdot 32-20 \cdot 8)\right]\\ &=\frac{2 \pi}{1215}(247 \sqrt{13}+64). \end{aligned} $$ The exact area of the surface obtained by rotating the given curve about the x-axis is $$ S=\int 2 \pi y d s=\int_{0}^{1} 2 \pi t^{2} \sqrt{9 t^{4}+4 t^{2} } d t=\frac{2 \pi}{1215}(247 \sqrt{13}+64). $$
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