Calculus 8th Edition

Published by Cengage
ISBN 10: 1285740629
ISBN 13: 978-1-28574-062-1

Chapter 8 - Further Applications of Integration - 8.2 Area of a Surface of Revolution - 8.2 Exercises - Page 595: 7

Answer

$$\frac{\pi}{27}(145 \sqrt{145}-1) $$

Work Step by Step

Given $$y=x^ 3, \ \ \ \ \ 0 \leq x \leq 2$$ Since $$\frac{dy}{dx}= 3x^2$$ Then \begin{aligned} S&= 2 \pi \int_{0}^{2} y\sqrt{1+\left[\frac{dy}{d x}\right]^{2}} d x\\ S&=2 \pi \int_{0}^{2} x^{3} \sqrt{1+\left(3 x^{2}\right)^{2}} d x\\ S&=2 \pi \int_{0}^{2} x^{3} \sqrt{1+9 x^{4}} d x \end{aligned} Let $$u=1+9x^4\ \ \ \ \ du=18xdx$$ and $$x=0\ \to\ u=1,\ \ x=2 \to \ u=145 $$ Then \begin{aligned} S&=2 \pi \int_{1}^{145} \sqrt{u} \frac{d u}{36}\\ & =\frac{\pi}{18} \int_{1}^{145} u^{1 / 2} d u\\ &=\frac{\pi}{18}\left[\frac{2}{3} u^{3 / 2}\right]_{1}^{145}\\ &=\frac{\pi}{18}\left[\frac{2}{3}(145)^{3 / 2}-\frac{2}{3}\right]\\ &=\frac{\pi}{18} \cdot \frac{2}{3}[145 \sqrt{145}-1]\\ &=\frac{\pi}{27}(145 \sqrt{145}-1) \end{aligned}
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