Calculus: Early Transcendentals 8th Edition

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

Chapter 8 - Section 8.2 - Area of a Surface of Revolution - 8.2 Exercises - Page 556: 19

Answer

$$ y= \frac{1}{5}x^{5}, \quad \quad 0 \leq x \leq 5 $$ The area of the surface obtained by rotating the curve about the x-axis is $$ \begin{aligned} S&=\int 2 \pi y d s \\ &=\int_{0}^{5} \frac{2}{5}\pi x^{5}\sqrt{1+x^{8} } dx\\ & \approx 1,230,507 \quad\quad\left[\text {by using Simpson’s Rule } \right] \\ &\approx 1,227,192. \quad\quad\left[\text {by using a calculator } \right] \\ \end{aligned} $$

Work Step by Step

$$ y= \frac{1}{5}x^{5}, \quad \quad 0 \leq x \leq 5 $$ $\Rightarrow$ $$ y^{\prime} =\frac{dy}{dx}= x^{4} $$ $\Rightarrow$ $$ \begin{aligned} ds &= \sqrt {1+\left(y^{\prime}\right)^{2}} dx \\ &=\sqrt{1+\left(x^{4} \right)^{2}} dx\\ &=\sqrt{1+\left(x^{8} \right)} dx\\ \end{aligned} $$ So, the area of the surface obtained by rotating the curve about the x-axis $$ \begin{aligned} S&=\int 2 \pi y d s \\ &=\int_{0}^{5} 2 \pi ( \frac{1}{5}x^{5})(\sqrt{1+\left(x^{8} \right)} )dx\\ &=\int_{0}^{5} \frac{2}{5}\pi x^{5}\sqrt{1+x^{8} } dx\\ \end{aligned} $$ Simpson’s Rule to approximate the given integral with the specified value of $ n=10$ , with $ n =10 , a = 0$, and $b = 5$ we have $$ \Delta x=\frac{b-a}{n}=\frac{5-0}{10}=\frac{1}{2} $$ Let $$ f(x)=\frac{2}{5}\pi x^{5}\sqrt{1+x^{8} } $$ then $$ \begin{aligned} S &= \int_{0}^{5} \frac{2}{5}\pi x^{5}\sqrt{1+x^{8} } dx\\ & = \frac{\Delta x}{3}\left[f\left(x_{0}\right)\right. +4 f\left(x_{1}\right)+2 f\left(x_{2}\right)+4 f\left(x_{3}\right)+\cdots \\ & \quad \left.+2 f\left(x_{n-2}\right)+4 f\left(x_{n-1}\right)+f\left(x_{n}\right)\right] \\ &\approx S_{10}\\ & \approx \frac{1 / 2}{3}[f(0)+4 f(0.5)+2 f(1)+\\ & \quad\quad+4 f(1.5)+2 f(2)+4 f(2.5)+2 f(3)+\\ & \quad\quad +4 f(3.5)+2 f(4)+4 f(4.5)+f(5)] \\ &\approx 1,230,507 \end{aligned} $$ But the surface area by using the a calculator is approximately $ 1,227,192.$
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