Calculus 8th Edition

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

Chapter 5 - Applications of Integration - Review - Exercises - Page 393: 7

Answer

$\frac{64}{15} \pi$

Work Step by Step

Given $$ y=x^2,\ \ \ \ y= 2x$$ First, we find the intersection points \begin{aligned} x^2&=2x\\ x^2-2x&=0\\ x(x-2)&=0 \end{aligned} Then $$x=0,\ \ x=2 $$ Since $$2x\geq x^2,\ \ \ \ 0\leq x\leq 2 $$ Then the outer radius $R= 2x$ and the inner $r= x^2$ Hence, by using washer method, we have \begin{aligned} V &=\pi\int_a^b[R^2-r^2]dx\\ & =\pi \int_0^2\left[(2 x)^2-\left(x^2\right)^2\right] d x\\ &=\pi \int_0^2\left(4 x^2-x^4\right) d x \\ & =\pi\left[\frac{4}{3} x^3-\frac{1}{5} x^5\right]_0^2\\ &=\pi\left(\frac{32}{3}-\frac{32}{5}\right) \\ & =32 \pi \cdot \frac{2}{15}\\ &=\frac{64}{15} \pi \end{aligned}
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