Multivariable Calculus, 7th Edition

Published by Brooks Cole
ISBN 10: 0-53849-787-4
ISBN 13: 978-0-53849-787-9

Chapter 15 - Multiple Integrals - 15.2 Exercises - Page 1012: 35

Answer

The average value of $f$ is $\frac{5}{6}$.

Work Step by Step

The region $R$ is a rectangle with vertices $(-1,0)$, $(-1,5)$, $(1,5)$, and $(1,0)$ or it can be written as the set of points $R=\{(x,y)|-1\leq x\leq 1, 0\leq y\leq 5\}$. The area of $R$ is $A=length\times width=\Delta x\cdot\Delta y=(1-(-1))\cdot (5-0)=10$. Find the average value of $f$ over $R$: $\overline{f}=\frac{1}{A}\iint_R f(x,y)dA$ $=\frac{1}{10}\int_{-1}^1\int_0^5x^2ydydx$ $=\frac{1}{10}\int_{-1}^1\frac{x^2y^2}{2}]_0^5dx$ $=\frac{1}{10}\int_{-1}^1\frac{25x^2}{2}dx$ $=\frac{1}{10}[\frac{25x^3}{6}]_{-1}^1$ $=\frac{1}{10}(\frac{25}{6}-(-\frac{25}{6}))$ $=\frac{5}{6}$
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