Calculus: Early Transcendentals (2nd Edition)

Published by Pearson
ISBN 10: 0321947347
ISBN 13: 978-0-32194-734-5

Chapter 6 - Applications of Integration - 6.6 Surface Area - 6.6 Exercises - Page 457: 19

Answer

$$15 \sqrt {17} \pi$$

Work Step by Step

Let us consider that $f(x)$ is a non-negative function which defines a continuous first derivative on the interval $[a,b]$. The area of the surface obtained when the graph of a function $f(x)$ on the interval $\left[ {a,b} \right]$ is revolved about the $x$- axis can be expressed as: $$S = \int_a^b 2\pi f(x) \sqrt {1 + [f'(x)]^2} dx $$ Rewrite the function as: $y=4x-1 \implies f(y)=\dfrac{y}{4}+\dfrac{1}{4}$ We have: $f(y)=\dfrac{y}{4}+\dfrac{1}{4} \implies f'(y)= \dfrac{1}{4}$ Then $$S=\int_{3}^{15} \dfrac{\pi}{2} (y+1) \sqrt {1+(\dfrac{1}{4})^2} \ dy \\= \dfrac{\pi}{2} \times \int_3^{15} (y+1) \dfrac{\sqrt {17}}{4} \ dy \\=\dfrac{\sqrt {17} \pi}{8} \times (\dfrac{(y+1)^2}{2}) \\=15 \sqrt {17} \pi$$
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