# Volumes for Two Functions

If the region bounded by the upper function $y_{upper}=f(x)$ and the lower funciton $y_{lower}=g(x)$, and the lines $x=a$ and $x=b$ is revolved about the $x$-axis, then its volume of revolution is given by:

$$ \begin{align} \displaystyle

V &= \int_{a}^{b}{\Big([f(x)]^2-[g(x)]^2\Big)}dx \\

&= \int_{a}^{b}{\Big(y_{upper}^2-y_{lower}^2\Big)}dx

\end{align} $$

### Example 1

Find the volume of revolution generated by revolving the region between $y=x^2$ and $y=\sqrt{x}$ about the $x$-axis.

\( \begin{align} \displaystyle

x^2 &= \sqrt{x} \\

x^4 &= x \\

x^4-x &= 0 \\

x(x^3-1) &= 0 \\

x &= 0 \text{ and } x=1 \\

V &= \pi \int_{0}^{1}{\big[\sqrt{x}^2-(x^2)^2\big]}dx \\

&= \pi \int_{0}^{1}{(x-x^4)}dx \\

&= \pi \bigg[\dfrac{x^2}{2}-\dfrac{x^5}{5}\bigg]_{0}^{1} \\

&= \pi \bigg[\dfrac{1^2}{2}-\dfrac{1^5}{5}\bigg]-\pi \bigg[\dfrac{0^2}{2}-\dfrac{0^5}{5}\bigg] \\

&= \dfrac{3 \pi}{10} \text{ units}^3

\end{align} \)

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