disks washer method

A method (two methods?) for solving for the volume of a solid of revolution.

The volume of the solid can be thought of as the sum of the areas of infinitely many circles that are infinitely thin. (Disk method)

$V=\pi {\int }_a^b{r}^2(x)dx$

When the bounded area doesn’t touch the axis of revolution: (Washer method: outer circle area - inner empty circle area) $V=2\pi {\int }_a^b[R^2(x)-r^2(x)]dx$ where $R(x)$ is the function representing the outer curve and $r(x)$ is the one representing the inner curve.