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= Rotated Volume
Consider an arbitrary given function
$ y = f(x) $
// TODO: add picture here
and we are about to rotate the function about $x$-axis. After the rotation, the 
under-curve area becomes solid 3 dimensional body's volume. The volume can be 
computed by bring the idea when we derive the definition of under-curve area,
that is, integration.

Let's take a close look of the 3D body. Consider an arbitrary slice of the 3D 
body, 
// TODO: add picture here
as you can observe, the slice is a disc, whose $V$ volume is characterized by 
the formula 
$ V = pi r^2 h, $
where $r$ is the radius of circle in the cross section, $h$ is the height of the
slice.