Problem $3$: $(10$ points$)$ If we revolve the right$-$angled triangle OAB around the $x-$axis, we get a cone of base radius $r$ and height $h.$

Use the disk method for finding the volume of a solid of revolution to show that the volume of a cone with radius $r$ and height $h$ is

$V=\frac{1}{3}{r}^{2}h$