How do you find the volume of a conical frustum?

First look at the volume of a cone, V = (piRH)/3. It is clear the volume of our conical frustum will be that of a large cone minus that of a smaller one on top. Also note that the smaller cone will simply be a scaled down version of the larger one as both cones share the same angles at thier verticies. Now we have all we need to work out the problem, V= Vl - Vs,  Vl = (piRlHl)/3 and Vs = (piRsHs)/3, Rl/Hl = Rs/Hs, H = Hl - Hs and Rl and Rs will be the radius of the bottom and the top of the frustrum respectively.

Now let's try an example; consider the conical frustrum with hieght = 4cm, upper radius = 6cm and lower radius = 10cm. By Rl/Hl = Rs/Hs we know by multiplication RlHs = RsHl and by H = Hl - Hs, then Hl = H + Hs; so 10Hs = 6(H+Hs) -> 10Hs = 24+6Hs -> 4Hs = 24 -> Hs = 6 -> Hl = 6+4 = 10. So, our volume V = (piRlHl)/3 - (piRsHs)/3 = (pi1010)/3 - (pi66)/3 = pi*(100-36)/3 = 64pi/3!