r=7
l=25
V=\frac{1}{3}P_p*h
P_p=\Pi*r^2
V=\frac{1}{3}*\Pi*r^2*h
h^2=l^2-r^2=25^2-7^2=625-49=576
h=\sqrt{576}=24
V=\frac{1}{3}*\Pi*7^2*24=\frac{49*24}{3}*\Pi=\frac{1176}{3}*\Pi=392\Pi
…
h=21
l=29
r^2=l^2-h^2=29^2-21^2=841-441=400
r=20
V=\frac{1}{3}*\Pi*20^2*21=\frac{400*21}{3}\Pi=\frac{8400}{3}\Pi=2800\Pi