>>3
(4/3)πr^3 = πa^2h

2π(a^2+ah)
= 2π{a^2+(ah/2)+(ah/2)}
≧ 3π{(a^2h)^2/4}^(1/3)
= 3π{(4/9)r^6}^(1/3)
= 3πr^2*{4/9}^(1/3)
≧ 4πr^2