V=πr²h
h=V/πr²(r>0)
表面积S=2πr²+2πrh=2πr²+2πr(V/πr²)=2πr²+2V/r (r>0)
令S'=4πr-2V/r²=0
求出r,再代入V=πr²h求出h,就可以了.