求描述气球膨胀状态的函数r(V)=(3V/4π)^1/3
r(V)=[3V/(4pi)]^(1/3)
=[3/(4pi)]^(1/3)*V^(1/3)
r'(V)=[3/(4pi)]^(1/3)*(1/3)V^(1/3-1)
=[1/(36pi)]^(1/3)*V^(-2/3)
=1/[(36pi)^(1/3)*V^(2/3)]
=1/(36piV^2)^(1/3) 其中的36pi怎么算出来的