令x=1-r^2则dx = -2rdr
所以上式=(-1/2)∫(1-x)[（x）^1/2]dx=(-1/2)∫(x）^1/2 - (x）^3/2dx
=(-1/2)[(2/3)x^(3/2) - (5/2) x^(5/2)] +C
再吧x=1-r^2代入上式即可