刚刚看到
......= 2π ∫ <0, 1> r(1-r^2)(3r^2+7) dr= 2π ∫ <0, 1> (7r-4r^3-3r^5) dr= 2π [7r^2/2-r^4-r^6/2] <0, 1>= 2π(7/2-1-1/2) = 4π
