令：z= rtanu ,dz= rsec^2 udu ,sinu=z/√(r^2+z^2)
∫ 1/(r^3*sec^3 u)*rsec^2 udu
= 1/r^2 ∫cosudu
=sinu/r^2 + C
=1/r^2 *z/√(r^2+z^2)] +C