取球坐标，z = rcosφ, x^2 + y^2 = r^2(sinφ)^2, 得
I = ∫<0, 2π>dθ ∫<0, π>dφ ∫<0, 1> r^4 (cosφ)^2 (sinφ)^2 r^2sinφ dr
= 2π ∫<0, π>(cosφ)^2 (sinφ)^3 dφ ∫<0, 1> r^6 dr
= - (2π/7) ∫<0, π>(cosφ)^2 [1-(cosφ)^2] dcosφ
= - (2π/7) [(1/3)(cosφ)^3 - (1/5)(cosφ)^5]<0, π>
= (2π/7)(4/15) = 8π/105
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