用球坐标变换x=rsinφcosθ，y=rsinφsinθ，z=rcosφ，0<=r<=a，0<=φ<=π，0<=θ<=2π原式=∫(0,2π)dθ∫(0,π)dφ∫(0,a)r^2*r^2*sinφdr=2π*∫(0,π)sinφdφ*∫(0,a)r^4dr=(4π/5)*r^5|(0,a)=(4π/5)*a^5
