用球坐标 ∫∫∫√(x^2+y^2+z^2)dxdydz =∫∫∫ r*r²sinφ drdφdθ=∫[0--->2π] dθ∫[0---->π] sinφdφ∫[0--->√2] r³ dr =2π(-cosφ)(1/4)r⁴ φ:[0---->π],r:[0--->√2] =4π