D : x²+y² = z/2 , 极坐标是 r = √(z/2)∫∫ (x²+y²)dxdy = ∫<0, 2π>dt∫<0, √(z/2)> r^3dr= (π/2)[r^4]<0, √(z/2)> = πz^2/8
