用极坐标：
∫∫D|xy|dxdy=∫(0,2π)dθ∫(0,1)r^3|sinθcosθ|dr
=(1/8)∫(0,2π) |sin2θ| dθ
=(1/8)(∫(0,π/2) sin2θdθ-∫(π/2,π)sin2θdθ+∫(π,3π/2)sin2θdθ-∫(3π/2,2π)sin2θdθ)
=(1/16)(2+2+2+2)=1/2