极坐标代换吧
边界：0≤r≤1,0≤θ≤π/2
∫∫(D)(X²+Y²)dxdy
=∫∫(D)r²drdθ(积分变量和θ无关,可分离)
=∫(0到π/2)dθ∫(0到1)r²dr
=π/2*(1/3)=π/6