P=-x^2y Q=xy^2∂P/∂y=-x^2 ∂Q/∂x=y^2根据格林公式：∫(L)fxy^2dy-x^2ydx=∫∫(D)[y^2-(-x^2)]dxdy=∫(0,2π)dθ∫(0,a)r^3dr=πa^4/2
