∮xy^2dy-x^2ydx=∫∫x^2+y^2dxdy=∫(0,2π)dθ∫(0,a)r^3dr=2π(1/4)r^4︱(0,a)=(1/2)πa^4注意：∫∫x^2+y^2dxdy是二重积分,在D上x^2+y^2≤a^2
