转化为极坐标，D变为r^2<=2rsinθ，即r<=2sinθ原积分=∫(0,π)dθ∫(0,2sinθ)(1-r^2)rdθ=∫(0,π)2(sinθ)^2-4(sinθ)^4dθ=∫(1-cos2θ)-(1-cos2θ)^2dθ=∫-(cos4θ-1)/2+cos2θdθ=π/2
