【俊狼猎英】团队为您解答~直接极坐标换元，x^2+y^2=r^2，区域D是0<=θ<=2π，0<=r<=1原积分=∫(0,2π)dθ∫(0,1)r√(1-r^2)dr=π∫(0,1)√(1-r^2)dr^2=-2π/3(1-r^2)^(3/2)|(0,1)=2π/3
