举报
女人是月亮
=π/2*∫(0,1)√[(1-r^2)/(1+r^2)]*1/2dr^2=π/4*∫(0,1)√[(1-r^2)/(1+r^2)]dr^2 算∫√[(1-r^2)/(1+r^2)]dr^2 =∫√[(1-y)/(1+y)]dy =∫(1-y)/√(1-y^2)dy =∫1√(1-y^2)dy+∫d(1-y^2)/[2√(1-y^2)] =arcsiny+√(1-y^2)+c =arcsinr^2+√(1-r^4)+c 分别取r为1或0代入求得∫(0,1)√[(1-r^2)/(1+r^2)]dr^2=π/2-1 再乘π/4就是最后结果:π/4*(π/2-1)