设x = cost因为x从0积到1,所以t从pi/2积到0(注意顺序不能换)dx = -sint dt所以原积分 = 从pi/2积到0 {-sinx*根号[1-(cost)^2]} dt = 从pi/2积到0 [-(sint)^2] dt = -1/2 从pi/2积到0 (1-cos2t) dt = pi/4...