sin^2 x +cos^2 x =1 得到 sin^2 x= 1- cos^2 x cos^2 x - sin^2 x =cos2x将上式代入可得 cos^2 x -( 1- cos^2 x)=cos2xcos^2 x -1+cos^2 x = cos2x2cos^2 x -1=cos2x2cos^2x =cos2x+1