cos(x+y)-cos x是如何等于-2sin (2x+y/2) sin (y/2)的,求解题步骤

sin(x+y)sin(x-y)=k,求cos^2x-cos^2y y =(cos^2) x - sin (3^x),求y' Sin x-sin y=2/3 cos x-cos y=1/2 求cos(x-y) sin(x+y)-sinx=2cos(x+1/2y）sin(1/2y)的详细证明步骤. 证明COS（X+Y）COS（X-Y）=COS^2X-SIN^2Y 化简sin(2x-y)*sin y+cos(2x-y)*sin y 已知函数y=(sin x+ cos x)(sin x+cos x)+2cos x*cos x ,求它的递减区间 求函数y =cos 2x +sin 2x /cos 2x -sin 2x 的最小正周期 2sin^2x+cos^2y=1,求sin^2x+cos^2y的取值范围 函数y=sin^2x+sin x cos x的值域是? 设sin(x+y)sin(x-y)=m,则cos^2x-cos^2y的值 sin^2x+cos^2y=1/2 求3sin^2x+sin^2y的最值