2sin(x/2)cos(x/2)=sinx请问这是个公式吗?还是推导出来的呢?如果是推导出来的,麻烦写下过程,

已知tan=2,求(cos x+sin x)/(cos x-sin x)+sin^2x y=2cos x (sin x+cos x) 已知sin(x)+3cos(x)=2,求sin(x)-cos(x)/sin(x)+cos(x)=? sin(x/2)cos(x/2)= y=(sin x + COS x)^2 2sin^2x - 5sin x cos x=3 cos^2x,求X, 已知:tan x=2 求:sin x+cos x/sin x-cos x=? sin^2 x-cos^2 x=cos 化简y=sin^2(x)+2sin(x)cos(x)+3cos^2(x) 证明sin^2(x)+cos^2(x+30)+sin(x)cos(x+30)=3/4 怎么证明2(1-sin x)(1+cos x)=(1-sin x+cos x)^2 证明sin^4x-cos^4x=sin^2x-cos^2x 求证sin^4x-cos^4x=sin^2x-cos^2x 求证（cos^2 x-sin^2 x)(cos^4 x+sin^4 x)+1/4 sin 2x sin 4x=cos 2x sin^3 x -cos^3 x sin^3 x -cos^3 x =(sin x -cos x )(sin^2 x +sin x*cos x +cos^2 x )问下这个是个公式还是什么?怎么样算出来的? 化简(sin x+cos x-1)(sin x-cos x+1)/sin 2x sin(2x+45)=cos sin(x+y)sin(x-y)=k,求cos^2x-cos^2y