已知π/2已知π/2
已知π/2判定(a+b),(a-b)的象限,求出sin(a+b)和sin(a-b)分别求出,cos2a=cos(a+b+a-b)=cos(a+b)cos(a-b)+sin(a+b)sin(a-b)=?带入求解就可以