请问这个三角函数表达式是怎么变换为另一个函数表达式的，请给出推翻过程

2sin(b/2)cos(a+b/2)=2sin(b/2)[cosacos(b/2)-sinasin(b/2)] =2sin(b/2)cosacos(b/2)-2sin(b/2)sinasin(b/2) =sinbcosa-2sin^2(b/2)sina