sin（a+B）cosa-1/2[ sin（2a+B）-sinB]
=sin（a+B）cosa-1/2[ sin（a+B+a）-sinB]
=sin(a+B)cosa-(1/2)[sin(a+B)cosa+cos(a+B)sina-sinB]
=(1/2)[sin(a+B)cosa-cos(a+B)sina+sinB]
=(1/2)[sin(a+B-B)+sinB]=sinB