sin(a+b)cosa - (1/2)[sin(2a+b) - 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-a) + (1/2)sinb
=sinb