y=sin(A+B/2+B/2) - sin(A+B/2-B/2)
y=sin(A+B/2)cos(B/2)+cos(A+B/2)sin(B/2)-sin(A+B/2)cos(B/2)+cos(A+B/2)sin(B/2)
即y=2sin(B/2)cos(A+B/2)
公式就是sin(a+b)=sin(a)cos(b)+cos(a) sin(b)