>>69
f(θ)=sin(θ−a)−sin(θ)=sin{(θ-a/2)-a/2}-sin{(θ-a/2)+a/2}
=sin(θ-a/2)*cos(a/2)-cos(θ-a/2)*sin(a/2)
-{sin(θ-a/2)cos(a/2)+cos(θ-a/2)sin(a/2)}
=-2cos(θ-a/2)sin(a/2)