Answer
See below.
Work Step by Step
We must verify $\sin A\sin B=0.5((\cos(A-B)-\cos(A+B))$ with $A=30,B=120$, plugging in these we get: $\sin 30\sin 120=0.5((\cos(30-120)-\cos(30+120))=0.5((\cos(-90)-\cos(150))\\0.5\cdot\sqrt3/2=0.5(0-(-\sqrt3/2))$
This is true, hence we verified what we had to.