Answer
See below.
Work Step by Step
We must verify $\sin A\cos B=0.5((\sin(A-B)+\sin(A+B))$ with $B=30,A=120$, plugging in these we get: $\sin 120\cos 30=0.5((\sin(90)+\sin(150))\\\sqrt3/2\sqrt3/2=0.5(1+0.5)=3/4$
This is true, hence we verified what we had to.