Answer
See below.
Work Step by Step
We must verify $\sin A-\sin B=2\cos\frac{A+B}{2}\sin\frac{A-B}{2}$ with $A=30,B=90$, plugging in these we get: $\sin 30-\sin 90=2\cos\frac{30+90}{2}\sin\frac{30-90}{2}\\0.5-1=2\cos60\sin(-30)\\-0.5=2(0.5)(-0.5)$
This is true, hence we verified what we had to.