Do you mean:
?
If so, then here's a proof:
Substitute the new numerator.
QED.
The left-hand-side is expressed in terms of sine and cosine.
We can also express the right-hand side in terms of sine and cosine...
?
is common to both sides, so divide both sides by
?
Divide both sides by 2.
?
Now you only need determine if the denominators are equal...