In the question, we are asked to prove that

We can find the proof below.
Explanation
To find the proof we must recall some fundamental principles of trigonometry.

We will then simplify the left-hand side. If it gives the same value as the right-hand side, it implies that the proof is complete.
Therefore, from the left-hand side,
Proof

Since the left-hand side has been proven to be equal to the right-hand side when simplified, that concludes the prove