There are some trigonometric identities which you must remember in order to simplify trigonometric expressions when required. When you are asked to prove something, ignore the right hand side of ...

Given any expression of the form \(a\cos x + b\sin x\) it is better to rewrite it into one of the forms \(k\cos (x \pm \alpha )\) or \(k\sin (x \pm \alpha )\) before answering the question.