use the formula { a couple of times} cos 2 Θ = 2 cos ² Θ - 1 = 1 - 2 sin ² Θ....your answer will likely have a cos 2x , a cos 4x , and a cos 6x in it.....note : cos ² Θ - sin ²Φ = cos ( Θ + Φ) cos (Θ - Φ) will also be needed for appropriate choices of Θ & Φ in terms of x
