Yet again another hard question I don't know how to do!
Prove that 1+ cos 2θ + cos 4θ+ cos 6θ = 4 cos θ cos 2θ cos 3θ
Hence, find all the angles between 0 degrees and 180 degrees inclusive, which satisfy the equation(below)
cos 2θ+ cos 4θ+ cos 6θ= -1
Thank you all in advance