2 cos x + cos 2x + cos 4x = cos x + cos x + cos 2x + cos 2x = 2cosx(1+cosx) + 2cos2x(1+cosx) = 2cosx(1+cosx+2cosx) = 2cosx(1+3cosx)
2 cos x + cos 2x + cos 4x = cos x + cos x + cos 2x + cos 2x = 2cosx(1+cosx) + 2cos2x(1+cosx) = 2cosx(1+cosx+2cosx) = 2cosx(1+3cosx)