2cos ( x+ П/4) cos( 2x + П/4) + sin3x

2*(1/2)[cos(x + π/4 - 2x - π/4) + cos(x + π/4 + 2x + π/4)] + sin3x = = cosx + cos(3x + π/2) + sin3x = cosx  - sin3x + sin3x = cosx