cos^4 a-(1/4)cos^2 2a-(1/2)cos 2a
=[(1/2)(cos2a+1)]²-(1/4)cos²2a-(1/2)cos2a
=(1/4)(cos²2a+2cos2a+1)-(1/4)cos²2a-(1/2)cos2a
=(1/4)cos²2a+(1/2)cos2a+1/4-(1/4)cos²2a-(1/2)cos2a
=1/4