cos(4x)-cos(8x)=cos2x-cos8x cos4x=cos2x 2cos^2x-1-cos2x=0 cos2x=t 2t^2-t-1=0 t=-1/2 t=1 cos2x=1 x=Пk 2x=(П+-П/3)+2Пk x=П/2+-П/6+Пk