1+cos8x=1+cos2*4x=2cos^2 4x 2cos^2 4x=2cos4x 2cos^2 4x-2cos4x=0 cos4x(c0s4x-1)=0 cos4x=0 4x=pi/2+pi n x=pi/8+1/4pi n cos4x=1 4x=2pi k x=1/2pi k, n,keZ