Cosx+cos3x=cos2x

Cosx+cos3x=cos2x 2cos ((x+3x) / 2) * cos ((x-3x) / 2) = cos2x 2cos2xcosx-cos2x=0 cos2x (2cosx-1) = 0 cos2x=0 ili 2cosx-1=0 2x=pi/2 + пn cosx=0.5 x = п/4 + пn/2, x = ±п/3 + 2 пn, n∈Z