1/2 (cos(4 x)-cos(6 x)) = 1/2 (cos(4 x)-cos(10 x)) sin^2(x) (2 cos(2 x)+2 cos(4 x)+1) = sin^2(x) (2 cos(2 x)+1) (2 cos(2 x)+2 cos(4 x)+2 cos(6 x)+1) x = 0 x = pi n, n element Z x = -pi x = pi n-pi/2, n element Z