cos2x =2cosx - 1 решить уравнение
cos^2x - sin^2x - 2cosx + (sin^2x + cos^2x) = 0 2cos^2x - 2cosx = 0 cosx(2cosx - 2) = 0 cosx = 0, cosx = 2/2 = 1 x = 2Pik x = Pi/2 + Pik