cos2x = (cosx)^2 - (sinx)^2
sin(x+п/2) = cosx
(cosx)^2 - (sinx)^2 - cosx = 0
(cosx)^2 - (1-(cosx)^2) - cosx = 0
2(cosx)^2 - cosx - 1 = 0
cosx = y
2y^2 - y - 1 = 0
D = 1-4*2*(-1) = 9
y1 = (1+3)/4 y1 = 1 cosx1 = 1 x1 = 2пN
y2 = (1-3)/4 y2 = -1/2 cosx2 = -1/2 x2 = +-2п/3 + 2пN
