cos^2x - (1-cos^2x) + cos^2x = 0
cos^2x + cos^2x + cos^2x - 1 = 0
3cos^2x = 1
воспользуемся формулой понижения степени
3 (1+cos2x)/2 = 1
3 + 3cos2x = 2
3cos2x = -1
cos2x = -1/3
2x = arccos(-1/3) + 2Pin
x = arccos(1/3)/2 + Pin