Cos2x=cos^2 (x) - sin^2 (x)
cos^2 (x) - sin^2 (x) + sin^2 (x) = 2cos(x)
cos^2 (x)-2cos(x) = 0
cosx(cosx - 2) = 0
cosx = 0 или cosx - 2 = 0
x = пи/2 + пиk cosx 
2
-15пи/8<пи/2+пиk<25пи/8 | - (пи/2)<br>-15пи/8 - пи/2<пиk<25пи/8 - пи/2<br>-19пи/8<пиk<21пи/8 | / пи<br>-19/8-2
ответ 5 корней (-3пи/2, -пи/2, пи/2, 3пи/2, 5пи/2)