Question

Решите уравнение : cos2x=-1/2

Answer 1

cos2x = -1/2

-1/2 = cos 120 градусов
2х = 120 градусов      х = 60 градусов 

Answer 2

2x=+- 2π/3+2πk     k∈Z
x= +- π/3+πk   k∈Z