Question

Cos 2x -7cos^2 x +4=0
Решите уравнение

Answer 1

2cos²x - 1 - 7cos²x + 4 = 0
5cos²x = 3
cos²x = 3/5
cosx = + - √(3/5)
x = + - arccos√(3/5)        x = + - (π - arccos√(3/5))