persamaan trigonometri mohon dibantu, tolong caranya trims. cos 2x = cos 120

diketahui :
cos 2x = cos 120

jawab :
cos 2x = cos 120
2cos^2 x - 1 = cos 120
2cos^2 x - cos 120 - 1 = 0
misal :
cos x = A

maka :
2cos^2 x - cos 120 - 1 = 0
2A^2 - A - 1 = 0
(A + 1)(2A - 1)
A = -1 V A = 1/2
...................................kembalikan
●→A = -1
cos x = -1
cos x = cos 180
>>> x = 180

●→ A = 1/2
cos x = 1/2
cos x = cos 60
>>>> x = 60

Hp : {60, 180}

silahka di check kembali, semoga membantu