2 sin²x + 5 cos x +1=0

2 sin²x + 5 cos x + 1 = 0
2(1 - cos²x) + 5 cos x + 1 = 0
2 - 2cos²x + 5 cos x + 1 = 0
2cos²x - 5 cos x - 3 = 0
cos x = p
2p² - 5p - 3 = 0
(2p - 6) (2p + 1) = 0

p = 3 (tm)
p = -1/2

cos x = -1/2
cos x = cos 120°
x = 120 + k.360
k = 0, x = 120°

x = -120 + k.360
k = 0, x = 240°

Hp = {120° , 240°}