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1) 2 cos^2 x - 5 cos x = 3
2 (cos x)^2 - 5 cos x - 3 = 0
Misal cos x = a
2a^2 - 5a - 3 = 0
(2a + 1)(a - 3) = 0
a = -1/2 atau a = 3
Yang diambil a = -1/2 karena nilai dari cos x : -1 ≤ cos x ≤ 1
a = -1/2
Cos x = -1/2 ===> cos bernilai negatif dikuadran 2 dan 3
Kuadran 2 => x = (180° - 60°) = 120°
Kuadran 3 => x = (180° + 60°) = 240°
HP = {120°, 240°}

2) 6 cos x - 3 = 0
6 cos x = 3
Cos x = 1/2 ====> cos bernilai positif dikuadran 1 dan 4
Kuadran 1 : x = 60°
Kuadran 4 : x = (360° - 60°) = 300°
HP = {60°, 300°}