Cos 2x - 7 cos x + 4 = 0

Interval : 0° ≤ x ≤ 360°

cos 2x - 7cos x + 4 = 0
2cos² x - 7cos x + 3 = 0
(2cos x - 1)(cos x - 3) = 0
2cos x - 1 = 0
cos x = 1/2
x = 30° dan 330°

HP = {30° , 330°}

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cos 2x = 2cos² x - 1