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2 cos x - = 1
2 cos² x - 1 = cos x
2 cos² x - cos x - 1 = 0
Misal, cos x = a
2a² - a - 1 = 0
(a - 1)(2a + 1) = 0
a = 1 ⇔ cos x = 1 ⇔ x = 90°, 270° (ada sudut lain?)
2a + 1 = 0 ⇔ a = - ⇔ x = 120°, 240° (ada sudut lain?)