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cos 2x = 1 - 2sin²x
2 cos 2x = 2 - 4sin²x
Maka,
4 sin x = 1 + 2cos 2x
4 sin x = 1 + 2 - 4sin²x
4 sin²x + 4sin x - 3 = 0
(2sin x + 3)(2sin x-1) = 0
sin x = -3/2 (Tidak memenuhi)
sin x = 1/2
sin x = sin 30
Pilihan : [A]