sin^2 = 1 - cos^2
2 - 2cos^2 x + 3 cos x = 0
t= cos x
t1,2 = ( 3 + - sqrt(9+16)) / 4
t1 = 2 - косинус не бывает больше 1
t2 = -0,5 => cos x = -0,5 => x=2пи/3 + 2пи n
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5 - 5 sin^2 x + 6 sin x -6 = 0
t=sin x
5 t^2 - 6t +1 = 0
t1,2 = (6 + - sqrt(36-20)) / 10
t1 = 1 => sin x = 1 => x = пи/2 + 2 пи m
t2 = - 0,2 => sin x = -0,2 => x = arcsin(-0,2) + пи n