Question 1098062
cos(2x) = 1 - 2sin^2(x)


Rewrite as a quadratic in terms of sin(x). Let sin(x) = y if you like.
1 - 2sin^2(x) - sin(x) = 0
-2y^2 - y + 1 = 0
2y^2 + y - 1 = 0
(2y - 1)(y + 1) = 0


sin(x) = 1/2 --> x = pi/6
sin(x) = -1 --> x = 3pi/2