Recall that
cos²(x) + sin²(x) = 1
Then in the equation
1 - cos(x) = 2 - 2 sin²(x)
we can rewrite as
1 - cos(x) = 2 (1 - sin²(x))
1 - cos(x) = 2 cos²(x)
2 cos²(x) + cos(x) - 1 = 0
Factorize the left side as
(2 cos(x) - 1) (cos(x) + 1) = 0
so that
2 cos(x) - 1 = 0 or cos(x) + 1 = 0
cos(x) = 1/2 or cos(x) = -1
On the interval (-π, π) (note that this interval is open, so we don't allow x = π), we have
• cos(x) = 1/2 for x = π/3 and x = -π/3
• cos(x) = -1 for x = π
