Question

How do you solve `cos^2 2x + 1 = -2cos2x` from [-2pi, pi/2]#?

Answer 1

Solve `cos^2 2x + 1 = -2cos 2x`

Ans: `+- pi/2`

Explanation:

cos 2x = t, we get the quadratic equation:

`t^2 + 2t + 1 = (t + 1)^2`

`t = cos 2x = -1` --> `2x = +- pi` --> `x = +- pi/2`