Find all real numbers in the interval [0,2pi) that satisfy the equation.

Question

cos^2 x+2 cos x+1 = 0

Answer 1

let z = cos x
then
z^2 + 2 z + 1 = 0
(z+1)(z+1) = 0
z = -1
so
cos x = -1
that is at x = 180

Answer 2

You can convert those degrees to radians I trust.