Question

cos^2x-2cosx-1=0

Answer 1

Let \(u=\cos(x)\) giving: \[
u^2-2u-1=0
\]Then solve for the roots of \(u\). Suppose they're \(r_1,r_2\).
You just have to solve \[
\cos(x) = r_1,\cos(x)=r_2
\]

Answer 2

You'll need to use the quadratic formula to find the roots of \(u\).