I know i'm supposed to use double angle formula, but i still don't get how to get my answer. I am to Express cos^2 (2x) in terms of cos 4x.
Do you know that $\displaystyle \displaystyle \begin{align*} \cos{2\theta} &\equiv 2\cos^2{\theta} - 1 \end{align*}$? Let $\displaystyle \displaystyle \begin{align*} \theta = 2x \end{align*}$.