Answer
The required solution is $2\cos 3x\cos x$.
Work Step by Step
One of the sum-to-product formula is $\cos \alpha +\cos \beta =2\cos \frac{\alpha +\beta }{2}\cos \frac{\alpha -\beta }{2}$. So, in this question, according to the above-mentioned formula, the value of $\alpha $ is $4x$ and the value of $\beta $ is $2x$.
Thus, the expression can be evaluated as provided below:
$\begin{align}
& \cos 4x+\cos 2x=2\cos \frac{4x+2x}{2}\cos \frac{4x-2x}{2} \\
& =2\cos \frac{6x}{2}\cos \frac{2x}{2} \\
& =2\cos 3x\cos x
\end{align}$
Hence, the given expression can be written as $2\cos 3x\cos x$. So, it is not possible to find the exact value.
