Q.Express the following as a sum or difference of two trigonometric functions: $2\cos4\theta\cos2\theta$

Factorization (also called transformation) formulae convert a SUM or DIFFERENCE of two sines or cosines into a PRODUCT, and, conversely, convert a PRODUCT of two sines/cosines into a sum or difference. Substituting $A=\dfrac{C+D}{2}$, $B=\dfrac{C-D}{2}$ into the four compound-angle identities gives $\sin C+\sin D=2\sin\dfrac{C+D}{2}\cos\dfrac{C-D}{2}$, $\sin C-\sin D=2\cos\dfrac{C+D}{2}\sin\dfrac{C-D}{2}$, $\cos C+\cos D=2\cos\dfrac{C+D}{2}\cos\dfrac{C-D}{2}$ and $\cos C-\cos D=-2\sin\dfrac{C+D}{2}\sin\dfrac{C-D}{2}$. Reading the same four identities the other way around gives the product-to-sum forms $2\sin A\cos B=\sin(A+B)+\sin(A-B)$, $2\cos A\sin B=\sin(A+B)-\sin(A-B)$, $2\cos A\cos B=\cos(A+B)+\cos(A-B)$ and $2\sin A\sin B=\cos(A-B)-\cos(A+B)$. These are the standard tool whenever a proof needs to CANCEL or TELESCOPE terms that a plain expansion cannot, for example reducing a four-term sum of cosines at unrelated-looking angles to a single value, or turning a ratio of sums into a ratio of a single sine/cosine pair that simplifies to a tangent or cotangent.