Can $\cos(2x)+8\cos(4x)$ be simplified into $0.5A(\cos(a+b) + \cos(a-b))$? I know that there isn't a direct transformation, however can it be done using other identities?
I have tried the double angle formulas but ultimately they don't really provide me help as it introduces more functions.
EDIT: Im trying to eventually make it into Acos(a)cos(b)
can this be done?

No. Your purported right hand side has magnitude at most 2, but your left hand side ranges from -9 to 9.
