#### Answer

$2\cos 6x\cdot\cos 2x$

#### Work Step by Step

Sum-to-Product:
$\displaystyle \cos A+\cos B=2\cos(\frac{A+B}{2})\cos(\frac{A-B}{2})$
$\cos(-A)=\cos A$
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$\displaystyle \frac{A+B}{2}=\frac{4x+8x}{2}=6x$
$\displaystyle \frac{A-B}{2}=\frac{4x-8x}{2}=-2x$
$\cos A+\cos B=2\cos 6x\cos(-2x)$
$= 2\cos 6x\cdot\cos 2x$