Answer
The formula $\cos \alpha +\cos \beta =2\cos \frac{\alpha +\beta }{2}\cos \frac{\alpha -\beta }{2}$ can be used to change the sum between two cosines into the product of two cosines expressions.
Work Step by Step
$\cos \alpha +\cos \beta =2\cos \frac{\alpha +\beta }{2}\cos \frac{\alpha -\beta }{2}$
Thus, the above identity sum to product formula reflects that the sum of two cosines is equal to the twice of the product of the two cosines expression.