Answer
The formula $\cos \alpha \cos \beta =\frac{1}{2}\left[ \cos \left( \alpha -\beta \right)+\cos \left( \alpha +\beta \right) \right]$ can be used to change the product of two cosines into the sum of two cosines expression.
Work Step by Step
Let us consider the given formula:
$\cos \alpha \cos \beta =\frac{1}{2}\left[ \cos \left( \alpha -\beta \right)+\cos \left( \alpha +\beta \right) \right]$
Thus, the above formula is a product-to-sum formula, which reflects that the product of two cosines is equal to half of the sum of the two cosines expression.
