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 expressions.
Work Step by Step
$\cos \alpha \cos \beta =\frac{1}{2}\left[ \cos \left( \alpha -\beta \right)+\cos \left( \alpha +\beta \right) \right]$
Thus, the above identity or product sum formula reflects that the products of two cosines are equal to half of the sum of the two cosines expression.
