Question

Cosacosb-cos(a-b)=? - - - - - - -

Answer 1

Cos(a-b)=cos(a) cos(b)+sin(a)sin(b)
cos(a) cos(b)-cos(a-b)=-sin(a)sin(b)

Answer 2

The trigonometric identity you're referring to is the difference of angles formula for cosine, also known as the cosine of a difference of angles formula:

$\cos(A - B) = \cos A \cdot \cos B + \sin A \cdot \sin B$

In your case, the formula becomes:

$\cos(a - b) = \cos a \cdot \cos b + \sin a \cdot \sin b$

So, $\cos(a - b) = \cos a \cdot \cos b + \sin a \cdot \sin b$.