\(\cos(a) \cdot \cos(b)  =\frac {1}{2} \left( \sin(a+b) + \sin(a-b) \right)\)

\(\cos(a) \cdot \cos(b)  =\frac {1}{2} \left( \cos(a+b) - \cos(a-b) \right)\)

\(\cos(a) \cdot \cos(b)  =\frac {1}{2} \left( \sin(a+b) - \sin(a-b) \right)\)

\(\cos(a) \cdot \cos(b)  =\frac {1}{2} \left( \cos(a+b) + \cos(a-b) \right)\)