Answer:
Cos(a)
cos(a)*cos(b)=(1/2) (cos(a+b)+cos(a-b))
Step-by-step explanation:
First we need to remember that
cos(a+b)=cos(a)*cos(b)-sen(a)*sen(b) eq.1
cos(a-b)=cos(a)*cos(b)+sen(a)*sen(b) eq.2
If we sum eq.1 with eq.2 we have:
cos(a+b)+cos(a-b)= 2 cos(a)*cos(b)
By solving we have the following
cos(a)*cos(b)= [cos(a+b)+cos(a-b)] *(1/2)
Thus, by comparing, we have our desire answer
