What is the Formula of Cos C + Cos D ?

Solution :

The formula of cos C + cos D is 2 cos($C + D\over 2$) cos($C – D\over 2$).

Example : Express cos $4\theta$ + cos $8\theta$ as a product.

Solution : We have,

cos $4\theta$ + cos $8\theta$ = 2 cos($8\theta + 4\theta\over 2$) cos($8\theta – 4\theta\over 2$)

= 2 cos $6\theta$ cos $2\theta$