# 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 $$2\theta$$

## Similar Formulas

What is the Formula of Sin C + Sin D ?

What is the Formula of Sin C – Sin D ?

What is the Formula of Cos C – Cos D ?