Solution :
The formula of cos C + cos D is 2 cos(\(C + D\over 2\)) cos(\(C โ D\over 2\)).
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(\(8\theta โ 4\theta\over 2\))
= 2 cos \(6\theta\) cos \(2\theta\)
Similar Formulas
What is the Formula of Sin C + Sin D ?
