Question

Simplify `cos9xcos2y+sin9xsin2y`?

Answer 1

`cos9xcos2y+sin9xsin2y=cos(9x-2y)`

Explanation:

As `cos(A-B)=cosAcosB+sinAsinB`

`cos9xcos2y+sin9xsin2y`

= `cos(9x-2y)`