Answer :

pepe11
cos9x - cos5x = -2sin2x.sin7x
sin17x - sin3x = 2sin7x.cos10x 
so f(x) = [tex] \frac{-2sin2x.sin7x}{2sin7x.cos10x} [/tex] =[tex] \frac{-sin2x}{cos10x} [/tex]