cos$5x/9$ cos$13x/9$ - sin$5x/9$ sin$13x/9$

Using the identity cos$a$cos$b$ - sin$a$sin$b$ = cos$a+b$:

= cos$(5x/9)+(13x/9)$ = cos$18x/9$ = cos$2x$

Therefore, the expression simplifies to cos$2x$.