AT-OEMO-FED1 2002 Problem 3

Let $f(x)=\frac{9^x}{9^x+3}$ . Compute $\sum_{k} f \biggl( \frac{k}{2002} \biggr)$ , where $k$ goes over all integers $k$ between $0$ and $2002$ which are coprime to $2002$ .