Well, start in the usual way, by writing a proposition about , and let take care of itself. So:
Then the propositional function is defined as:
Then write down the sum as the term you get when .
You'll find you can then take out lots of common factors, and eventually express this as , thus showing that .
Finally, you'll need to prove that is true for any .
I hope I've given you enough to go on. Let me know if you need more help.