Hello -

Tricky to know where to start, isn't it? Is it that we vary, or ?

Well, start in the usual way, by writing a proposition about , and let take care of itself. So:

Let

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.

Grandad