Combinations

This is a little complicated, i'll try to explain as best i can.

Say I have n different numbers, http://www.mathhelpforum.com/math-he...4d00b6cb-1.gif
I wish to find the summation of every possible way of multiplying http://www.mathhelpforum.com/math-he...155c67a6-1.gif different numbers together.

For clarification: if http://www.mathhelpforum.com/math-he...ec82816c-1.gif I would want:
http://www.mathhelpforum.com/math-he...e0c084ff-1.gif
If http://www.mathhelpforum.com/math-he...73fb80de-1.gif i would want:
http://www.mathhelpforum.com/math-he...472ed53c-1.gif

Basically, I just need a function http://www.mathhelpforum.com/math-he...802e72e4-1.gif that gives this result.

Choosing elements out of set of elements ( ) and disregarding their order is the definition of combinations.

Except you obviously haven't read my post at all. That's not what I'm after. I'm fairly sure I explained what I'm looking for fairly clearly.
Ok, you've shown how many different terms the function http://www.mathhelpforum.com/math-he...802e72e4-1.gif will have in terms of x and n. It isn't a formula that will calculate the value of each term and add them together.

Could somebody please read my post and help me with this?