Cumulative Sum

Hopefully my title isn't misleading (or just plain incorrect, its been a while since i did maths).
Honestly I'm fairly new to excel, but after breezing through the new users forum i belive this question goes a bit beyond that.

I have, for my formula, 2 constant values a and x, I then also have a variable, n.

The formula I'm trying to convert into excel terms is

SIGMA (with respect from 1 upto n) ((1-x)^(1-n) * x)*a

Unfortunatly thats not drastically inteligable so i've attached a picture file that should explain it in more understandable terms.

Now the problem im having is, sigma is a cummulative funtion, which in this example is relative to the variable n, firstly I don't know what formulas I would have to use in order to get a sigma/summination function, and secondly I am uncertain of how to link it to a variable range (i would guess along the count, offset side of things?)

Hopefully something in their is understandable.
(annoyingly the neater picture I had preped is too large a file to be used, so I had to resort to a scanned pdf, sorry about that)

Thanks,
Fraser