Repeating a formula based on a cell value

Hi everyone, first post, and stumped =)

Is there any way in excel to have a formula "repeat" based off of a value in a cell? for example (the formula)

(100)*.9 [from a cell value of 1
((100*.9)+100)*9) [from a cell value of 2]
((((100*.9)+100)*9)+100)*9) [from a cell value of 3]

and so on and so forth (cell value from 1 to 20 at the moment) Any experts lend a hand? Help is greatly appreciated!

Thanks,
Chris

Wikipedia shows a simple formula for finding the sum of a finite geometric series. Can you use that?