Geometric Series - Simple Question [closed]

The geometric series formula I am using is for an algorithmic analysis problem: $$\frac{a(1-a^n)}{1-a}.$$

If $a = -\frac{5}{6}$, what is the solution in its most simplified form?

Edit:

   [-5/6.(1 - (-5/6)^n)] / [1 - (-5/6)]
=> [-5/6.(1 + (5/6)^n)] / [1+(5/6)]
=> -5/6.(1+(5/6)^n) * 6/11
=> -5/11.(1+(5/6))^n)


This is what I think the answer is - and I know this isn't difficult - but I haven't done maths in over 10 years and need someone to take me through this example so I can study it and learn from it. Please don't close.

-

closed as too localized by Aryabhata, Eric Naslund, t.b., Jonas Meyer, Ｊ. Ｍ.Apr 17 '11 at 0:43

This question is unlikely to help any future visitors; it is only relevant to a small geographic area, a specific moment in time, or an extraordinarily narrow situation that is not generally applicable to the worldwide audience of the internet. For help making this question more broadly applicable, visit the help center.If this question can be reworded to fit the rules in the help center, please edit the question.

This is not particularly difficult. Would you like to show us what you think it might be, and then we can check? –  Henry Apr 12 '11 at 14:50
Plug in, simplify. You can then ask us if that's the right answer; but this is basic arithmetic (not even algebra at this point). –  Arturo Magidin Apr 12 '11 at 14:59
-1: And I have voted to close as too localized. –  Aryabhata Apr 12 '11 at 15:15
Your calculation is OK except for the fact that you can't take the minus sign out of $(-5/6)^n$; this is $(-1)^n(5/6)^n$, which is $-(5/6)^n$ if $n$ is odd, but $+(5/6)^n$ if $n$ is even. –  joriki Apr 12 '11 at 16:21
If it's a calculation you want to verify, just plug it into wolframalpha.com –  Brian Vandenberg Apr 12 '11 at 16:33