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:

My answer is :

   [-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.

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

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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
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