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Possible Duplicate:
Value of $\sum\limits_n x^n$

Given $a\in\mathbb{R}$ and $0

let $(X_n)$ be a sequence defined: $X_n=1+a+a^2+...+a^n$, $\forall n\in\mathbb{N}$.

How do I show that $X_n=\frac{1-a^{n+1}}{1-a}$

Thanks.

  • 0
    The answer is given in http://math.stackexchange.com/a/29024/24682012-04-05

2 Answers 2