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