Geometric series

Let $x$ be a real number. If $|x|\ge 1$, then the series $\sum _{n=0}^{\mathrm{\infty }}{x}^n$ is divergent. If however $|x|\le 1$, then the series is absolutely convergent and the sum is given by:

On the reverse side, there is some geometric visual reasoning; can you remember what it looks like?