<s>[INST] Partial sum of a geometric series Consider the geometric series $\sum_{n=0}^\infty ar^n$  where $a=1$
and $r=-\frac{1}{2}$. Since $|r| < 1$, the series converges to 
$S = \sum_{n=0}^\infty ar^n = \frac{1}{1+\frac{1}{2}} = \frac{2}{3}$. 
I would like to arrive at the same sum by computing
$\lim_{N \to \infty} S_N$ where $S_N$ is the partial sum of $N$ terms of
the goemetric series.
First few terms of the geometric series are: $1, -\frac{1}{2}, \frac{1}{4}, 
-\frac{1}{8}, \frac{1}{16}, -\frac{1}{32}, \cdots$.
Here are my attempts to find $S_N$:
\begin{align*}
  S_1 &= 1 = \frac{2^{(1-1)}}{2^{(1-1)}} \\
  S_2 &= 1 - \frac{1}{2} = \frac{1}{2} = \frac{2^{(2-1)} - 1}{2^{(2-1)}} \\
  S_3 &= \frac{1}{2} + \frac{1}{4} = \frac{3}{4} =
  \frac{2^{(3-1)} - 1}{2^{(3-1)}} \\
  S_4 &= \frac{3}{4} - \frac{1}{8} = \frac{5}{8} = 
  \frac{2^{(4-1)} - 3}{2^{(4-1)}} \\
  \cdots \\
  S_N &= ??
\end{align*}
\begin{align*}
  S_1 &= 1 - \frac{0}{1} \\
  S_2 &= 1 - \frac{1}{2} \\
  S_3 &= 1 - \frac{1}{2} + \frac{1}{4} = 1 - \frac{1}{4} \\
  S_4 &= 1 - \frac{1}{4} - \frac{1}{8} = 1 - \frac{3}{8} \\
  \cdots \\
  S_N &= ??
\end{align*}
I cannot find a pattern that will help me find $S_N$.
 [/INST] Hint: 
$$a + ar + ar^2 + \ldots + ar^{n-1} = \frac{ a( 1- r^n) } { (1-r) }. $$
</s>