Adding and Subtracting Complex Numbers

Recall the formula for the sum of an infinite geometric series: $$S=\frac{a_1}{1-r}$$ where $a_1$ is the first element of the series and $r$ is the common ratio between any two terms ($\frac{a_{n+1}}{a_n}$ for any $n$). This formula only applies for $-1<r<1$.

For more information, check out Infinite Geometric Series.