The unit I needed to recall the most was the Sequences and Series unit. Specifically Infinite Geometric Series. These are the types of infinite geometric series:

Diverging series expand each time so its impossible to determine a sum because the numbers grow so much each time they are multipled by the common ratio. In a diverging series the common ratio has to be negative.

Unlike diverging series, converging series do have a sum, converging series converge, and eventually the numbers will be so small that you could determine a sum. The ratio for a converging series needs to be bigger than -1 and smaller than 0.

This is the formula for finding the sum of a converging series .