# How do you find the sum of the infinite geometric series given a_1=18 and r=-1.5?

This Geometric series will not have a "sum", since the $| - 1.5 |$ = 1.5 which is greater than 1.
If the $| r | < 1$, you can then compute the sum with the formula: ${S}_{\infty} = \frac{{a}_{1}}{1 - r}$.