Chapter 12 Sequences and Series - 12.5 Use Recursive Rules with Sequences and Functions - 12.5 Exercises - Quiz for Lessons 12.4-12.5 - Page 833: 3

no sum exists

An infinite geometric series has a sum if and only if $|r|\lt1$, where $r$ is the common ratio. If it exists, then it equals $\frac{a_1}{1-r}$ where $a_1$ is the first term. Here $r=2.5$ $|2.5|\gt1$, thus no sum exists