## Calculus (3rd Edition)

This is a geometric series in which $c=1$ and $r=3$ with the sum $$\frac{1}{1-3}=-\frac{1}{2}.$$ It is impossible for the sum to be negative, since all the terms are positive. The formula is not valid because we have $r=3$ and we must have $|r|\lt 1$ to use the formula.