Solution
= Solution
The initial term at $n=-1$ is three, and the common ratio is $1/3$. The convergent <geometric series> therefore sums to
$$
\boxed{\sum_{n=-1}^{\infty}\left(\frac13\right)^n=\frac3{1-1/3}=\frac92.}
$$
= Solution
The initial term at $n=-1$ is three, and the common ratio is $1/3$. The convergent <geometric series> therefore sums to
$$
\boxed{\sum_{n=-1}^{\infty}\left(\frac13\right)^n=\frac3{1-1/3}=\frac92.}
$$