Fuente TeX:

\frac{ 3^n\left(1+(\frac{-2}{3})^n \right) }{3^{n+1}\left(1+(\frac{-2}{3})^{n+1}\right)} = \frac{ 1+(\frac{-2}{3})^n }{3\left(1+(\frac{-2}{3})^{n+1}\right)} \rightarrow \frac{1}{3}