Referencia: Lathi Sec. B p54
B.8-3 Sumatorias
\sum_{k=m}^{n} r^k = \frac{r^{n+1}-r^m}{r-1} \texttt{ , } r\neq 1 \sum_{k=0}^{n} k = \frac{n(n+1)}{2} \sum_{k=0}^{n} k^2 = \frac{n(n+1)(2n+1)}{6} \sum_{k=0}^{n} k r^k = \frac{r+[n(r-1)-1]r^{n+1}}{(r-1)^2} \texttt{ , } r\neq 1 \sum_{k=0}^{n} k^2 r^k = \frac{r[(1+r)(1-r^n)-2n(1-r)r^n - n^2(1-r)^2 r^n] }{(1-r)^3} \texttt{ , } r\neq 1