UVa/11481 - Arrange the Numbers/explicación/e

   1 \documentclass[10pt,letterpaper]{article}
   2
   3 %---------------------------------------------------------------
   4 \usepackage[utf8]{inputenc}
   5 \usepackage[spanish]{babel}
   6 \usepackage{listings}
   7 \usepackage[usenames,dvipsnames]{color}
   8 \usepackage{amsmath}
   9 %\usepackage{color}
  10 %---------------------------------------------------------------
  11
  12 \setlength{\topmargin}{-1.0in}
  13 \setlength{\textheight}{9.5in}
  14 \setlength{\evensidemargin}{0.0in}
  15 \setlength{\oddsidemargin}{0.0in}
  16 \setlength{\textwidth}{6.5in}
  17
  18 \begin{document}
  19
  20 \emph{Explicación:} La respuesta es
  21 $$
  22 m \choose{k} \times \displaystyle \sum_{i=0}^{m-k} (-1)^{i}  m - k \choose{i} (n-k-i)!
  23 $$
  24
  25 \end{document}