[texhax] Please help with a Mathematical Symbol

[Robert Sherry]

     I would like to write n choose 2 using the standard mathematical symbol
of parenthesis and n written above 2 with nothing between them. From reading
the documentation, I thought the following would do it:

            \begin{displaymath}
            \binom{r}{2}
            \end{displaymath}

However, LaTeX says that \binom is an undefined control sequence. I am using
the MikTex version of LaTeX. Could somebody please help?

                                      Robert Sherry