## Precalculus (6th Edition) Blitzer

The required solution is \left( \begin{align} & n \\ & r \\ \end{align} \right)=\underline{\frac{n!}{r!\left( n-r \right)!}}
We know that for non-negative integers n and r, with$n\ge r$, the expression \left( \begin{align} & n \\ & r \\ \end{align} \right) (read as “n above r”) is called the binomial coefficient and is defined by: \left( \begin{align} & n \\ & r \\ \end{align} \right)=\frac{n!}{r!\left( n-r \right)!}