Chu-Vandermonde Identity/Examples/3 from 4 + 5

Example of Chu-Vandermonde Identity
Let $r = 4$, $s = 5$ and $n = 3$

We know:
 * $\ds \binom {r + s} n = \sum_{k \mathop = 0}^n \binom r k \binom s {n - k}$

Therefore: