Chu-Vandermonde Identity

Theorem
Let $r, s \in \R, n \in \Z$.

Then:
 * $\displaystyle \sum_k \binom r k \binom s {n - k} = \binom {r + s} n$

where $\dbinom r k$ is a binomial coefficient.

Also known as
When $r$ and $s$ are integers, it is more commonly known as Vandermonde's Identity or Vandermonde's Convolution.