Chu-Vandermonde Identity/Falling Factorial Variant

Theorem
Let $r, s \in \R, n \in \Z_{\ge 0}$.

Then:
 * $\ds \sum_{k \mathop = 0}^n \dbinom n k r^{\underline k} s^{\underline {n-k} } = \paren {r + s}^{\underline n}$

Also known as
This identity is also known as Vandermonde's formula.