Falling Factorial of Sum of Integers

Theorem
Let $r \in \R$ be a real number.

Let $a, b \in \Z$ be (positive) integers.

Then:
 * $r^{\underline {a + b} } = r^{\underline a} \left({r - a}\right)^{\underline b}$

where $r^{\underline a}$ denotes the $a$th falling factorial of $r$.