Sean $r$, $s$ y $t$ enteros con $0 \leq r$, $0 \leq s$ y $r+s \leq t$. Demuestra que \[ \frac{\binom s0}{\binom tr} + \frac{\binom s1}{\binom{t}{r+1}} + \cdots + \frac{\binom ss}{\binom{t}{r+s}} = \frac{t+1}{(t+1-s)\binom{t-s}{r}}. \]