In this case the meaning is $\displaystyle { - r \choose k} = \frac{{\left( { - r} \right)\left( { - r - 1} \right)\left( { - r - 2} \right) \cdots \left( { - r - k + 1} \right)}}{{k!}}$.
The proof is tedious. Expand both sides and compare.
In this case the meaning is $\displaystyle { - r \choose k} = \frac{{\left( { - r} \right)\left( { - r - 1} \right)\left( { - r - 2} \right) \cdots \left( { - r - k + 1} \right)}}{{k!}}$.
The proof is tedious. Expand both sides and compare.