# Proof of combination

• February 19th 2008, 07:19 AM
hockey777
Proof of combination
Prove the following equality

$\binom{-r}{k}$= $(-1)^{k}$ $\binom{r+k-1}{k}$

Any help would greatly appreciated.
• February 19th 2008, 08:46 AM
topsquark
Quote:

Originally Posted by hockey777
Prove the following equality

$\binom{-r}{k}$= $(-1)^{k}$ $\binom{r+k-1}{k}$

Any help would greatly appreciated.

How are you defining ${-r \choose k}$? Normally the combinitorial function is not defined for negative values.

-Dan
• February 19th 2008, 09:18 AM
Plato
In this case the meaning is ${ - r \choose k} = \frac{{\left( { - r} \right)\left( { - r - 1} \right)\left( { - r - 2} \right) \cdots \left( { - r - k + 1} \right)}}{{k!}}$.
The proof is tedious. Expand both sides and compare.
• February 19th 2008, 10:15 AM
hockey777
Quote:

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

= $-1^k$ $\frac{n(n+1)...(n+k-1)}{k!}$

= $-1^k$ $\binom{n+k-1}{k}$

Would that work?