# Math Help - Proof help

1. ## Proof help

Prove that r[(r-1)Ck] = rCk(r-k)

This may be hard to read... Prove that r times (r-1) choose k = r choose k time (r-k)

I don't have a clue how to show this. Any help is greatly appreciated!!

2. Just to aid in reading...

Prove that $\displaystyle r{{r-1}\choose{k}} = (r - k){r\choose{k}}$.

Surely you can prove this by evaluating the combinations using the formula

$\displaystyle {n\choose{k}} = \frac{n!}{k!(n-k)!}$...