Equivalent form for Binomial Expression?

1. Jul 17, 2013

eddybob123

Is there a way to express ${n\choose k}{n\choose r}$ in another form without n as the "top" of the binomial coefficient? I remember seeing it once but I forgot what it was.

2. Jul 17, 2013

LCKurtz

Maybe something here will do:

http://www.cs.columbia.edu/~cs4205/files/CM4.pdf