MATH (to Reiny)

I tried to simplify it with the second fraction by multiplying the top and bottom by "r" and i didn't work. Well at least I don't understand how to get there. Can you please just explain how...
n!r/(n-r-1)!(r-1)!r
goes to
n!r/(n-r+1)!r!

I get one put of it, that r x (r-1)! = r! but what about the (n-r-1)! -> (n-r+1)!

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asked by Alex
  1. I checked over my previous post and the only thing I see wrong is that I forgot to put brackets in the denominator of the second fraction of the RS.
    I had it on my paper but typing it out here one has to be sooooo careful.

    so the RS should have said:

    RS = n!/((n-r)!r!) + n!/((r-1)!(n-(r-1))!)
    which is then
    RS = n!/((n-r)!r!) + n!/((r-1)!(n-r+1)!)

    I hope you can follow it now

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    posted by Reiny
  2. yes i got now, thanks for the help.

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    posted by Alex

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