This is a question from my first proofs homework and I am confused about the combinatorial argument aspect. I already did the algebraic proof. I think I am supposed to put into words what both sides represent? So the LHS would be the number of ways of picking a committee of $k$ members from $n$ people, or number of subsets of size $k$ from a total of $n$, but I don't know how to articulate what is happening on the RHS.


Here's a hint. Try multiplying through by $k$, so it's at least obvious that both sides are integers. Then what you want to show is that $k\binom{n}{k} = n\binom{n-1}{k-1}$. The LHS is now the number of ways to select a committee of $k$ members from $n$ people along with a decision of which of those $k$ will be chairperson. Can you reinterpret this to get the RHS?.

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    $\begingroup$ Okay, that helped a lot. So then the RHS would be the number of ways of selecting the chairperson first, and then forming the committee of k-1 from a total of n-1 people, because we assume the chairperson is a part of the committee? $\endgroup$ – Marla Mar 5 '13 at 16:05
  • $\begingroup$ Exactly right. I'm glad I could help. $\endgroup$ – Noah Stein Mar 5 '13 at 16:08

Your expression can be rewritten as $$ \binom{n}{k} \binom{k}{1}=\binom{n}{1}\binom{n-1}{k-1} $$ Hence LHS can be interpreted as the number of ways to select $k$ unique items out of $n$ AND then one out of these $k$. Then RHS is the number of ways to select one item out of $n$ AND then $k-1$ out of the remaining $n-1$.

  • $\begingroup$ It seems worth pointing out that this argument works more generally, in the sense that you can replace all the $1$'s by $m$ for any $m\leq k$. $\endgroup$ – Andreas Blass Mar 5 '13 at 17:39
  • $\begingroup$ A generalization of this idea leads to multinomial coefficients. $\endgroup$ – Lord_Farin Apr 18 '13 at 7:58

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