How do I prove the identity with combinatorics?

Question

I want to prove the following identity

$$r+(r+1)\binom{r}{r-1}+(r+2)\binom{r+1}{r-1}+...+n\binom{n-1}{r-1}=r\binom{n+1}{r+1} \tag{1}$$

I can prove by induction but I want to prove with combinatorics.

I consider number of different $n+1$ people grouping (each person is different ages) with 2 rules that are

1. $r+1$ people in a group.

2. the group consist of $1$ head and $r$ members that head must not be the youngest people in group.

I can find number of different is RHS of (1) but I can't find it is equal to LHS of (1).

SHOW RHS :

Start with choosing $r+1$ people of $n+1$ people so I have the $\binom{n+1}{r+1}$ number that is possible.

Then, I choose the head, I can choose only $r$ people in group (ignoring the youngest people).

So, I have RHS.

I can't find the LHS. All help would be appreciated.

Answer 1

Your start is good. You want to choose $r+1$ people of $n+1$, and a head that isn't the youngest of those. Suppose you first choose who is going to be the youngest in your group. Since there are $r$ others in your group, that must be the 1st, 2nd, ... or $(n-r+1)$th youngest overall (so there are enough older people).

If you've chosen the $k$th youngest overall to be the youngest in the group, you now have $n+1-k$ choices for your head (all the older people) and then you have to choose the remaining $r-1$ members of the group from the remaining $n-k$ people who are older than the designated youngest. That should give you the LHS.