$\displaystyle \sum^{10}_{k=0}\binom{20+k}{20}\cdot \binom{20-k}{10}$

Question

The value of $\displaystyle \sum^{10}_{k=0}\binom{20+k}{20}\cdot \binom{20-k}{10}$

Using $\displaystyle \binom{n}{r}=\binom{n}{n-r}$

So we have $\displaystyle \sum^{10}_{k=0}\binom{20+k}{k}\cdot \binom{20-k}{10-k}=\binom{40}{10}$

(Above using $2$ way counting ,

we have total $40$ students and we have to select $10$ students)

But answer given as $\displaystyle \binom{41}{10}$

Please have a look on that problem

Answer 1

Their answer is right.

Consider the bijection: Choose $31$ from $41$ objects. The location of the $21$st object (at location $21+k$) splits the rest into a selection of $20$ objects from the first $20+k$ and a selection of $10$ from the last $20-k$. Similarly, a selection of the latter sort can be subsumed into a selection of $31$ from $41$ objects.

Alternatively, $1/(1-z)^{n+1}=\sum_{k=0} ((n+k)Cn )x^k$, so the first binomial is the coefficients of $z^k$ in $1/(1-z)^{21}$, the second of $z^{10-k}$ in $1/(1-z)^{11}$, and the total sumproduct is the coefficient of $z^{10}$, which gives that you want the coefficient of $z^{10}$ of $1/(1-z)^{32}$ which gives $(32+10-1)C31=41C10$.

Answer 2

Here we have a Chu-Vandermonde Identity in disguise.

We obtain: \begin{align*} \color{blue}{\sum^{10}_{k=0}\binom{20+k}{k}\binom{20-k}{10-k}} &=\sum^{10}_{k=0}\binom{-21}{k}\binom{-11}{10-k}\tag{1}\\ &=\binom{-32}{10}\tag{2}\\ &\,\,\color{blue}{=\binom{41}{10}}\tag{3} \end{align*} and the claim follows.

Comment:

• In (1) we use the binomial identity $\binom{-p}{q}=\binom{p+q-1}{q}(-1)^q$ twice.

• In (2) we apply the Chu-Vandermonde identity.

• In (3) we apply the binomial identity used in (1) again.