# How to calculate the sum of binomials? [closed]

I want to prove below:

n is natural number.

$$\sum_{k=1}^n k \binom{2n}{n+k} =\frac{1}{2}(n+1) \binom{2n}{n+1}$$

Please tell me above proof.

## closed as off-topic by Grigory M, user91500, Namaste, Davide Giraudo, HakimMay 25 '14 at 13:36

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• @takoika I hope I didn't mess up your question. – Santosh Linkha May 25 '14 at 8:00
• Wolframalpha solves (1/2)(n+1)(2n_C_(n+1)).I want to know its process. – takoika May 25 '14 at 8:01
• without $k$ this is partial sum of rows of Pascal triangle, which doesn't have a closed-form expression. This one probably doesn't have one either. You can asymptotics though. – Alex May 25 '14 at 9:58

## 1 Answer

You can use $$\sum_{k=0}^{2n}\binom{2n}{k}=(1+1)^{2n}=4^n$$ and $$\binom{m}{j}=\binom{m}{m-j}$$ and $$j\binom{m}{j}=m\binom{m-1}{j-1}$$ to simplify your expression.

For instance, combining them to \begin{align} 2k\binom{2n}{n+k}&=(n+k)\binom{2n}{n+k}-(n-k)\binom{2n}{n-k}\\[1em] &=2n\binom{2n-1}{n+k-1}-2n\binom{2n-1}{n-k-1}\\[1em] &=2n\binom{2n-1}{n+k-1}-2n\binom{2n-1}{n+k} \end{align} Perhaps this can be used in a telescoping sum.

• Is first fomula wrong? – takoika May 25 '14 at 14:18
• Sorry, but yes. Shameful mistake. – LutzL May 25 '14 at 14:48