# Evaluate $\sum_{k=0}^nk(n-k)$

Evaluate $\sum_{k=0}^nk(n-k)$

I have $\sum_{k=0}^nkn-\sum_{k=0}^nk^2$

then im suppose to use the identity of $\left( \begin{array}{c} r \\ r \end{array} \right)+\left( \begin{array}{c} r+1 \\ r \end{array} \right)+...+\left( \begin{array}{c} n \\ r \end{array} \right)=\left( \begin{array}{c} n+1 \\ r+1 \end{array} \right)$ to solve this , but i have no idea how this works .

can someone explain in detail of how binomial identities substitute into this ?

I hadn’t thought of using that identity, but it works quite nicely:

\begin{align*} \sum_{k=0}^nkn&=n\sum_{k=0}^nk\\\\ &=n\sum_{k=0}^n\binom{k}1\;, \end{align*}

and

\begin{align*} \sum_{k=0}^nk^2&=\sum_{k=0}^n\Big(k(k-1)+k\Big)\\\\ &=\sum_{k=0}^nk(k-1)+\sum_{k=0}^nk\\\\ &=2\sum_{k=0}^n\binom{k}2+\sum_{k=0}^n\binom{k}1\;.\tag{1} \end{align*}

I’ll leave the rest to you.

Added: You’ve been given the identity $$\binom{r}r+\binom{r+1}r+\ldots+\binom{n}r=\binom{n+1}{r+1}\;,$$ or in summation notation $$\sum_{k=r}^n\binom{k}r=\binom{n+1}{r+1}\;.$$ Since $\binom{k}r=0$ when $0\le k<r$, this can just as well be written $$\sum_{k=0}^n\binom{k}r=\binom{n+1}{r+1}$$ or, in expanded form, $$\binom0r+\binom1r+\ldots+\binom{n}r=\binom{n+1}{r+1}\;.$$ $(1)$ has two summations of this form, one with $r=2$ and one with $r=1$. For the former, for instance, the identity says that $$\sum_{k=0}^n\binom{k}2=\binom{n+1}{2+1}=\binom{n+1}3\;.$$ If you want, you can convert this to a polynomial in $n$:

\begin{align*} \binom{n+1}3&=\frac{(n+1)!}{3!\big((n+1)-3\big)!}\\ &=\frac{(n+1)!}{3!(n-2)!}\\ &=\frac{(n+1)n(n-1)}{3!}\\ &=\frac16n(n-1)(n+1)\\ &=\frac16\left(n^3-n\right)\;. \end{align*}

• I still have no idea how to use the identity.and only n or numbers can be put in the front of sum sign ? I have no knowledge of using binomial identities, the book I have just sort of skips in how the identities are used in equations. – cindy Oct 22 '13 at 1:06
• @cindy: I can pull the $n$ out of that first sum because it doesn’t depend on $k$: it’s the same factor in every term, and $$\sum_{k=0}^nkn=0\cdot n+1\cdot n+2\cdot n+\ldots+n\cdot n=n(0+1+2+\ldots+n)=n\sum_{k=0}^nk\;.$$ Using the identity that you were given is just a matter of substituting the values from this problem into the identity, but I’ll add one of the evaluations to my answer in a minute. – Brian M. Scott Oct 22 '13 at 1:13