Show that $$\frac{2n+2}{2n+1}\cdot\frac{1}{\binom{2n}{k}}=\frac{1}{\binom{2n+1}{k}}+\frac{1}{\binom{2n+1}{k+1}}$$

Calculate the value of $$\sum_{k=1}^{2n-1}(-1)^{k-1}\binom{2n}{k}^{-1}$$

Let $n$ be a positive integer. Show that $$\sum_{k=0}^{n}k^2\binom{n}{k}^2=n^2\binom{2n-2}{n-1}$$

Find the value of $$\sum_{k=0}^{n-1}\binom{2n-1}{k}$$

Calculate the value of $$\displaystyle\sum_{k=0}^{n}(-1)^{k}k\binom{n}{k}$$

Show that $$\left(\sum_{k=0}^{\infty}x^k\right)^2=\sum_{k=0}^{\infty}(k+1)x^k$$

Let $f(x)$ be the generating function for $a_0$, $a_1$, $a_2$, $\cdots$. Find the generating function for $$a_0, a_0 + a_1, a_0+a_1+a_2, \cdots$$

Show that $$\sqrt{1+x}=1+\sum_{n=1}^{\infty}\frac{(-1)^{n-1}}{n\cdot 2^{2n-1}}\binom{2n-2}{n-1}x^n$$

Let $n$ be a positive integer greater than $1$. Show $$\sum_{k=1}^{n-1}\frac{1}{k(n-k)}\binom{2(k-1)}{k-1}\binom{2(n-k-1)}{n-k-1}=\frac{1}{n}\binom{2(n-1)}{n-1}$$

Show that $$\frac{1}{\sqrt{1-4x}}=\sum_{n=0}^{\infty}\binom{2n}{n}x^n$$

(Generalized binomial expansion) If $a$, $b$, and $r$ are some real or complex numbers, then $$(a+b)^r = \sum_{k=0}^{\infty}\binom{r}{k}a^{r-k}b^k$$

Here, the following definition still holds when $r$ is a real or complex number: $$\binom{r}{k}=\frac{r(r-1)\cdots(r-k+1)}{1\cdot 2\cdots k}$$

Show $$\sum_{k=0}^{n}(-1)^k\binom{n}{k}\binom{m+k}{q}=(-1)^n\binom{m}{q-n}$$

Let $N$ be the value of the following expression. $$\sum_{k=0}^{n-1}\left(\binom{n}{0}+\binom{n}{1}+\cdots+\binom{n}{k}\right)\left(\binom{n}{k+1}+\binom{n}{k+2}+\cdots+\binom{n}{n}\right)$$

Show $$N=\frac{n}{2}\binom{2n}{n}$$

Show that $$\sum_{k=1}^{n}(-1)^k\binom{n}{k}\left(1+\frac{1}{2}+\cdots+\frac{1}{k}\right)=-\frac{1}{n}$$

Prove $$\sum_{k=0}^{n}(-1)^k\frac{{n \choose k}}{\binom{m+k}{k}}=\frac{m}{m+n}$$

Evaluate the value of $$\sum_{m=0}^{2009}\sum_{n=0}^{m}\binom{2009}{m}\binom{m}{n}$$

Find the sum of all $n$ such that $$\binom{n}{0}-\binom{n}{1}+\binom{n}{2}-\binom{n}{3}+\cdots +\binom{n}{2018} = 0$$

Given randomly selected $5$ distinct positive integers not exceeding $90$, what is the expected average value of the fourth largest number?

For every integer $n$ from $0$ to $6$, we have $3$ identical weights with weight $2^n$ grams. How many total ways are there to form a total weight of $263$ grams using only these weights?

Let $\mathbb{S}$ be a set of integers, $\max(\mathbb{S})$ be the largest element in $\mathbb{S}$, and $\mid\mathbb{S}\mid$ be the number of elements in $\mathbb{S}$. Find the number of non-empty set $\mathbb{S}\in\{1,2,\cdots,10\}$ satisfying $\max(\mathbb{S})\le\mid\mathbb{S}\mid + 2$.

Let $m$ and $n$ be positive integers. Show that $$\frac{(m+n)!}{(m+n)^{m+n}}<\frac{m!}{m^m}\frac{n!}{n^n}$$

How many different ways are there to make a payment of $n$ dollars using any number of $\$1$ and $\$2$ bills?

How many $4$-digit integers are there whose sum of all digits equals $12$?

There are $10$ red, $10$ blue, and $10$ white balls. How many different ways are there to retrieve $16$ balls with all the three colors present.