CGMO 2020

In the quadrilateral $ABCD$ , $AB=AD$ , $CB=CD$ , $\angle ABC =90^\circ$ . $E$ , $F$ are on $AB$ , $AD$ and $P$ , $Q$ are on $EF$ ( $P$ is between $E, Q$ ), satisfy $\frac{AE}{EP}=\frac{AF}{FQ}$ . $X, Y$ are on $CP, CQ$ that satisfy $BX \perp CP, DY \perp CQ$ . Prove that $X, P, Q, Y$ are concyclic.

Attachments:

Let $n$ be an integer and $n \geq 2$ , $x_1, x_2, \cdots , x_n$ are arbitrary real number, find the maximum value of $$2\sum_{1\leq i<j \leq n}\left \lfloor x_ix_j \right \rfloor-\left ( n-1 \right )\sum_{i=1}^{n}\left \lfloor x_i^2 \right \rfloor$$

There are $3$ classes with $n$ students in each class, and the heights of all $3n$ students are pairwise distinct. Partition the students into groups of $3$ such that in each group, there is one student from each class. In each group, call the tallest student the tall guy. Suppose that for any partition of the students, there are at least 10 tall guys in each class, prove that the minimum value of $n$ is $40$ .

Let $p,q$ be primes, where $p>q$ . Define $t=\gcd(p!-1,q!-1)$ . Prove that $t\le p^{\frac{p}{3}}$ .

Find all the real number sequences $\{b_n\}_{n \geq 1}$ and $\{c_n\}_{n \geq 1}$ that satisfy the following conditions: (i) For any positive integer $n$ , $b_n \leq c_n$ ; (ii) For any positive integer $n$ , $b_{n+1}$ and $c_{n+1}$ is the two roots of the equation $x^2+b_nx+c_n=0$ .

Let $p, q$ be integers and $p, q > 1$ , $gcd(p, 6q)=1$ . Prove that: $$\sum_{k=1}^{q-1}\left \lfloor \frac{pk}{q}\right\rfloor^2 \equiv 2p \sum_{k=1}^{q-1}k\left\lfloor \frac{pk}{q} \right\rfloor (mod q-1)$$

Let $n$ be a given positive integer. Let $\mathbb{N}_+$ denote the set of all positive integers.

Determine the number of all finite lists $(a_1,a_2,\cdots,a_m)$ such that:(1) $m\in \mathbb{N}_+$ and $a_1,a_2,\cdots,a_m\in \mathbb{N}_+$ and $a_1+a_2+\cdots+a_m=n$ .(2) The number of all pairs of integers $(i,j)$ satisfying $1\leq i<j\leq m$ and $a_i>a_j$ is even.

For example, when $n=4$ , the number of all such lists $(a_1,a_2,\cdots,a_m)$ is $6$ , and these lists are $(4),$ $(1,3),$ $(2,2),$ $(1,1,2),$ $(2,1,1),$ $(1,1,1,1)$ .