Let $N$ be a positive integer. Prove that there exist three permutations $a_1,\dots,a_N$, $b_1,\dots,b_N$, and $c_1,\dots,c_N$ of $1,\dots,N$ such that \[\left|\sqrt{a_k}+\sqrt{b_k}+\sqrt{c_k}-2\sqrt{N}\right|<2023\] for every $k=1,2,\dots,N$.

\[\int \frac{1}{(1+x)\sqrt{x}}\,\mathrm dx\] [i]Proposed by Connor Gordon[/i]

Denote by $\mathbb{Q}^+$ the set of positive rational numbers. A function $f : \mathbb{Q}^+ \to \mathbb{Q}$ satisfies • $f(p) = 1$ for all primes $p$, and • $f(ab) = af(b) + bf(a)$ for all $ a,b \in \mathbb{Q}^+ $. For which positive integers $n$ does the equation $nf(c) = c$ have at least one solution $c$ in $\mathbb{Q}^+$?

Let $ABC$ be a triangle with incircle $O$ and side lengths $5, 8$, and $9$. Consider the other tangent line to $O$ parallel to $BC$, which intersects $AB$ at $B_a$ and $AC$ at $C_a$. Let $r_a$ be the inradius of triangle $AB_aC_a$, and define $r_b$ and $r_c$ similarly. Find $r_a + r_b + r_c$.

$24! = 620,448,401,733,239,439,360,000$ ends in four zeros, and $25!=15,511,210,043,330,985,984,000,000$ ends in six zeros. Thus, there is no integer $n$ such that $n!$ ends in exactly five zeros. Let $S$ be the set of all $k$ such that for no integer n does $n!$ end in exactly $k$ zeros. If the numbers in $S$ are listed in increasing order, 5 will be the first number. Find the 100th number in that list.

Find all solutions $x,y \in \mathbb{Z}$, $x,y \geq 0$, to the equation \[ 1 + 3^x = 2^y. \]

The nine-point Euler circle of triangle $ABC$ is tangent to the excircles in the angle $A,B,C$ at $Fa, Fb, Fc$ respectively. Prove that $AF_a$ bisects the angle $\angle CAB$ if and only if $AFa$ bisects the angle $\angle F_bAF_c$. Đỗ Thanh Sơn

Solve over the real numbers the equation $$3^{\log_5(5x-10)}-2=5^{-1+\log_3x}.$$

In convex pentagon $ABCDE$, the sides $AE,BC$ are parallel and $\angle ADE=\angle BDC$. The diagonals $AC$ and $BE$ intersect at $P$. Prove that $\angle EAD=\angle BDP$ and $\angle CBD=\angle ADP$.

For how many integer values of $x$ is $|2x|\leq 7\pi?$ $\textbf{(A) }16 \qquad\textbf{(B) }17\qquad\textbf{(C) }19\qquad\textbf{(D) }20\qquad\textbf{(E) }21$

Given the circle of radius $1$ and several its chords with the sum of lengths $1$. Prove that one can be inscribe a regular hexagon into that circle so that its sides don’t intersect those chords.

Suppose that $ n$ is the product of three consecutive integers and that $ n$ is divisible by $ 7$. Which of the following is not necessarily a divisor of $ n$? $ \textbf{(A)}\ 6 \qquad \textbf{(B)}\ 14 \qquad \textbf{(C)}\ 21 \qquad \textbf{(D)}\ 28 \qquad \textbf{(E)}\ 42$

Given that two real numbers $x$ and $y$ satisfy $x^2 - 6xy + 9y^2 + |x - 3| = 0$, calculate $x + y$. $\textbf{(A) }1\qquad\textbf{(B) }3\qquad\textbf{(C) }4\qquad\textbf{(D) }16\qquad\textbf{(E) }\text{impossible to determine}$

Given a triangle $ABC$ and a point $K$ . The lines $AK$,$BK$,$CK$ hit the opposite side of the triangle at $D,E,F$ respectively. On the exterior of $ABC$, we construct three pairs of similar triangles: $BDM$,$DCN$ on $BD$,$DC$, $CEP$,$EAQ$ on $CE$,$EA$, and $AFR$,$FBS$ on $AF$, $FB$. The lines $MN$,$PQ$,$RS$ intersect each other form a triangle $XYZ$. Prove that $AX$,$BY$,$CZ$ are concurrent.

Let $ABC$ be an acute angled triangle and point $M$ chosen differently from $A,B,C$. Prove that $M$ is the orthocenter of triangle $ABC$ if and only if \[\frac{BC}{MA}\vec{MA}+\frac{CA}{MB}\vec{MB}+\frac{AB}{MC}\vec{MC}= \vec{0}\]

A directed line segment, starting point is $P(-1,1)$, finishing point is $Q(2,2)$. If line $l:x+my+m=0$ intersects $PQ$ at its extended line, then the range value of $m$ is________.

On a plane there is an invisible [color=#eee]rabbit[/color] (rabbit) hiding on a lattice point. We want to put $n$ hunters on some lattice points to catch the rabbit. In a turn each hunter can choose to shoot to left/right or top/bottom direction. On the $i$th turn there will be these steps in order 1. The rabbit jumps to an adjacent lattice point on the top, bottom, left, or right. 2. item Each hunter moves to an adjacent lattice point on the top, bottom, left or right (each hunter can move to different direction). Then they shoot a bullet which travels $\frac{334111214}{334111213}i$ units on the directions they chose. If a bullet hits the rabbit then it is caught. Find the smallest number $n$ such that the rabbit would definitely be caught in a finite number of turns. [i]Proposed by tob8y[/i]

On a circle there are $n$ red and $n$ blue arcs given in such a way that each red arc intersects each blue one. Prove that some point is contained by at least $n$ of the given coloured arcs.

There are $8436$ steel balls, each with radius $1$ centimeter, stacked in a tetrahedral pile, with one ball on top, $3$ balls in the second layer, $6$ in the third layer, $10$ in the fourth, and so on. Determine the height of the pile in centimeters.

Sir Alex plays the following game on a row of 9 cells. Initially, all cells are empty. In each move, Sir Alex is allowed to perform exactly one of the following two operations: [list=1] [*] Choose any number of the form $2^j$, where $j$ is a non-negative integer, and put it into an empty cell. [*] Choose two (not necessarily adjacent) cells with the same number in them; denote that number by $2^j$. Replace the number in one of the cells with $2^{j+1}$ and erase the number in the other cell. [/list] At the end of the game, one cell contains $2^n$, where $n$ is a given positive integer, while the other cells are empty. Determine the maximum number of moves that Sir Alex could have made, in terms of $n$. [i]Proposed by Warut Suksompong, Thailand[/i]

Consider infinite diagrams [asy] import graph; size(90); real lsf = 0.5; pen dp = linewidth(0.7) + fontsize(10); defaultpen(dp); pen ds = black; label("$n_{00} \ n_{01} \ n_{02} \ldots$", (1.14,1.38), SE*lsf); label("$n_{10} \ n_{11} \ n_{12} \ldots$", (1.2,1.8), SE*lsf); label("$n_{20} \ n_{21} \ n_{22} \ldots$", (1.2,2.2), SE*lsf); label("$\vdots \quad \vdots \qquad \vdots $", (1.32,2.72), SE*lsf); draw((1,1)--(3,1)); draw((1,1)--(1.02,2.62)); clip((-4.3,-10.94)--(-4.3,6.3)--(16.18,6.3)--(16.18,-10.94)--cycle); [/asy] where all but a finite number of the integers $n_{ij} , i = 0, 1, 2, \ldots, j = 0, 1, 2, \ldots ,$ are equal to $0$. Three elements of a diagram are called [i]adjacent[/i] if there are integers $i$ and $j$ with $i \geq 0$ and $j \geq 0$ such that the three elements are [b](i)[/b] $n_{ij}, n_{i,j+1}, n_{i,j+2},$ or [b](ii)[/b] $n_{ij}, n_{i+1,j}, n_{i+2,j} ,$ or [b](iii)[/b] $n_{i+2,j}, n_{i+1,j+1}, n_{i,j+2}.$ An elementary operation on a diagram is an operation by which three [i]adjacent[/i] elements $n_{ij}$ are changed into $n_{ij}'$ in such a way that $|n_{ij}-n_{ij}'|=1.$ Two diagrams are called equivalent if one of them can be changed into the other by a finite sequence of elementary operations. How many inequivalent diagrams exist?

Let $a,b,c$ be positive real numbers such that $ab+bc+ca\le 3abc$. Prove that \[\sqrt{\frac{a^2+b^2}{a+b}}+\sqrt{\frac{b^2+c^2}{b+c}}+\sqrt{\frac{c^2+a^2}{c+a}}+3\le \sqrt{2} (\sqrt{a+b}+\sqrt{b+c}+\sqrt{c+a})\]

Many Gothic cathedrals have windows with portions containing a ring of congruent circles that are circumscribed by a larger circle, In the figure shown, the number of smaller circles is four. What is the ratio of the sum of the areas of the four smaller circles to the area of the larger circle? [asy]unitsize(6mm); defaultpen(linewidth(.8pt)); draw(Circle((0,0),1+sqrt(2))); draw(Circle((sqrt(2),0),1)); draw(Circle((0,sqrt(2)),1)); draw(Circle((-sqrt(2),0),1)); draw(Circle((0,-sqrt(2)),1));[/asy]$ \textbf{(A)}\ 3\minus{}2\sqrt2 \qquad \textbf{(B)}\ 2\minus{}\sqrt2 \qquad \textbf{(C)}\ 4(3\minus{}2\sqrt2) \qquad \textbf{(D)}\ \frac12(3\minus{}\sqrt2)$ $ \textbf{(E)}\ 2\sqrt2\minus{}2$

$2020$ magicians are divided into groups of $2$ for the Lexington Magic Tournament. After every $5$ days, which is the duration of one match, teams are rearranged so no $2$ people are ever on the same team. If the longest tournament is $n$ days long, what is the value of $n?$ [i]Proposed by Ephram Chun[/i]

a) A certain committee has gathered $40$ times. There were $10$ members on every meeting. Not a single couple has met on the meetings twice. Prove that there were no less then $60$ members in the committee. b) Prove that you can not construct more then $30$ subcommittees of $5$ members from the committee of $25$ members, with no couple of subcommittees having more than one common member.