Do there exist two polynomials $P$ and $Q$ with integer coefficient such that i) both $P$ and $Q$ have a coefficient with absolute value bigger than $2021$, ii) all coefficients of $P \cdot Q$ by absolute value are at most $1$.

Let $ABC$ be an acute, non-isosceles triangle with $H$ the orthocenter and $M$ the midpoint of $AH$. Denote $O_1$,$O_2$ as the centers of circles pass through $H$ and respectively tangent to $BC$ at $B$, $C$. Let $X$, $Y$ be the ex-centers which respect to angle $H$ in triangles $HMO_1$,$HMO_2$. Prove that $XY$ is parallel to $O_1O_2$.

Let $x$, $y$ and $z$ be odd positive integers such that $\gcd \ (x, y, z) = 1$ and the sum $x^2 +y^2 +z^2$ is divisible by $x+y+z$. Prove that $x+y+z- 2$ is not divisible by $3$.

In the popular game of Minesweeper, some fields of an $a \times b$ board are marked with a mine and on all the remaining fields the number of adjacent fields that contain a mine is recorded. Two fields are considered adjacent if they share a common vertex. For which $k \in \{0, 1, 2, 3, 4, 5, 6, 7, 8\}$ is it possible for some $a$ and $b$ , $ab > 2021$, to create a board whose fields are covered in mines, except for $2021$ fields who are all marked with $k$?

There are $n \ge 2$ positive integers written on the whiteboard. A move consists of three steps: calculate the least common multiple $N$ of all numbers then choose any number $a$ and replace $a$ by $N/a$ . Prove that, using a finite number of moves, you can always make all the numbers on the whiteboard equal to $ 1$.

Let $ABC$ be an acute triangle with $AB < AC$ and inscribed in the circle $(O)$. Denote $I$ as the incenter of $ABC$ and $D$, $E$ as the intersections of $AI$ with $BC$, $(O)$ respectively. Take a point $K$ on $BC$ such that $\angle AIK = 90^o$ and $KA$, $KE$ meet $(O)$ again at M,N respectively. The rays $ND$, $NI$ meet the circle $(O)$ at $Q$,$P$. 1. Prove that the quadrilateral $MPQE$ is a kite. 2. Take $J$ on $IO$ such that $AK \perp AJ$. The line through $I$ and perpendicular to $OI$ cuts $BC$ at $R$ ,cuts $EK$ at $S$ .Prove that $OR \parallel JS$.

Let $a$, $b$, and $c$ be positive real numbers. Prove that $$(a^5 - a^2 +3)(b^5 - b^2 +3)(c^5 - c^2 +3)\ge (a+b+c)^3$$

A set of $n$ points in space is given, no three of which are collinear and no four of which are co-planar (on a single plane), and each pair of points is connected by a line segment. Initially, all the line segments are colorless. A positive integer $b$ is given and Alice and Bob play the following game. In each turn Alice colors one segment red and then Bob colors up to $b$ segments blue. This is repeated until there are no more colorless segments left. If Alice colors a red triangle, Alice wins. If there are no more colorless segments and Alice hasn’t succeeded in coloring a red triangle, Bob wins. Neither player is allowed to color over an already colored line segment. 1. Prove that if $b < \sqrt{2n - 2} -\frac32$ , then Alice has a winning strategy. 2. Prove that if $b \ge 2\sqrt{n}$, then Bob has a winning strategy.