Given real numbers $x,y,z,a$ satisfying: $$ x+y+z = a$$$$ \frac{1}{x}+\frac{1}{y}+\frac{1}{z} = \frac{1}{a} $$Prove that at least one of the numbers $x,y,z$ is equal to $a$.

Prove it is possible to find $2^{2021}$ different pairs of positive integers $(a_i,b_i)$ such that: $$ \frac{1}{a_ib_i}+\frac{1}{a_2b_2} + \ldots + \frac{1}{a_{2^{2021}}b_{2^{2021}}} = 1 $$$$ a_1+a_2 +\ldots a_{2^{2021}} +b_1+b_2 + \ldots +b_{2^{2021}} = 3^{2022} $$Note: Pairs $(a,b)$ and $(c,d)$ are different if $a \neq c$ or $b \neq d$

Given isosceles $\triangle ABC$ with $AB = AC$ and $\angle BAC = 22^{\circ}$. On the side $BC$ point $D$ is chosen such that $BD = 2CD$. The foots of perpendiculars from $B$ to lines $AD$ and $AC$ are points $E$, $F$ respectively. Find with the proof value of the angle $\angle CEF$.

Initially, on the board, all integers from $1$ to $400$ are written. Two players play a game alternating their moves. In one move it is allowed to erase from the board any 3 integers, which form a triangle. The player, who can not perform a move loses. Who has a winning strategy?

Find all positive integers $n,k$ satisfying: $$ n^3 -5n+10 =2^k $$

Let $ABC$ be an isosceles triangle with $BC=CA$, and let $D$ be a point inside side $AB$ such that $AD< DB$. Let $P$ and $Q$ be two points inside sides $BC$ and $CA$, respectively, such that $\angle DPB = \angle DQA = 90^{\circ}$. Let the perpendicular bisector of $PQ$ meet line segment $CQ$ at $E$, and let the circumcircles of triangles $ABC$ and $CPQ$ meet again at point $F$, different from $C$. Suppose that $P$, $E$, $F$ are collinear. Prove that $\angle ACB = 90^{\circ}$.

For each prime $p$, construct a graph $G_p$ on $\{1,2,\ldots p\}$, where $m\neq n$ are adjacent if and only if $p$ divides $(m^{2} + 1-n)(n^{2} + 1-m)$. Prove that $G_p$ is disconnected for infinitely many $p$

Let $\mathcal{A}$ denote the set of all polynomials in three variables $x, y, z$ with integer coefficients. Let $\mathcal{B}$ denote the subset of $\mathcal{A}$ formed by all polynomials which can be expressed as \begin{align*} (x + y + z)P(x, y, z) + (xy + yz + zx)Q(x, y, z) + xyzR(x, y, z) \end{align*}with $P, Q, R \in \mathcal{A}$. Find the smallest non-negative integer $n$ such that $x^i y^j z^k \in \mathcal{B}$ for all non-negative integers $i, j, k$ satisfying $i + j + k \geq n$.

In a regular 100-gon, 41 vertices are colored black and the remaining 59 vertices are colored white. Prove that there exist 24 convex quadrilaterals $Q_{1}, \ldots, Q_{24}$ whose corners are vertices of the 100-gon, so that the quadrilaterals $Q_{1}, \ldots, Q_{24}$ are pairwise disjoint, and every quadrilateral $Q_{i}$ has three corners of one color and one corner of the other color.

Suppose that $a,b,c,d$ are positive real numbers satisfying $(a+c)(b+d)=ac+bd$. Find the smallest possible value of $$\frac{a}{b}+\frac{b}{c}+\frac{c}{d}+\frac{d}{a}.$$Israel

Let $ABCD$ be a convex quadrilateral with $\angle ABC>90$, $CDA>90$ and $\angle DAB=\angle BCD$. Denote by $E$ and $F$ the reflections of $A$ in lines $BC$ and $CD$, respectively. Suppose that the segments $AE$ and $AF$ meet the line $BD$ at $K$ and $L$, respectively. Prove that the circumcircles of triangles $BEK$ and $DFL$ are tangent to each other. $\emph{Slovakia}$