Numbers $7^2$,$8^2,\ldots,2023^2$,$2024^2$ are written on the board. Is it possible to add to one of them $7$, to some other one $8$, $\ldots$, to the remaining $2024$ such that all numbers became prime M. Zorka

Let $S$ be the set of all non-increasing sequences of numbers $a_1 \geq a_2 \geq \ldots \geq a_{101}$ such that $a_i \in \{ 0,1,\ldots ,101 \}$ for all $1 \leq i \leq 101$ For every sequence $s \in S$ let $$f(s)=\lceil \frac{a_1}{2} \rceil+\lfloor \frac{a_2}{2} \rfloor + \lceil \frac{a_3}{2} \rceil + \ldots + \lfloor \frac{a_{100}}{2} \rfloor + \lceil \frac{a_{101}}{2} \rceil$$where $\lfloor x \rfloor$ is the greatest integer, not exceeding $x$, and $\lceil x \rceil$ is the least integer at least $x$. Prove that the number of sequences $s \in S$ for which $f(s)$ is even is the same, as the number of sequences $s$ for which $f(s)$ is odd M. Zorka

Do there exist positive integer numbers $a$ and $b$, for which the number $(\sqrt{1+\frac{4}{a}}-1)(\sqrt{1+\frac{4}{b}}-1)$ is rational V. Kamianetski

In a convex hexagon $ABCDEF$ equalities $\angle ABC= \angle CDE= \angle EFA$ hold, and the angle bisectors of angles $ABC$, $CDE$ and $EFA$ intersect in one point. Rays $AB$ and $DC$ intersect at $P$, rays $BC$ and $ED$ - at $Q$, rays $CD$ and $FE$ - at $R$, rays $DE$ and $AF$ - at $S$. Prove that $PR=QS$ M. Zorka

Polina wrote on the first page of her notebook $n$ different positive integers. On the second page she wrote all pairwise sums of the numbers from the first page, and on the third - absolute values of pairwise differences of number from the second page. After that she kept doing same operations, i.e. on the page $2k$ she wrote all pairwise sums of numbers from page $2k-1$, and on the page $2k+1$ absolute values of differences of numbers from page $2k$. At some moment Polina noticed that there exists a number $M$ such that, no matter how long she does her operations, on every page there are always at most $M$ distinct numbers. What is the biggest $n$ for which it is possible? M. Karpuk

For each number $x$ we denote by $S(x)$ the sum of digits from its decimal representation. Find all positive integers $m$ for each of which there exists a positive integer $n$, such that $$S(n^2-2n+10)=m$$Chernov S.

On the diagonal $AC$ of the convex quadrilateral $ABCD$ points $P$,$Q$ are chosen such that triangles $ABD$,$PCD$ and $QBD$ are similar to each other in this order. Prove that $AQ=PC$ M. Zorka

A right $100$-gon $P$ is given, which has $x$ vertices coloured in white and all other in black. If among some vertices of a right polygon, all the vertices of which are also vertices of $P$, there is exactly one white vertex, then you are allowed to colour this vertex in black. Find all positive integers $x \leq 100$ for which for all initial colourings it is not possible to make all vertices black. A. Vaidzelevich,M. Shutro

Find all triples $(x,y,z)$ of positive real numbers such that $$ \begin{cases} 2x^2+y^3=3 \\ 3y^2+z^3=4 \\ 4z^2+x^3=5 \\ \end{cases} $$M. Zorka

A set $X=\{ x_1,x_2,\ldots,x_n \}$ consisting of $n$ positive integers is given. It is known that the greatest common divisor of any four different elements of $X$ is $1$. For every number $x_i$ let $m_i$ be the number of elements of $X$, which are divisible by $x_i$ For every $n \geq 4$, find the maximal possible value of the sum $m_1+\ldots+m_n$ A. Vaidzelevich

On the side $AC$ of triangle $ABC$ point $D$ is chosen. The perpendicular bisector of segment $BD$ intersects the circumcircle $\Omega$ of triangle $ABC$ at $P$, $Q$. Point $E$ lies on the arc $AC$ of circle $\Omega$, that doesn't contain point $B$, such that $\angle ABD=\angle CBE$. Prove that the orthocenter of the triangle $PQE$ lies on the line $AC$ M. Zorka

In some company, consisting of $n$ people, any two have at most $k \geq 2$ common friends. Lets call group of people working in the company unsocial if everyone in the group has at most one friend from the group. Prove that there exists an unsocial group consisting of at least $\sqrt{\frac{2n}{k}}$ people M. Zorka

Yuri and Vlad are playing a game on the table $4 \times 100$. Firstly, Yuri chooses $73$ squares $2 \times 2$ (squares can intersect, but cannot be equal). Then Vlad colours the cells of the table in $4$ colours such that in any row and in any column, and in any square chosen by Yuri, there were cells of all 4 colours. After that Vlad pays 2 rubles for every square $2 \times 2$, not chosen by Yuri, which cells of all 4 colours. What is the maximum possible number of rubles Yuri can get regardless of Vlad's actions M. Shutro

Given pairs $(a_1,b_1)$, $(a_2,b_2),\ldots, (a_n,b_n)$ of non-negative real numbers such that for any real $x$ and $y$ the equality $$\sqrt{a_1x^2+b_1y^2}+\sqrt{a_2x^2+b_2y^2}+\ldots+\sqrt{a_nx^2+b_ny^2}=\sqrt{x^2+y^2}$$Prove that $a_1=b_1,a_2=b_2,\ldots$,$a_n=b_n$ A. Vaidzelevich

Find all pairs of positive integers $(m,n)$, for which $$(m^n-n)^m=n!+m$$D. Volkovets

Given right hexagon $H$ with side length $1$. On the sides of $H$ points $A_1$,$A_2$,$\ldots$,$A_k$ such that at least one of them is the midpoint of some side and for every $1 \leq i \leq k$ lines $A_{i-1}A_i$ and $A_iA_{i+1}$ form equal angles with the side, that contains the point $A_i$ (let $A_0=A_k$ and $A_{k+1}=A_1$. It is known that the length of broken line $A_1A_2\ldots A_kA_1$ is a positive integer Prove that $n$ is divisible by $3$ M. Zorka

Let $1=d_1<d_2<\ldots<d_k=n$ be all divisors of $n$. It turned out that numbers $d_2-d_1,\ldots,d_k-d_{k-1}$ are $1,3,\ldots,2k-3$ in some order. Find all possible values of $n$ M. Zorka

Some vertices of a regular $2024$-gon are marked such that for any regural polygon, all of whose vertices are vertices of the $2024$-gon, at least one of his vertices is marked. Find the minimal possible number of marked vertices A. Voidelevich

Find all functions $f: \mathbb{R} \to \mathbb{R}$ such that for every $x,y \in \mathbb{R}$ the following equation holds:$$1+f(xy)=f(x+f(y))+(y-1)f(x-1)$$M. Zorka

A parallelogram $ABCD$ is given. The incircle of triangle $ABC$ with center $I$ touches $AB,BC,CA$ at $R,P,Q$. Ray $DI$ intersects segment $AB$ at $S$. It turned out that $\angle DPR=90$ Prove that the circle with diameter $AS$ is tangent to the circumcircle of triangle $DPQ$ M. Zorka

Let $n$ be a positive integer. On the blackboard all quadratic polynomials with positive integer coefficients, that do not exceed $n$, without real roots are written Find all $n$ for which the number of written polynomials is even A. Voidelevich

Let $\omega$ be the circumcircle of triangle $ABC$. Tangent lines to $\omega$ at points $A$ and $C$ intersect at $K$. Line $BK$ intersects $\omega$ for the second time at $M$. On the line $BC$ point $N$ is chosen such that $\angle BAN = 90$. Line $MN$ intersects $\omega$ for the second time at $D$. Prove that $BD=BC$ P. Chernikova

Let's call a pair of positive integers $(k,n)$ interesting if $n$ is composite and for every divisor $d<n$ of $n$ at least one of $d-k$ and $d+k$ is also a divisor of $n$ Find the number of interesting pairs $(k,n)$ with $k \leq 100$ M. Karpuk

A right hexagon with side length $n$ is divided into tiles of three types, which are shown in the image, which are rhombuses with side length $1$ each and the acute angle $60$. In one move you can choose three tiles, arranged as shown in the image on the left, and rearrange them, as shown in the image on the right Moves are made until it is impossible to make a move. a) Prove that for the fixed initial arrangement of tiles the same amount of moves would be made independent of the moves. b) For each positive integer $n$ find the maximum number of moves among all possible initial arrangements M. Zorka

Let $m$ and $n$ be two integers bigger than one $1$. $m+n$ positive integers not exceeding $mn-1$ are chosen. Prove that among them one can find $x \neq y$, that satisfy $\lfloor \frac{x}{n} \rfloor = \lfloor \frac{y}{n} \rfloor$ and $\lfloor \frac{x}{m} \rfloor = \lfloor \frac{y}{m} \rfloor$ A. Voidelevich

$29$ quadratic polynomials $f_1(x), \ldots, f_{29}(x)$ and $15$ real numbers $x_1<x_2<\ldots<x_{15}$ are given. Prove that for some two given polynomials $f_i(x)$ and $f_j(x)$ the following inequality holds: $$\sum_{k=1}^{14} (f_i(x_{k+1})-f_i(x_k))(f_j(x_{k+1})-f_j(x_k))>0$$A. Voidelevich

In a triangle $ABC$ point $I$ is the incenter, $I_A$ - excenter, $W$ - midpoint of the arc $BAC$ of circumcircle $\omega$ of $ABC$. Point $H$ is the projection of $I_A$ on $IW$. The tangent line to the circumcircle $BIC$ in point $I$ intersects $\omega$ in $E, F$. Prove that the perpendicular bisector to $AI$ is tangent to the circumcircle $EFH$ M. Zorka

Non-empty set $M$, that consists of positive integer numbers, has the following property: if for some(not necessarily distinct) positive integers $a_1,\ldots,a_{2024}$ the number $a_1\ldots a_{2024}$ is in $M$, then the number $a_1+a_2+\ldots+a_{2024}$ is also in $M$ Prove that all positive integer numbers, starting from $2049$, are in the $M$ M. Zorka

On the chord $AB$ of the circle $\omega$ points $C$ and $D$ are chosen such that $AC=BD$ and point $C$ lies between $A$ and $D$. On $\omega$ point $E$ and $F$ are marked, they lie on different sides with respect to line $AB$ and lines $EC$ and $FD$ are perpendicular to $AB$. The angle bisector of $AEB$ intersects line $DF$ at $R$ Prove that the circle with center $F$ and radius $FR$ is tangent to $\omega$ V. Kamenetskii, D. Bariev

Let $2=p_1<p_2<\ldots<p_n<\ldots$ be all prime numbers. Prove that for any positive integer $n \geq 3$ there exist at least $p_n+n-1$ prime numbers, that do not exceed $p_1p_2\ldots p_n$ I. Voronovich

Positive real numbers $a_1,a_2,\ldots, a_n$ satisfy the equation $$2a_1+a_2+\ldots+a_{n-1}=a_n+\frac{n^2-3n+2}{2}$$For every positive integer $n \geq 3$ find the smallest possible value of the sum $$\frac{(a_1+1)^2}{a_2}+\ldots+\frac{(a_{n-1}+1)^2}{a_n}$$M. Zorka

Projector emits rays in space. Consider all acute angles between the rays. It is known that no matter what ray we remove, the number of acute angles decreases by exactly $2$ What is the maximal number of rays the projector can emit? M. Karpuk, E. Barabanov