Olympiad problems

2020 JBMO Shortlist, 3

Tags: Junior , Balkan , shortlist , 2020 , number theory

Find the largest integer $k$ ($k \ge 2$), for which there exists an integer $n$ ($n \ge k$) such that from any collection of $n$ consecutive positive integers one can always choose $k$ numbers, which verify the following conditions: 1. each chosen number is not divisible by $6$, by $7$, nor by $8$; 2. the positive difference of any two distinct chosen numbers is not divisible by at least one of the numbers $6$, $7$, and $8$.

2007 Junior Macedonian Mathematical Olympiad, 4

Tags: Junior , JMMO , 2007 , Macedonia , algebra , Inequality

The numbers $a_{1}, a_{2}, ..., a_{20}$ satisfy the following conditions: $a_{1} \ge a_{2} \ge ... \ge a_{20} \ge 0$ $a_{1} + a_{2} = 20$ $a_{3} + a_{4} + ... + a_{20} \le 20$ . What is maximum value of the expression: $a_{1}^2 + a_{2}^2 + ... + a_{20}^2$ ? For which values of $a_{1}, a_{2}, ..., a_{20}$ is the maximum value achieved?

2023 Junior Balkan Team Selection Tests - Romania, P2

Tags: Inequality , Junior , Balkan , shortlist , algebra

Suppose that $a, b,$ and $c$ are positive real numbers such that $$a + b + c \ge \frac{1}{a} + \frac{1}{b} + \frac{1}{c}.$$ Find the largest possible value of the expression $$\frac{a + b - c}{a^3 + b^3 + abc} + \frac{b + c - a}{b^3 + c^3 + abc} + \frac{c + a - b}{c^3 + a^3 + abc}.$$

2020 JBMO Shortlist, 4

Tags: Junior , Balkan , shortlist , 2020 , number theory , prime numbers

Find all prime numbers $p$ such that $(x + y)^{19} - x^{19} - y^{19}$ is a multiple of $p$ for any positive integers $x$, $y$.

2022 JBMO Shortlist, C3

Tags: combinatorics , Junior , Balkan , shortlist

There are $200$ boxes on the table. In the beginning, each of the boxes contains a positive integer (the integers are not necessarily distinct). Every minute, Alice makes one move. A move consists of the following. First, she picks a box $X$ which contains a number $c$ such that $c = a + b$ for some numbers $a$ and $b$ which are contained in some other boxes. Then she picks a positive integer $k > 1$. Finally, she removes $c$ from $X$ and replaces it with $kc$. If she cannot make any mobes, she stops. Prove that no matter how Alice makes her moves, she won't be able to make infinitely many moves.

2016 Macedonia JBMO TST, 4

Tags: JMMO , Macedonia , Junior , 2016 , algebra , Inequality

Let $x$, $y$, and $z$ be positive real numbers. Prove that $\sqrt {\frac {xy}{x^2 + y^2 + 2z^2}} + \sqrt {\frac {yz}{y^2 + z^2 + 2x^2}}+\sqrt {\frac {zx}{z^2 + x^2 + 2y^2}} \le \frac{3}{2}$. When does equality hold?

2023 Bulgaria JBMO TST, 4

Tags: geometry , orthocenter , Junior , Balkan , shortlist

Given is an acute angled triangle $ABC$ with orthocenter $H$ and circumcircle $k$. Let $\omega$ be the circle with diameter $AH$ and $P$ be the point of intersection of $\omega$ and $k$ other than $A$. Assume that $BP$ and $CP$ intersect $\omega$ for the second time at points $Q$ and $R$, respectively. If $D$ is the foot of the altitude from $A$ to $BC$ and $S$ is the point of the intersection of $\omega$ and $QD$, prove that $HR = HS$.

2002 Flanders Junior Olympiad, 1

Tags: inequalities , Junior , algebra

Prove that for all $a,b,c \in \mathbb{R}^+_0$ we have \[\frac{a}{bc}+\frac{b}{ac}+\frac{c}{ab} \ge \frac2a+\frac2b-\frac2c\] and determine when equality occurs.

2021 JBMO TST - Turkey, 1

Tags: geometry , circles , perpendicular lines , Turkey , Junior

In an acute-angled triangle $ABC$, the circle with diameter $[AB]$ intersects the altitude drawn from vertex $C$ at a point $D$ and the circle with diameter $[AC]$ intersects the altitude drawn from vertex $B$ at a point $E$. Let the lines $BD$ and $CE$ intersect at $F$. Prove that $$AF\perp DE$$

2022 JBMO Shortlist, G2

Tags: geometry , Junior , Balkan , shortlist , parallelograms , concentric

Let $ABC$ be a triangle with circumcircle $k$. The points $A_1, B_1,$ and $C_1$ on $k$ are the midpoints of arcs $\widehat{BC}$ (not containing $A$), $\widehat{AC}$ (not containing $B$), and $\widehat{AB}$ (not containing $C$), respectively. The pairwise distinct points $A_2, B_2,$ and $C_2$ are chosen such that the quadrilaterals $AB_1A_2C_1, BA_1B_2C_1,$ and $CA_1C_2B_1$ are parallelograms. Prove that $k$ and the circumcircle of triangle $A_2B_2C_2$ have a common center. [b]Comment.[/b] Point $A_2$ can also be defined as the reflection of $A$ with respect to the midpoint of $B_1C_1$, and analogous definitions can be used for $B_2$ and $C_2$.

2019 Macedonia Junior BMO TST, 4

Tags: JMMO , Macedonia , 2019 , Junior , algebra

Let the real numbers $a$, $b$, and $c$ satisfy the equations $(a+b)(b+c)(c+a)=abc$ and $(a^9+b^9)(b^9+c^9)(c^9+a^9)=(abc)^9$. Prove that at least one of $a$, $b$, and $c$ equals $0$.

2023 Greece JBMO TST, 4

Tags: number theory , factorial , sum of integers , Junior , Balkan , shortlist

Determine all pairs $(k, n)$ of positive integers that satisfy $$1! + 2! + ... + k! = 1 + 2 + ... + n.$$

2022 Azerbaijan Junior National Olympiad, C4

Tags: combinatorics , Junior , Azerbaijan , Bound

There is a $8*8$ board and the numbers $1,2,3,4,...,63,64$. In all the unit squares of the board, these numbers are places such that only $1$ numbers goes to only one unit square. Prove that there is atleast $4$ $2*2$ squares such that the sum of the numbers in $2*2$ is greater than $100$.

2022 Azerbaijan JBMO TST, C5?

Tags: Junior , Balkan , shortlist , 2021 , combinatorics , Coloring , board

Alice and Bob play a game together as a team on a $100 \times 100$ board with all unit squares initially white. Alice sets up the game by coloring exactly $k$ of the unit squares red at the beginning. After that, a legal move for Bob is to choose a row or column with at least $10$ red squares and color all of the remaining squares in it red. What is the smallest $k$ such that Alice can set up a game in such a way that Bob can color the entire board red after finitely many moves? Proposed by [i]Nikola Velov, Macedonia[/i]

2021 JBMO Shortlist, C3

Tags: Junior , Balkan , shorlist , 2021 , combinatorics , Process

We have a set of $343$ closed jars, each containing blue, yellow and red marbles with the number of marbles from each color being at least $1$ and at most $7$. No two jars have exactly the same contents. Initially all jars are with the caps up. To flip a jar will mean to change its position from cap-up to cap-down or vice versa. It is allowed to choose a triple of positive integers $(b; y; r) \in \{1; 2; ...; 7\}^3$ and flip all the jars whose number of blue, yellow and red marbles differ by not more than $1$ from $b, y, r$, respectively. After $n$ moves all the jars turned out to be with the caps down. Find the number of all possible values of $n$, if $n \le 2021$.

2016 Macedonia JBMO TST, 5

Tags: Junior , JMMO , 2016 , number theory , Macedonia , greatest common divisor

Solve the following equation in the set of positive integers $x + y^2 + (GCD(x, y))^2 = xy \cdot GCD(x, y)$.

2022 JBMO Shortlist, C6

Tags: combinatorics , board , Junior , Balkan , shortlist

Let $n \ge 2$ be an integer. In each cell of a $4n \times 4n$ table we write the sum of the cell row index and the cell column index. Initially, no cell is colored. A move consists of choosing two cells which are not colored and coloring one of them in red and one of them in blue. Show that, however Alex perfors $n^2$ moves, Jane can afterwards perform a number of moves (eventually none) after which the sum of the numbers written in the red cells is the same as the sum of the numbers written in the blue ones.

2021 JBMO Shortlist, C6

Tags: Junior , Balkan , shortlist , 2021 , combinatorics , game , board coloring

Given an $m \times n$ table consisting of $mn$ unit cells. Alice and Bob play the following game: Alice goes first and the one who moves colors one of the empty cells with one of the given three colors. Alice wins if there is a figure, such as the ones below, having three different colors. Otherwise Bob is the winner. Determine the winner for all cases of $m$ and $n$ where $m, n \ge 3$. Proposed by [i]Toghrul Abbasov, Azerbaijan[/i]

2021 JBMO Shortlist, N6

Tags: Junior , Balkan , shortlist , 2021 , number theory , modulo

Given a positive integer $n \ge 2$, we define $f(n)$ to be the sum of all remainders obtained by dividing $n$ by all positive integers less than $n$. For example dividing $5$ with $1, 2, 3$ and $4$ we have remainders equal to $0, 1, 2$ and $1$ respectively. Therefore $f(5) = 0 + 1 + 2 + 1 = 4$. Find all positive integers $n \ge 3$ such that $f(n) = f(n - 1) + (n - 2)$.

2023 Azerbaijan JBMO TST, 1

Tags: number theory , LCM , Junior , Balkan , shortlist , AZE JBMO TST , Inequality

Let $a < b < c < d < e$ be positive integers. Prove that $$\frac{1}{[a, b]} + \frac{1}{[b, c]} + \frac{1}{[c, d]} + \frac{2}{[d, e]} \le 1$$ where $[x, y]$ is the least common multiple of $x$ and $y$ (e.g., $[6, 10] = 30$). When does equality hold?

2007 Junior Macedonian Mathematical Olympiad, 2

Tags: Junior , Macedonia , JMMO , 2007 , geometry , parallelogram

Let $ABCD$ be a parallelogram and let $E$ be a point on the side $AD$, such that $\frac{AE}{ED} = m$. Let $F$ be a point on $CE$, such that $BF \perp CE$, and the point $G$ is symmetrical to $F$ with respect to $AB$. If point $A$ is the circumcenter of triangle $BFG$, find the value of $m$.

2016 Macedonia JBMO TST, 1

Tags: JMMO , Junior , Macedonia , 2016 , number theory

Solve the following equation in the set of integers $x_{1}^4 + x_{2}^4 +...+ x_{14}^4=2016^3 - 1$.

2007 Junior Macedonian Mathematical Olympiad, 5

Tags: Junior , JMMO , Macedonia , 2007 , geometry , combinatorics

We are given an arbitrary $\bigtriangleup ABC$. a) Can we dissect $\bigtriangleup ABC$ in $4$ pieces, from which we can make two triangle similar to $\bigtriangleup ABC$ (each piece can be used only once)? Justify your answer! b) Is it possible that for every positive integer $n \ge 2$ , we are able to dissect $\bigtriangleup ABC$ in $2n$ pieces, from which we can make two triangles similar to $\bigtriangleup ABC$ (each piece can be used only once)? Justify your answer!

2021 JBMO Shortlist, A2

Tags: Junior , Balkan , shortlist , 2021 , algebra , Sequence

Let $n > 3$ be a positive integer. Find all integers $k$ such that $1 \le k \le n$ and for which the following property holds: If $x_1, . . . , x_n$ are $n$ real numbers such that $x_i + x_{i + 1} + ... + x_{i + k - 1} = 0$ for all integers $i > 1$ (indexes are taken modulo $n$), then $x_1 = . . . = x_n = 0$. Proposed by [i]Vincent Jugé and Théo Lenoir, France[/i]

2022 Azerbaijan Junior National Olympiad, N2

Tags: number theory , modulo , Azerbaijan , Junior

If $x,y,z \in\mathbb{N}$ and $2x^2+3y^3=4z^4$, Prove that $6|x,y,z$