Olympiad problems

2025 Kyiv City MO Round 1, Problem 1

Tags: algebra

You are given $ 11 $ numbers with an arithmetic mean of $ 10 $. Each of the first $ 4 $ numbers is increased by $ 20 $, and each of the last $ 7 $ numbers is decreased by $ 24 $. What is the arithmetic mean of the new $ 11 $ numbers?

1992 IMO Longlists, 49

Tags: recurrence relation , inequality , sequence , algebra

Given real numbers $x_i \ (i = 1, 2, \cdots, 4k + 2)$ such that \[\sum_{i=1}^{4k +2} (-1)^{i+1} x_ix_{i+1} = 4m \qquad ( \ x_1=x_{4k+3} \ )\] prove that it is possible to choose numbers $x_{k_{1}}, \cdots, x_{k_{6}}$ such that \[\sum_{i=1}^{6} (-1)^{i} k_i k_{i+1} > m \qquad ( \ x_{k_{1}} = x_{k_{7}} \ )\]

Russian TST 2022, P3

Tags: algebra , inequality

Let $n\geqslant 3$ be an integer and $x_1>x_2>\cdots>x_n$ be real numbers. Suppose that $x_k>0\geqslant x_{k+1}$ for an index $k{}$. Prove that \[\sum_{i=1}^k\left(x_i^{n-2}\prod_{j\neq i}\frac{1}{x_i-x_j}\right)\geqslant 0.\]

2008 Postal Coaching, 3

Tags: inequalities , algebra , complex

Let $a$ and $b$ be two complex numbers. Prove the inequality $$|1 + ab| + |a + b| \ge \sqrt{|a^2 - 1| \cdot |b^2 - 1|}$$

V Soros Olympiad 1998 - 99 (Russia), 10.2

Tags: algebra , trigonometry

Solve the equation $$ |\cos 3x - tgt| + |\cos 3x + tgt| = |tg^2t -3|.$$

2012 Korea Junior Math Olympiad, 7

Tags: inequalities , algebra , maximum , positive real , sum

If all $x_k$ ($k = 1, 2, 3, 4, 5)$ are positive reals, and $\{a_1,a_2, a_3, a_4, a_5\} = \{1, 2,3 , 4, 5\}$, find the maximum of $$\frac{(\sqrt{s_1x_1} +\sqrt{s_2x_2}+\sqrt{s_3x_3}+\sqrt{s_4x_4}+\sqrt{s_5x_5})^2}{a_1x_1 + a_2x_2 + a_3x_3 + a_4x_4 + a_5x_5}$$ ($s_k = a_1 + a_2 +... + a_k$)

2025 Harvard-MIT Mathematics Tournament, 9

Tags: algebra , number theory

Let $f$ be the unique polynomial of degree at most $2026$ such that for all $n \in \{1,2, 3, \ldots, 2027\},$ $$f(n)=\begin{cases} 1 & \text{if } $n$ \text{ is a perfect square}, \\ 0 & \text{otherwise.} \end{cases}$$ Suppose that $\tfrac{a}{b}$ is the coefficient of $x^{2025}$ in $f,$ where $a$ and $b$ are integers such that $\gcd(a,b)=1.$ Compute the unique integer $r$ between $0$ and $2026$ (inclusive) such that $a-rb$ is divisible by $2027.$ (Note that $2027$ is prime.)

2022 SG Originals, Q2

Tags: function , algebra , functional equation , set , integer

Find all functions $f$ mapping non-empty finite sets of integers, to integers, such that $$f(A+B)=f(A)+f(B)$$ for all non-empty sets of integers $A$ and $B$. $A+B$ is defined as $\{a+b: a \in A, b \in B\}$.

2009 Hanoi Open Mathematics Competitions, 6

Tags: algebra , system of equations

Suppose that $4$ real numbers $a, b,c,d$ satisfy the conditions $\begin{cases} a^2 + b^2 = 4\\ c^2 + d^2 = 4 \\ ac + bd = 2 \end{cases}$ Find the set of all possible values the number $M = ab + cd$ can take.

PEN M Problems, 27

Tags: abstract algebra , polynomial , induction , vector , modular arithmetic , calculus , algebra

Let $ p \ge 3$ be a prime number. The sequence $ \{a_{n}\}_{n \ge 0}$ is defined by $ a_{n}=n$ for all $ 0 \le n \le p-1$, and $ a_{n}=a_{n-1}+a_{n-p}$ for all $ n \ge p$. Compute $ a_{p^{3}}\; \pmod{p}$.

2012 Canadian Mathematical Olympiad Qualification Repechage, 6

Tags: algebra , derivative , quadratic , calculus

Determine whether there exist two real numbers $a$ and $b$ such that both $(x-a)^3+ (x-b)^2+x$ and $(x-b)^3 + (x-a)^2 +x$ contain only real roots.

2007 Hungary-Israel Binational, 2

Tags: algebra , inequalities

Let $ a,b,c,d$ be real numbers, such that $ a^2\le 1, a^2 \plus{} b^2\le 5, a^2 \plus{} b^2 \plus{} c^2\le 14, a^2 \plus{} b^2 \plus{} c^2 \plus{} d^2\le 30$. Prove that $ a \plus{} b \plus{} c \plus{} d\le 10$.

2019 Serbia National MO, 6

Tags: algebra , sequence

Sequences $(a_n)_{n=0}^{\infty}$ and $(b_n)_{n=0}^{\infty}$ are defined with recurrent relations : $$a_0=0 , \;\;\; a_1=1, \;\;\;\; a_{n+1}=\frac{2018}{n} a_n+ a_{n-1}\;\;\; \text {for }\;\;\; n\geq 1$$ and $$b_0=0 , \;\;\; b_1=1, \;\;\;\; b_{n+1}=\frac{2020}{n} b_n+ b_{n-1}\;\;\; \text {for }\;\;\; n\geq 1$$ Prove that :$$\frac{a_{1010}}{1010}=\frac{b_{1009}}{1009}$$

2021 Bosnia and Herzegovina Team Selection Test, 1

Tags: inequalities , max , algebra

Let $x,y,z$ be real numbers from the interval $[0,1]$. Determine the maximum value of expression $$W=y\cdot \sqrt{1-x}+z\cdot\sqrt{1-y}+x\cdot\sqrt{1-z}$$

2004 India National Olympiad, 2

Tags: algebra , quadratic

$p > 3$ is a prime. Find all integers $a$, $b$, such that $a^2 + 3ab + 2p(a+b) + p^2 = 0$.

2018 Bulgaria EGMO TST, 3

Tags: algebra , function , induction

Find all one-to-one mappings $f:\mathbb{N}\to\mathbb{N}$ such that for all positive integers $n$ the following relation holds: \[ f(f(n)) \leq \frac {n+f(n)} 2 . \]

2020 Canadian Junior Mathematical Olympiad, 1

Tags: algebra , sequence

Let $a_1, a_2, a_3, . . .$ be a sequence of positive real numbers that satisfies $a_1 = 1$ and $a^2_{n+1} + a_{n+1} = a_n$ for every natural number $n$. Prove that $a_n \ge \frac{1}{n}$ for every natural number $n$.

2016 Latvia Baltic Way TST, 4

Tags: functional equation , functional , algebra

Find all functions $f : R \to R$ defined for real numbers, take real values and for all real $x$ and $y$ the equality holds: $$f(2^x+2y) =2^yf(f(x))f(y).$$

2018 Brazil Team Selection Test, 6

Tags: inequalities , algebra

An integer $n \geq 3$ is given. We call an $n$-tuple of real numbers $(x_1, x_2, \dots, x_n)$ [i]Shiny[/i] if for each permutation $y_1, y_2, \dots, y_n$ of these numbers, we have $$\sum \limits_{i=1}^{n-1} y_i y_{i+1} = y_1y_2 + y_2y_3 + y_3y_4 + \cdots + y_{n-1}y_n \geq -1.$$ Find the largest constant $K = K(n)$ such that $$\sum \limits_{1 \leq i < j \leq n} x_i x_j \geq K$$ holds for every Shiny $n$-tuple $(x_1, x_2, \dots, x_n)$.

2014 IberoAmerican, 2

Tags: polynomial , algebra

Find all polynomials $P(x)$ with real coefficients such that $P(2014) = 1$ and, for some integer $c$: $xP(x-c) = (x - 2014)P(x)$

2016 Azerbaijan BMO TST, 4

Tags: function , functional equation , algebra

Find all functions $f:\mathbb{N}\to\mathbb{N}$ such that \[f(f(n))=n+2015\] where $n\in \mathbb{N}.$

1991 Federal Competition For Advanced Students, 2

Tags: algebra

Solve in real numbers the equation: $ \frac{1}{x}\plus{}\frac{1}{x\plus{}2}\minus{}\frac{1}{x\plus{}4}\minus{}\frac{1}{x\plus{}6}\minus{}\frac{1}{x\plus{}8}\minus{}\frac{1}{x\plus{}10}\plus{}\frac{1}{x\plus{}12}\plus{}\frac{1}{x\plus{}14}\equal{}0.$

2011 Princeton University Math Competition, A2

Tags: algebra

A function $S(m, n)$ satisfies the initial conditions $S(1, n) = n$, $S(m, 1) = 1$, and the recurrence $S(m, n) = S(m - 1, n)S(m, n - 1)$ for $m\geq 2, n\geq 2$. Find the largest integer $k$ such that $2^k$ divides $S(7, 7)$.

2007 Estonia Math Open Junior Contests, 1

Tags: algebra

The escalator of the department store, which at any given time can be seen at $75$ steps section, moves up one step in $2$ seconds. At time $0$, Juku is standing on an escalator step equidistant from each end, facing the direction of travel. He goes by a certain rule: one step forward, two steps back, then again one step forward, two back, etc., taking one every second in increments of one step. Which end will Juku finally get out and at what point will it happen?

2017 BMT Spring, 1

Tags: algebra

$10$ students take the Analysis Round. The average score was a $3$ and the high score was a $7$. If no one got a $0$, what is the maximum number of students that could have achieved the high score?