Olympiad problems

2018 Mexico National Olympiad, 4

Let $n\geq 2$ be an integer. For each $k$-tuple of positive integers $a_1, a_2, \ldots, a_k$ such that $a_1+a_2+\cdots +a_k=n$, consider the sums $S_i=1+2+\ldots +a_i$ for $1\leq i\leq k$. Determine, in terms of $n$, the maximum possible value of the product $S_1S_2\cdots S_k$. [i]Proposed by Misael Pelayo[/i]

1957 AMC 12/AHSME, 34

Tags: parameterization , graphing lines , slope , function , algebra , analytic geometry , inequalities

The points that satisfy the system $ x \plus{} y \equal{} 1,\, x^2 \plus{} y^2 < 25,$ constitute the following set: $ \textbf{(A)}\ \text{only two points} \qquad \\ \textbf{(B)}\ \text{an arc of a circle}\qquad \\ \textbf{(C)}\ \text{a straight line segment not including the end\minus{}points}\qquad \\ \textbf{(D)}\ \text{a straight line segment including the end\minus{}points}\qquad \\ \textbf{(E)}\ \text{a single point}$

2020 HMIC, 2

Tags: chessboard , combinatorics

Some bishops and knights are placed on an infinite chessboard, where each square has side length $1$ unit. Suppose that the following conditions hold: [list] [*] For each bishop, there exists a knight on the same diagonal as that bishop (there may be another piece between the bishop and the knight). [*] For each knight, there exists a bishop that is exactly $\sqrt{5}$ units away from it. [*] If any piece is removed from the board, then at least one of the above conditions is no longer satisfied. [/list] If $n$ is the total number of pieces on the board, find all possible values of $n$. [i]Sheldon Kieren Tan[/i]

2023 ISL, C2

Tags: combinatorics

Determine the maximal length $L$ of a sequence $a_1,\dots,a_L$ of positive integers satisfying both the following properties: [list=disc] [*]every term in the sequence is less than or equal to $2^{2023}$, and [*]there does not exist a consecutive subsequence $a_i,a_{i+1},\dots,a_j$ (where $1\le i\le j\le L$) with a choice of signs $s_i,s_{i+1},\dots,s_j\in\{1,-1\}$ for which \[s_ia_i+s_{i+1}a_{i+1}+\dots+s_ja_j=0.\] [/list]

2001 Junior Balkan MO, 4

Tags: perimeter , geometry , trigonometry , combinatorics

Let $N$ be a convex polygon with 1415 vertices and perimeter 2001. Prove that we can find 3 vertices of $N$ which form a triangle of area smaller than 1.

2015 NIMO Problems, 5

Tags: algebra , complex number , polynomial

Let $a, b, c, d, e,$ and $f$ be real numbers. Define the polynomials \[ P(x) = 2x^4 - 26x^3 + ax^2 + bx + c \quad\text{ and }\quad Q(x) = 5x^4 - 80x^3 + dx^2 + ex + f. \] Let $S$ be the set of all complex numbers which are a root of [i]either[/i] $P$ or $Q$ (or both). Given that $S = \{1,2,3,4,5\}$, compute $P(6) \cdot Q(6).$ [i]Proposed by Michael Tang[/i]

2007 IMO Shortlist, 7

Tags: reflection , incenter , circumcircle , geometry

Given an acute triangle $ ABC$ with $ \angle B > \angle C$. Point $ I$ is the incenter, and $ R$ the circumradius. Point $ D$ is the foot of the altitude from vertex $ A$. Point $ K$ lies on line $ AD$ such that $ AK \equal{} 2R$, and $ D$ separates $ A$ and $ K$. Lines $ DI$ and $ KI$ meet sides $ AC$ and $ BC$ at $ E,F$ respectively. Let $ IE \equal{} IF$. Prove that $ \angle B\leq 3\angle C$. [i]Author: Davoud Vakili, Iran[/i]

1992 All Soviet Union Mathematical Olympiad, 579

Tags: game , vector , combinatorics , game strategy

$1992$ vectors are given in the plane. Two players pick unpicked vectors alternately. The winner is the one whose vectors sum to a vector with larger magnitude (or they draw if the magnitudes are the same). Can the first player always avoid losing?

1986 China Team Selection Test, 3

Tags: inequalities , quadratic

Let $x_i,$ $1 \leq i \leq n$ be real numbers with $n \geq 3.$ Let $p$ and $q$ be their symmetric sum of degree $1$ and $2$ respectively. Prove that: i) $p^2 \cdot \frac{n-1}{n}-2q \geq 0$ ii) $\left|x_i - \frac{p}{n}\right| \leq \sqrt{p^2 - \frac{2nq}{n-1}} \cdot \frac{n-1}{n}$ for every meaningful $i$.

2024 Centroamerican and Caribbean Math Olympiad, 5

Tags: algebra , system of equations

Let $x$ and $y$ be positive real numbers satisfying the following system of equations: \[ \begin{cases} \sqrt{x}\left(2 + \dfrac{5}{x+y}\right) = 3 \\\\ \sqrt{y}\left(2 - \dfrac{5}{x+y}\right) = 2 \end{cases} \] Find the maximum value of $x + y$.

1998 Romania Team Selection Test, 4

Tags: combinatorics , function , trigonometry , analytic geometry

Consider in the plane a finite set of segments such that the sum of their lengths is less than $\sqrt{2}$. Prove that there exists an infinite unit square grid covering the plane such that the lines defining the grid do not intersect any of the segments. [i]Vasile Pop[/i]

2016 Kosovo National Mathematical Olympiad, 2

Tags: number theory

Evaluate the sum of all three digits number which are not divisible by $13$ .

2024 Philippine Math Olympiad, P4

Tags: combinatorics , number theory , function

Let $n$ be a positive integer. Suppose for any $\mathcal{S} \subseteq \{1, 2, \cdots, n\}$, $f(\mathcal{S})$ is the set containing all positive integers at most $n$ that have an odd number of factors in $\mathcal{S}$. How many subsets of $\{1, 2, \cdots, n\}$ can be turned into $\{1\}$ after finitely many (possibly zero) applications of $f$?

1953 Miklós Schweitzer, 5

Tags: number theory

Show that any positive integer has at least as many positive divisors of the form $3k+1$ as of the form $3k-1$. [b](N. 7)[/b]

1979 Bulgaria National Olympiad, Problem 5

Tags: geometry , triangle , pentagon

A convex pentagon $ABCDE$ satisfies $AB=BC=CA$ and $CD=DE=EC$. Let $S$ be the center of the equilateral triangle $ABC$ and $M$ and $N$ be the midpoints of $BD$ and $AE$, respectively. Prove that the triangles $SME$ and $SND$ are similar.

2019 District Olympiad, 2

Tags: function , integrability , calculus

Let $n$ be a positive integer and $f:[0,1] \to \mathbb{R}$ be an integrable function. Prove that there exists a point $c \in \left[0,1- \frac{1}{n} \right],$ such that [center] $ \int\limits_c^{c+\frac{1}{n}}f(x)\mathrm{d}x=0$ or $\int\limits_0^c f(x) \mathrm{d}x=\int\limits_{c+\frac{1}{n}}^1f(x)\mathrm{d}x.$ [/center]

JBMO Geometry Collection, 2015

Tags: geometry

Let $ABC$ be an acute triangle.The lines $l_1$ and $l_2$ are perpendicular to $AB$ at the points $A$ and $B$, respectively.The perpendicular lines from the midpoint $M$ of $AB$ to the lines $AC$ and $BC$ intersect $l_1$ and $l_2$ at the points $E$ and $F$, respectively.If $D$ is the intersection point of the lines $EF$ and $MC$, prove that \[\angle ADB = \angle EMF.\]

2021/2022 Tournament of Towns, P6

Tags: combinatorics

There were made 7 golden, 7 silver and 7 bronze for a tournament. All the medals of the same material should weigh the same and the medals of different materials should have different weight. However, it so happened that exactly one medal had a wrong weight. If this medal is golden, it is lighter than a standard golden medal; if it is bronze, it is heavier than a standard bronze one; if it is silver, it may be lighter or heavier than a standard silver one. Is it possible to find the nonstandard one for sure, using three weighings on a balance scale with no weights?

2000 Nordic, 1

Tags: sum , integer , combinatorics

In how many ways can the number $2000$ be written as a sum of three positive, not necessarily different integers? (Sums like $1 + 2 + 3$ and $3 + 1 + 2$ etc. are the same.)

2017 Assam Mathematics Olympiad, 1

Tags:

1)$k, l, m\in\mathbb{N}$ $2^{k+l} +2^{l+m}+2^{m+k}\le 2^{k+l+m+1} +1$ [color=#00f]Moved to HSO. ~ oVlad[/color]

2023 Malaysian Squad Selection Test, 5

Tags: algebra

Find the maximal value of $c>0$ such that for any $n\ge 1$, and for any $n$ real numbers $x_1, \cdots, x_n$ there exists real numbers $a ,b$ such that $$\{x_i-a\}+\{x_{i+1}-b\}\le \frac{1}{2024}$$ for at least $cn$ indices $i$. Here, $x_{n+1}=x_1$ and $\{x\}$ denotes the fractional part of $x$. [i]Proposed by Wong Jer Ren[/i]

2019 Mexico National Olympiad, 2

Tags: perpendicular lines , geometry , orthocenter , perpendicular bisector

Let $H$ be the orthocenter of acute-angled triangle $ABC$ and $M$ be the midpoint of $AH$. Line $BH$ cuts $AC$ at $D$. Consider point $E$ such that $BC$ is the perpendicular bisector of $DE$. Segments $CM$ and $AE$ intersect at $F$. Show that $BF$ is perpendicular to $CM$. [i]Proposed by Germán Puga[/i]

2016 Saudi Arabia IMO TST, 1

Tags: collinear , geometry , orthocenter , circumcircle , incircle

Let $ABC$ be a triangle whose incircle $(I)$ touches $BC, CA, AB$ at $D, E, F$, respectively. The line passing through $A$ and parallel to $BC$ cuts $DE, DF$ at $M, N$, respectively. The circumcircle of triangle $DMN$ cuts $(I)$ again at $L$. a) Let $K$ be the intersection of $N E$ and $M F$. Prove that $K$ is the orthocenter of the triangle $DMN$. b) Prove that $A, K, L$ are collinear.

1974 IMO Longlists, 7

Tags: number theory

Let $p$ be a prime number and $n$ a positive integer. Prove that the product \[{N=\frac{1}{p^{n^2}}} \prod_{i=1;2 \nmid i}^{2n-1} \biggl[ \left( (p-1)! \right) \binom{p^2 i}{pi}\biggr]\] Is a positive integer that is not divisible by $p.$

2020 ASDAN Math Tournament, 12

Tags: team test

Let $S_n$ be the number of subsets of the first $n$ positive integers that have the same number of even values and odd values; the empty set counts as one of these subsets. Compute the smallest positive integer $n$ such that $S_n$ is a multiple of $2020$.