Olympiad problems

1949-56 Chisinau City MO, 44

Determine the locus of points, for each of which the difference between the squares of the distances to two given points is a constant value.

1991 National High School Mathematics League, 1

Tags: arithmetic sequence

Set $S=\{1,2,\cdots,n\}$. $A$ is an increasing arithmetic sequence (at least two numbers), and all numbers are in $S$. Also, we can't add any number in $S$ to $A$ without changing its tolerance. Find the number of such sequence $A$.

2017 Peru IMO TST, 4

Tags: factorial , number theory , perfect square

The product $1\times 2\times 3\times ...\times n$ is written on the board. For what integers $n \ge 2$, we can add exclamation marks to some factors to convert them into factorials, in such a way that the final product can be a perfect square?

2022 Malaysia IMONST 2, 2

Tags: algebra

It is known that there are $n$ integers $a_1, a_2, \cdots, a_n$ such that $$a_1 + a_2 + \cdots + a_n = 0 \qquad \text{and} \qquad a_1 \times a_2 \times \cdots \times a_n = n.$$ Determine all possible values of $n$.

2014 Poland - Second Round, 2.

Tags: geometry , radii

Distinct points $A$, $B$ and $C$ lie on a line in this order. Point $D$ lies on the perpendicular bisector of the segment $BC$. Denote by $M$ the midpoint of the segment $BC$. Let $r$ be the radius of the incircle of the triangle $ABD$ and let $R$ be the radius of the circle with center lying outside the triangle $ACD$, tangent to $CD$, $AC$ and $AD$. Prove that $DM=r+R$.

1986 Putnam, A4

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A [i]transversal[/i] of an $n\times n$ matrix $A$ consists of $n$ entries of $A$, no two in the same row or column. Let $f(n)$ be the number of $n \times n$ matrices $A$ satisfying the following two conditions: (a) Each entry $\alpha_{i,j}$ of $A$ is in the set $\{-1,0,1\}$. (b) The sum of the $n$ entries of a transversal is the same for all transversals of $A$. An example of such a matrix $A$ is \[ A = \left( \begin{array}{ccc} -1 & 0 & -1 \\ 0 & 1 & 0 \\ 0 & 1 & 0 \end{array} \right). \] Determine with proof a formula for $f(n)$ of the form \[ f(n) = a_1 b_1^n + a_2 b_2^n + a_3 b_3^n + a_4, \] where the $a_i$'s and $b_i$'s are rational numbers.

2002 India IMO Training Camp, 3

Tags: quadratic , algebra

Let $X=\{2^m3^n|0 \le m, \ n \le 9 \}$. How many quadratics are there of the form $ax^2+2bx+c$, with equal roots, and such that $a,b,c$ are distinct elements of $X$?

2018 Thailand TST, 2

Tags: number theory , decimal representation , digit

Call a rational number [i]short[/i] if it has finitely many digits in its decimal expansion. For a positive integer $m$, we say that a positive integer $t$ is $m-$[i]tastic[/i] if there exists a number $c\in \{1,2,3,\ldots ,2017\}$ such that $\dfrac{10^t-1}{c\cdot m}$ is short, and such that $\dfrac{10^k-1}{c\cdot m}$ is not short for any $1\le k<t$. Let $S(m)$ be the set of $m-$tastic numbers. Consider $S(m)$ for $m=1,2,\ldots{}.$ What is the maximum number of elements in $S(m)$?

2000 China Team Selection Test, 3

Tags: number theory

For positive integer $a \geq 2$, denote $N_a$ as the number of positive integer $k$ with the following property: the sum of squares of digits of $k$ in base a representation equals $k$. Prove that: a.) $N_a$ is odd; b.) For every positive integer $M$, there exist a positive integer $a \geq 2$ such that $N_a \geq M$.

2017 Azerbaijan BMO TST, 4

Tags: string , combinatorics , binary

The leader of an IMO team chooses positive integers $n$ and $k$ with $n > k$, and announces them to the deputy leader and a contestant. The leader then secretly tells the deputy leader an $n$-digit binary string, and the deputy leader writes down all $n$-digit binary strings which differ from the leader’s in exactly $k$ positions. (For example, if $n = 3$ and $k = 1$, and if the leader chooses $101$, the deputy leader would write down $001, 111$ and $100$.) The contestant is allowed to look at the strings written by the deputy leader and guess the leader’s string. What is the minimum number of guesses (in terms of $n$ and $k$) needed to guarantee the correct answer?

MOAA Team Rounds, 2021.20

Tags: team

Compute the sum of all integers $x$ for which there exists an integer $y$ such that \[x^3+xy+y^3=503.\] [i]Proposed by Nathan Xiong[/i]

1976 Spain Mathematical Olympiad, 3

Tags: geometry , geometric transformation

Through a lens that inverts the image we look at the rearview mirror of our car. If it reflects the license plate of the car that follows us, $CS-3965-EN$, draw the image we receive. Also draw the one obtained by permuting previous transformations, that is, reflecting in the mirror the image that the license plate gives the lens. Is the product of both transformations , reflection in the mirror and refraction through the lens, commutative?

2010 Contests, 1

Tags: rearrangement inequality , inequalities

Let $a,b$ and $c$ be positive real numbers. Prove that \[ \frac{a^2b(b-c)}{a+b}+\frac{b^2c(c-a)}{b+c}+\frac{c^2a(a-b)}{c+a} \ge 0. \]

2009 Kyiv Mathematical Festival, 4

Tags: geometry , convex polygon , perimeter , square

Two convex polygons can be placed into a square with the side $1$ without intersection. Prove that at least one polygon has the perimeter that is less than or equal to $3,5$ .

2010 Singapore Junior Math Olympiad, 2

Tags: digit , multiple , number theory

Find the sum of all the $5$-digit integers which are not multiples of $11$ and whose digits are $1, 3, 4, 7, 9$.

2010 Harvard-MIT Mathematics Tournament, 4

Tags: calculus , limit , trigonometry

Compute $\displaystyle\lim_{n\to\infty}\dfrac{\sum_{k=1}^n|\cos(k)|}{n}$.

2018 Sharygin Geometry Olympiad, 24

Tags: geometry

A crystal of pyrite is a parallelepiped with dashed faces. The dashes on any two adjacent faces are perpendicular. Does there exist a convex polytope with the number of faces not equal to 6, such that its faces can be dashed in such a manner?

2016 Azerbaijan Team Selection Test, 2

Tags: algebra , polynomial

Find all polynomials $P(x)$ with real coefficents, such that for all $x,y,z$ satisfying $x+y+z=0$, the equation below is true: \[P(x+y)^3+P(y+z)^3+P(z+x)^3=3P((x+y)(y+z)(z+x))\]

2020 EGMO, 5

Tags: geometry , incenter

Consider the triangle $ABC$ with $\angle BCA > 90^{\circ}$. The circumcircle $\Gamma$ of $ABC$ has radius $R$. There is a point $P$ in the interior of the line segment $AB$ such that $PB = PC$ and the length of $PA$ is $R$. The perpendicular bisector of $PB$ intersects $\Gamma$ at the points $D$ and $E$. Prove $P$ is the incentre of triangle $CDE$.

2017 CMIMC Team, 2

Tags: team

Suppose $x$, $y$, and $z$ are nonzero complex numbers such that $(x+y+z)(x^2+y^2+z^2)=x^3+y^3+z^3$. Compute \[(x+y+z)\left(\dfrac1x+\dfrac1y+\dfrac1z\right).\]

2017 Purple Comet Problems, 22

Tags: function , combinatorics

Find the number of functions $f$ that map the set $\{1,2, 3,4\}$ into itself such that the range of the function $f(x)$ is the same as the range of the function $f(f(x))$.

2023 Kurschak Competition, 2

Tags: combinatorics , graph theory , grid , analytic geometry

Let $n\geq 2$ be a positive integer. We call a [i]vertex[/i] every point in the coordinate plane, whose $x$ and $y$ coordinates both are from the set $\{1,2,3,...,n\}$. We call a segment between two vertices an [i]edge[/i], if its length if $1$. We've colored some edges red, such that between any two vertices, there is a unique path of red edges (a path may contain each edge at most once). The red edge $f$ is [i]vital[/i] for an edge $e$, if the path of red edges connecting the two endpoints of $e$ contain $f$. Prove that there is a red edge, such that it is vital for at least $n$ edges.

2018 Peru Cono Sur TST, 1

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Consider 2016 distinct points on a circle. It is allowed to move from one point to another on the circle by jumping 2 or 3 points forward in a clockwise direction. What is the minimum number of jumps required to visit all points and return to the starting point?

2023 Kyiv City MO Round 1, Problem 1

Tags: geometry , rectangle

The rectangle is cut into 6 squares, as shown on the figure below. The gray square in the middle has a side equal to 1. What is the area of the rectangle? [img]https://i.ibb.co/gg1tBTN/Kyiv-MO-2023-7-1.png[/img]

May Olympiad L2 - geometry, 2010.2

Tags: geometry , rectangle , equal segments

Let $ABCD$ be a rectangle and the circle of center $D$ and radius $DA$, which cuts the extension of the side $AD$ at point $P$. Line $PC$ cuts the circle at point $Q$ and the extension of the side $AB$ at point $R$. Show that $QB = BR$.