Olympiad problems

2012 239 Open Mathematical Olympiad, 6

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In an $n$-element set $S$, several subsets $A_1, A_2, \ldots , A_k$ are distinguished, each consists of at least two, but not all elements of $S$. What is the largest $k$ that it’s possible to write down the elements of $S$ in a row in the order such that we don’t find all of the element of an $A_i$ set in the consecutive elements of the row?

2009 Czech-Polish-Slovak Match, 6

Let $n\ge 16$ be an integer, and consider the set of $n^2$ points in the plane: \[ G=\big\{(x,y)\mid x,y\in\{1,2,\ldots,n\}\big\}.\] Let $A$ be a subset of $G$ with at least $4n\sqrt{n}$ elements. Prove that there are at least $n^2$ convex quadrilaterals whose vertices are in $A$ and all of whose diagonals pass through a fixed point.

2002 Romania National Olympiad, 2

Tags: ratio , geometry , geometric transformation , reflection , circumcircle , inradius , trigonometry

Let $ABC$ be a right triangle where $\measuredangle A = 90^\circ$ and $M\in (AB)$ such that $\frac{AM}{MB}=3\sqrt{3}-4$. It is known that the symmetric point of $M$with respect to the line $GI$ lies on $AC$. Find the measure of $\measuredangle B$.

2010 Contests, 1

Tags: trigonometry , geometry

Let $ABCDEF$ be a convex hexagon in which diagonals $AD, BE, CF$ are concurrent at $O$. Suppose $[OAF]$ is geometric mean of $[OAB]$ and $[OEF]$ and $[OBC]$ is geometric mean of $[OAB]$ and $[OCD]$. Prove that $[OED]$ is the geometric mean of $[OCD]$ and $[OEF]$. (Here $[XYZ]$ denotes are of $\triangle XYZ$)

2022-2023 OMMC, 14

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Find \[ \left\lfloor \frac{10000}{1} \right\rfloor + \left\lfloor \frac{10000}{2} \right\rfloor + \cdots + \left\lfloor \frac{10000}{100} \right\rfloor - \left\lfloor \frac{10000}{101} \right\rfloor - \cdots - \left\lfloor \frac{10000}{10000} \right\rfloor. \]

2016 Moldova Team Selection Test, 2

Tags: floor function , quadratic , function

Let $p$ be a prime number of the form $4k+1$. Show that \[\sum^{p-1}_{i=1}\left( \left \lfloor \frac{2i^{2}}{p}\right \rfloor-2\left \lfloor \frac{i^{2}}{p}\right \rfloor \right) = \frac{p-1}{2}.\]

2023 MOAA, 4

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A two-digit number $\overline{ab}$ is [i]self-loving[/i] if $a$ and $b$ are relatively prime positive integers and $\overline{ab}$ is divisible by $a+b$. How many self-loving numbers are there? [i]Proposed by Anthony Yang and Andy Xu[/i]

2019 Irish Math Olympiad, 8

Tags: geometry , circumcircle , parallelogram , concurrency

Consider a point $G$ in the interior of a parallelogram $ABCD$. A circle $\Gamma$ through $A$ and $G$ intersects the sides $AB$ and $AD$ for the second time at the points $E$ and $F$ respectively. The line $FG$ extended intersects the side $BC$ at $H$ and the line $EG$ extended intersects the side $CD$ at $I$. The circumcircle of triangle $HGI$ intersects the circle $\Gamma$ for the second time at $M \ne G$. Prove that $M$ lies on the diagonal $AC$.

2020 AMC 10, 9

Tags: algebra

How many ordered pairs of integers $(x, y)$ satisfy the equation$$x^{2020}+y^2=2y?$$ $\textbf{(A) } 1 \qquad\textbf{(B) } 2 \qquad\textbf{(C) } 3 \qquad\textbf{(D) } 4 \qquad\textbf{(E) } \text{infinitely many}$

2006 Miklós Schweitzer, 10

Tags: convex , compactness , real analysis , euclidean space

Let $K_1,...,K_d$ be convex, compact sets in $R^d$ with non-empty interior. Suppose they are strongly separated, which means for any choice of $x_1 \in K_1, x_2 \in K_2, ...$, their affine hull is a hyperplane in $R^d$. Also let $0< \alpha_i <1$. A half-space H is called an $\alpha$-cut if $vol(K_i \cap H) = \alpha_i\cdot vol(K_i)$ for all i. How many $\alpha$-cuts are there?

2002 Korea - Final Round, 2

Tags: angle chasing , geometry

Let $ABC$ be an acute triangle and let $\omega$ be its circumcircle. Let the perpendicular line from $A$ to $BC$ meet $\omega$ at $D$. Let $P$ be a point on $\omega$, and let $Q$ be the foot of the perpendicular line from $P$ to the line $AB$. Prove that if $Q$ is on the outside of $\omega$ and $2\angle QPB = \angle PBC$, then $D,P,Q$ are collinear.

2000 All-Russian Olympiad Regional Round, 11.5

Tags: algebra , inequalities

For non-negative numbers $x$ and $y$ not exceeding $1$, prove that $$\frac{1}{\sqrt{1+x^2}}+\frac{1}{\sqrt{1+y^2}} \le \frac{2}{\sqrt{1 + xy}},$$

2007 Bulgaria Team Selection Test, 4

Tags: inequalities , combinatorics

Let $G$ is a graph and $x$ is a vertex of $G$. Define the transformation $\varphi_{x}$ over $G$ as deleting all incident edges with respect of $x$ and drawing the edges $xy$ such that $y\in G$ and $y$ is not connected with $x$ with edge in the beginning of the transformation. A graph $H$ is called $G-$[i]attainable[/i] if there exists a sequece of such transformations which transforms $G$ in $H.$ Let $n\in\mathbb{N}$ and $4|n.$ Prove that for each graph $G$ with $4n$ vertices and $n$ edges there exists $G-$[i]attainable[/i] graph with at least $9n^{2}/4$ triangles.

2021 Math Prize for Girls Problems, 19

Tags: geometry , 3d geometry , tetrahedron

Let $T$ be a regular tetrahedron. Let $t$ be the regular tetrahedron whose vertices are the centers of the faces of $T$. Let $O$ be the circumcenter of either tetrahedron. Given a point $P$ different from $O$, let $m(P)$ be the midpoint of the points of intersection of the ray $\overrightarrow{OP}$ with $t$ and $T$. Let $S$ be the set of eight points $m(P)$ where $P$ is a vertex of either $t$ or $T$. What is the volume of the convex hull of $S$ divided by the volume of $t$?

1991 Mexico National Olympiad, 1

Tags: fraction , irreducible , number theory

Evaluate the sum of all positive irreducible fractions less than $1$ and having the denominator $1991$.

2018 Auckland Mathematical Olympiad, 3

Tags: geometry , paper folding , folding , rectangle , area

A rectangular sheet of paper whose dimensions are $12 \times 18$ is folded along a diagonal, creating the $M$-shaped region drawn in the picture (see below). Find the area of the shaded region. [img]https://cdn.artofproblemsolving.com/attachments/4/7/d82cde3e91ab83fa14cd6cefe9bba28462dde1.png[/img]

2022 Junior Balkan Team Selection Tests - Romania, P3

Tags: combinatorics

Find how many positive integers $k\in\{1,2,\ldots,2022\}$ have the following property: if $2022$ real numbers are written on a circle so that the sum of any $k$ consecutive numbers is equal to $2022$ then all of the $2022$ numbers are equal.

2011 Today's Calculation Of Integral, 713

Tags: calculus , integration , limit , calculus computations

If a positive sequence $\{a_n\}_{n\geq 1}$ satisfies $\int_0^{a_n} x^{n}\ dx=2$, then find $\lim_{n\to\infty} a_n.$

2019 Saint Petersburg Mathematical Olympiad, 5

Tags: combinatorics , minimum

A class has $25$ students. The teacher wants to stock $N$ candies, hold the Olympics and give away all $N$ candies for success in it (those who solve equally tasks should get equally, those who solve less get less, including, possibly, zero candies). At what smallest $N$ this will be possible, regardless of the number of tasks on Olympiad and the student successes?

2017 Harvard-MIT Mathematics Tournament, 4

Tags: geometry

Triangle $ABC$ has $AB=10$, $BC=17$, and $CA=21$. Point $P$ lies on the circle with diameter $AB$. What is the greatest possible area of $APC$?

2008 iTest Tournament of Champions, 1

Tags: floor function

Find $k$ where $2^k$ is the largest power of $2$ that divides the product \[2008\cdot 2009\cdot 2010\cdots 4014.\]

2004 Tuymaada Olympiad, 4

Tags: combinatorics

There are many opposition societies in the city of N. Each society consists of $10$ members. It is known that for every $2004$ societies there is a person belonging to at least $11$ of them. Prove that the government can arrest $2003$ people so that at least one member of each society is arrested. [i]Proposed by V.Dolnikov, D.Karpov[/i]

2024 Kyiv City MO Round 1, Problem 3

Tags: game , number theory , divisibility

Petro and Vasyl play the following game. They take turns making moves and Petro goes first. In one turn, a player chooses one of the numbers from $1$ to $2024$ that wasn't selected before and writes it on the board. The first player after whose turn the product of the numbers on the board will be divisible by $2024$ loses. Who wins if every player wants to win? [i]Proposed by Mykhailo Shtandenko[/i]

2011 Romania Team Selection Test, 1

Tags: parallelogram , geometric transformation , reflection , geometry

Let $ABCD$ be a cyclic quadrilateral. The lines $BC$ and $AD$ meet at a point $P$. Let $Q$ be the point on the line $BP$, different from $B$, such that $PQ=BP$. Consider the parallelograms $CAQR$ and $DBCS$. Prove that the points $C,Q,R,S$ lie on a circle.

Novosibirsk Oral Geo Oly VII, 2021.6

Tags: geometry , equal segments , equilateral

Inside the equilateral triangle $ABC$, points $P$ and $Q$ are chosen so that the quadrilateral $APQC$ is convex, $AP = PQ = QC$ and $\angle PBQ = 30^o$. Prove that $AQ = BP$.