Olympiad problems

STEMS 2021 Math Cat B, Q1

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An acute angled triangle $\mathcal{T}$ is inscribed in circle $\Omega$.Denote by $\Gamma$ the nine-point circle of $\mathcal{T}$.A circle $\omega$ passes through two of the vertices of $\mathcal{T}$, and centre of $\Omega$.Prove that the common external tangents of $\Gamma$ and $\omega$ meet on the external bisector of the angle at third vertex of $\mathcal{T}$.

2011 Morocco TST, 1

Find all pairs $(m,n)$ of nonnegative integers for which \[m^2 + 2 \cdot 3^n = m\left(2^{n+1} - 1\right).\] [i]Proposed by Angelo Di Pasquale, Australia[/i]

2002 All-Russian Olympiad, 2

Tags: geometry , angle bisector , incenter

Point $A$ lies on one ray and points $B,C$ lie on the other ray of an angle with the vertex at $O$ such that $B$ lies between $O$ and $C$. Let $O_1$ be the incenter of $\triangle OAB$ and $O_2$ be the center of the excircle of $\triangle OAC$ touching side $AC$. Prove that if $O_1A = O_2A$, then the triangle $ABC$ is isosceles.

1979 USAMO, 2

Tags: 3d geometry , geometry , reflection , angle bisector , sphere , geometric transformation

Let $S$ be a great circle with pole $P$. On any great circle through $P$, two points $A$ and $B$ are chosen equidistant from $P$. For any [i] spherical triangle [/i] $ABC$ (the sides are great circles ares), where $C$ is on $S$, prove that the great circle are $CP$ is the angle bisector of angle $C$. [b] Note. [/b] A great circle on a sphere is one whose center is the center of the sphere. A pole of the great circle $S$ is a point $P$ on the sphere such that the diameter through $P$ is perpendicular to the plane of $S$.

2004 Brazil National Olympiad, 2

Tags: geometry , combinatorics

Determine all values of $n$ such that it is possible to divide a triangle in $n$ smaller triangles such that there are not three collinear vertices and such that each vertex belongs to the same number of segments.

1997 Croatia National Olympiad, Problem 4

Tags: triangle , geometry , parallelogram

On the sides of a triangle $ABC$ are constructed similar triangles $ABD,BCE,CAF$ with $k=AD/DB=BE/EC=CF/FA$ and $\alpha=\angle ADB=\angle BEC=\angle CFA$. Prove that the midpoints of the segments $AC,BC,CD$ and $EF$ form a parallelogram with an angle $\alpha$ and two sides whose ratio is $k$.

1949-56 Chisinau City MO, 39

Tags: logarithm , algebra

Solve the equation: $\log_{x} 2 \cdot \log_{2x} 2 = \log_{4x} 2$.

2005 MOP Homework, 3

Tags: radical axis , geometry , power of a point

Circles $S_1$ and $S_2$ meet at points $A$ and $B$. A line through $A$ is parallel to the line through the centers of $S_1$ and $S_2$ and meets $S_1$ and $S_2$ again $C$ and $D$ respectively. Circle $S_3$ having $CD$ as its diameter meets $S_1$ and $S_2$ again at $P$ and $Q$ respectively. Prove that lines $CP$, $DQ$, and $AB$ are concurent.

2004 Tournament Of Towns, 4

Tags: sequence , integer , arithmetic progression , number theory

Arithmetical progression $a_1, a_2, a_3, a_4,...$ contains $a_1^2 , a_2^2$ and $a_3^2$ at some positions. Prove that all terms of this progression are integers.

2024 India IMOTC, 12

Tags: geometry

Let $ABC$ be an acute-angled triangle with $AB<AC$, and let $O,H$ be its circumcentre and orthocentre respectively. Points $Z,Y$ lie on segments $AB,AC$ respectively, such that \[\angle ZOB=\angle YOC = 90^{\circ}.\] The perpendicular line from $H$ to line $YZ$ meets lines $BO$ and $CO$ at $Q,R$ respectively. Let the tangents to the circumcircle of $\triangle AYZ$ at points $Y$ and $Z$ meet at point $T$. Prove that $Q, R, O, T$ are concyclic. [i]Proposed by Kazi Aryan Amin and K.V. Sudharshan[/i]

2013 Kosovo National Mathematical Olympiad, 5

Tags: trapezoid , function , similar triangles , geometry

A trapezium has parallel sides of length equal to $a$ and $b$ ($a <b$), and the distance between the parallel sides is the altitude $h$. The extensions of the non-parallel lines intersect at a point that is a vertex of two triangles that have as sides the parallel sides of the trapezium. Express the areas of the triangles as functions of $a,b$ and $h$.

2017 CCA Math Bonanza, L3.3

Tags: perimeter , arithmetic sequence , geometry

An acute triangle $ABC$ has side lenghths $a$, $b$, $c$ such that $a$, $b$, $c$ forms an arithmetic sequence. Given that the area of triangle $ABC$ is an integer, what is the smallest value of its perimeter? [i]2017 CCA Math Bonanza Lightning Round #3.3[/i]

2015 Indonesia MO Shortlist, N3

Tags: number theory , divisibility

Given positive integers $a,b,c,d$ such that $a\mid c^d$ and $b\mid d^c$. Prove that \[ ab\mid (cd)^{max(a,b)} \]

2016 ASDAN Math Tournament, 7

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What is $$\sum_{n=1996}^{2016}\lfloor\sqrt{n}\rfloor?$$

2021 Macedonian Mathematical Olympiad, Problem 4

Tags: combinatorics

For a fixed positive integer $n \geq 3$ we are given a $n$ $\times$ $n$ board with all unit squares initially white. We define a [i]floating plus [/i]as a $5$-tuple $(M,L,R,A,B)$ of unit squares such that $L$ is in the same row and left of $M$, $R$ is in the same row and right of $M$, $A$ is in the same column and above $M$ and $B$ is in the same column and below $M$. It is possible for $M$ to form a floating plus with unit squares that are not next to it. Find the largest positive integer $k$ (depending on $n$) such that we can color some $k$ squares black in such a way that there is no black colored floating plus. [i]Authored by Nikola Velov[/i]

2022 Bulgarian Spring Math Competition, Problem 8.4

Tags: permutations with restrictions , combinatorics

Let $p = (a_{1}, a_{2}, \ldots , a_{12})$ be a permutation of $1, 2, \ldots, 12$. We will denote \[S_{p} = |a_{1}-a_{2}|+|a_{2}-a_{3}|+\ldots+|a_{11}-a_{12}|\]We'll call $p$ $\textit{optimistic}$ if $a_{i} > \min(a_{i-1}, a_{i+1})$ $\forall i = 2, \ldots, 11$. $a)$ What is the maximum possible value of $S_{p}$. How many permutations $p$ achieve this maximum?$\newline$ $b)$ What is the number of $\textit{optimistic}$ permtations $p$? $c)$ What is the maximum possible value of $S_{p}$ for an $\textit{optimistic}$ $p$? How many $\textit{optimistic}$ permutations $p$ achieve this maximum?

1998 Miklós Schweitzer, 6

Tags: perimeter , calculus , geometry

Let U be the union of a finite number (not necessarily connected and not necessarily disjoint) of closed unit squares lying in the plane. Can the quotient of the perimeter and area of U be arbitrarily large? @below: i think "single" means "connected".

2019 Silk Road, 3

Tags: number theory , eulers function , diophantine equation , relatively prime

Find all pairs of $ (a, n) $ natural numbers such that $ \varphi (a ^ n + n) = 2 ^ n. $ ($ \varphi (n) $ is the Euler function, that is, the number of integers from $1$ up to $ n $, relative prime to $ n $)

2020 Tournament Of Towns, 6

Tags: combinatorics , game

Alice has a deck of $36$ cards, $4$ suits of $9$ cards each. She picks any $18$ cards and gives the rest to Bob. Now each turn Alice picks any of her cards and lays it face-up onto the table, then Bob similarly picks any of his cards and lays it face-up onto the table. If this pair of cards has the same suit or the same value, Bob gains a point. What is the maximum number of points he can guarantee regardless of Alice’s actions? Mikhail Evdokimov

2002 Irish Math Olympiad, 3

Tags: function , functional equation , algebra

Find all functions $ f: \mathbb{Q} \rightarrow \mathbb{Q}$ such that: $ f(x\plus{}f(y))\equal{}y\plus{}f(x)$ for all $ x,y \in \mathbb{Q}$.

1998 China National Olympiad, 1

Tags: geometry , trigonometry , incenter

Let $ABC$ be a non-obtuse triangle satisfying $AB>AC$ and $\angle B=45^{\circ}$. The circumcentre $O$ and incentre $I$ of triangle $ABC$ are such that $\sqrt{2}\ OI=AB-AC$. Find the value of $\sin A$.

2013 NZMOC Camp Selection Problems, 4

Tags: geometry , ratio , 3d geometry , octahedron

Let $C$ be a cube. By connecting the centres of the faces of $C$ with lines we form an octahedron $O$. By connecting the centers of each face of $O$ with lines we get a smaller cube $C'$. What is the ratio between the side length of $C$ and the side length of $C'$?

2006 Sharygin Geometry Olympiad, 4

Tags: geometry , regular , square , pentagon

a) Given two squares $ABCD$ and $DEFG$, with point $E$ lying on the segment $CD$, and points$ F,G$ outside the square $ABCD$. Find the angle between lines $AE$ and $BF$. b) Two regular pentagons $OKLMN$ and $OPRST$ are given, and the point $P$ lies on the segment $ON$, and the points $R, S, T$ are outside the pentagon $OKLMN$. Find the angle between straight lines $KP$ and $MS$.

1993 Tournament Of Towns, (378) 7

Tags: combinatorics

In a handbook of plants each plant is characterized by $100$ attributes (each attribute may either be present in a plant or not). Two plants are called [i]dissimilar [/i] if they differ by no less than $51$ attributes. (a) Prove that the handbook cannot describe more than $50$ pair-wise dissimilar plants. (b) Can it describe $50$ pairwise dissimilar plants? (Dima Tereshin)

2020 Latvia Baltic Way TST, 2

Tags: functional equation in r , algebra , function

Determine all functions $f:\mathbb R\to\mathbb R$ that satisfy equation: $$ f(x^3+y^3) =f(x^3) + 3x^2f(x)f(y) + 3f(x)f(y)^2 + y^6f(y) $$ for all reals $x,y$