Olympiad problems

2006 AMC 10, 18

Tags:

Let $ a_1, a_2, ...$ be a sequence for which \[a_1 \equal{} 2\,\hspace{.2in}a_2 \equal{} 3\, \hspace{.2in}\text{and}\hspace{.2in}a_n \equal{} \frac {a_{n \minus{} 1}}{a_{n \minus{} 2}} \text{ for each positive integer } n \ge 3.\]What is $ a_{2006}$? $\textbf{(A) } \frac 12 \qquad \textbf{(B) } \frac 23 \qquad \textbf{(C) } \frac 32 \qquad \textbf{(D) } 2 \qquad \textbf{(E) } 3$

2015 CCA Math Bonanza, I2

Tags: factorial

The operation $*$ is defined by the following: $a*b=a!-ab-b.$ Compute the value of $5*8.$ [i]2015 CCA Math Bonanza Individual Round #2[/i]

2012 Online Math Open Problems, 13

Tags:

A circle $\omega$ has center $O$ and radius $r$. A chord $BC$ of $\omega$ also has length $r$, and the tangents to $\omega$ at $B$ and $C$ meet at $A$. Ray $AO$ meets $\omega$ at $D$ past $O$, and ray $OA$ meets the circle centered at $A$ with radius $AB$ at $E$ past $A$. Compute the degree measure of $\angle DBE$. [i]Author: Ray Li[/i]

2023 Brazil Cono Sur TST, 1

Tags: number theory

Let $n = p_1p_2 \dots p_k$ be the product of distinct primes $p_1, p_2, \dots , p_k$, with $k > 1$. Find all $n$ such that $n$ is multiple of $p_1 - 1, p_2 - 1, \dots , p_k - 1$.

1962 All Russian Mathematical Olympiad, 018

Tags: geometry , geometric construction , triangle construction

Given two sides of the triangle. Construct that triangle, if medians to those sides are orthogonal.

1999 French Mathematical Olympiad, Problem 1

Tags: geometry , 3d geometry

What is the maximum possible volume of a cylinder inscribed in a cone and having the same axis of symmetry as the cone? What is the maximum possible volume of a ball inscribed in the cone with center on the axis of symmetry of the cone? Compare these three volumes.

2018 Cono Sur Olympiad, 1

Tags: geometry

Let $ABCD$ be a convex quadrilateral, where $R$ and $S$ are points in $DC$ and $AB$, respectively, such that $AD=RC$ and $BC=SA$. Let $P$, $Q$ and $M$ be the midpoints of $RD$, $BS$ and $CA$, respectively. If $\angle MPC + \angle MQA = 90$, prove that $ABCD$ is cyclic.

2016 Grand Duchy of Lithuania, 2

Tags: combinatorics

During a school year $44$ competitions were held. Exactly $7$ students won in each of the competitions. For any two competitions, there exists exactly $1$ student who won in both competitions. Is it true that there exists a student who won all of the competitions?

2010 Kyrgyzstan National Olympiad, 4

Tags: geometry

Point $O$ is chosen in a triangle $ABC$ such that ${d_a},{d_b},{d_c}$ are distance from point $O$ to sides $BC,CA,AB$, respectively. Find position of point $O$ so that product ${d_a} \cdot {d_b} \cdot {d_c}$ becomes maximum.

2020 Moldova EGMO TST, 4

Tags: geometry , incenter , angle bisector

The incircle of triangle $ABC$ touches $AC$ and $BC$ respectively $P$ and $Q$. Let $N$ and $M$ be the midpoints of the sides $AC$ and $BC$ respectively.$AM$ and $BP$,$BN$ and $AQ$ intersects at the points $X$ and $Y$ respectively. If the points $C,X$ and $Y$ are collinear , then prove that $CX$ is the angle bisector of $\angle ACB$.

2019 Hanoi Open Mathematics Competitions, 13

Tags: triangle inequality , geometry , equilateral , distance

Find all points inside a given equilateral triangle such that the distances from it to three sides of the given triangle are the side lengths of a triangle.

2025 Sharygin Geometry Olympiad, 24

Tags: geometry , 3d geometry , geometric inequality

The insphere of a tetrahedron $ABCD$ touches the faces $ABC$, $BCD$, $CDA$, $DAB$ at $D^{\prime}$, $A^{\prime}$, $B^{\prime}$, $C^{\prime}$ respectively. Denote by $S_{AB}$ the area of the triangle $AC^{\prime}B^{\prime}$. Define similarly $S_{AC}$, $S_{BC},$ $S_{AD}$, $S_{BD}$, $S_{CD}$. Prove that there exists a triangle with sidelengths $\sqrt{S_{AB}S_{CD}}$, $\sqrt{S_{AC}S_{BD}}$ , $\sqrt{S_{AD}S_{BC}}$. Proposed by: S.Arutyunyan

2012 Cuba MO, 6

Tags: geometry

Let $ABC$ be a right triangle at $A$, and let $AD$ be the relative height to the hypotenuse. Let $N$ be the intersection of the bisector of the angle of vertex $C$ with $AD$. Prove that $$AD \cdot BC = AB \cdot DC + BD \cdot AN.$$

2009 Switzerland - Final Round, 4

Tags: combinatorics

Let $n$ be a natural number. Each cell of a $n \times n$ square contains one of $n$ different symbols, such that each of the symbols is in exactly $n$ cells. Show that a row or a column exists that contains at least \sqrt{n} different symbols.

2019 Harvard-MIT Mathematics Tournament, 8

Tags: hmmt , geometry

In triangle $ABC$ with $AB < AC$, let $H$ be the orthocenter and $O$ be the circumcenter. Given that the midpoint of $OH$ lies on $BC$, $BC = 1$, and the perimeter of $ABC$ is 6, find the area of $ABC$.

1997 Akdeniz University MO, 3

Tags: equation , number theory , sequence

$(x_n)$ be a sequence with $x_1=0$, $$x_{n+1}=5x_n + \sqrt{24x_n^2+1}$$. Prove that for $k \geq 2$ $x_k$ is a natural number.

2006 AMC 12/AHSME, 22

Tags: floor function , function , inequalities , algebra

Suppose $ a, b,$ and $ c$ are positive integers with $ a \plus{} b \plus{} c \equal{} 2006$, and $ a!b!c! \equal{} m\cdot10^n$, where $ m$ and $ n$ are integers and $ m$ is not divisible by 10. What is the smallest possible value of $ n$? $ \textbf{(A) } 489 \qquad \textbf{(B) } 492 \qquad \textbf{(C) } 495 \qquad \textbf{(D) } 498 \qquad \textbf{(E) } 501$

2021 IMO Shortlist, C8

Tags: combinatorics

Determine the largest integer $N$ for which there exists a table $T$ of integers with $N$ rows and $100$ columns that has the following properties: $\text{(i)}$ Every row contains the numbers $1$, $2$, $\ldots$, $100$ in some order. $\text{(ii)}$ For any two distinct rows $r$ and $s$, there is a column $c$ such that $|T(r,c) - T(s, c)|\geq 2$. (Here $T(r,c)$ is the entry in row $r$ and column $c$.)

2009 Argentina Iberoamerican TST, 3

Tags: search , symmetry , combinatorics

Within a group of $ 2009$ people, every two people has exactly one common friend. Find the least value of the difference between the person with maximum number of friends and the person with minimum number of friends.

1962 AMC 12/AHSME, 31

Tags: ratio

The ratio of the interior angles of two regular polygons with sides of unit length is $ 3: 2$. How many such pairs are there? $ \textbf{(A)}\ 1 \qquad \textbf{(B)}\ 2 \qquad \textbf{(C)}\ 3 \qquad \textbf{(D)}\ 4 \qquad \textbf{(E)}\ \text{infinitely many}$

1992 Miklós Schweitzer, 5

Tags: number theory , real analysis

Prove that if the $a_i$'s are different natural numbers, then $\sum_ {j = 1}^n a_j ^ 2 \prod_{k \neq j} \frac{a_j + a_k}{a_j-a_k}$ is a square number.

Estonia Open Junior - geometry, 2012.1.3

Tags: ratio , geometry , rectangle , right triangle

A rectangle $ABEF$ is drawn on the leg $AB$ of a right triangle $ABC$, whose apex $F$ is on the leg $AC$. Let $X$ be the intersection of the diagonal of the rectangle $AE$ and the hypotenuse $BC$ of the triangle. In what ratio does point $X$ divide the hypotenuse $BC$ if it is known that $| AC | = 3 | AB |$ and $| AF | = 2 | AB |$?

1991 Vietnam National Olympiad, 1

Tags: combinatorics

$1991$ students sit around a circle and play the following game. Starting from some student $A$ and counting clockwise, each student on turn says a number. The numbers are $1,2,3,1,2,3,...$ A student who says $2$ or $3$ must leave the circle. The game is over when there is only one student left. What position was the remaining student sitting at the beginning of the game?

2014 Danube Mathematical Competition, 3

Tags: midpoint , geometry , right angle , orthocenter

Let $ABC$ be a triangle with $\angle A<90^o, AB \ne AC$. Denote $H$ the orthocenter of triangle $ABC$, $N$ the midpoint of segment $[AH]$, $M$ the midpoint of segment $[BC]$ and $D$ the intersection point of the angle bisector of $\angle BAC$ with the segment $[MN]$. Prove that $<ADH=90^o$

2021/2022 Tournament of Towns, P7

Tags: combinatorics , combinatorial geometry

A checkered square of size $2\times2$ is covered by two triangles. Is it necessarily true that: [list=a] [*]at least one of its four cells is fully covered by one of the triangles; [*]some square of size $1\times1$ can be placed into one of these triangles? [/list] [i]Alexandr Shapovalov[/i]