Olympiad problems

1985 Greece National Olympiad, 3

Tags: analytic geometry , geometry , distance , number theory , lattice points

Consider the line (E): $5x-10y+3=0$ . Prove that: a) Line $(E)$ doesn't pass through points with integer coordinates. b) There is no point $A(a_1,a_2)$ with $ a_1,a_2 \in \mathbb{Z}$ with distance from $(E)$ less then $\frac{\sqrt3}{20}$.

2023 JBMO TST - Turkey, 3

Tags: geometry , tangent

Let $ABC$ is triangle and $D \in AB$,$E \in AC$ such that $DE//BC$. Let $(ABC)$ meets with $(BDE)$ and $(CDE)$ at the second time $K,L$ respectively. $BK$ and $CL$ intersect at $T$. Prove that $TA$ is tangent to the $(ABC)$

2019 Flanders Math Olympiad, 2

Tags: algebra , sum

Calculate the sum of all unsimplified fractions whose numerator and denominator are positive divisors of $1000$.

1989 IMO Shortlist, 28

Tags: geometry , area of a triangle , geometric inequality , point set

Consider in a plane $ P$ the points $ O,A_1,A_2,A_3,A_4$ such that \[ \sigma(OA_iA_j) \geq 1 \quad \forall i, j \equal{} 1, 2, 3, 4, i \neq j.\] where $ \sigma(OA_iA_j)$ is the area of triangle $ OA_iA_j.$ Prove that there exists at least one pair $ i_0, j_0 \in \{1, 2, 3, 4\}$ such that \[ \sigma(OA_iA_j) \geq \sqrt{2}.\]

2022 Germany Team Selection Test, 1

Tags: algebra , real analysis , inequalities

Let $n\geq 2$ be an integer and let $a_1, a_2, \ldots, a_n$ be positive real numbers with sum $1$. Prove that $$\sum_{k=1}^n \frac{a_k}{1-a_k}(a_1+a_2+\cdots+a_{k-1})^2 < \frac{1}{3}.$$

2023 Regional Olympiad of Mexico Southeast, 3

Tags: grid , coloring

Let $n$ be a positive integer. A grid of $n\times n$ has some black-colored cells. Drini can color a cell if at least three cells that share a side with it are also colored black. Drini discovers that by repeating this process, all the cells in the grid can be colored. Prove that if there are initially $k$ colored cells, then $$k\geq \frac{n^2+2n}{3}.$$

2018 HMNT, 6

Tags: geometry

Triangle $\triangle PQR$, with $PQ=PR=5$ and $QR=6$, is inscribed in circle $\omega$. Compute the radius of the circle with center on $\overline{QR}$ which is tangent to both $\omega$ and $\overline{PQ}$.

2004 Estonia National Olympiad, 3

Tags: bisects area , convex , parallel , geometry

On the sides $AB , BC$ of the convex quadrilateral $ABCD$ lie points $M$ and $N$ such that $AN$ and $CM$ each divide the quadrilateral $ABCD$ into two equal area parts. Prove that the line $MN$ and $AC$ are parallel.

2006 Bulgaria Team Selection Test, 1

Tags: circumcircle , parallelogram , symmetry , angle bisector , power of a point , geometry

[b]Problem 1.[/b] Points $D$ and $E$ are chosen on the sides $AB$ and $AC$, respectively, of a triangle $\triangle ABC$ such that $DE\parallel BC$. The circumcircle $k$ of triangle $\triangle ADE$ intersects the lines $BE$ and $CD$ at the points $M$ and $N$ (different from $E$ and $D$). The lines $AM$ and $AN$ intersect the side $BC$ at points $P$ and $Q$ such that $BC=2\cdot PQ$ and the point $P$ lies between $B$ and $Q$. Prove that the circle $k$ passes through the point of intersection of the side $BC$ and the angle bisector of $\angle BAC$. [i]Nikolai Nikolov[/i]

2022 Costa Rica - Final Round, 5

Tags: number theory

The $1$st edition of OLCOMA was organized in $1989$, so in $2022$ the $34$th edition will be celebrated. Suppose that the Olympics will continue to be held annually without interruption. We say that a year $N$ is [i]good [/i] if the OLCOMA edition number of that year divides the product $N(N +1)$. For example, the year $2022$ is good because $34$ divides $2022 \cdot 2023$. Determine the last year $N$ in the $21$st century, $2000\le N \le 2099$, which is good.

1974 Poland - Second Round, 4

Tags: geometry , area

In a convex quadrilateral $ ABCD $ with area $ S $, each side was divided into 3 equal parts and segments were drawn connecting the appropriate points of division of the opposite sides in such a way that the quadrilateral was divided into 9 quadrilaterals. Prove that the sum of the areas of the following three quadrilaterals resulting from the division: the one containing the vertex $ A $, the middle one and the one containing the vertex $ C $ is equal to $ \frac{S}{3} $.

2018 South East Mathematical Olympiad, 2

Tags: combinatorics

In a Cartesian plane, if both horizontal coordinate and vertical coordinate of a point are rational numbers, we call the point [i]rational point[/i]. Otherwise, we call it [i]irrational point[/i]. Consider an arbitrary regular pentagon on the Cartesian plane. Please compare the number of rational point and the number of irrational point among the five vertices of the pentagon. Prove your conclusion.

2014 German National Olympiad, 3

Tags: coloring , sum , additive combinatorics , combinatorics

Given two positive integers $n$ and $k$, we say that $k$ is [i]$n$-ergetic[/i] if: However the elements of $M=\{1,2,\ldots, k\}$ are coloured in red and green, there exist $n$ not necessarily distinct integers of the same colour whose sum is again an element of $M$ of the same colour. For each positive integer $n$, determine the least $n$-ergetic integer, if it exists.

2005 Rioplatense Mathematical Olympiad, Level 3, 3

Tags: integration , combinatorics

Let $k$ be a positive integer. Show that for all $n>k$ there exist convex figures $F_{1},\ldots, F_{n}$ and $F$ such that there doesn't exist a subset of $k$ elements from $F_{1},..., F_{n}$ and $F$ is covered for this elements, but $F$ is covered for every subset of $k+1$ elements from $F_{1}, F_{2},....., F_{n}$.

2010 Greece Team Selection Test, 3

Tags: circumcircle , geometric transformation , reflection , geometry

Let $ABC$ be a triangle,$O$ its circumcenter and $R$ the radius of its circumcircle.Denote by $O_{1}$ the symmetric of $O$ with respect to $BC$,$O_{2}$ the symmetric of $O$ with respect to $AC$ and by $O_{3}$ the symmetric of $O$ with respect to $AB$. (a)Prove that the circles $C_{1}(O_{1},R)$, $C_{2}(O_{2},R)$, $C_{3}(O_{3},R)$ have a common point. (b)Denote by $T$ this point.Let $l$ be an arbitary line passing through $T$ which intersects $C_{1}$ at $L$, $C_{2}$ at $M$ and $C_{3}$ at $K$.From $K,L,M$ drop perpendiculars to $AB,BC,AC$ respectively.Prove that these perpendiculars pass through a point.

2008 JBMO Shortlist, 3

Tags: trigonometry , angle bisector , perpendicular bisector , power of a point , geometry

The vertices $ A$ and $ B$ of an equilateral triangle $ ABC$ lie on a circle $k$ of radius $1$, and the vertex $ C$ is in the interior of the circle $ k$. A point $ D$, different from $ B$, lies on $ k$ so that $ AD\equal{}AB$. The line $ DC$ intersects $ k$ for the second time at point $ E$. Find the length of the line segment $ CE$.

2009 Stanford Mathematics Tournament, 9

Tags:

All of the roots of $x^3+ax^2+bx+c$ are positive integers greater than $2$, and the coefficients satisfy $a+b+c+1=-2009$. Find $a$

1987 Tournament Of Towns, (138) 3

Tags: combinatorics , chessboard

Nine pawns forming a $3$ by $3$ square are placed in the lower left hand corner of an $8$ by $8$ chessboard. Any pawn may jump over another one standing next to it into a free square, i .e. may be reflected symmetrically with respect to a neighb our's centre (jumps may be horizontal , vertical or diagonal) . It is required to rearrange the nine pawns in another corner of the chessboard (in another $3$ by $3$ square) by means of such jumps. Can the pawns be thus re-arranged in the (a) upper left hand corner? (b) upper right hand corner? (J . E . Briskin)

MOAA Gunga Bowls, 2023.20

Tags:

Big Bad Brandon is assigning groups of his Big Bad Burglars to attack 7 different towers. Each Burglar can only belong to one attack group and Brandon takes over a tower if the number of Burglars attacking the tower strictly exceeds the number of knights guarding it. He knows there the total number of knights guarding the towers is 99 but does not know the exact number of knights guarding each tower. What is the minimum number of Burglars that Brandon needs to guarantee he can take over at least 4 of the 7 towers? [i]Proposed by Eric Wang[/i]

1995 Polish MO Finals, 3

Tags: geometry

$PA, PB, PC$ are three rays in space. Show that there is just one pair of points $B', C$' with $B'$ on the ray $PB$ and $C'$ on the ray $PC$ such that $PC' + B'C' = PA + AB'$ and $PB' + B'C' = PA + AC'$.

2008 VJIMC, Problem 3

Tags: number theory

Find all pairs of natural numbers $(n,m)$ with $1<n<m$ such that the numbers $1$, $\sqrt[n]n$ and $\sqrt[m]m$ are linearly dependent over the field of rational numbers $\mathbb Q$.

2017 India Regional Mathematical Olympiad, 1

Tags: construction , geometry

Let $AOB$ be a given angle less than $180^{\circ}$ and let $P$ be an interior point of the angular region determined by $\angle AOB$. Show, with proof, how to construct, using only ruler and compass, a line segment $CD$ passing through $P$ such that $C$ lies on the way $OA$ and $D$ lies on the ray $OB$, and $CP:PD=1:2$.

2015 Brazil Team Selection Test, 3

Tags: combinatorics , algebra , prime number

Construct a tetromino by attaching two $2 \times 1$ dominoes along their longer sides such that the midpoint of the longer side of one domino is a corner of the other domino. This construction yields two kinds of tetrominoes with opposite orientations. Let us call them $S$- and $Z$-tetrominoes, respectively. Assume that a lattice polygon $P$ can be tiled with $S$-tetrominoes. Prove that no matter how we tile $P$ using only $S$- and $Z$-tetrominoes, we always use an even number of $Z$-tetrominoes. [i]Proposed by Tamas Fleiner and Peter Pal Pach, Hungary[/i]

2011 HMNT, 8

Tags: number theory

Find the number of integers $x$ such that the following three conditions all hold: $\bullet$ $x$ is a multiple of $5$ $\bullet$ $121 < x < 1331$ $\bullet$ When $x$ is written as an integer in base $11$ with no leading $0$s (i.e. no $0$s at the very left), its rightmost digit is strictly greater than its leftmost digit.

1984 Putnam, B3

Tags: abstract algebra , set

Prove or disprove the following statement: If $F$ is a finite set with two or more elements, then there exists a binary operation $*$ on $F$ such that for all $x,y,z$ in $F$, $(\text i)$ $x*z=y*z$ implies $x=y$ $(\text{ii})$ $x*(y*z)\ne(x*y)*z$