Olympiad problems

2000 Saint Petersburg Mathematical Olympiad, 11.1

An equilateral triangle with side length 9 is divided into 81 congruent triangles with segments which are parallel to the sides of the triangle. Prove that it cannot be divided into more than 18 parallelograms with sides 1 and 2. [I]Proposed by O.Vanyushina[/i]

2022 IMO Shortlist, G1

Tags: geometry , congruent triangles , parallel , computer problems

Let $ABCDE$ be a convex pentagon such that $BC=DE$. Assume that there is a point $T$ inside $ABCDE$ with $TB=TD,TC=TE$ and $\angle ABT = \angle TEA$. Let line $AB$ intersect lines $CD$ and $CT$ at points $P$ and $Q$, respectively. Assume that the points $P,B,A,Q$ occur on their line in that order. Let line $AE$ intersect $CD$ and $DT$ at points $R$ and $S$, respectively. Assume that the points $R,E,A,S$ occur on their line in that order. Prove that the points $P,S,Q,R$ lie on a circle.

1999 Chile National Olympiad, 6

Tags: geometry , congruent triangles , similar triangles

Prove that there are infinite pairs of non-congruent triangles that have the same angles and two of their equal sides. Develop an algorithm or rule to obtain these pairs of triangles and indicate at least one pair that satisfies the asserted.

2012 Tournament of Towns, 5

Tags: reflection , angle bisector , congruent triangles , incircle , tangent , geometry

Let $\ell$ be a tangent to the incircle of triangle $ABC$. Let $\ell_a,\ell_b$ and $\ell_c$ be the respective images of $\ell$ under reflection across the exterior bisector of $\angle A,\angle B$ and $\angle C$. Prove that the triangle formed by these lines is congruent to $ABC$.

2003 Indonesia MO, 6

Tags: geometry , inequalities , congruent triangles , combinatorics

The hall in a castle is a regular hexagon where its sides' length is 6 meters. The floor of the hall is to be tiled with equilateral triangular tiles where its sides' length is 50 centimeters. Each tile is divided into three congruent triangles by their altitudes up to its orthocenter (see below). Each of these small triangles are colored such that each tile has different colors and no two tiles have identical colorings. How many colors at least are required? A tile's pattern is: [asy] draw((0,0.000)--(2,0.000)); draw((2,0.000)--(1,1.732)); draw((1,1.732)--(0,0.000)); draw((1,0.577)--(0,0.000)); draw((1,0.577)--(2,0.000)); draw((1,0.577)--(1,1.732)); [/asy]

2021 Israel TST, 3

Tags: geometry , congruent triangles

Consider a triangle $ABC$ and two congruent triangles $A_1B_1C_1$ and $A_2B_2C_2$ which are respectively similar to $ABC$ and inscribed in it: $A_i,B_i,C_i$ are located on the sides of $ABC$ in such a way that the points $A_i$ are on the side opposite to $A$, the points $B_i$ are on the side opposite to $B$, and the points $C_i$ are on the side opposite to $C$ (and the angle at A are equal to angles at $A_i$ etc.). The circumcircles of $A_1B_1C_1$ and $A_2B_2C_2$ intersect at points $P$ and $Q$. Prove that the line $PQ$ passes through the orthocenter of $ABC$.

2008 Sharygin Geometry Olympiad, 1

Tags: congruent triangles , geometry

(B.Frenkin) Does a convex quadrilateral without parallel sidelines exist such that it can be divided into four congruent triangles?

2001 India Regional Mathematical Olympiad, 5

Tags: geometry , circumcircle , trigonometry , congruent triangles

In a triangle $ABC$, $D$ is a point on $BC$ such that $AD$ is the internal bisector of $\angle A$. Suppose $\angle B = 2 \angle C$ and $CD =AB$. prove that $\angle A = 72^{\circ}$.

2007 Sharygin Geometry Olympiad, 8

Tags: geometry , collinear , congruent triangles , concurrency

Three circles pass through a point $P$, and the second points of their intersection $A, B, C$ lie on a straight line. Let $A_1 B_1, C_1$ be the second meets of lines $AP, BP, CP$ with the corresponding circles. Let $C_2$ be the intersections of lines $AB_1$ and $BA_1$. Let $A_2, B_2$ be defined similarly. Prove that the triangles $A_1B_1C_1$ and $A_2B_2C_2$ are equal,

Ukrainian TYM Qualifying - geometry, II.2

Tags: geometry , congruent triangles , 3d geometry , tetrahedron

Is it true that when all the faces of a tetrahedron have the same area, they are congruent triangles?

2022 IMO, 4

Tags: geometry , congruent triangles , parallel , computer problems

Let $ABCDE$ be a convex pentagon such that $BC=DE$. Assume that there is a point $T$ inside $ABCDE$ with $TB=TD,TC=TE$ and $\angle ABT = \angle TEA$. Let line $AB$ intersect lines $CD$ and $CT$ at points $P$ and $Q$, respectively. Assume that the points $P,B,A,Q$ occur on their line in that order. Let line $AE$ intersect $CD$ and $DT$ at points $R$ and $S$, respectively. Assume that the points $R,E,A,S$ occur on their line in that order. Prove that the points $P,S,Q,R$ lie on a circle.

1993 Brazil National Olympiad, 3

Tags: circumcircle , congruent triangles , geometry

Given a circle and its center $O$, a point $A$ inside the circle and a distance $h$, construct a triangle $BAC$ with $\angle BAC = 90^\circ$, $B$ and $C$ on the circle and the altitude from $A$ length $h$.

VII Soros Olympiad 2000 - 01, 8.8

Tags: geometry , congruent triangles

Is there a quadrilateral, any vertex of which can be moved to another location so that the new quadrilateral is congruent to the original one?

Indonesia MO Shortlist - geometry, g3

Tags: geometry , congruent triangles , circles

Suppose $L_1$ is a circle with center $O$, and $L_2$ is a circle with center $O'$. The circles intersect at $ A$ and $ B$ such that $\angle OAO' = 90^o$. Suppose that point $X$ lies on the circumcircle of triangle $OAB$, but lies inside $L_2$. Let the extension of $OX$ intersect $L_1$ at $Y$ and $Z$. Let the extension of $O'X$ intersect $L_2$ at $W$ and $V$ . Prove that $\vartriangle XWZ$ is congruent with $\vartriangle XYV$.

2010 Math Prize For Girls Problems, 10

Tags: analytic geometry , geometry , geometric transformation , congruent triangles

The triangle $ABC$ lies on the coordinate plane. The midpoint of $\overline{AB}$ has coordinates $(-16, -63)$, the midpoint of $\overline{AC}$ has coordinates $(13, 50)$, and the midpoint of $\overline{BC}$ has coordinates $(6, -85)$. What are the coordinates of point $A$?

2024 Czech and Slovak Olympiad III A, 6

Tags: geometry , congruent triangles , side bash , prime number

Find all right triangles with integer side lengths in which two congruent circles with prime radius can be inscribed such that they are externally tangent, both touch the hypotenuse, and each is tangent to another leg of the right triangle.

1982 AMC 12/AHSME, 22

Tags: geometry , congruent triangles

In a narrow alley of width $w$ a ladder of length $a$ is placed with its foot at point $P$ between the walls. Resting against one wall at $Q$, the distance $k$ above the ground makes a $45^\circ$ angle with the ground. Resting against the other wall at $R$, a distance $h$ above the ground, the ladder makes a $75^\circ$ angle with the ground. The width $w$ is equal to $\textbf {(A) } a \qquad \textbf {(B) } RQ \qquad \textbf {(C) } k \qquad \textbf {(D) } \frac{h+k}{2} \qquad \textbf {(E) } h$

1986 IMO Longlists, 11

Tags: geometry , 3d geometry , tetrahedron , trigonometry , congruent triangles

Prove that the sum of the face angles at each vertex of a tetrahedron is a straight angle if and only if the faces are congruent triangles.

2005 Flanders Junior Olympiad, 2

Tags: ratio , symmetry , geometry , congruent triangles

Starting with two points A and B, some circles and points are constructed as shown in the figure:[list][*]the circle with centre A through B [*]the circle with centre B through A [*]the circle with centre C through A [*]the circle with centre D through B [*]the circle with centre E through A [*]the circle with centre F through A [*]the circle with centre G through A[/list] [i][size=75](I think the wording is not very rigorous, you should assume intersections from the drawing)[/size][/i] Show that $M$ is the midpoint of $AB$. [img]https://cdn.artofproblemsolving.com/attachments/d/4/2352ab21cc19549f0381e88ddde9dce4299c2e.png[/img]

1991 Greece National Olympiad, 3

Tags: combinatorics , combinatorial geometry , geometry , congruent triangles

Prove that exists triangle that can be partitions in $2050$ congruent triangles.

VI Soros Olympiad 1999 - 2000 (Russia), 10.4

Tags: geometry , congruent triangles

Can we say that two triangles are congruent if the radii of the inscribed circles, the radii of the circumscribed circles, and the areas of these triangles are equal?

2017 Balkan MO Shortlist, C2

Tags: lattice points , combinatorics , number theory , congruent triangles , coloring

Let $n,a,b,c$ be natural numbers. Every point on the coordinate plane with integer coordinates is colored in one of $n$ colors. Prove there exists $c$ triangles whose vertices are colored in the same color, which are pairwise congruent, and which have a side whose lenght is divisible by $a$ and a side whose lenght is divisible by $b$.

2023 Irish Math Olympiad, P1

Tags: geometry , parallelogram , congruent triangles

We are given a triangle $ABC$ such that $\angle BAC < 90^{\circ}$. The point $D$ is on the opposite side of the line $AB$ to $C$ such that $|AD| = |BD|$ and $\angle ADB = 90^{\circ}$. Similarly, the point $E$ is on the opposite side of $AC$ to $B$ such that $|AE| = |CE|$ and $\angle AEC = 90^{\circ}$. The point $X$ is such that $ADXE$ is a parallelogram. Prove that $|BX| = |CX|$.

2021 Iberoamerican, 6

Tags: geometry , congruent triangles

Consider a $n$-sided regular polygon, $n \geq 4$, and let $V$ be a subset of $r$ vertices of the polygon. Show that if $r(r-3) \geq n$, then there exist at least two congruent triangles whose vertices belong to $V$.

2012 Sharygin Geometry Olympiad, 4

Tags: analytic geometry , parallelogram , ratio , perpendicular bisector , congruent triangles , geometry

Given triangle $ABC$. Point $M$ is the midpoint of side $BC$, and point $P$ is the projection of $B$ to the perpendicular bisector of segment $AC$. Line $PM$ meets $AB$ in point $Q$. Prove that triangle $QPB$ is isosceles.