geometry problems from International Mathematical Olympiads (also known as IMO)

aops links are in the names

by Cezar Cosnita

An isosceles trapezoid with
bases a and b and altitude h is given.

by Gheorghe D. Simionescu

by Gheorghe D. Simionescu

by Tullio Viola

by Ðorde Dugošija

by Jan van de Craats

by Jan van de Craats

by Murray Klamkin

by Murray Klamkin

by Nikolai Vasil'ev and Igor F. Sharygin

by Murray Klamkin

by David Monk

by Jan van de Craats

by Jan van de Craats

by Igor F. Sharygin

by Laurentiu Panaitopol

by Frank Budden

by Igor F. Sharygin

by Gengzhe Chang and Dongxu Qi

by Sven Sigurðsson

by I.A. Kushnir

by Lucien Kieffer

by Dimitris Kontogiannis

by Esther Szekeres

by Eggert Briem

by Arkadii Skopenkov

by Johan Yebbou

by Johan Yebbou

2006 IMO Shortlist G10 (SRB) problem 6

To each side $a$ of a convex polygon we assign the maximum area of a triangle contained in the polygon and having $a$ as one of its sides. Show that the sum of the areas assigned to all sides of the polygon is not less than twice the area of the polygon.

In triangle $ABC$, the angle bisector at vertex $C$ intersects the circumcircle and the perpendicular bisectors of sides $BC$ and $CA$ at points $R, P$, and $Q$, respectively. The midpoints of $BC$ and $CA$ are $S$ and $T$, respectively. Prove that triangles $RQT$ and $RPS$ have the same area.

Consider five points $A, B, C, D, E$ such that $ABCD$ is a parallelogram and BCED is a cyclic quadrilateral. Let $\ell$ be a line passing through $A$, and let $\ell $ intersect segment $ DC$ and line $BC$ at points $F$ and $G$, respectively. Suppose that $EF = EG = EC$. Prove that $\ell$ is the bisector of angle $DAB$.

In an acute-angled triangle $ABC$, point $H$ is the orthocentre and $A_o, B_o, C_o$ are the midpoints of the sides $BC, CA, AB$, respectively. Consider three circles passing through $H$: $\omega_a$ around $A_o, \omega_b$ around $B_o$ and $\omega_c$ around $C_o$. The circle $\omega_a$ intersects the line $BC$ at $A_1$ and $A_2$; $\omega_b$ intersects$ CA$ at $B_1$ and $B_2$; $\omega_c$ intersects $AB$ at $C_1$ and $C_2$. Show that the points $A_1, A_2, B_1, B_2, C_1, C_2$ lie on a circle.

Let $ABCD$ be a convex quadrilateral with $AB \ne BC$. Denote by $\omega_1$ and $\omega_2$ the incircles of triangles $ABC$ and $ADC$. Suppose that there exists a circle $ \omega$ inscribed in angle $ABC$, tangent to the extensions of line segments $AD$ and $CD$. Prove that the common external tangents of $\omega_1 $ and $\omega_2 $ intersect on $ \omega$.

Let ABC be a triangle with $AB = AC$. The angle bisectors of $A$ and $B$ meet the sides $BC$ and $AC$ in $D$ and $E$, respectively. Let $K$ be the incenter of triangle $ADC$. Suppose that $\angle BEK = 45^o$. Find all possible values of $\angle BAC$.

Let $ABC$ be a triangle with circumcenter $O$. The points $P$ and $Q$ are interior points of the sides $CA$ and $AB$, respectively. The circle $k $ passes through the midpoints of the segments $BP, CQ$, and $PQ$. Prove that if the line $PQ $ is tangent to circle $ k$ then $ OP = OQ$.

Point $P$ lies inside triangle $ABC$. Lines $AP, BP, CP$ meet the circumcircle of $ABC$ again at points $K, L, M$, respectively. The tangent to the circumcircle at $C$ meets line $AB$ at $S$. Prove that $SC = SP$ if and only if $MK = ML$.

Let $I$ be the incenter of a triangle $ABC$ and $ \Gamma $ be its circumcircle. Let the line $AI$ intersect $ \Gamma$ at a point $D \ne A$. Let $F$ and $E$ be points on side $BC$ and arc $BDC$ respectively such that $\angle BAF = \angle CAE < \frac{1}{2} \angle BAC$. Finally, let $G $ be the midpoint of the segment $IF$. Prove that the lines $DG$ and $EI$ intersect on $\Gamma$.

The IMO Compendium A Collection of Problems Suggested for The International Mathematical-Olympiads 1959-2009, 2nd Edition

aops links are in the names

__1959 - 2018__

Construct a right triangle
with given hypotenuse c such that the median drawn to the hypotenuse is the
geometric mean of the two legs of the triangle.

An arbitrary point M is selected in the interior of
the segment AB. The squares AMCD and MBEF are constructed on the same side of
AB, with the segments AM and MB as their respective bases. The circles
circumscribed about these squares, with centers P and Q, intersect at M and
also at another point N. Let N΄ denote the
point of intersection of the straight lines AF and BC.

a) Prove that the points N and N΄ coincide.

b) Prove that the straight lines MN pass through a
fixed point S independent of the choice of M.

c) Find the locus of the midpoints of the segments PQ
as M varies between A and B.

by Cezar Cosnita

Two planes, P and Q, intersect
along the line p. The point A is given in the plane P, and the point C in the
plane Q, neither of these points lies on the straight line p: Construct an
isosceles trapezoid ABCD (with AB parallel to CD) in which a circle can be
inscribed, and with vertices B and D lying in the planes P and Q respectively.

In a given right triangle ABC, the hypotenuse BC, of
length a, is divided into n equal parts (n an odd integer). Let ω be the acute angle subtending, from A, that segment which contains the
midpoint of the hypotenuse. Let h be the length of the altitude to the
hypotenuse of the triangle. Prove $\tan \omega =\frac{4nh}{\left( {{n}^{2}}-1
\right)\alpha }$

by Gheorghe D. Simionescu

by Gheorghe D. Simionescu

Construct triangle ABC, given h

_{a}, h_{b}(the altitudes from A and B) and m_{a}, the median from vertex A.
Consider the cube ABCDA'B'C'D' (with
face ABCD directly above face A'B'C'D').

a) Find the locus of the
midpoints of segments XY , where X is any point of AC and Y is any point of
B'D'.

b) Find the locus of points Z
which lie on the segments XY of part (a) such that ΖΥ = 2 ΧΖ .

a) On the axis of symmetry of
this trapezoid, find all points P such that both legs of
the trapezoid subtend right angles at P.

b) Calculate the distance of
P from either base.

c) Determine under what
conditions such points P actually exist. (Discuss various cases that might
arise).

Consider triangle P

_{1}P_{2}P_{3}and a point P within the triangle. Lines P_{1}P , P_{2}P , P_{3}P intersect the opposite sides in points Q_{1}, Q_{2}, Q_{3}respectively. Prove that, of the numbers $\frac{{{P}_{1}}P}{P{{Q}_{1}}},\frac{{{P}_{2}}P}{P{{Q}_{2}}},\frac{{{P}_{3}}P}{P{{Q}_{3}}}$_{ }at least one is ≤ 2 and at least one is ≥ 2.
Construct triangle ABC if AC = b, AB = c and <AMB =
ω, where M is the midpoint of segment BC and ω < 90

^{o}. Prove that a solution exists if and only if $b\varepsilon \varphi \frac{\omega }{2}\le c<b$ . In what case does the equality hold?
Consider a plane ε and three non-collinear points A, B, C on the
same side of ε, suppose the plane determined by these three points is not
parallel to ε. In plane a take three arbitrary points A΄, B΄, C΄. Let L, M, N
be the midpoints of segments AA΄, BB΄, CC΄΄, let G be the centroid of triangle LMN. (We will not consider positions of
the points A΄, B΄, C΄such that the points L, M, N do not form a triangle.) What is the locus of
point G as A΄, B΄, C΄. range independently over the plane ε ?

by Gheorghe D. Simionescu

Consider the cube ABCDA΄B΄C΄D΄ (ABCD and A΄B΄C΄D΄ are the
upper and lower bases, respectively, and edges AA΄, BB΄, CC΄, DD΄ are
parallel). The point X moves at constant speed along the perimeter of the
square ABCD in the direction ABCDA, and the point Y moves at the same rate
along the perimeter of the square B΄C΄CB in the
direction B΄C΄CB΄ Β. Points X
and Y begin their motion at the same instant from the starting positions A and
B΄, respectively. Determine and draw the locus of the midpoints of the
segments XY.

On the circle K there are
given three distinct points A,B,C. Construct (using only straightedge and
compasses) a fourth point D on K such that a circle can be inscribed in the
quadrilateral thus obtained.

Consider an isosceles
triangle. Let r be the radius of its circumscribed circle and ρ the radius
of its inscribed circle. Prove that the distance d between the centers of these
two circles is

The tetrahedron SABC has the
following property: there exist five spheres, each tangent to the edges SA, SB,
SC, BC, CA, AB or to their extensions.

(a) Prove that the tetrahedron
SABC is regular.

(b) Prove conversely that for
every regular tetrahedron five such spheres exist.

Point Α and segment BC are given. Determine the locus
of points in space which are vertices of right angles with one side passing
through Α, and the other side intersecting the segment BC.

In an n-gon all of whose interior angles are equal,
the lengths of consecutive sides satisfy the relation α

_{1}≥ α_{2}≥ ... ≥ α_{n}. Prove that α_{1}= α_{2}= ... = α_{n}.
A circle is inscribed in
triangle ABC with sides a, b, c, Tangents to the circle parallel to the sides
of the triangle are constructed. Each of these tangents cuts off a triangle
from ∆ ABC. In each of these triangles, a circle is inscribed. Find the sum of
the areas of all four inscribed circles (in terms of a, b, c).

In tetrahedron ABCD, vertex D
is connected with D

_{o}the centroid of ∆ ABC. Lines parallel to D D_{o}are drawn through A, B and C: These lines intersect the planes BCD, CAD and ABD in points A_{1}, B_{1}and C_{1}, respectively. Prove that the volume of ABCD is one third the volume of A_{1}B_{1}C_{1}D_{o}. Is the result true if point D_{o}is selected anywhere within ∆ ABC
Given the tetrahedron ABCD
whose edges AB and CD have lengths a and b respectively. The distance between
the skew lines AB and CD is d, and the angle between them is ω.
Tetrahedron ABCD is divided into two solids by plane ε, parallel
to lines AB and CD. The ratio of the distances of ε from AB and
CD is equal to k. Compute the ratio of the volumes of the two solids obtained.

Consider ∆ OAB with acute
angle AOB: Through a point M ≠ O perpendiculars are drawn to OA and OB, the
feet of which are P and Q respectively. The point of intersection of the
altitudes of ∆ OPQ is H. What is the locus of H if M is permitted to range over
(a) the side AB, (b) the interior of ∆ OAB?

by Gheorghe D. Simionescu

Prove: The sum of the
distances of the vertices of a regular tetrahedron from the center of its
circumscribed sphere is less than the sum of the distances of these vertices
from any other point in space.

In the interior of sides BC,
CA, AB of triangle ABC, any points K, L, M
respectively, are selected. Prove that the area of at least one of the
triangles AML, BKM, CLK is less than or equal to one quarter of the area of
triangle ABC.

Let ABCD be a parallelogram with side lengths AB = α, AD = 1, and with <BAD = ω. If ∆ABD is acute, prove that the four circles of
radius 1 with centers A,B,C,D cover the parallelogram if and only if $a\le \cos \omega +\sqrt{3}\sin \omega$

Prove that if one and only one edge of a tetrahedron
is greater than 1, then its
volume is ≤ 1 / 8.

Let A

_{o}B_{o}C_{o}and A_{1}B_{1}C_{1 }be any two acute-angled triangles. Consider all triangles ABC that are similar to ∆ A_{1}B_{1}C_{1}(so that vertices A_{1},B_{1},C_{1}correspond to vertices A, B, C respectively) and circumscribed about triangle A_{o}B_{o}C_{o}(where A_{o }lies on BC, B_{o}on CA, and C_{o}on AB). Of all such possible triangles, determine the one with maximum area, and construct it.by Tullio Viola

Prove that in every tetrahedron there is a vertex such
that the three edges meeting there have lengths which are the sides of a
triangle.

For each value of k =1,2,3,4,5 find
necessary and sufficient conditions on the number a > 0 so that there exists a tetrahedron with k edges of length a, and the remaining 6-k edges of length 1.

A semicircular arc γ is drawn on AB as diameter. C is a point on γ other
than A and B, and D is the
foot of the perpendicular from C to
AB. We consider three circles, γ1, γ2, γ3, all
tangent to the line ΑΒ. Of these, γ1 is
inscribed in ∆ ABC, while γ2 and γ3 are both tangent to CD
and to γ, one on each
side of CD . Prove that γ1, γ2 and γ3 have
a second tangent in common.

Let M be
a point on the side AB of ∆ABC. Let r

_{1}, r_{2}and r be the radii of the inscribed circles of triangles AMC,BMC and ABC. Let q_{1}, q_{2 }and q be the radii of the excircles of the same triangles that lie in the angle ACB. Prove that $\frac{{{r}_{1}}}{{{q}_{1}}}\cdot \frac{{{r}_{2}}}{{{q}_{2}}}=\frac{r}{q}$
In the tetrahedron ABCD, angle BDC is a right angle. Suppose that
the foot H of the perpendicular
from D to the plane ABC is the intersection of the
altitudes of ∆ABC. Prove that (AB + BC + CA)

^{2}≤ 6 (AD^{2}+ BD^{2}+ CD^{2}). For what tetrahedra does equality hold?
Consider a convex polyhedron P

_{1}with nine vertices A_{1}A_{2}, ... ,A_{9}. Let P_{i}be the polyhedron obtained from P_{1}by a translation that moves vertex A_{1}to A_{i}(i = 2, 3, ... , 9): Prove that at least two of the polyhedra P_{1},P_{2}, … , P_{9}have an interior point in common.
All the faces of tetrahedron ABCD are acute-angled
triangles. We consider all closed polygonal paths of the form XYZTX defined as
follows: X is a point on edge AB distinct from A and B; similarly, Y,Z, T are
interior points of edges BC,CD,DA respectively. Prove:

a) If <DAB + <BCD ≠ <CDA + <ABC ,then
among the polygonal paths, there is none of minimal length.

b) If <DAB + <BCD = <CDA + <ABC, then
there are infinitely many shortest polygonal paths, their common length being
2AC sin(a/2), where a = <BAC + <CAD + <DAB.

Prove that if n
≥ 4, every quadrilateral
that can be inscribed in a circle can be dissected into n quadrilaterals each of which is inscribable in a circle.

Given four distinct parallel planes, prove that there
exists a regular tetrahedron with a vertex on each plane.

Determine whether or not there exists a finite set M of points in space not lying in the
same plane such that, for any two points A
and B of M, one can select two other points C and D of M so that
lines AB and CD are parallel and not coincident.

A soldier needs to check on the presence of mines in a
region having the shape of an equilateral triangle. The radius of action of his
detector is equal to half the altitude of the triangle. The soldier leaves from
one vertex of the triangle. What path shouid he follow in order to travel the
least possible distance and still accomplish his mission?

by Ðorde Dugošija

On the sides of an arbitrary triangle ABC triangles
ABR, BCP, CAQ are constructed
externally with <CBP = <CAQ = 45

^{0}, <BCP = <ACQ = 30^{0}, <ABR = <BAR = 15^{0}_{ }. Prove that <QRP = 90^{0}and QR = RP.by Jan van de Craats

In a plane convex quadrilateral of area 32, the
sum of the lengths of two opposite sides and one diagonal is 16. Determine all possible lengths of
the other diagonal.

Equilateral triangles ABK,BCL,CDM,DAN are constructed inside the square ABCD: Prove that the midpoints of the
four segments KL,LM,MN,NK and
the midpoints of the eight segments AK,
BK,BL,CL,CM,DM,DN,AN are the twelve vertices of a regular dodecagon.

by Jan van de Craats

P is a given point inside a given sphere. Three mutually perpendicular
rays from P intersect the
sphere at points U, V, and W, Q denotes the vertex
diagonally opposite to P in the
parallelepiped determined by PU, PV, and
PW. Find the locus of Q for all such triads of rays from P.

by Murray Klamkin

In triangle ABC,
AB = AC. A circle is
tangent internally to the circumcircle of triangle ABC and also to sides AB,
AC at P, Q, respectively. Prove that the midpoint of segment PQ is the center of the incircle of
triangle ABC.

by Murray Klamkin

Two circles in a plane intersect. Let A be one of the points of
intersection. Starting simultaneously from A two points move with constant speeds, each point travelling
along its own circle in the same sense. The two points return to A
simultaneously after one revolution. Prove that there is a fixed point P in the plane such that, at any
time, the distances from P to the
moving points are equal.

by Nikolai Vasil'ev and Igor F. Sharygin

Given a plane π, a point P in this plane
and a point Q not in π, find all points R in π such that the ratio $\frac{QP+PR}{QR}$ is a maximum.

by Murray Klamkin

P is a point inside a given triangle ABC. D,E, F are the feet of the perpendiculars from P to the lines BC, CA, AB respectively. Find all P for which $\frac{BC}{PD}+\frac{CA}{PE}+\frac{AB}{PF}$ is least.

by David Monk

Three congruent circles have a common point O and lie inside a given triangle.
Each circle touches a pair of sides of the triangle. Prove that the incenter
and the circumcenter of the triangle and the point O are collinear.

A non-isosceles triangle A

_{1}A_{2}A_{3}is given with sides a_{1}, a_{2}, a_{3}(a_{i}is the side opposite Ai). For all i = 1, 2, 3, M_{i}is the midpoint of side a_{i}, and T_{i}is the point where the incircle touches side a_{i}. Denote by S_{i }the reflection of Ti in the interior bisector of angle A_{i}. Prove that the lines M_{1}S_{1}, M_{2}S_{2}and M_{3}S_{3 }are concurrent.by Jan van de Craats

The diagonals AC
and CE of the regular
hexagon ABCDEF are divided by
the inner points M and N, respectively, so that $\frac{AM}{AC}=\frac{CN}{CE}=r$. Determine r if B,M, and N are collinear.

by Jan van de Craats

Let A be one of the two
distinct points of intersection of two unequal coplanar circles C

_{1}and C_{2}with centers O_{1 }and O_{2}, respectively. One of the common tangents to the circles touches C_{1 }at P_{1}and C_{2 }at P_{2}, while the other touches C_{1 }at Q_{1 }and C_{2 }at Q_{2}. Let M_{1}be the midpoint of P_{1}Q_{1 }and M_{2 }be the midpoint of P_{2}Q_{2}. Prove that <O_{1}AO_{2 }= <M_{1}AM_{2}.by Igor F. Sharygin

Let ABC be
an equilateral triangle and E the
set of all points contained in the three segments AB,BC and CA (including
A, B and C). Determine whether, for every
partition of E into two
disjoint subsets, at least one of the two subsets contains the vertices of a
right-angled triangle. Justify your answer.

Let ABCD be
a convex quadrilateral such that the line CD is a tangent to the circle on AB as diameter. Prove that the line AB is a tangent to the circle on CD as diameter if and only if the lines BC and AD are
parallel.

by Laurentiu Panaitopol

A circle has center on the side AB of the cyclic quadrilateral ABCD. The other three sides are
tangent to the circle. Prove that AD +
BC = AB.

by Frank Budden

A circle with center O passes through the vertices A and C of
triangle ABC and intersects the
segments AB and BC again at distinct points K and N respectively. The circumscribed circles of the triangles ABC and EBN intersect at exactly two distinct points B and M. Prove that angle <OMB
is a right angle.

by Igor F. Sharygin

A triangle A

_{1}A_{2}A_{3}and a point P_{0}are given in the plane. We define A_{s}= A_{s-}_{3}for all s ≥ 4. We construct a set of points P_{1}, P_{2}, P_{3}, . . . , such that P_{k}_{+1}is the image of P_{k}under a rotation with center A_{k}_{+1}through angle 120^{o}clockwise (for k = 0, 1, 2, . . . ). Prove that if P_{1986}= P_{0}, then the triangle A_{1}A_{2}A_{3 }is equilateral.by Gengzhe Chang and Dongxu Qi

Let A,
B be adjacent vertices of a
regular n-gon (n ≥ 5) in the plane having center at O. A triangle XY Z, which is congruent to and initially coincides with OAB, moves in the plane in such a way
that Y and Z each trace out the whole boundary
of the polygon, X remaining
inside the polygon. Find the locus of X.

by Sven Sigurðsson

In an acute-angled triangle ABC the interior bisector of the angle A intersects BC at
L and intersects the
circumcircle of ABC again at N. From point L perpendiculars are drawn to AB and AC, the
feet of these perpendiculars being K and
M respectively. Prove that the
quadrilateral AKNM and the
triangle ABC have equal areas.

by I.A. Kushnir

Consider two concentric circles of radii R and r (R > r)
with the same center. Let P be
a fixed point on the smaller circle and B
a variable point on the larger circle. The line BP meets the larger circle again at C. The perpendicular l to
BP at P meets the smaller circle again at A. (If l is
tangent to the circle at P then
A ≡ P.)

(i) Find the set of values of BC

_{}^{2}+ CA_{}^{2}+ AB_{}^{2}.
(ii) Find the locus of the midpoint of BC.

by Lucien Kieffer

ABC is a triangle right-angled at A, and D is the
foot of the altitude from A.
The straight line joining the incenters of the triangles ABD, ACD intersects the sides AB,
AC at the points K, L respectively. S and
T denote the areas of the
triangles ABC and AKL respectively. Show that S ≥ 2T.

by Dimitris Kontogiannis

In an acute-angled triangle ABC the internal bisector of angle A meets the circumcircle of the triangle again at A1. Points B

_{1}and C_{1 }are defined similarly. Let A_{0}be the point of intersection of the line AA_{1}with the external bisectors of angles B and C. Points B_{0}and C_{0}are defined similarly. Prove that:
(i) The area of the triangle A

_{0}B_{0}C_{0}is twice the area of the hexagon AC_{1}BA_{1}CB_{1}.
(ii) The area of the triangle A

_{0}B_{0}C_{0}is at least four times the area of the triangle ABC.by Esther Szekeres

Let ABCD be
a convex quadrilateral such that the sides AB, AD, BC satisfy AB = AD + BC. There exists a point P inside the quadrilateral at a
distance h from the line CD such that AP
= h + AD and BP = h + BC. Show that: $\frac{1}{\sqrt{h}}\ge
\frac{1}{\sqrt{AD}}+\frac{1}{\sqrt{BC}}$.

by Eggert Briem

Chords AB and
CD of a circle intersect at a
point E inside the circle. Let M be an interior point of the segment
EB. The tangent line at E to the circle through D, E, and M intersects
the lines BC and AC at F and G,
respectively. If $\frac{AM}{AB}=t$ ,
find $\frac{EF}{EG}$ in terms
of t.

by C.R. Pranesachar

Given a triangle ABC, let I be
the center of its inscribed circle. The internal bisectors of the angles A,B,C meet the opposite sides in A’, B’ , C’ respectively. Prove that $\frac{1}{4}<\frac{AI\cdot
BI\cdot CI}{AA'\cdot BB'\cdot CC'}\le \frac{8}{27}$

by Arkadii Skopenkov

Let ABC be
a triangle and P an interior
point of ABC . Show that at
least one of the angles <PAB, <PBC, <PCA is less than or equal
to 30

^{0}.by Johan Yebbou

In the plane let C be a circle, L a
line tangent to the circle C and M a point on L.
Find the locus of all points P with
the following property: there exists two
points Q,R on L such that M is the midpoint of QR and C is the inscribed circle of triangle PQR.

by Johan Yebbou

1993 IMO Shortlist ISL15 (MKD 1) problem 4

For three points $A,B,C$ in the plane we define $m(ABC)$ to be the smallest length of the three altitudes of the triangle $ABC$, where in the case of $A,B,C$ collinear, $m(ABC)=0$. Let $A,B,C$ be given points in the plane. Prove that for any point $X$ in the plane, $m(ABC) \le m(ABX)+m(AXC)+m(XBC)$.

$A,B,C,D$ are four points in the plane, with $C,D$ on the same side of the line $AB$, such that $AC\cdot BD = AD\cdot BC$ and $\angle ADB = 90^o +\angle ACB$. Find the ratio $\frac{AB\cdot CD}{ AC\cdot BD }$, and prove that circles $ACD,BCD$ are orthogonal.

(Intersecting circles are said to be orthogonal if at either common point their tangents are perpendicular.)

$N$ is an arbitrary point on the bisector of $\angle BAC$. $P$ and $O$ are points on the lines $AB$ and $AN$, respectively, such that $\angle ANP=90^o =\angle APO$. $Q$ is an arbitrary point on $NP$, and an arbitrary line through $Q$ meets the lines $AB$ and $AC$ at $E$ and $F$ respectively. Prove that $\angle OQE= 90^o$ if and only if $QE=QF$.

For three points $A,B,C$ in the plane we define $m(ABC)$ to be the smallest length of the three altitudes of the triangle $ABC$, where in the case of $A,B,C$ collinear, $m(ABC)=0$. Let $A,B,C$ be given points in the plane. Prove that for any point $X$ in the plane, $m(ABC) \le m(ABX)+m(AXC)+m(XBC)$.

by D. Dimovski

1993 IMO Shortlist ISL22 (UNK 2) problem 2$A,B,C,D$ are four points in the plane, with $C,D$ on the same side of the line $AB$, such that $AC\cdot BD = AD\cdot BC$ and $\angle ADB = 90^o +\angle ACB$. Find the ratio $\frac{AB\cdot CD}{ AC\cdot BD }$, and prove that circles $ACD,BCD$ are orthogonal.

(Intersecting circles are said to be orthogonal if at either common point their tangents are perpendicular.)

by David Monk

1994 IMO Shortlist G4 (AUS-ARM) problem 2$N$ is an arbitrary point on the bisector of $\angle BAC$. $P$ and $O$ are points on the lines $AB$ and $AN$, respectively, such that $\angle ANP=90^o =\angle APO$. $Q$ is an arbitrary point on $NP$, and an arbitrary line through $Q$ meets the lines $AB$ and $AC$ at $E$ and $F$ respectively. Prove that $\angle OQE= 90^o$ if and only if $QE=QF$.

by H. Lausch & G. Tonoyan

1995 IMO Shortlist G1 (BGR) problem 1

Let $A,B,C$, and $D$ be distinct points on a line, in that order. The circles with diameters $AC$ and $BD$ intersect at $X$ and $Y$. $O$ is an arbitrary point on the line $XY$ but not on $AD$. $CO$ intersects the circle with diameter $AC$ again at $M$, and $BO$ intersects the other circle again at $N$. Prove that the lines $AM,DN$, and $XY$ are concurrent.

Let $ABCDEF$ be a convex hexagon with $AB = BC =CD, DE = EF = FA$, and $ \angle BCD = \angle EFA = \pi /3$ (that is, $60^o$). Let $G$ and $H$ be two points interior to the hexagon such that angles $AGB$ and $DHE$ are both $2\pi /3$ (that is,$120^o$). Prove that $AG+GB+GH+DH +HE \ge CF$.

Let $A,B,C$, and $D$ be distinct points on a line, in that order. The circles with diameters $AC$ and $BD$ intersect at $X$ and $Y$. $O$ is an arbitrary point on the line $XY$ but not on $AD$. $CO$ intersects the circle with diameter $AC$ again at $M$, and $BO$ intersects the other circle again at $N$. Prove that the lines $AM,DN$, and $XY$ are concurrent.

by B. Mihailov

1995 IMO Shortlist G5 (NZL) problem 5Let $ABCDEF$ be a convex hexagon with $AB = BC =CD, DE = EF = FA$, and $ \angle BCD = \angle EFA = \pi /3$ (that is, $60^o$). Let $G$ and $H$ be two points interior to the hexagon such that angles $AGB$ and $DHE$ are both $2\pi /3$ (that is,$120^o$). Prove that $AG+GB+GH+DH +HE \ge CF$.

by A. McNaughton

1996 IMO Shortlist G2 (CAN) problem 2

Let $P$ be a point inside △ABC such that $\angle APB - \angle C = \angle APC- \angle B$. Let $D,E$ be the incenters of $\triangle APB,\triangle APC$ respectively. Show that $AP,BD$ and $CE$ meet in a point.

by J.P. Grossman

1996 IMO Shortlist G5 (ARM) problem 5

Let $ABCDEF$ be a convex hexagon such that $AB$ is parallel to $DE, BC$ is parallel to $EF$, and $CD$ is parallel to $AF$. Let $R_A,R_C,R_E$ be the circumradii of triangles $FAB,BCD,DEF$ respectively, and let $P$ denote the perimeter of the hexagon. Prove that $R_A+R_C+R_E \ge \frac{P}{2}$.

by N.M. Sedrakyan

1997 IMO Shortlist ISL8 (UNK) problem 2

In triangle $ABC$ the angle at $A $ is the smallest. A line through $A$ meets the circumcircle again at the point $U$ lying on the arc $BC$ opposite to $A$.

The perpendicular bisectors of $CA$ and $AB$ meet $AU$ at $V$ and $W$, respectively, and the lines $CV,BW$ meet at $T$. Show that $AU = TB+TC$.

__Original formulation__

Four different points $A,B,C,D$ are chosen on a circle $G$ such that the triangle $BCD$ is not right-angled. Prove that:

a) The perpendicular bisectors of $AB$ and $AC$ meet the line $AD$ at certain points $W$ and $V$, respectively, and that the lines $CV$ and $BW$ meet at a certain point $T$.

b) The length of one of the line segments $AD, BT$, and $CT$ is the sum of the lengths of the other two.

by David Monk

1998 IMO Shortlist ISL1 (LUX) problem 1

A convex quadrilateral $ABCD$ has perpendicular diagonals. The perpendicular bisectors of $AB$ and $CD$ meet at a unique point $P$ inside $ABCD$. Prove that $ABCD$ is cyclic if and only if triangles $ABP$ and $CDP$ have equal areas.

by Charles Leytem

1998 IMO Shortlist ISL3 (UKR) problem 5

Let $I$ be the incenter of triangle $ABC$. Let $K, L$, and $M$ be the points of tangency of the incircle of $ABC$ with $AB, BC$, and $CA$, respectively. The line $t$ passes through $B$ and is parallel to $KL$. The lines $MK$ and $ML$ intersect $t$ at the points $R$ and $S$. Prove that $\angle RIS$ is acute.

by V. Yasinskiy

1999 IMO Shortlist G3 (EST) problem 1

A set $S$ of points in space will be called completely symmetric if it has at least three elements and satisfies the following condition: For every two distinct points $A,B$ from S the perpendicular bisector of the segment $AB$ is an axis of symmetry for $S$. Prove that if a completely symmetric set is finite, then it consists of the vertices of either a regular polygon, a regular tetrahedron, or a regular octahedron.

Two circles $\Omega_1$ and $\Omega_2$ touch internally the circle $\Omega$ in $M$ and $N$, and the center of $\Omega_2$ is on $\Omega_1$. The common chord of the circles $\Omega_1$ and $\Omega_2$ intersects $\Omega$ in $A$ and $B$. $MA$ and $MB$ intersect $\Omega_1$ in $C$ and $D$. Prove that $\Omega_2$ is tangent to $CD$.

A set $S$ of points in space will be called completely symmetric if it has at least three elements and satisfies the following condition: For every two distinct points $A,B$ from S the perpendicular bisector of the segment $AB$ is an axis of symmetry for $S$. Prove that if a completely symmetric set is finite, then it consists of the vertices of either a regular polygon, a regular tetrahedron, or a regular octahedron.

by Jan Villemson

1999 IMO Shortlist G6 (RUS) problem 5Two circles $\Omega_1$ and $\Omega_2$ touch internally the circle $\Omega$ in $M$ and $N$, and the center of $\Omega_2$ is on $\Omega_1$. The common chord of the circles $\Omega_1$ and $\Omega_2$ intersects $\Omega$ in $A$ and $B$. $MA$ and $MB$ intersect $\Omega_1$ in $C$ and $D$. Prove that $\Omega_2$ is tangent to $CD$.

by P. Kozhevnikov

2000 IMO Shortlist G2 (RUS) problem 1
Two circles $G_1$ and $G_2$ intersect at $M$ and $N$. Let $AB$ be the line tangent to these circles at $A$ and $B$, respectively, such that $M$ lies closer to $AB$ than N. Let $CD$ be the line parallel to $AB$ and passing through $M$, with $C$ on $G_1$ and $D$ on $G_2$. Lines $AC$ and $BD$ meet at $E$, lines $AN$ and $CD$ meet at $P$; lines $BN$ and $CD$ meet at $Q$. Show that $EP = EQ$.

by Sergey Berlov

2000 IMO Shortlist G8 (RUS) problem 6

$A_1A_2A_3$ is an acute-angled triangle. The foot of the altitude from $A_i$ is $K_i$, and the incircle touches the side opposite $A_i $ at $L_i$. The line $K_1K_2$ is reflected in the line $L_1L_2$. Similarly, the line K2K3 is reflected in $L_2L_3$, and $K_3K_1$ is reflected in $L_3L_1$. Show that the three new lines form a triangle with vertices on the incircle.

by L. Emelyanov, T. Emelyanova

2001 IMO Shortlist G2 (KOR) problem 1

In acute triangle $ABC$ with circumcenter $O$ and altitude $AP, \angle C \ge \angle B + 30^\circ$. Prove that $\angle A + \angle COP < 90^\circ$.

by Hojoo Lee

2001 IMO Shortlist G8 (ISR) problem 5

Let $ABC$ be a triangle with $\angle BAC = 60^\circ$. Let $AP$ bisect $\angle BAC$ and let $BQ$ bisect $\angle ABC$, with $P$ on $BC$ and $Q$ on $AC$. If $AB +BP = AQ+QB$, what are the angles of the triangle?

by Shay Gueron

2002 IMO Shortlist G3 (KOR) problem 2

The circle $S$ has centre $O$, and $BC$ is a diameter of $S$. Let $A$ be a point of $S$ such that $\angle AOB < 120^\circ$. Let $D$ be the midpoint of the arc $AB$ which does not contain $C$. The line through $O$ parallel to $DA$ meets the line $AC$ at I. The perpendicular bisector of $OA$ meets $S$ at $E$ and at $F$. Prove that $I$ is the incentre of the triangle $CEF$.

by Hojoo Lee

2002 IMO Shortlist G6 (UKR) problem 6

Let $n \ge 3$ be a positive integer. Let $C_1, C_2, C_3, … , C_n$ be unit circles in the plane, with centres $O_1, O_2, O_3, … , O_n$ respectively. If no line meets more than two of the circles, prove that $ \sum_{1\le I < j \le n} \frac{1}{O_ i O_ j} \le frac{(n- 1) \pi}{4}$.

by V. Yasinskiy

2003 IMO Shortlist G1 (FIN) problem 4

Let $ABCD$ be a cyclic quadrilateral. Let $P , Q, R$ be the feet of the perpendiculars from $D$ to the lines $BC, CA, AB$, respectively. Show that $PQ = QR$ if and only if the bisectors of $\angle ABC$ and $\angle ADC$ are concurrent with $AC$.

by Matti Lehtinen

2003 IMO Shortlist G6 (POL) problem 3

Each pair of opposite sides of a convex hexagon has the following property:

the distance between their midpoints is equal to $ \sqrt{3} / 2 $ times the sum of their lengths. Prove that all the angles of the hexagon are equal.

by Waldemar Pompe

2004 IMO Shortlist G1 (ROU) problem 1

Let $ABC$ be an acute-angled triangle with $AB \ne AC$. The circle with diameter $BC$ intersects the sides $AB$ and $AC$ at $M$ and $N$ , respectively. Denote by $O$ the midpoint of $BC$. The bisectors of the angles $BAC$ and $MON$ intersect at $R$. Prove that the circumcircles of the triangles $BMR$ and $CNR$ have a common point lying on the line segment $BC$.
by D. Serbanescu & V. Vornicu

2004 IMO Shortlist G4 (POL) problem 5
In a convex quadrilateral $ABCD$ the diagonal $BD$ does not bisect the angles $ABC$ and $CDA$. The point $P$ lies inside $ABCD$ and satisfies $\angle PBC = \angle DBA$ and $\angle PDC = \angle BDA$. Prove that $ABCD$ is a cyclic quadrilateral if and only if $AP = CP$ .

2005 IMO Shortlist G2 (ROU) problem 1
by Waldemar Pompe

Six points are chosen on the sides of an equilateral triangle $ABC$: $A_1, A_2$ on $BC, B_1, B_2$ on $CA$, and $C_1, C_2$ on $AB$, so that they are the vertices of a convex hexagon $A_1A_2B_1B_2C_1C_2$ with equal side lengths. Prove that the lines $A_1B_2, B_1C_2$ and $C_1A_2$ are concurrent.

by Bogdan Enescu

2005 IMO Shortlist G4 (POL) problem 5
Let$ ABCD$ be a fixed convex quadrilateral with $BC = DA$ and $BC$ not parallel to $DA$. Let two variable points $E$ and $F$ lie on the sides $BC$ and $DA$, respectively, and satisfy $BE = DF$ . The lines $AC$ and $BD$ meet at $P$ , the lines $BD$ and $EF$ meet at $Q$, the lines $EF$ and $AC$ meet at $R$. Prove that the circumcircles of triangles $PQR$, as $E$ and $F$ vary, have a common point other than $P$ .

by Waldemar Pompe

2006 IMO Shortlist G1 (KOR) problem 1
Let $ABC$ be a triangle with incentre $I$. A point $P$ in the interior of the triangle satisfies $\angle PBA + \angle PCA = \angle PBC + \angle PCB$. Show that $AP \ge AI$ and that equality holds if and only if $P$ coincides with $I$.

by Hojoo Lee

2006 IMO Shortlist G10 (SRB) problem 6

To each side $a$ of a convex polygon we assign the maximum area of a triangle contained in the polygon and having $a$ as one of its sides. Show that the sum of the areas assigned to all sides of the polygon is not less than twice the area of the polygon.

by Dušan Ðukic

2007 IMO Shortlist G1 (CZE) problem 4In triangle $ABC$, the angle bisector at vertex $C$ intersects the circumcircle and the perpendicular bisectors of sides $BC$ and $CA$ at points $R, P$, and $Q$, respectively. The midpoints of $BC$ and $CA$ are $S$ and $T$, respectively. Prove that triangles $RQT$ and $RPS$ have the same area.

by Marek Pechal

2007 IMO Shortlist G4 (LUX) problem 2Consider five points $A, B, C, D, E$ such that $ABCD$ is a parallelogram and BCED is a cyclic quadrilateral. Let $\ell$ be a line passing through $A$, and let $\ell $ intersect segment $ DC$ and line $BC$ at points $F$ and $G$, respectively. Suppose that $EF = EG = EC$. Prove that $\ell$ is the bisector of angle $DAB$.

by Charles Leytem

2008 IMO Shortlist G1 (RUS) problem 1In an acute-angled triangle $ABC$, point $H$ is the orthocentre and $A_o, B_o, C_o$ are the midpoints of the sides $BC, CA, AB$, respectively. Consider three circles passing through $H$: $\omega_a$ around $A_o, \omega_b$ around $B_o$ and $\omega_c$ around $C_o$. The circle $\omega_a$ intersects the line $BC$ at $A_1$ and $A_2$; $\omega_b$ intersects$ CA$ at $B_1$ and $B_2$; $\omega_c$ intersects $AB$ at $C_1$ and $C_2$. Show that the points $A_1, A_2, B_1, B_2, C_1, C_2$ lie on a circle.

by A. Gavrilyuk

2008 IMO Shortlist G7 (RUS) problem 6Let $ABCD$ be a convex quadrilateral with $AB \ne BC$. Denote by $\omega_1$ and $\omega_2$ the incircles of triangles $ABC$ and $ADC$. Suppose that there exists a circle $ \omega$ inscribed in angle $ABC$, tangent to the extensions of line segments $AD$ and $CD$. Prove that the common external tangents of $\omega_1 $ and $\omega_2 $ intersect on $ \omega$.

by V. Shmarov

2009 IMO Shortlist G1 (BEL) problem 4Let ABC be a triangle with $AB = AC$. The angle bisectors of $A$ and $B$ meet the sides $BC$ and $AC$ in $D$ and $E$, respectively. Let $K$ be the incenter of triangle $ADC$. Suppose that $\angle BEK = 45^o$. Find all possible values of $\angle BAC$.

by H. Lee, P. Vandendriessche & J. Vonk,

2009 IMO Shortlist G2 (RUS) problem 2Let $ABC$ be a triangle with circumcenter $O$. The points $P$ and $Q$ are interior points of the sides $CA$ and $AB$, respectively. The circle $k $ passes through the midpoints of the segments $BP, CQ$, and $PQ$. Prove that if the line $PQ $ is tangent to circle $ k$ then $ OP = OQ$.

by Sergei Berlov

2010 IMO Shortlist G2 (POL) problem 4Point $P$ lies inside triangle $ABC$. Lines $AP, BP, CP$ meet the circumcircle of $ABC$ again at points $K, L, M$, respectively. The tangent to the circumcircle at $C$ meets line $AB$ at $S$. Prove that $SC = SP$ if and only if $MK = ML$.

by Marcin E. Kuczma

2010 IMO Shortlist G4 (HKG) problem 2Let $I$ be the incenter of a triangle $ABC$ and $ \Gamma $ be its circumcircle. Let the line $AI$ intersect $ \Gamma$ at a point $D \ne A$. Let $F$ and $E$ be points on side $BC$ and arc $BDC$ respectively such that $\angle BAF = \angle CAE < \frac{1}{2} \angle BAC$. Finally, let $G $ be the midpoint of the segment $IF$. Prove that the lines $DG$ and $EI$ intersect on $\Gamma$.

by Tai Wai Ming & Wang Chongli

2011 IMO Shortlist G8 (JPN) problem 6

Let $ABC$ be an acute triangle with circumcircle $\omega$. Let $t$ be a tangent line to $ \omega.$ Let $t_a, t_b$, and $t_c$ be the lines obtained by reflecting $t$ in the lines $BC, CA$, and $AB$, respectively. Show that the circumcircle of the triangle determined by the lines $t_a, t_b$, and $t_c$ is tangent to the circle $\omega$.

2012 IMO Shortlist G1 (HEL) problem 1

In the triangle $ABC$ the point $J$ is the center of the excircle opposite to $A$. This excircle is tangent to the side $BC$ at $M$, and to the lines $AB$ and $AC$ at $K$ and $L$ respectively. The lines $LM$ and $BJ$ meet at $F$, and the lines $KM$ and $CJ$ meet at $G$. Let $S$ be the point of intersection of the lines $AF$ and $BC$, and let $T$ be the point of intersection of the lines $AG$ and $BC$. Prove that $M$ is the midpoint of $ST$.

Let $ABC$ be a triangle with $\angle BCA = 90^o$, and let $C_0$ be the foot of the altitude from $C$. Choose a point $X$ in the interior of the segment $CC_0$, and let $K, L$ be the points on the segments $AX,BX$ for which $BK = BC$ and $AL = AC$ respectively. Denote by $M$ the intersection of $AL$ and $BK$. Show that $MK = ML$.

Let $ABC$ be an acute-angled triangle with orthocenter $H$, and let $W$ be a point on side $BC$. Denote by $M$ and $N$ the feet of the altitudes from $B$ and $C$, respectively. Denote by $\omega_1$ the circumcircle of $BWN$, and let $X$ be the point on $\omega_1$ which is diametrically opposite to $W$. Analogously, denote by $\omega_2$ the circumcircle of $CWM$, and let $Y$ be the point on $\omega_2$ which is diametrically opposite to $W$. Prove that $X, Y$ and $H$ are collinear.

Let the excircle of the triangle $ABC$ lying opposite to $A$ touch its side $BC$ at the point $A_1$. Define the points $B_1$ and $C_1$ analogously. Suppose that the circumcentre of the triangle $A_1B_1C_1$ lies on the circumcircle of the triangle $ABC$. Prove that the triangle $ABC$ is right-angled.

The points $P$ and $Q$ are chosen on the side $BC$ of an acute-angled triangle $ABC$ so that $\angle PAB =\angle ACB$ and $\angle QAC =\angle CBA$. The points $M$ and $N$ are taken on the rays $AP$ and $AQ$, respectively, so that $AP = PM$ and $AQ =QN$. Prove that the lines $BM$ and $CN$ intersect on the circumcircle of the triangle $ABC$.

Let $ABCD$ be a convex quadrilateral with $\angle B = \angle D =90^o$. Point H is the foot of the perpendicular from $A$ to $BD$. The points $S$ and $T$ are chosen on the sides $AB$ and $AD$, respectively, in such a way that $H$ lies inside triangle $SCT$ and $ \angle SHC - \angle BSC= 90^o , \angle THC - \angle DTC = 90^o$ . Prove that the circumcircle of triangle $SHT$ is tangent to the line $BD$.

Let $ABC$ be an acute triangle with circumcircle $\omega$. Let $t$ be a tangent line to $ \omega.$ Let $t_a, t_b$, and $t_c$ be the lines obtained by reflecting $t$ in the lines $BC, CA$, and $AB$, respectively. Show that the circumcircle of the triangle determined by the lines $t_a, t_b$, and $t_c$ is tangent to the circle $\omega$.

2012 IMO Shortlist G1 (HEL) problem 1

In the triangle $ABC$ the point $J$ is the center of the excircle opposite to $A$. This excircle is tangent to the side $BC$ at $M$, and to the lines $AB$ and $AC$ at $K$ and $L$ respectively. The lines $LM$ and $BJ$ meet at $F$, and the lines $KM$ and $CJ$ meet at $G$. Let $S$ be the point of intersection of the lines $AF$ and $BC$, and let $T$ be the point of intersection of the lines $AG$ and $BC$. Prove that $M$ is the midpoint of $ST$.

by Evangelos Psychas

2012 IMO Shortlist G5 (CZE) problem 5Let $ABC$ be a triangle with $\angle BCA = 90^o$, and let $C_0$ be the foot of the altitude from $C$. Choose a point $X$ in the interior of the segment $CC_0$, and let $K, L$ be the points on the segments $AX,BX$ for which $BK = BC$ and $AL = AC$ respectively. Denote by $M$ the intersection of $AL$ and $BK$. Show that $MK = ML$.

by Josef Tkadlec

2013 IMO Shortlist G1 (THA) problem 4Let $ABC$ be an acute-angled triangle with orthocenter $H$, and let $W$ be a point on side $BC$. Denote by $M$ and $N$ the feet of the altitudes from $B$ and $C$, respectively. Denote by $\omega_1$ the circumcircle of $BWN$, and let $X$ be the point on $\omega_1$ which is diametrically opposite to $W$. Analogously, denote by $\omega_2$ the circumcircle of $CWM$, and let $Y$ be the point on $\omega_2$ which is diametrically opposite to $W$. Prove that $X, Y$ and $H$ are collinear.

by Warut Suksompong & Potcharapol Suteparuk

2013 IMO Shortlist G6 (RUS) problem 3Let the excircle of the triangle $ABC$ lying opposite to $A$ touch its side $BC$ at the point $A_1$. Define the points $B_1$ and $C_1$ analogously. Suppose that the circumcentre of the triangle $A_1B_1C_1$ lies on the circumcircle of the triangle $ABC$. Prove that the triangle $ABC$ is right-angled.

by Alexander A. Polyansky

2014 IMO Shortlist G1 (GEO) problem 4The points $P$ and $Q$ are chosen on the side $BC$ of an acute-angled triangle $ABC$ so that $\angle PAB =\angle ACB$ and $\angle QAC =\angle CBA$. The points $M$ and $N$ are taken on the rays $AP$ and $AQ$, respectively, so that $AP = PM$ and $AQ =QN$. Prove that the lines $BM$ and $CN$ intersect on the circumcircle of the triangle $ABC$.

by Giorgi Arabidze

2014 IMO Shortlist G5 (IRN) problem 3Let $ABCD$ be a convex quadrilateral with $\angle B = \angle D =90^o$. Point H is the foot of the perpendicular from $A$ to $BD$. The points $S$ and $T$ are chosen on the sides $AB$ and $AD$, respectively, in such a way that $H$ lies inside triangle $SCT$ and $ \angle SHC - \angle BSC= 90^o , \angle THC - \angle DTC = 90^o$ . Prove that the circumcircle of triangle $SHT$ is tangent to the line $BD$.

2015 IMO Shortlist G2 (HEL) problem 4

Let $ABC$ be a triangle inscribed into a circle $\Omega$ with center $O$. A circle $\Gamma$ with center $A$ meets the side $BC$ at points $D$ and $E$ such that $D$ lies between $B$ and $E$. Moreover, let $F$ and $G$ be the common points of $\Gamma$ and $\Omega$. We assume that $F$ lies on the arc $AB$ of $\Omega$ not containing $C$, and $G$ lies on the arc $AC$ of $\Omega$ not containing $B$. The circumcircles of the triangles $BDF$ and $CEG$ meet the sides $AB$ and $AC$ again at $K$ and $L$, respectively. Suppose that the lines $FK$ and $GL$ are distinct and intersect at $X$. Prove that the points $A, X$, and $O$ are collinear.

Let $ABC$ be an acute triangle with $AB >AC$, and let $\Gamma$ be its circumcircle. Let $H, M$, and $F$ be the orthocenter of the triangle, the midpoint of $BC$, and the foot of the altitude from $A$, respectively. Let $Q$ and $K$ be the two points on $\Gamma$ that satisfy $ \angle AQH= 90^o$ and $\angle QKH= 90^o$. Prove that the circumcircles of the triangles $KQH$ and $KFM$ are tangent to each other.

Let $ABC$ be a triangle inscribed into a circle $\Omega$ with center $O$. A circle $\Gamma$ with center $A$ meets the side $BC$ at points $D$ and $E$ such that $D$ lies between $B$ and $E$. Moreover, let $F$ and $G$ be the common points of $\Gamma$ and $\Omega$. We assume that $F$ lies on the arc $AB$ of $\Omega$ not containing $C$, and $G$ lies on the arc $AC$ of $\Omega$ not containing $B$. The circumcircles of the triangles $BDF$ and $CEG$ meet the sides $AB$ and $AC$ again at $K$ and $L$, respectively. Suppose that the lines $FK$ and $GL$ are distinct and intersect at $X$. Prove that the points $A, X$, and $O$ are collinear.

by Evangelos Psychas & Silouanos Brazitikos

2015 IMO Shortlist G6 (UKR) problem 3Let $ABC$ be an acute triangle with $AB >AC$, and let $\Gamma$ be its circumcircle. Let $H, M$, and $F$ be the orthocenter of the triangle, the midpoint of $BC$, and the foot of the altitude from $A$, respectively. Let $Q$ and $K$ be the two points on $\Gamma$ that satisfy $ \angle AQH= 90^o$ and $\angle QKH= 90^o$. Prove that the circumcircles of the triangles $KQH$ and $KFM$ are tangent to each other.

2016 IMO Shortlist G1 (BEL) problem 1

In a convex pentagon $ABCDE$, let $F$ be a point on $AC$ such that $\angle FBC = 90^o$. Suppose triangles ABF, ACD and ADE are similar isosceles triangles with $\angle FAB = \angle FBA = \angle DAC = \angle DCA = \angle EAD = \angle EDA$. Let M be the midpoint of $CF$. Point $X$ is chosen such that $AMXE$ is a parallelogram. Show that $BD,EM$ and $FX$ are concurrent.

2017 IMO Shortlist G2 (LUX) problem 4

Let $R$ and $S$ be distint points on circle $\Omega$, and let $t$ denote the tangent line to $\Omega$ at $R$. Point $R' $ is the reflection of $R$ with respect to $S$. A point $I$ is chosen on the smaller arc $RS$ of $\Omega$ so that the circumcircle $\Gamma$ of triangle $ ISR' $ intersects $t$ at two different points. Denote by $A$ the common point of $ \Gamma$ and $t$ that is closest to $R$. Line $AI$ meets $\Omega$ again at $J$. Show that $JR' $ is tangent to $ \Gamma$.

by Charles Leytem

2018 IMO problem 1 (HEL)

Let $\Gamma$ be the circumcircle of acute triangle $ABC$. Points $D$ and $E$ are on segments $AB$ and $AC$ respectively such that $AD = AE$. The perpendicular bisectors of $BD$ and $CE$ intersect minor arcs $AB$ and $AC$ of $\Gamma$ at points $F$ and $G$ respectively. Prove that lines $DE$ and $FG$ are either parallel or they are the same line.

by Silouanos Brazitikos, Vangelis Psyxas & Michael Sarantis

2018 IMO problem 6 (POL)

A convex quadrilateral $ABCD$ satisfies $AB\cdot CD = BC\cdot DA$. Point $X$ lies inside $ABCD$ so that $\angle{XAB} = \angle{XCD}\quad\,\,\text{and}\quad\,\,\angle{XBC} = \angle{XDA}.$ Prove that $\angle{BXA} + \angle{DXC} = 180^\circ$

__source for shortlists__:

The IMO Compendium A Collection of Problems Suggested for The International Mathematical-Olympiads 1959-2009, 2nd Edition

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