Olympic Revenge 2002-21 (Brazil) 25p (-06,-08)

geometry problems from (Brazilian) Olympic Revenge
with aops links in the names

It is a competition for teachers,  written by students.

inside aops here

2002 - 2021 
missing 2006 , 2008

2002 Olympic Revenge P2
$ABCD$ is a inscribed quadrilateral. $P$ is the intersection point of its diagonals. $O$ is its circumcenter. $\Gamma$ is the circumcircle of $ABO$. $\Delta$ is the circumcircle of $CDO$.
$M$ is the midpoint of arc $AB$ on $\Gamma$ who doesn't contain $O$. $N$ is the midpoint of arc $CD$ on $\Delta$ who doesn't contain $O$. Show that $M,N,P$ are collinear.

2003 Olympic Revenge P1
Let $ABC$ be a triangle with circumcircle $\Gamma$. $D$ is the midpoint of arc $BC$ (this arc does not contain $A$). $E$ is the common point of $BC$ and the perpendicular bisector of $BD$. $F$ is the common point of $AC$ and the parallel to $AB$ containing $D$. $G$ is the common point of $EF$ and $AB$. $H$ is the common point of $GD$ and $AC$. Show that $GAH$ is isosceles.

2003 Olympic Revenge P3

Let $ABC$ be a triangle with $\angle BAC =60^\circ$. $A'$ is the symmetric point of $A$ wrt $\overline{BC}$. $D$ is the point in $\overline{AC}$ such that $\overline{AB}=\overline{AD}$. $H$ is the orthocenter of triangle $ABC$. $l$ is the external angle bisector of $\angle BAC$. $\{M\}=\overline{A'D}\cap l$,$\{N\}=\overline{CH} \cap l$. Show that $\overline{AM}=\overline{AN}$

2004 Olympic Revenge P1
$ABC$ is a triangle and $D$ is an internal point such that  $\angle DAB=\angle  DBC =\angle DCA$. $O_a$ is the circumcenter of $DBC$. $O_b$ is the circumcenter of $DAC$. $O_c$ is the circumcenter of $DAB$. Show that if the area of $ABC$ and $O_aO_bO_c$ are equal then $ABC$ is equilateral.

2004 Olympic Revenge P3

$ABC$ is a triangle and $\omega$ its incircle. Let $P,Q,R$ be the intersections with $\omega$ and the sides $BC,CA,AB$ respectively. $AP$ cuts $\omega$ in $P$ and $X$. $BX,CX$ cut $\omega$ in $M,N$ respectively. Show that $MR,NQ,AP$ are parallel or concurrent.

2005 Olympic Revenge P2

Let $\Gamma$ be a circumference, and $A,B,C,D$ points of $\Gamma$ (in this order).$r$ is the tangent to $\Gamma$ at point A.$s$ is the tangent to $\Gamma$ at point D. Let $E=r \cap BC,F=s \cap BC$. Let $X=r \cap s,Y=AF \cap DE,Z=AB \cap CD$. Show that the points $X,Y,Z$ are collinear.

Note: assume the existence of all above point

2006 missing

2007 Olympic Revenge P3
The triangles $BCD$ and $ACE$ are externally constructed to sides $BC$ and $CA$ of a triangle $ABC$ such that $AE = BD$ and $\angle BDC+\angle AEC = 180^\circ$. Let $F$ be a point on segment $AB$ such that ${AF\over FB}={CD\over CE}$. Prove that ${DE\over CD+CE}={EF\over BC}={FD\over AC}$.

2008 might be missing

2008 Olympic Revenge P
Let $ABC$ be a triangle and $M$ the midpoint of $BC$. Let $D$ and $E$ be points such that $ABC$, $DBA$ and $EAC$ are similar (in this order). The symmedians of $D$ with respect to $DBA$ and $E$ with respect to $EAC$ meet at $K$. Suppose that AM/BC=√3/2 Prove that $KAB$ and $KCA$ are similar (in this order).

2009 Olympic Revenge P1
Given a scalene triangle $ABC$ with circuncenter $O$ and circumscribed circle $\Gamma$. Let $D, E ,F$ the midpoints of $BC, AC, AB$. Let $M=OE \cap AD$, $N=OF \cap AD$ and $P=CM \cap BN$. Let $X=AO \cap PE$, $Y=AP \cap OF$. Let $r$ the tangent of $\Gamma$ through $A$. Prove that $r, EF, XY$ are concurrent.

2009 Olympic Revenge P3
Let $ABC$ to be a triangle with incenter $I$. $\omega_{A}$, $\omega_{B}$ and $\omega_{C}$ are the incircles of the triangles $BIC$, $CIA$ and $AIB$, repectively. After all, $T$ is the tangent point between $\omega_{A}$ and $BC$. Prove that the other internal common tangent to $\omega_{B}$ and $\omega_{C}$ passes through the point $T$.

2010 Olympic Revenge P6
Let $ABC$ to be a triangle and $\Gamma$ its circumcircle. Also, let $D, F, G$ and $E$, in this order, on the arc $BC$ which does not contain $A$ satisfying $\angle BAD = \angle CAE$ and $\angle BAF = \angle CAG$. Let $D`, F`, G`$ and $E`$ to be the intersections of $AD, AF, AG$ and $AE$ with $BC$, respectively. Moreover, $X$ is the intersection of $DF`$ with $EG`$, $Y$ is the intersection of $D`F$ with $E`G$, $Z$ is the intersection of $D`G$ with $E`F$ and $W$ is the intersection of $EF`$ with $DG`$. Prove that $X, Y$ and $A$ are collinear, such as $W, Z$ and $A$. Moreover, prove that $\angle BAX = \angle CAZ$.

2011 Olympic Revenge P4
Let $ABCD$ to be a quadrilateral inscribed in a circle $\Gamma$. Let $r$ and $s$ to be the tangents to $\Gamma$ through $B$ and $C$, respectively, $M$ the intersection between the lines $r$ and $AD$ and $N$ the intersection between the lines $s$ and $AD$. After all, let $E$ to be the intersection between the lines $BN$ and $CM$, $F$ the intersection between the lines $AE$ and $BC$ and $L$ the midpoint of $BC$. Prove that the circuncircle of the triangle $DLF$ is tangent to $\Gamma$.

2012 Olympic Revenge P6
Let $ABC$ be an scalene triangle and $I$ and $H$ its incenter, ortocenter respectively. The incircle touchs $BC$, $CA$ and $AB$ at $D,E$ an $F$. $DF$ and $AC$ intersects at $K$ while $EF$ and $BC$ intersets at $M$. Shows that $KM$ cannot be paralel to $IH$.

2013 Olympic Revenge P2
Let $ABC$ to be an acute triangle. Also, let $K$ and $L$ to be the two intersections of the perpendicular from $B$ with respect to side $AC$ with the circle of diameter  $AC$, with $K$ closer to $B$ than $L$. Analogously, $X$ and $Y$ are the two intersections of the perpendicular from $C$ with respect to side $AB$ with the circle of diamter $AB$, with $X$ closer to $C$ than $Y$. Prove that the intersection of $XL$ and $KY$ lies on $BC$.

2014 Olympic Revenge P1
Let $ABC$ an acute triangle and $\Gamma$ its circumcircle. The bisector of $BAC$ intersects $\Gamma$ at $M\neq A$. A line $r$ parallel to $BC$ intersects $AC$ at $X$ and $AB$ at $Y$. Also, $MX$ and $MY$ intersect $\Gamma$ again at $S$ and $T$, respectively. If $XY$ and $ST$ intersect at $P$, prove that $PA$ is tangent to $\Gamma$.

2015 Olympic Revenge P5

Given a triangle $A_1 A_2 A_3$, let $a_i$ denote the side opposite to $A_i$, where indices are taken modulo 3. Let $D_1 \in a_1$. For $D_i \in A_i$, let $\omega_i$ be the incircle of the triangle formed by lines $a_i, a_{i+1}, A_iD_i$, and $D_{i+1} \in a_{i+1}$ with $A_{i+1} D_{i+1}$ tangent to $\omega_i$. Show that the set $\{D_i: i \in \mathbb{N}\}$ is finite.

2016 Olympic Revenge P3
Let $\Gamma$ be a fixed circle. Find all finite sets $S$ of points on $\Gamma$ with the following property: For any point $P$ on $\Gamma$, one can partition $S$ into two sets $A$ and $B$ such that the sum of distances from $P$ to the points in $A$ equals the sum of distances from $P$ to the points in $B$.

2016 Olympic Revenge P4

Let $\Omega$ and $\Gamma$ be two circles such that $\Omega$ is inside $\Gamma$. Let $P$ be a point in $\Gamma$. Let the the two lines tangent to $\Omega$ passing through $P$ meet again $\Gamma$ at $A$ and $B$ ($A,B \neq P$). Prove that as we vary the point $P$ in $\Gamma$, the line $AB$ remains tangent to a fixed circle.

2017 Olympic Revenge P2
Let $\triangle$$ABC$ a triangle with circumcircle $\Gamma$. Suppose there exist points $R$ and $S$ on sides $AB$ and $AC$, respectively, such that $BR=RS=SC$. A tangent line through $A$ to $\Gamma$ meet the line $RS$ at $P$. Let $I$ the incenter of triangle $\triangle$$ARS$. Prove that $PA=PI$

2018 Olympic Revenge P2
Let $ABC$ be a scalene triangle with incenter $I$ and circumcircle $\Gamma$, of center $O$. Let the inscribed circle to $ABC$ touch the sides at $D$, $E$ and $F$. Let $AI$ intersect $EF$ at $N$, and $\Gamma$ at $M \neq A$. Let $MD$ intersect $\Gamma$ at $L\neq M$. Let $IL$ intersect $EF$ at $K$. And let the circle with diameter $MN$ intersect $\Gamma$ at $P \neq M$. Prove that $AK, PN$ and $OI$ meet at a point.

2018 Olympic Revenge P4
Let $ABC$ be a triangle with acute angles with incircle $\omega$ and incenter $I$. Let $T_A,T_B$ and $T_C$ be the points where $\omega$ touches the sides of $ABC$. Let $l_A$ be a line through $A$ parallel to $BC$, and let $l_B$ and $l_C$ be analogously defined. Let $L_A$ be the second intersection of $AI$ to the circumcircle of $ABC$, and define $L_B$ and $L_C$ analogously. Let $P_A = T_BT_C \cap l_A$ and $P_B, P_C$ be defined analogously. Let $S_A = P_BT_B \cap P_CT_C$ and define $S_B, S_C$ analogously. Prove that $S_AL_A, S_BL_B$ and $S_CL_C$ meet at a point.

2019 Olympic Revenge P1
Let $ABC$ be a scalene acute-angled triangle and $D$ be the point on its circumcircle such that $AD$ is a symmedian of triangle $ABC$. Let $E$ be the reflection of  $D$ about $BC$, $C_0$ the reflection of $E$ about $AB$ and $B_0$ the reflection of $E$ about $AC$. Prove that the lines $AD$, $BB_0$ and $CC_0$ are concurrent if and only if $\angle BAC = 60^{\circ}.$

2019 Olympic Revenge P3
Let $\Gamma$ be a circle centered at $O$ with radius $R$. Let $X$ and $Y$ be points on $\Gamma$ such that $XY<R$. Let $I$ be a point such that $IX = IY$ and $XY = OI$. Describe how to construct with ruler and compass a triangle which has circumcircle $\Gamma$, incenter $I$ and Euler line $OX$. Prove that this triangle is unique.

2020 Olympic Revenge P3
Let $ABC$ be a triangle and $\omega$ its circumcircle. Let $D$ and $E$ be the feet of the angle bisectors relative to $B$ and $C$, respectively. The line $DE$ meets $\omega$ at $F$ and $G$. Prove that the tangents to $\omega$ through $F$ and $G$ are tangents to the excircle of $\triangle ABC$ opposite to $A$.

2021 Olympic Revenge P3

Let $I, C, \omega$ and $\Omega$ be the incenter, circumcenter, incircle and circumcircle, respectively,

of the scalene triangle $XYZ$ with $XZ > YZ > XY$. The incircle $\omega$ is tangent to the sides

$YZ, XZ$ and $XY$ at the points $D, E$ and $F$. Let $S$ be the point on $\Omega$ such that $XS,

CI$and$YZ$are concurrent. Let$(XEF) \cap \Omega = R$,$(RSD) \cap (XEF) = U$,$SU \cap CI = N$,

$EF \cap YZ = A$, $EF \cap CI = T$ and $XU \cap YZ = O$. Prove that $NARUTO$ is cyclic.