### Bulgaria 1997 - 2019 34p

geometry problems from Bulgarian Mathematical Olympiads (final round)

(in Bulgarian: Национална олимпиада по математика)

1997 - 2019

1997 Bulgaria P2
Let $M$ be the centroid of $\Delta ABC$ . Prove the inequality $\sin \angle CAM + \sin\angle CBM \le \frac{2}{\sqrt 3}$
(a) if the circumscribed circle of $\Delta AMC$ is tangent to the line $AB$
(b) for any $\Delta ABC$

1997 Bulgaria P5
Given a triangle $ABC$. Let $M$ and $N$ be the points where the angle bisectors of the angles $ABC$ and $BCA$ intersect the sides $CA$ and $AB$, respectively. Let $D$ be the point where the ray $MN$ intersects the circumcircle of triangle $ABC$. Prove that $\frac{1}{BD}=\frac{1}{AD}+\frac{1}{CD}$.

1998 Bulgaria P3
On the sides of a non-obtuse triangle $ABC$ a square, a regular $n$-gon and a regular $m$-gon ($m$,$n > 5$) are constructed externally, so that their centers are vertices of a regular triangle. Prove that $m = n = 6$ and find the angles of $\triangle ABC$.

1999 Bulgaria P3
The vertices of a triangle have integer coordinates and one of its sides is of length $\sqrt{n}$, where $n$ is a square-free natural number. Prove that the ratio of the circumradius and the inradius is an irrational number.

1999 Bulgaria P5
The vertices $A,B,C$ of an acute-angled triangle $ABC$ lie on the sides $B_1C_1, C_1A_1, A_1B_1$ respectively of a triangle $A_1B_1C_1$ similar to the triangle $ABC$ ($\angle A = \angle A_1$, etc.). Prove that the orthocenters of triangles $AB$C and $A_1B_1C_1$ are equidistant from the circumcenter of △ABC.

2000 Bulgaria P2
Let be given an acute triangle $ABC$. Show that there exist unique points $A_1 \in BC$, $B_1 \in CA$, $C_1 \in AB$ such that each of these three points is the midpoint of the segment whose endpoints are the orthogonal projections of the other two points on the corresponding side. Prove that the triangle $A_1B_1C_1$ is similar to the triangle whose side lengths are the medians of $\triangle ABC$.

Let $D$ be the midpoint of the base $AB$ of the isosceles acute triangle $ABC$. Choose point $E$ on segment $AB$, and let $O$ be the circumcenter of triangle $ACE$. Prove that the line through $D$ perpendicular to $DO$, the line through $E$ perpendicular to $BC$, and the line through $B$ parallel to $AC$ are concurrent.

2001 Bulgaria P2
Suppose that $ABCD$ is a parallelogram such that $DAB>90$. Let the point $H$ to be on $AD$ such that $BH$ is perpendicular to $AD$. Let the point $M$ to be the midpoint of $AB$. Let the point $K$ to be the intersecting point of the line $DM$ with the circumcircle of $ADB$. Prove that $HKCD$ is concyclic.

2002 Bulgaria P2
Consider the orthogonal projections of the vertices $A$, $B$ and $C$ of triangle $ABC$ on external bisectors of $\angle ACB$, $\angle BAC$ and $\angle ABC$, respectively. Prove that if $d$ is the diameter of the circumcircle of the triangle, which is formed by the feet of projections, while $r$ and $p$ are the inradius and the semiperimeter of triangle $ABC$, prove that $r^2+p^2=d^2$

2002 Bulgaria P4
Let $I$ be the incenter of a non-equilateral triangle $ABC$ and $T_1$, $T_2$, and $T_3$ be the tangency points of the incircle with the sides $BC$, $CA$ and $AB$, respectively. Prove that the orthocenter of triangle $T_1T_2T_3$ lies on the line $OI$, where $O$ is the circumcenter of triangle $ABC$.

2003 Bulgaria P2
Let $H$ be an arbitrary point on the altitude $CP$ of the acute triangle $ABC$. The lines $AH$ and $BH$ intersect $BC$ and $AC$ in $M$ and $N$, respectively.
(a) Prove that $\angle NPC =\angle MPC$.
(b) Let $O$ be the common point of $MN$ and $CP$. An arbitrary line through $O$ meets the sides of quadrilateral $CNHM$ in $D$ and $E$. Prove that $\angle EPC =\angle DPC$.

2004 Bulgaria P1
Let $I$ be the incenter of triangle $ABC$, and let $A_1$, $B_1$, $C_1$ be arbitrary points on the segments $(AI)$, $(BI)$, $(CI)$, respectively. The perpendicular bisectors of $AA_1$, $BB_1$, $CC_1$ intersect each other at $A_2$, $B_2$, and $C_2$. Prove that the circumcenter of the triangle $A_2B_2C_2$ coincides with the circumcenter of the triangle $ABC$ if and only if $I$ is the orthocenter of triangle $A_1B_1C_1$.

2005 Bulgaria P2
Consider two circles $k_{1},k_{2}$ touching externally at point $T$. a line touches $k_{2}$ at point $X$ and intersects $k_{1}$ at points $A$ and $B$. Let $S$ be the second intersection point of $k_{1}$ with the line $XT$ . On the arc $\widehat{TS}$ not containing $A$ and $B$ is chosen a point $C$ . Let $\ CY$ be the tangent line to $k_{2}$ with $Y\in k_{2}$ , such that the segment
$CY$ does not intersect the segment $ST$ . If $I=XY\cap SC$ . Prove that :
(a) the points $C,T,Y,I$ are concyclic.
(b) $I$ is the excenter of triangle $ABC$ with respect to the side $BC$.

2005 Bulgaria P4
Let $ABC$ be a triangle with $AC\neq BC$, and let $A^{\prime }B^{\prime }C$ be a triangle obtained from $ABC$ after some rotation centered at $C$. Let $M,E,F$ be the midpoints of the segments $BA^{\prime },AC$ and $CB^{\prime }$ respectively. If $EM=FM$, find $\widehat{EMF}$.

2006 Bulgaria P5
The triangle $ABC$ is such that $\angle BAC=30^{\circ},\angle ABC=45^{\circ}$. Prove that if $X$ lies on the ray $AC$, $Y$ lies on the ray $BC$ and $OX=BY$, where $O$ is the circumcentre of triangle $ABC$, then $S_{XY}$ passes through a fixed point.

2007 Bulgaria P1
The quadrilateral $ABCD$, where $\angle BAD+\angle ADC>\pi$, is inscribed a circle with centre $I$. A line through $I$ intersects $AB$ and $CD$ in points $X$ and $Y$ respectively such that $IX=IY$. Prove that $AX\cdot DY=BX\cdot CY$.
Let $ABC$ be an acute triangle and $CL$ be the angle bisector of $\angle ACB$. The point $P$ lies on the segment $CL$ such that $\angle APB=\pi-\frac{_1}{^2}\angle ACB$. Let $k_1$ and $k_2$ be the circumcircles of the triangles $APC$ and $BPC$. $BP\cap k_1=Q, AP\cap k_2=R$. The tangents to $k_1$ at $Q$ and $k_2$ at $B$ intersect at $S$ and the tangents to $k_1$ at $A$ and $k_2$ at $R$ intersect at $T$. Prove that $AS=BT.$

2009 Bulgaria P2
In the triangle $ABC$ its incircle with center $I$ touches its sides $BC, CA$ and $AB$ in the points $A_1, B_1, C_1$ respectively. Through $I$ is drawn a line $\ell$. The points $A', B'$ and $C'$ are reflections of $A_1, B_1, C_1$ with respect to the line $\ell$. Prove that the lines $A_1A', B_1B'$ and $C_1C'$ intersects at a common point.

2010 Bulgaria P2
Each of two different lines parallel to the the axis $Ox$ have exactly two common points on the graph of the function $f(x)=x^3+ax^2+bx+c$. Let $\ell_1$ and $\ell_2$ be two lines parallel to $Ox$ axis which meet the graph of $f$ in points $K_1, K_2$ and $K_3, K_4$, respectively. Prove that the quadrilateral formed by $K_1, K_2, K_3$ and $K_4$ is a rhombus if and only if its area is equal to $6$ units.

Let $k$ be the circumference of the triangle $ABC.$ The point $D$ is an arbitrary point on the segment $AB.$ Let $I$ and $J$ be the centers of the circles which are tangent to the side $AB,$ the segment $CD$ and the circle $k.$ We know that the points $A, B, I$ and $J$ are concyclic. The excircle of the triangle $ABC$ is tangent to the side $AB$ in the point $M.$ Prove that $M \equiv D.$

Point $O$ is inside $\triangle ABC$. The feet of perpendicular from $O$ to $BC,CA,AB$ are $D,E,F$. Perpendiculars from $A$ and $B$ respectively to $EF$ and $FD$ meet at $P$. Let $H$ be the foot of perpendicular from $P$ to $AB$. Prove that $D,E,F,H$ are concyclic.

We are given an acute-angled triangle $ABC$ and a random point $X$ in its interior, different from the centre of the circumcircle $k$ of the triangle. The lines $AX,BX$ and $CX$ intersect $k$ for a second time in the points $A_1,B_1$ and $C_1$ respectively. Let $A_2,B_2$ and $C_2$ be the points that are symmetric of  $A_1,B_1$ and $C_1$ in respect to $BC,AC$ and $AB$ respectively. Prove that the circumcircle of the triangle $A_2,B_2$ and $C_2$ passes through a constant point that does not depend on the choice of $X$.

2013 Bulgaria P5
Consider acute $\triangle ABC$ with altitudes $AA_1, BB_1$ and $CC_1$ ($A_1 \in BC,B_1 \in AC,C_1 \in AB$). A point $C'$ on the extension of $B_1A_1$ beyond $A_1$ is such that  $A_1C' = B_1C_1$. Analogously, a point $B'$ on the extension of A$_1C_1$ beyond $C_1$ is such that $C_1B' = A_1B_1$ and a point $A'$ on the extension of $C_1B_1$ beyond $B_1$ is such that $B_1A' = C_1A_1$. Denote by $A'', B'', C''$ the symmetric points of $A' , B' , C'$ with respect to $BC, CA$ and $AB$ respectively. Prove that if $R, R'$ and R'' are circumradiii of $\triangle ABC, \triangle A'B'C'$ and $\triangle A''B''C''$, then $R, R'$ and $R''$ are sidelengths of a triangle with area equals one half of the area of $\triangle ABC$.

Let $k$ be a given circle and $A$ is a fixed point outside $k$. $BC$ is a diameter of $k$. Find the locus of the orthocentre of $\triangle ABC$ when $BC$ varies.

Let $ABCD$ be a quadrilateral inscribed in a circle $k$. $AC$ and $BD$ meet at $E$. The rays $\overrightarrow{CB}, \overrightarrow{DA}$ meet at $F$. Prove that the line through the incenters of $\triangle ABE\,,\, \triangle ABF$ and the line through the incenters of $\triangle CDE\,,\, \triangle CDF$ meet at a point lying on the circle $k$.

The hexagon $ABLCDK$ is inscribed and the line $LK$ intersects the segments $AD, BC, AC$ and $BD$ in points $M, N, P$ and $Q$, respectively. Prove that $NL \cdot KP \cdot MQ = KM \cdot PN \cdot LQ$.

In a triangle $\triangle ABC$ points $L, P$ and $Q$ lie on the segments $AB, AC$ and $BC$, respectively, and are such that $PCQL$ is a parallelogram. The circle with center the midpoint $M$ of the segment $AB$ and radius $CM$ and the circle of diameter $CL$ intersect for the second time at the point $T$. Prove that the lines $AQ, BP$ and $LT$ intersect in a point.

Let $\triangle {ABC}$ be isosceles triangle with $AC=BC$ . The point $D$ lies on the extension of $AC$ beyond $C$ and is that $AC>CD$. The angular bisector of $\angle BCD$ intersects $BD$ at point $N$ and let $M$ be the midpoint of $BD$. The tangent at $M$ to the circumcircle of triangle $AMD$ intersects the side $BC$ at point  $P$. Prove that points $A,P,M$ and $N$ lie on a circle.

An convex qudrilateral $ABCD$ is given. $O$ is the intersection point of the diagonals $AC$ and $BD$. The points $A_1,B_1,C_1, D_1$ lie respectively on $AO, BO, CO, DO$ such that $AA_1=CC_1, BB_1=DD_1$. The circumcircles of $\triangle AOB$ and $\triangle COD$ meet at second time at $M$ and the the circumcircles of $\triangle AOD$ and $\triangle BOC$ - at $N$. The circumcircles of $\triangle A_1OB_1$ and $\triangle C_1OD_1$ meet at second time at $P$ and the the circumcircles of $\triangle A_1OD_1$ and $\triangle B_1OC_1$ - at $Q$. Prove that the quadrilateral $MNPQ$ is cyclic.

An acute non-isosceles $\triangle ABC$ is given. $CD, AE, BF$ are its altitudes. The points $E', F'$ are symetrical of $E, F$ with respect accordingly to $A$ and $B$. The point $C_1$ lies on $\overrightarrow{CD}$, such that $DC_1=3CD$. Prove that $\angle E'C_1F'=\angle ACB$

Let $ABCD$ be a cyclic quadrilateral. Let $H_{1}$ be the orthocentre of triangle $ABC$. Point $A_{1}$ is the image of $A$ after reflection about $BH_{1}$. Point $B_{1}$ is the image of of $B$ after reflection about $AH_{1}$. Let $O_{1}$ be the circumcentre of $(A_{1}B_{1}H_{1})$. Let $H_{2}$ be the orthocentre of triangle $ABD$. Point $A_{2}$ is the image of $A$ after reflection about $BH_{2}$. Point $B_{2}$ is the image of of $B$ after reflection about $AH_{2}$. Let $O_{2}$ be the circumcentre of $(A_{2}B_{2}H_{2})$. Lets denote by $\ell_{AB}$ be the line through $O_{1}$ and $O_{2}$. $\ell_{AD}$ ,$\ell_{BC}$ ,$\ell_{CD}$ are defined analogously. Let $M=\ell_{AB} \cap \ell_{BC}$, $N=\ell_{BC} \cap \ell_{CD}$, $P=\ell_{CD} \cap \ell_{AD}$,$Q=\ell_{AD} \cap \ell_{AB}$. Prove that $MNPQ$ is cyclic.

Let $ABCD$ be a quadrilateral ,circumscribed about a circle. Let $M$ be a point on the side $AB$. Let $I_{1}$,$I_{2}$ and $I_{3}$ be the incentres of triangles $AMD$, $CMD$ and $BMC$ respectively. Prove that $I_{1}I_{2}I_{3}M$ is circumscribed.

Let $ABC$ be an acute triangle with orthocenter $H$ and circumcenter $O.$ Let the intersection points of the perpendicular bisector of $CH$ with $AC$ and $BC$ be $X$ and $Y$ respectively. Lines $XO$ and $YO$ cut $AB$ at $P$ and $Q$ respectively. If $XP+YQ=AB+XY,$ determine $\measuredangle OHC.$

A hexagon $ABCDEF$ is inscribed in a circle and $AB\cdot CD\cdot EF=BC\cdot DE\cdot FA$. The points $B$ and $B_1$ are symmetrical over the line $AC$; $D$ and $D_1$ - symmetrical over $CE$, and $F, F_1$ symmetrical over $EA$.  Prove that $\triangle BDF\sim \triangle B_1D_1F_1$.

source:
https://klasirane.com/OLI.asp