SHARYGIN 2012

In triangle $ABC$ , given lines $l_{b}$ and $l_{c}$ containing the bisectors of angles $B$ and $C$ , and the foot $L_{1}$ of the bisector of angle $A$ . Restore triangle $ABC$ .

In a convex quadrilateral all sidelengths and all angles are pairwise different. a) Can the greatest angle be adjacent to the greatest side and at the same time the smallest angle be adjacent to the smallest side? b) Can the greatest angle be non-adjacent to the smallest side and at the same time the smallest angle be non-adjacent to the greatest side?

Given triangle $ABC$ and point $P$ . Points $A', B', C'$ are the projections of $P$ to $BC, CA, AB$ . A line passing through $P$ and parallel to $AB$ meets the circumcircle of triangle $PA'B'$ for the second time in point $C_{1}$ . Points $A_{1}, B_{1}$ are defined similarly. Prove that a) lines $AA_{1}, BB_{1}, CC_{1}$ concur; b) triangles $ABC$ and $A_{1}B_{1}C_{1}$ are similar.

Let $O$ be the circumcenter of an acute-angled triangle $ABC$ . A line passing through $O$ and parallel to $BC$ meets $AB$ and $AC$ in points $P$ and $Q$ respectively. The sum of distances from $O$ to $AB$ and $AC$ is equal to $OA$ . Prove that $PB + QC = PQ$ .

Points $A, B$ are given. Find the locus of points $C$ such that $C$ , the midpoints of $AC, BC$ and the centroid of triangle $ABC$ are concyclic.

In a convex quadrilateral $ABCD$ suppose $AC \cap BD = O$ and $M$ is the midpoint of $BC$ . Let $MO \cap AD = E$ . Prove that $\frac{AE}{ED} = \frac{S_{\triangle ABO}}{S_{\triangle CDO}}$ .

Given triangle $ABC$ . Consider lines $l$ with the next property: the reflections of $l$ in the sidelines of the triangle concur. Prove that all these lines have a common point.

Given right-angled triangle $ABC$ with hypothenuse $AB$ . Let $M$ be the midpoint of $AB$ and $O$ be the center of circumcircle $\omega$ of triangle $CMB$ . Line $AC$ meets $\omega$ for the second time in point $K$ . Segment $KO$ meets the circumcircle of triangle $ABC$ in point $L$ . Prove that segments $AL$ and $KM$ meet on the circumcircle of triangle $ACM$ .

A square $ABCD$ is inscribed into a circle. Point $M$ lies on arc $BC$ , $AM$ meets $BD$ in point $P$ , $DM$ meets $AC$ in point $Q$ . Prove that the area of quadrilateral $APQD$ is equal to the half of the area of the square.

A triangle and two points inside it are marked. It is known that one of the triangle's angles is equal to $58^{\circ}$ , one of two remaining angles is equal to $59^{\circ}$ , one of two given points is the incenter of the triangle and the second one is its circumcenter. Using only the ruler without partitions determine where is each of the angles and where is each of the centers.

Two circles with radii 1 meet in points $X, Y$ , and the distance between these points also is equal to $1$ . Point $C$ lies on the first circle, and lines $CA, CB$ are tangents to the second one. These tangents meet the first circle for the second time in points $B', A'$ . Lines $AA'$ and $BB'$ meet in point $Z$ . Find angle $XZY$ .

Point $D$ lies on side $AB$ of triangle $ABC$ . Let $\omega_1$ and $\Omega_1,\omega_2$ and $\Omega_2$ be the incircles and the excircles (touching segment $AB$ ) of triangles $ACD$ and $BCD.$ Prove that the common external tangents to $\omega_1$ and $\omega_2,$ $\Omega_1$ and $\Omega_2$ meet on $AB$ .

Two perpendicular lines pass through the orthocenter of an acute-angled triangle. The sidelines of the triangle cut on each of these lines two segments: one lying inside the triangle and another one lying outside it. Prove that the product of two internal segments is equal to the product of two external segments.

Nikolai Beluhov and Emil Kolev

A circle $\omega$ with center $I$ is inscribed into a segment of the disk, formed by an arc and a chord $AB$ . Point $M$ is the midpoint of this arc $AB$ , and point $N$ is the midpoint of the complementary arc. The tangents from $N$ touch $\omega$ in points $C$ and $D$ . The opposite sidelines $AC$ and $BD$ of quadrilateral $ABCD$ meet in point $X$ , and the diagonals of $ABCD$ meet in point $Y$ . Prove that points $X, Y, I$ and $M$ are collinear.

An arbitrary point is selected on each of twelve diagonals of the faces of a cube.The centroid of these twelve points is determined. Find the locus of all these centroids.

Given are $n$ $(n > 2)$ points on the plane such that no three of them are collinear. In how many ways this set of points can be divided into two non-empty subsets with non-intersecting convex envelops?