IGO 2020

By a fold of a polygon-shaped paper, we mean drawing a segment on the paper and folding the paper along that. Suppose that a paper with the following figure is given. We cut the paper along the boundary of the shaded region to get a polygon-shaped paper. Start with this shaded polygon and make a rectangle-shaped paper from it with at most 5 number of folds. Describe your solution by introducing the folding lines and drawing the shape after each fold on your solution sheet. (Note that the folding lines do not have to coincide with the grid lines of the shape.) Proposed by Mahdi Etesamifard

Attachments:

A parallelogram $ABCD$ is given ( $AB \neq BC$ ). Points $E$ and $G$ are chosen on the line $\overline{CD}$ such that $\overline{AC}$ is the angle bisector of both angles $\angle EAD$ and $\angle BAG$ . The line $\overline{BC}$ intersects $\overline{AE}$ and $\overline{AG}$ at $F$ and $H$ , respectively. Prove that the line $\overline{FG}$ passes through the midpoint of $HE$ . Proposed by Mahdi Etesamifard

According to the figure, three equilateral triangles with side lengths $a,b,c$ have one common vertex and do not have any other common point. The lengths $x, y$ , and $z$ are defined as in the figure. Prove that $3(x+y+z)>2(a+b+c)$ . Proposed by Mahdi Etesamifard

Attachments:

Let $P$ be an arbitrary point in the interior of triangle $\triangle ABC$ . Lines $\overline{BP}$ and $\overline{CP}$ intersect $\overline{AC}$ and $\overline{AB}$ at $E$ and $F$ , respectively. Let $K$ and $L$ be the midpoints of the segments $BF$ and $CE$ , respectively. Let the lines through $L$ and $K$ parallel to $\overline{CF}$ and $\overline{BE}$ intersect $\overline{BC}$ at $S$ and $T$ , respectively; moreover, denote by $M$ and $N$ the reflection of $S$ and $T$ over the points $L$ and $K$ , respectively. Prove that as $P$ moves in the interior of triangle $\triangle ABC$ , line $\overline{MN}$ passes through a fixed point. Proposed by Ali Zamani

We say two vertices of a simple polygon are visible from each other if either they are adjacent, or the segment joining them is completely inside the polygon (except two endpoints that lie on the boundary). Find all positive integers $n$ such that there exists a simple polygon with $n$ vertices in which every vertex is visible from exactly $4$ other vertices. (A simple polygon is a polygon without hole that does not intersect itself.) Proposed by Morteza Saghafian

A trapezoid $ABCD$ is given where $AB$ and $CD$ are parallel. Let $M$ be the midpoint of the segment $AB$ . Point $N$ is located on the segment $CD$ such that $\angle ADN = \frac{1}{2} \angle MNC$ and $\angle BCN = \frac{1}{2} \angle MND$ . Prove that $N$ is the midpoint of the segment $CD$ .

Proposed by Alireza Dadgarnia

Let $ABC$ be an isosceles triangle ( $AB = AC$ ) with its circumcenter $O$ . Point $N$ is the midpoint of the segment $BC$ and point $M$ is the reflection of the point $N$ with respect to the side $AC$ . Suppose that $T$ is a point so that $ANBT$ is a rectangle. Prove that $\angle OMT = \frac{1}{2} \angle BAC$ .

Proposed by Ali Zamani

In acute-angled triangle $ABC$ ( $AC > AB$ ), point $H$ is the orthocenter and point $M$ is the midpoint of the segment $BC$ . The median $AM$ intersects the circumcircle of triangle $ABC$ at $X$ . The line $CH$ intersects the perpendicular bisector of $BC$ at $E$ and the circumcircle of the triangle $ABC$ again at $F$ . Point $J$ lies on circle $\omega$ , passing through $X, E,$ and $F$ , such that $BCHJ$ is a trapezoid ( $CB \parallel HJ$ ). Prove that $JB$ and $EM$ meet on $\omega$ .

Proposed by Alireza Dadgarnia

Triangle $ABC$ is given. An arbitrary circle with center $J$ , passing through $B$ and $C$ , intersects the sides $AC$ and $AB$ at $E$ and $F$ , respectively. Let $X$ be a point such that triangle $FXB$ is similar to triangle $EJC$ (with the same order) and the points $X$ and $C$ lie on the same side of the line $AB$ . Similarly, let $Y$ be a point such that triangle $EYC$ is similar to triangle $FJB$ (with the same order) and the points $Y$ and $B$ lie on the same side of the line $AC$ . Prove that the line $XY$ passes through the orthocenter of the triangle $ABC$ .

Proposed by Nguyen Van Linh - Vietnam

Find all numbers $n \geq 4$ such that there exists a convex polyhedron with exactly $n$ faces, whose all faces are right-angled triangles. (Note that the angle between any pair of adjacent faces in a convex polyhedron is less than $180^\circ$ .)

Proposed by Hesam Rajabzadeh

Let $M,N,P$ be midpoints of $BC,AC$ and $AB$ of triangle $\triangle ABC$ respectively. $E$ and $F$ are two points on the segment $\overline{BC}$ so that $\angle NEC = \frac{1}{2} \angle AMB$ and $\angle PFB = \frac{1}{2} \angle AMC$ . Prove that $AE=AF$ . Proposed by Alireza Dadgarnia

Assume three circles mutually outside each other with the property that every line separating two of them have intersection with the interior of the third one. Prove that the sum of pairwise distances between their centers is at most $2\sqrt{2}$ times the sum of their radii. (A line separates two circles, whenever the circles do not have intersection with the line and are on different sides of it.) Note. Weaker results with $2\sqrt{2}$ replaced by some other $c$ may be awarded points depending on the value of $c>2\sqrt{2}$ Proposed by Morteza Saghafian

Convex circumscribed quadrilateral $ABCD$ with its incenter $I$ is given such that its incircle is tangent to $\overline{AD},\overline{DC},\overline{CB},$ and $\overline{BA}$ at $K,L,M,$ and $N$ . Lines $\overline{AD}$ and $\overline{BC}$ meet at $E$ and lines $\overline{AB}$ and $\overline{CD}$ meet at $F$ . Let $\overline{KM}$ intersects $\overline{AB}$ and $\overline{CD}$ at $X,Y$ , respectively. Let $\overline{LN}$ intersects $\overline{AD}$ and $\overline{BC}$ at $Z,T$ , respectively. Prove that the circumcircle of triangle $\triangle XFY$ and the circle with diameter $EI$ are tangent if and only if the circumcircle of triangle $\triangle TEZ$ and the circle with diameter $FI$ are tangent. Proposed by Mahdi Etesamifard

Consider an acute-angled triangle $\triangle ABC$ ( $AC>AB$ ) with its orthocenter $H$ and circumcircle $\Gamma$ .Points $M$ , $P$ are midpoints of $BC$ and $AH$ respectively.The line $\overline{AM}$ meets $\Gamma$ again at $X$ and point $N$ lies on the line $\overline{BC}$ so that $\overline{NX}$ is tangent to $\Gamma$ . Points $J$ and $K$ lie on the circle with diameter $MP$ such that $\angle AJP=\angle HNM$ ( $B$ and $J$ lie one the same side of $\overline{AH}$ ) and circle $\omega_1$ , passing through $K,H$ , and $J$ , and circle $\omega_2$ passing through $K,M$ , and $N$ , are externally tangent to each other. Prove that the common external tangents of $\omega_1$ and $\omega_2$ meet on the line $\overline{NH}$ . Proposed by Alireza Dadgarnia