$(a)$ A plane $\pi$ passes through the vertex $O$ of the regular tetrahedron $OPQR$. We define $p, q, r$ to be the signed distances of $P,Q,R$ from $\pi$ measured along a directed normal to $\pi$. Prove that \[p^2 + q^2 + r^2 + (q - r)^2 + (r - p)^2 + (p - q)^2 = 2a^2,\] where $a$ is the length of an edge of a tetrahedron. $(b)$ Given four parallel planes not all of which are coincident, show that a regular tetrahedron exists with a vertex on each plane. [u]Note:[/u] Part $(b)$ is [url=http://www.artofproblemsolving.com/Forum/viewtopic.php?f=49&t=60825&start=0]IMO 1972 Problem 6[/url]

Given an integer $d \ge 2$ and a circle $\omega$. Hansel drew a finite number of chords of circle $\omega$. The following condition is fulfilled: each end of each chord drawn is at least an end of $d$ different drawn chords. Prove that there is a drawn chord which intersects at least $\tfrac{d^2}{4}$ other drawn chords. Here we assume that the chords with a common end intersect. Note: Proof that a certain drawn chord crosses at least $\tfrac{d^2}{8}$ other drawn chords will be awarded two points.

There are $50$ male and $50$ female members registered in the QED-DB, who are also there are numbered from $1$ to $100$. In $100$ rounds, Andreas chooses at random one member for the seminar in Bad Tolz, whereupon Katharina already has two each time selected QED members of different sexes may or may not be paired up. Of course QED members cannot be coupled multiple times, ignoring relationships from the time before but both conscientiously. The stability of a relationship between two QED members is the amount of the difference between their numbers in DB and the sum of all stabilities is the promotion of young talent in the QED. What is the greatest possible demand of offspring guaranteed to achieve orgasm? [hide=original wording]In der QED-DB sind 50 m¨annliche und 50 weibliche Mitglieder eingetragen, welche dort mit den Zahlen von 1 bis 100 durchnummeriert sind. In 100 Runden w¨ahlt Andreas jeweils zuf¨allig ein Mitglied fu¨r das Seminar in Bad T¨olz aus, woraufhin jedes Mal Katharina zwei bereits ausgew¨ahlte QEDler unterschiedlichen Geschlechts verkuppeln darf, aber nicht muss. Natu¨rlich k¨onnen QEDler nicht mehrfach verkuppelt werden, Beziehungen aus der Zeit davor ignorieren beide aber gewissenhaft. Die Stabilit¨at einer Beziehung zwischen zwei QEDlern ist der Betrag der Differenz ihrer Zahlen in der DB und die Summe aller Stabilit¨aten ist die Nachwuchsf¨orderung im QED. Was ist die gr¨oßtm¨ogliche Nachwuchsf¨orderung, welche die Orgas garantiert erreichen k¨onnen¿[/hide]

Let the quadratic $P(x)=x^2+5x+1$. Two distinct real numbers $a,b$ satisfy \[P(a+b)=ab\] \[P(ab)=a+b\] Find the sum of all possible values of $a^2$. [i]Proposed by Harry Kim[/i]

On a flat plane in Camelot, King Arthur builds a labyrinth $\mathfrak{L}$ consisting of $n$ walls, each of which is an infinite straight line. No two walls are parallel, and no three walls have a common point. Merlin then paints one side of each wall entirely red and the other side entirely blue. At the intersection of two walls there are four corners: two diagonally opposite corners where a red side and a blue side meet, one corner where two red sides meet, and one corner where two blue sides meet. At each such intersection, there is a two-way door connecting the two diagonally opposite corners at which sides of different colours meet. After Merlin paints the walls, Morgana then places some knights in the labyrinth. The knights can walk through doors, but cannot walk through walls. Let $k(\mathfrak{L})$ be the largest number $k$ such that, no matter how Merlin paints the labyrinth $\mathfrak{L},$ Morgana can always place at least $k$ knights such that no two of them can ever meet. For each $n,$ what are all possible values for $k(\mathfrak{L}),$ where $\mathfrak{L}$ is a labyrinth with $n$ walls?

Let $O$ be the circumcenter of an acute triangle $ABC$. Line $OA$ intersects the altitudes of $ABC$ through $B$ and $C$ at $P$ and $Q$, respectively. The altitudes meet at $H$. Prove that the circumcenter of triangle $PQH$ lies on a median of triangle $ABC$.

A plane intersects a sphere in a circle $C$. The points $A$ and $B$ lie on the sphere on opposite sides of the plane. The line joining $A$ to the center of the sphere is normal to the plane. Another plane $p$ intersects the segment $AB$ and meets $C$ at $P$ and $Q$. Show that $BP\cdot BQ$ is independent of the choice of $p$.

There are $25$ IMO participants attending a party. Every two of them speak to each other in some language, and they use only one language even if they both know some other language as well. Among every three participants there is a person who uses the same language to speak to the other two (in that group of three). Prove that there is an IMO participant who speaks the same language to at least $10$ other participants

Given a positive integer $k$ show that there exists a prime $p$ such that one can choose distinct integers $a_1,a_2\cdots, a_{k+3} \in \{1, 2, \cdots ,p-1\}$ such that p divides $a_ia_{i+1}a_{i+2}a_{i+3}-i$ for all $i= 1, 2, \cdots, k$. [i]South Africa [/i]

A convex quadrilateral $ABCD$ is given in which the bisectors of the interior angles $\angle ABC$ and $\angle ADC$ have a common point on the diagonal $AC$. Prove that the bisectors of the interior angles $\angle BAD$ and $\angle BCD$ have a common point on the diagonal $BD$.

Which compound contains the highest percentage of nitrogen by mass? $ \textbf{(A)} \text{NH}_2\text{OH} (M=33.0) \qquad\textbf{(B)}\text{NH}_4\text{NO}_2 (M=64.1)\qquad$ $\textbf{(C)}\text{N}_2\text{O}_3 (M=76.0)\qquad\textbf{(D)}\text{NH}_4\text{NH}_2\text{CO}_2 (M=78.1)\qquad $

In a tetrahedron $ABCD$ the edges $AB$ and $CD$ are perpendicular and $\angle ACB =\angle ADB$. Prove that the plane through $AB$ and the midpoint of the edge $CD$, is perpendicular to $CD$.

P(x) is polynomial such that, polynomial P(P(x)) is strictly monotone in all real number line. Prove that polynomial P(x) is also strictly monotone in all real number line.

Let $a, b, c, d, m, n$ be positive integers such that $a^{2}+b^{2}+c^{2}+d^{2}=1989$, $n^{2}=max\left \{ a,b,c,d \right \}$ and $a+b+c+d=m^{2}$. Find the values of $m$ and $n$.

Let $ABC$ be a triangle with $CA \neq CB$. Let $D$, $F$, and $G$ be the midpoints of the sides $AB$, $AC$, and $BC$ respectively. A circle $\Gamma$ passing through $C$ and tangent to $AB$ at $D$ meets the segments $AF$ and $BG$ at $H$ and $I$, respectively. The points $H'$ and $I'$ are symmetric to $H$ and $I$ about $F$ and $G$, respectively. The line $H'I'$ meets $CD$ and $FG$ at $Q$ and $M$, respectively. The line $CM$ meets $\Gamma$ again at $P$. Prove that $CQ = QP$. [i]Proposed by El Salvador[/i]

Is there exist some positive integer $n$, such that the first decimal of $2^n$ (from left to the right) is $5$ while the first decimal of $5^n$ is $2$? [i](5 points)[/i]

Let $d\geq 13$ be an integer, and let $P(x) = a_dx^d + a_{d-1}x^{d-1} + \dots + a_1x+a_0$ be a polynomial of degree $d$ with complex coefficients such that $a_n = a_{d-n}$ for all $n\in\{0,1,\dots,d\}$. Prove that if $P$ has no double roots, then $P$ has two distinct roots $z_1$ and $z_2$ such that $|z_1-z_2|<1$.

A quadrilateral $ABCD$ is inscribed in a circle and the lenght of side $AD$ equals the sum of the lenghts of the sides $AB$ and $CD$. Prove that the angle bisectors of $\angle ABC$ and $\angle BCD$ meet on the side $AD$.

Let $I$ be the incentre of acute-angled triangle $ABC$. Let the incircle meet $BC, CA$, and $AB$ at $D, E$, and $F,$ respectively. Let line $EF$ intersect the circumcircle of the triangle at $P$ and $Q$, such that $F$ lies between $E$ and $P$. Prove that $\angle DPA + \angle AQD =\angle QIP$. (Slovakia)

Numbers from$ 1$ to $8$ were written at the vertices of the cube, and on each edge the absolute value of the difference between the numbers at its ends.. What is the smallest number of different numbers that can be written on the edges?

What is the area of the region enclosed by the graph of the equations $x^2 - 14x + 3y + 70 = 21 + 11y - y^2$ that lies below the line $y = x-3$? $\text{(A) }6\pi\qquad\text{(B) }7\pi\qquad\text{(C) }8\pi\qquad\text{(D) }9\pi\qquad\text{(E) }10\pi$

Let $a, b, c$ be positive real numbers such that $a^2 + b^2 + c^2 = 3$. Prove that $$\frac{1}{1+2ab}+\frac{1}{1+2bc}+\frac{1}{1+2ca}\ge 1$$

The figure obtained by removing one small unit square from the $2\times 2$ grid table is called an $L$ ''shape". .Put $k$ L-shapes in an $8\times 8$ grid table. Each $L$-shape can be rotated, but each $L$ shape is required to cover exactly three small unit squares in the grid table, and the common area covered by any two $L$ shapes is $0$, and except for these $k$ $L$ shapes, no other $L$ shapes can be placed. Find the minimum value of $k$.

Given a cyclic quadrilateral $ABCD$. Suppose $E, F, G, H$ are respectively the midpoint of the sides $AB, BC, CD, DA$. The line passing through $G$ and perpendicular on $AB$ intersects the line passing through $H$ and perpendicular on $BC$ at point $K$. Prove that $\angle EKF = \angle ABC$.

Let $P(x)=ax^3+(b-a)x^2-(c+b)x+c$ and $Q(x)=x^4+(b-1)x^3+(a-b)x^2-(c+a)x+c$ be polynomials of $x$ with $a,b,c$ non-zero real numbers and $b>0$.If $P(x)$ has three distinct real roots $x_0,x_1,x_2$ which are also roots of $Q(x)$ then: A)Prove that $abc>28$, B)If $a,b,c$ are non-zero integers with $b>0$,find all their possible values.