Found problems: 85335
2005 Tournament of Towns, 5
Prove that if a regular icosahedron and a regular dodecahedron have a common circumsphere, then they have a common insphere.
[i](7 points)[/i]
2024 AMC 12/AHSME, 15
A triangle in the coordinate plane has vertices $A(\log_21,\log_22)$, $B(\log_23,\log_24)$, and $C(\log_27,\log_28)$. What is the area of $\triangle ABC$?
$
\textbf{(A) }\log_2\frac{\sqrt3}7\qquad
\textbf{(B) }\log_2\frac3{\sqrt7}\qquad
\textbf{(C) }\log_2\frac7{\sqrt3}\qquad
\textbf{(D) }\log_2\frac{11}{\sqrt7}\qquad
\textbf{(E) }\log_2\frac{11}{\sqrt3}\qquad
$
2013 AIME Problems, 7
A rectangular box has width $12$ inches, length $16$ inches, and height $\tfrac{m}{n}$ inches, where $m$ and $n$ are relatively prime positive integers. Three faces of the box meet at a corner of the box. The center points of those three faces are the vertices of a triangle with an area of $30$ square inches. Find $m+n$.
2000 All-Russian Olympiad, 2
Tanya chose a natural number $X\le100$, and Sasha is trying to guess this number. He can select two natural numbers $M$ and $N$ less than $100$ and ask about $\gcd(X+M,N)$. Show that Sasha can determine Tanya's number with at most seven questions.
2015 NIMO Summer Contest, 7
The NIMO problem writers have invented a new chess piece called the [i]Oriented Knight[/i]. This new chess piece has a limited number of moves: it can either move two squares to the right and one square upward or two squares upward and one square to the right. How many ways can the knight move from the bottom-left square to the top-right square of a $16\times 16$ chess board?
[i] Proposed by Tony Kim and David Altizio [/i]
2003 Tournament Of Towns, 1
Two players in turns color the sides of an $n$-gon. The first player colors any side that has $0$ or $2$ common vertices with already colored sides. The second player colors any side that has exactly $1$ common vertex with already colored sides. The player who cannot move, loses. For which $n$ the second player has a winning strategy?
1998 Croatia National Olympiad, Problem 3
Ivan and Krešo started to travel from Crikvenica to Kraljevica, whose distance is $15$ km, and at the same time Marko started from Kraljevica to Crikvenica. Each of them can go either walking at a speed of $5$ km/h, or by bicycle with the speed of $15$ km/h. Ivan started walking, and Krešo was driving a bicycle until meeting Marko. Then Krešo gave the bicycle to Marko and continued walking to Kraljevica, while Marko continued to Crikvenica by bicycle. When Marko met Ivan, he gave him the bicycle and continued on foot, so Ivan arrived at Kraljevica by bicycle. Find, for each of them, the time he spent in travel as well as the time spent in walking.
2001 China Team Selection Test, 2.2
Given distinct positive integers \( g \) and \( h \), let all integer points on the number line \( OX \) be vertices. Define a directed graph \( G \) as follows: for any integer point \( x \), \( x \rightarrow x + g \), \( x \rightarrow x - h \). For integers \( k, l (k < l) \), let \( G[k, l] \) denote the subgraph of \( G \) with vertices limited to the interval \([k, l]\). Find the largest positive integer \( \alpha \) such that for any integer \( r \), the subgraph \( G[r, r + \alpha - 1] \) of \( G \) is acyclic. Clarify the structure of subgraphs \( G[r, r + \alpha - 1] \) and \( G[r, r + \alpha] \) (i.e., how many connected components and what each component is like).
2024 Iranian Geometry Olympiad, 3
Inside a convex quadrilateral $ABCD$ with $BC>AD$, a point $T$ is chosen. $S$ lies on the segment $AT$ such that $DT = BC, \angle TSD = 90^\circ$.
Prove that if $\angle DTA + \angle TAB + \angle ABC = 180^\circ$, then $AB + ST \geqslant CD + AS$.
[i]Proposed by Alexander Tereshin - Russia[/i]
2025 AIME, 13
Let the sequence of rationals $x_1,x_2,\dots$ be defined such that $x_1=\frac{25}{11}$ and
\[x_{k+1}=\frac{1}{3}\left(x_k+\frac{1}{x_k}-1\right).\]
$x_{2025}$ can be expressed as $\frac{m}{n}$ for relatively prime positive integers $m$ and $n$. Find the remainder when $m+n$ is divided by $1000$.
2024 China Team Selection Test, 10
Let $M$ be a positive integer. $f(x):=x^3+ax^2+bx+c\in\mathbb Z[x]$ satisfy $|a|,|b|,|c|\le M.$ $x_1,x_2$ are different roots of $f.$ Prove that $$|x_1-x_2|>\frac 1{M^2+3M+1}.$$
[i]Created by Jingjun Han[/i]
2002 AMC 10, 15
The positive integers $ A$, $ B$, $ A \minus{} B$, and $ A \plus{} B$ are all prime numbers. The sum of these four primes is
$ \textbf{(A)}\ \text{even} \qquad \textbf{(B)}\ \text{divisible by }3 \qquad \textbf{(C)}\ \text{divisible by }5 \qquad \textbf{(D)}\ \text{divisible by }7 \\ \textbf{(E)}\ \text{prime}$
2006 Baltic Way, 5
An occasionally unreliable professor has devoted his last book to a certain binary operation $*$. When this operation is applied to any two integers, the result is again an integer. The operation is known to satisfy the following axioms:
$\text{a})\ x*(x*y)=y$ for all $x,y\in\mathbb{Z}$;
$\text{b})\ (x*y)*y=x$ for all $x,y\in\mathbb{Z}$.
The professor claims in his book that
$1.$ The operation $*$ is commutative: $x*y=y*x$ for all $x,y\in\mathbb{Z}$.
$2.$ The operation $*$ is associative: $(x*y)*z=x*(y*z)$ for all $x,y,z\in\mathbb{Z}$.
Which of these claims follow from the stated axioms?
2024 Taiwan TST Round 2, C
Find all functions $f:\mathbb{N}\to\mathbb{N}$ s.t. for all $A\subset \mathbb{N}$ with 2024 elements, the set $$S_A:=\{f^{(k)}(x)\mid k=1,...,2024,x\in A\}$$ also has 2024 elements. ($f^{(k)}=f\circ f\circ...\circ f$ is the $k$-th iteration of $f$.)
1972 IMO Longlists, 3
On a line a set of segments is given of total length less than $n$. Prove that every set of $n$ points of the line can be translated in some direction along the line for a distance smaller than $\frac{n}{2}$ so that none of the points remain on the segments.
2021 Indonesia TST, A
Given a polynomial $p(x) =Ax^3+x^2-A$ with $A \neq 0$. Show that for every different real number $a,b,c$, at least one of $ap(b)$, $bp(a)$, and $cp(a)$ not equal to 1.
2022 Thailand TST, 1
Consider a $100\times 100$ square unit lattice $\textbf{L}$ (hence $\textbf{L}$ has $10000$ points). Suppose $\mathcal{F}$ is a set of polygons such that all vertices of polygons in $\mathcal{F}$ lie in $\textbf{L}$ and every point in $\textbf{L}$ is the vertex of exactly one polygon in $\mathcal{F}.$ Find the maximum possible sum of the areas of the polygons in $\mathcal{F}.$
[i]Michael Ren and Ankan Bhattacharya, USA[/i]
2012 Greece Junior Math Olympiad, 4
On a plane $\Pi$ is given a straight line $\ell$ and on the line $\ell$ are given two different points $A_1, A_2$. We consider on the plane $\Pi$, outside the line $\ell$, two different points $A_3, A_4$. Examine if it is possible to put points $A_3$ and $A_4$ on such positions such the four points $A_1, A_2, A_3, A_4$ form the maximal number of possible isosceles triangles, in the following cases:
(a) when the points $A_3, A_4$ belong to dierent semi-planes with respect to $\ell$;
(b) when the points $A_3, A_4$ belong to the same semi-planes with respect to $\ell$.
Give all possible cases and explain how is possible to construct in each case the points $A_3$ and $A_4$.
2019 Belarus Team Selection Test, 8.2
Let $\mathbb Z$ be the set of all integers. Find all functions $f:\mathbb Z\to\mathbb Z$ satisfying the following conditions:
1. $f(f(x))=xf(x)-x^2+2$ for all $x\in\mathbb Z$;
2. $f$ takes all integer values.
[i](I. Voronovich)[/i]
2009 Bosnia and Herzegovina Junior BMO TST, 1
Lengths of sides of triangle $ABC$ are positive integers, and smallest side is equal to $2$. Determine the area of triangle $P$ if $v_c = v_a + v_b$, where $v_a$, $v_b$ and $v_c$ are lengths of altitudes in triangle $ABC$ from vertices $A$, $B$ and $C$, respectively.
1993 Bundeswettbewerb Mathematik, 1
In a regular nonagon, each vertex is colored either red or green. Three corners of the nonagon determine a triangle. Such a triangle is called [i]red [/i] or [i]green [/i] if all its vertices are red or green if all are green. Prove that for each such coloring of the nonagon there are at least two different ones , that are congruent triangles of the same color.
2006 Thailand Mathematical Olympiad, 7
Let $x, y, z$ be reals summing to $1$ which minimizes $2x^2 + 3y^2 + 4z^2$. Find $x$.
2025 Harvard-MIT Mathematics Tournament, 1
Let $a,b,$ and $c$ be pairwise distinct positive integers such that $\tfrac{1}{a}, \tfrac{1}{b}, \tfrac{1}{c}$ is an increasing arithmetic sequence in that order. Prove that $\gcd(a,b)>1.$
2024 All-Russian Olympiad Regional Round, 10.6
Do there exist distinct reals $x, y, z$, such that $\frac{1}{x^2+x+1}+\frac{1}{y^2+y+1}+\frac{1}{z^2+z+1}=4$?
2018 Morocco TST., 3
In triangle $ABC$, let $\omega$ be the excircle opposite to $A$. Let $D, E$ and $F$ be the points where $\omega$ is tangent to $BC, CA$, and $AB$, respectively. The circle $AEF$ intersects line $BC$ at $P$ and $Q$. Let $M$ be the midpoint of $AD$. Prove that the circle $MPQ$ is tangent to $\omega$.