Olympiad problems

1977 IMO Longlists, 38

Tags: inequalities

Let $m_j > 0$ for $j = 1, 2,\ldots, n$ and $a_1 \leq \cdots \leq a_n < b_1 \leq \cdots \leq b_n < c_1 \leq \cdots \leq c_n$ be real numbers. Prove that \[\Biggl( \sum_{j=1}^{n} m_j(a_j+b_j+c_j) \Biggr)^2 > 3 \Biggl( \sum_{j=1}^{n} m_j \Biggr) \Biggl( \sum_{j=1}^{n} m_j(a_jb_j+b_jc_j+c_ja_j) \Biggr).\]

2012 Albania Team Selection Test, 2

Tags: ratio , circumcircle , geometry

It is given an acute triangle $ABC$ , $AB \neq AC$ where the feet of altitude from $A$ its $H$. In the extensions of the sides $AB$ and $AC$ (in the direction of $B$ and $C$) we take the points $P$ and $Q$ respectively such that $HP=HQ$ and the points $B,C,P,Q$ are concyclic. Find the ratio $\tfrac{HP}{HA}$.

2015 IFYM, Sozopol, 3

Tags: game strategy , combinatorics

A cube 10x10x10 is constructed from 1000 white unit cubes. Polly and Velly play the following game: Velly chooses a certain amount of parallelepipeds 1x1x10, no two of which have a common vertex or an edge, and repaints them in black. Polly can choose an arbitrary number of unit cubes and ask Velly for their color. What’s the least amount of unit cubes she has to choose so that she can determine the color of each unit cube?

2023 Bulgarian Autumn Math Competition, 12.4

Tags: combinatorics

A set of points in the plane is called $\textit{good}$ if the distance between any two points in it is at most $1$. Let $f(n, d)$ be the largest positive integer such that in any $\textit{good}$ set of $3n$ points, there is a circle of diameter $d$, which contains at least $f(n, d)$ points. Prove that there exists a positive real $\epsilon$, such that for all $d \in (1-\epsilon, 1)$, the value of $f(n, d)$ does not depend on $d$ and find that value as a function of $n$.

2010 Contests, 1

Tags: algebra

For a real number $t$ and positive real numbers $a,b$ we have \[2a^2-3abt+b^2=2a^2+abt-b^2=0\] Find $t.$

2003 Turkey Team Selection Test, 3

Tags: arithmetic sequence , number theory

Is there an arithmetic sequence with a. $2003$ b. infinitely many terms such that each term is a power of a natural number with a degree greater than $1$?

2015 Dutch Mathematical Olympiad, 5

Tags: algebra , inequalities

Given are (not necessarily positive) real numbers $a, b$, and $c$ for which $|a - b| \ge |c| , |b - c| \ge |a|$ and $|c - a| \ge |b|$ . Prove that one of the numbers $a, b$, and $c$ is the sum of the other two.

2015 Peru IMO TST, 13

Tags: geometry

Let $ABC$ be a triangle with circumcircle $\Omega$ and incentre $I$. Let the line passing through $I$ and perpendicular to $CI$ intersect the segment $BC$ and the arc $BC$ (not containing $A$) of $\Omega$ at points $U$ and $V$ , respectively. Let the line passing through $U$ and parallel to $AI$ intersect $AV$ at $X$, and let the line passing through $V$ and parallel to $AI$ intersect $AB$ at $Y$ . Let $W$ and $Z$ be the midpoints of $AX$ and $BC$, respectively. Prove that if the points $I, X,$ and $Y$ are collinear, then the points $I, W ,$ and $Z$ are also collinear. [i]Proposed by David B. Rush, USA[/i]

2014 Contests, 1

Tags: ratio , angle bisector , geometry

In a triangle $ABC$, the external bisector of $\angle BAC$ intersects the ray $BC$ at $D$. The feet of the perpendiculars from $B$ and $C$ to line $AD$ are $E$ and $F$, respectively and the foot of the perpendicular from $D$ to $AC$ is $G$. Show that $\angle DGE + \angle DGF = 180^{\circ}$.

1988 Bundeswettbewerb Mathematik, 3

Tags: geometry , perimeter , orthic triangle

Prove that all acute-angled triangles with the equal altitudes $h_c$ and the equal angles $\gamma$ have orthic triangles with same perimeters.

2010 Contests, 1

Tags: floor function , modular arithmetic , combinatorics

Let $D$ be the set of all pairs $(i,j)$, $1\le i,j\le n$. Prove there exists a subset $S \subset D$, with $|S|\ge\left \lfloor\frac{3n(n+1)}{5}\right \rfloor$, such that for any $(x_1,y_1), (x_2,y_2) \in S$ we have $(x_1+x_2,y_1+y_2) \not \in S$. (Peter Cameron)

2019 China Team Selection Test, 3

Tags: inequalities

Let $n$ be a given even number, $a_1,a_2,\cdots,a_n$ be non-negative real numbers such that $a_1+a_2+\cdots+a_n=1.$ Find the maximum possible value of $\sum_{1\le i<j\le n}\min\{(i-j)^2,(n+i-j)^2\}a_ia_j .$

2015 Mexico National Olympiad, 4

Tags: number theory , combinatorics

Let $n$ be a positive integer. Mary writes the $n^3$ triples of not necessarily distinct integers, each between $1$ and $n$ inclusive on a board. Afterwards, she finds the greatest (possibly more than one), and erases the rest. For example, in the triple $(1, 3, 4)$ she erases the numbers 1 and 3, and in the triple $(1, 2, 2)$ she erases only the number 1, Show after finishing this process, the amount of remaining numbers on the board cannot be a perfect square.

Geometry Mathley 2011-12, 11.3

Tags: geometry , equal angles , equal ratio , angle

Let $ABC$ be a triangle such that $AB = AC$ and let $M$ be a point interior to the triangle. If $BM$ meets $AC$ at $D$. show that $\frac{DM}{DA}=\frac{AM}{AB}$ if and only if $\angle AMB = 2\angle ABC$. Michel Bataille

2009 Purple Comet Problems, 9

Tags:

One plant is now $44$ centimeters tall and will grow at a rate of $3$ centimeters every $2$ years. A second plant is now $80$ centimeters tall and will grow at a rate of $5$ centimeters every $6$ years. In how many years will the plants be the same height?

2012 NIMO Problems, 5

Tags: number theory , relatively prime

A number is called [i]purple[/i] if it can be expressed in the form $\frac{1}{2^a 5^b}$ for positive integers $a > b$. The sum of all purple numbers can be expressed as $\frac{a}{b}$ for relatively prime positive integers $a, b$. Compute $100a + b$. [i]Proposed by Eugene Chen[/i]

2006 Victor Vâlcovici, 2

Tags: geometry , circumcircle , analytic geometry

Consider a point $ B $ on a segment $ AC. $ Find the locus of the points $ M $ that have the property that the circumcircles of $ ABM $ and $ BCM $ have equal radii. [i]Nicolae Soare[/i]

Kyiv City MO Seniors Round2 2010+ geometry, 2018.10.3.1

Tags: circumcircle , perpendicular , geometry

The point $O$ is the center of the circumcircle of the acute triangle $ABC$. The line $AC$ intersects the circumscribed circle $\Delta ABO$ for second time at the point $X$. Prove that $XO \bot BC$.

1990 Austrian-Polish Competition, 3

Tags: algebra , system of equations

Show that there are two real solutions to: $$\begin{cases} x + y^2 + z^4 = 0 \\ y + z^2 + x^4 = 0 \\ z + x^2 + y^5 = 0\end {cases}$$

2022 Dutch IMO TST, 2

Tags: algebra

Let $\lfloor x \rfloor$ denote the greatest integer less than or equal to $x.$ Let $\lambda \geq 1$ be a real number and $n$ be a positive integer with the property that $\lfloor \lambda^{n+1}\rfloor, \lfloor \lambda^{n+2}\rfloor ,\cdots, \lfloor \lambda^{4n}\rfloor$ are all perfect squares$.$ Prove that $\lfloor \lambda \rfloor$ is a perfect square$.$

2018 AMC 10, 24

Tags: geometry

Let $ABCDEF$ be a regular hexagon with side length $1$. Denote by $X, Y,$ and $Z$ the midpoints of $\overline{AB}$, $\overline{CD}$, and $\overline{EF}$, respectively. What is the area of the convex hexagon whose interior is the intersection of the interiors of $\triangle ACE$ and $\triangle XYZ$? $\textbf{(A) }\dfrac{3}{8}\sqrt{3}\qquad\textbf{(B) }\dfrac{7}{16}\sqrt{3}\qquad\textbf{(C) }\dfrac{15}{32}\sqrt{3}\qquad\textbf{(D) }\dfrac{1}{2}\sqrt{3}\qquad\textbf{(E) }\dfrac{9}{16}\sqrt{3}$

2003 Finnish National High School Mathematics Competition, 5

Tags: symmetry , arithmetic sequence , combinatorics

Players Aino and Eino take turns choosing numbers from the set $\{0,..., n\}$ with $n\in \Bbb{N}$ being fixed in advance. The game ends when the numbers picked by one of the players include an arithmetic progression of length $4.$ The one who obtains the progression wins. Prove that for some $n,$ the starter of the game wins. Find the smallest such $n.$

2024 Canadian Mathematical Olympiad Qualification, 1

Tags: algebra , functional equation

Find all functions $f : R \to R$ that satisfy the functional equation $$f(x + f(xy)) = f(x)(1 + y).$$

2013 Middle European Mathematical Olympiad, 3

Tags: perpendicular bisector , angle chasing , geometry

Let $ABC$ be an isosceles triangle with $AC=BC$. Let $N$ be a point inside the triangle such that $2 \angle ANB = 180 ^\circ + \angle ACB $. Let $ D $ be the intersection of the line $BN$ and the line parallel to $AN$ that passes through $C$. Let $P$ be the intersection of the angle bisectors of the angles $CAN$ and $ABN$. Show that the lines $DP$ and $AN$ are perpendicular.

2016 ISI Entrance Examination, 2

Tags: algebra , polynomial

Consider the polynomial $ax^3+bx^2+cx+d$ where $a,b,c,d$ are integers such that $ad$ is odd and $bc$ is even.Prove that not all of its roots are rational..