Olympiad problems

2008 Germany Team Selection Test, 1

Tags: inequalities , algebra

Determine $ Q \in \mathbb{R}$ which is so big that a sequence with non-negative reals elements $ a_1 ,a_2, \ldots$ which satisfies the following two conditions: [b](i)[/b] $ \forall m,n \geq 1$ we have $ a_{m \plus{} n} \leq 2 \left(a_m \plus{} a_n \right)$ [b](ii)[/b] $ \forall k \geq 0$ we have $ a_{2^k} \leq \frac {1}{(k \plus{} 1)^{2008}}$ such that for each sequence element we have the inequality $ a_n \leq Q.$

2016 Harvard-MIT Mathematics Tournament, 8

Tags: hmmt

Let $P_1P_2 \ldots P_8$ be a convex octagon. An integer $i$ is chosen uniformly at random from $1$ to $7$, inclusive. For each vertex of the octagon, the line between that vertex and the vertex $i$ vertices to the right is painted red. What is the expected number times two red lines intersect at a point that is not one of the vertices, given that no three diagonals are concurrent?

2021 BMT, 8

Tags: expected value

Consider the randomly generated base 10 real number $r = 0.\overline{p_0p_1p_2\ldots}$, where each $p_i$ is a digit from $0$ to $9$, inclusive, generated as follows: $p_0$ is generated uniformly at random from $0$ to $9$, inclusive, and for all $i \geq 0$, $p_{i + 1}$ is generated uniformly at random from $p_i$ to $9$, inclusive. Compute the expected value of $r$.

1998 Belarus Team Selection Test, 2

Tags: algebra , set

Find all finite sets $M \subset R$ containing at least two elements such that $(2a/3 -b^2) \in M$ for any two different elements $a,b \in M$.

2018 Silk Road, 2

Tags: functional equation , algebra

Find all functions $f:\ \mathbb{R}\rightarrow\mathbb{R}$ such that for any real number $x$ the equalities are true: $f\left(x+1\right)=1+f(x)$ and $f\left(x^4-x^2\right)=f^4(x)-f^2(x).$ [url=http://matol.kz/comments/3373/show]source[/url]

2023 Macedonian Team Selection Test, Problem 3

Tags: function , algebra

Let $f:\mathbb{N} \rightarrow \mathbb{N}$ be a monotonically increasing function over the natural numbers, such that $f(f(n))=n^{2}$. What is the smallest, and what is the largest value that $f(2023)$ can take? [i]Proposed by Ilija Jovcheski[/i]

2022-23 IOQM India, 19

Tags: combinatorics , counting

Consider a string of $n$ $1's$. We wish to place some $+$ signs in between so that the sum is $1000$. For instance, if $n=190$, one may put $+$ signs so as to get $11$ ninety times and $1$ ten times , and get the sum $1000$. If $a$ is the number of positive integers $n$ for which it is possible to place $+$ signs so as to get the sum $1000$, then find the sum of digits of $a$.

2014 Sharygin Geometry Olympiad, 4

Tags: circles , geometry , common tangent

Let $H$ be the orthocenter of a triangle $ABC$. Given that $H$ lies on the incircle of $ABC$ , prove that three circles with centers $A, B, C$ and radii $AH, BH, CH$ have a common tangent. (Mahdi Etesami Fard)

2007 Harvard-MIT Mathematics Tournament, 18

Tags: geometry , circumcircle

Convex quadrilateral $ABCD$ has right angles $\angle A$ and $\angle C$ and is such that $AB=BC$ and $AD=CD$. The diagonals $AC$ and $BD$ intersect at point $M$. Points $P$ and $Q$ lie on the circumcircle of triangle $AMB$ and segment $CD$, respectively, such that points $P$, $M$, and $Q$ are collinear. Suppose that $m\angle ABC=160^\circ$ and $m\angle QMC=40^\circ$. Find $MP\cdot MQ$, given that $MC=6$.

2013 Costa Rica - Final Round, 2

Tags: number theory , odd , even , sum , composite number

Determine all even positive integers that can be written as the sum of odd composite positive integers.

2000 Tournament Of Towns, 5

Tags: rectangle table , table , combinatorics

Each of the cells of an $m \times n$ table is coloured either black or white. For each cell, the total number of the cells which are in the same row or in the same column and of the same colour as this cell is strictly less than the total number of the cells which are in the same row or in the same column and of the other colour as this cell. Prove that in each row and in each column the number of white cells is the same as the number of black ones. (A Shapovalov)

1984 IMO Longlists, 25

Tags: number theory , equation , product , perfect square

Prove that the product of five consecutive positive integers cannot be the square of an integer.

2012 Korea Junior Math Olympiad, 8

Tags: probability , combinatorics

Let there be $n$ students, numbered $1$ through $n$. Let there be $n$ cards with numbers $1$ through $n$ written on them. Each student picks a card from the stack, and two students are called a pair if they pick each other's number. Let the probability that there are no pairs be $p_n$. Prove that $p_n - p_{n-1}=0$ if $n$ is odd, and prove that $p_n - p_{n-1}= \frac{1}{(-2)^kk^{1-k}}$ if $n = 2k$.

2024 Auckland Mathematical Olympiad, 4

Tags: angle chasing , geometry

The altitude $AH$ and the bisector $CL$ of triangle $ABC$ intersect at point $O$. Find the angle $BAC$, if it is known that the difference between angle $COH$ and half of angle $ABC$ is $46$.

2012 NIMO Problems, 3

Tags: factorial

Let \[ S = \sum_{i = 1}^{2012} i!. \] The tens and units digits of $S$ (in decimal notation) are $a$ and $b$, respectively. Compute $10a + b$. [i]Proposed by Lewis Chen[/i]

2016 ASDAN Math Tournament, 5

Tags: algebra test

Let $f(x)$ be a real valued function. Recall that if the inverse function $f^{-1}(x)$ exists, then $f^{-1}(x)$ satisfies $f(f^{-1}(x))=f^{-1}(f(x))=x$. Given that the inverse of the function $f(x)=x^3-12x^2+48x-60$ exists, find all real $a$ that satisfy $f(a)=f^{-1}(a)$.

1988 Greece Junior Math Olympiad, 1

Tags: algebra

i) Simplify $\left(a-\frac{4ab}{a+b}+b\right): \left(\frac{a}{a+b}-\frac{b}{b-a}-\frac{2ab}{a^2-b^2}\right)$ ii) Simplify $\frac{2x^2-(3a+b)x+a^2+ab}{2x^2-(a+3b)x+ab+b^2}$

2010 Albania National Olympiad, 5

Tags: pigeonhole principle , combinatorics

All members of the senate were firstly divided into $S$ senate commissions . According to the rules, no commission has less that $5$ senators and every two commissions have different number of senators. After the first session the commissions were closed and new commissions were opened. Some of the senators now are not a part of any commission. It resulted also that every two senators that were in the same commission in the first session , are not any more in the same commission. [b](a)[/b]Prove that at least $4S+10$ senators were left outside the commissions. [b](b)[/b]Prove that this number is achievable. Albanian National Mathematical Olympiad 2010---12 GRADE Question 5.

2017 Purple Comet Problems, 20

Tags:

A right circular cone has a height equal to three times its base radius and has volume 1. The cone is inscribed inside a sphere as shown. The volume of the sphere is $\frac{m}{n}$, where $m$ and $n$ are relatively prime positive integers. Find $m + n$. [center][img]https://snag.gy/92ikv3.jpg[/img][/center]

2011 Laurențiu Duican, 3

Tags: geometry , inequalities , circumradius , inradius

Prove that for a triangle $ ABC $ with $ \angle BAC \ge 90^{\circ } , $ having circumradius $ R $ and inradius $ r, $ the following inequality holds: $$ R\sin A>2r $$ [i]Romeo Ilie[/i]

2014 IMO, 3

Tags: geometry , circumcircle , quadrilateral , tangent , tangent circle

Convex quadrilateral $ABCD$ has $\angle ABC = \angle CDA = 90^{\circ}$. Point $H$ is the foot of the perpendicular from $A$ to $BD$. Points $S$ and $T$ lie on sides $AB$ and $AD$, respectively, such that $H$ lies inside triangle $SCT$ and \[ \angle CHS - \angle CSB = 90^{\circ}, \quad \angle THC - \angle DTC = 90^{\circ}. \] Prove that line $BD$ is tangent to the circumcircle of triangle $TSH$.

1971 Canada National Olympiad, 7

Tags: ratio

Let $n$ be a five digit number (whose first digit is non-zero) and let $m$ be the four digit number formed from $n$ by removing its middle digit. Determine all $n$ such that $n/m$ is an integer.

1985 Iran MO (2nd round), 4

Tags: floor function , number theory

Let $x$ and $y$ be two real numbers. Prove that the equations \[\lfloor x \rfloor + \lfloor y \rfloor =\lfloor x +y \rfloor , \quad \lfloor -x \rfloor + \lfloor -y \rfloor =\lfloor -x-y \rfloor\] Holds if and only if at least one of $x$ or $y$ be integer.

1995 Tuymaada Olympiad, 1

Tags: geometry

Give a geometric proof of the statement that the fold line on a sheet of paper is straight.

2021 Dutch IMO TST, 1

Tags: combinatorics , game , game strategy , winning strategy , domino

Let $m$ and $n$ be natural numbers with $mn$ even. Jetze is going to cover an $m \times n$ board (consisting of $m$ rows and $n$ columns) with dominoes, so that every domino covers exactly two squares, dominos do not protrude or overlap, and all squares are covered by a domino. Merlin then moves all the dominoe color red or blue on the board. Find the smallest non-negative integer $V$ (in terms of $m$ and $n$) so that Merlin can always ensure that in each row the number squares covered by a red domino and the number of squares covered by a blue one dominoes are not more than $V$, no matter how Jetze covers the board.