Olympiad problems

2015 Sharygin Geometry Olympiad, P12

Find the maximal number of discs which can be disposed on the plane so that each two of them have a common point and no three have it

2007 China Team Selection Test, 1

Tags: function , induction , algebra

Find all functions $ f: \mathbb{Q}^{\plus{}} \mapsto \mathbb{Q}^{\plus{}}$ such that: \[ f(x) \plus{} f(y) \plus{} 2xy f(xy) \equal{} \frac {f(xy)}{f(x\plus{}y)}.\]

2019 Brazil Undergrad MO, 1

Tags: matrices , linear algebra , polynomial

Let $ I $ and $ 0 $ be the square identity and null matrices, both of size $ 2019 $. There is a square matrix $A$ with rational entries and size $ 2019 $ such that: a) $ A ^ 3 + 6A ^ 2-2I = 0 $? b) $ A ^ 4 + 6A ^ 3-2I = 0 $?

1970 IMO Longlists, 31

Tags: inequalities , geometry

Prove that for any triangle with sides $a, b, c$ and area $P$ the following inequality holds: \[P \leq \frac{\sqrt 3}{4} (abc)^{2/3}.\] Find all triangles for which equality holds.

2007 Italy TST, 2

Tags: circumcircle , geometry

Let $ABC$ a acute triangle. (a) Find the locus of all the points $P$ such that, calling $O_{a}, O_{b}, O_{c}$ the circumcenters of $PBC$, $PAC$, $PAB$: \[\frac{ O_{a}O_{b}}{AB}= \frac{ O_{b}O_{c}}{BC}=\frac{ O_{c}O_{a}}{CA}\] (b) For all points $P$ of the locus in (a), show that the lines $AO_{a}$, $BO_{b}$ , $CO_{c}$ are cuncurrent (in $X$); (c) Show that the power of $X$ wrt the circumcircle of $ABC$ is: \[-\frac{ a^{2}+b^{2}+c^{2}-5R^{2}}4\] Where $a=BC$ , $b=AC$ and $c=AB$.

1990 AIME Problems, 10

Tags: trigonometry , number theory , least common multiple , greatest common divisor , modular arithmetic , complex number , relatively prime

The sets $A = \{z : z^{18} = 1\}$ and $B = \{w : w^{48} = 1\}$ are both sets of complex roots of unity. The set $C = \{zw : z \in A \ \text{and} \ w \in B\}$ is also a set of complex roots of unity. How many distinct elements are in $C$?

2000 Harvard-MIT Mathematics Tournament, 3

Tags: algebra

Find the sum of all integers from $1$ to $1000$ inclusive which contain at least one $7$ in their digits, i.e. find $$7 + 17 +... + 979 + 987 + 997.$$

1949 Putnam, A4

Tags: geometry , 3d geometry , tetrahedron

Given that $P$ is a point inside a tetrahedron with vertices at $A, B, C$ and $D$, such that the sum of the distances $PA+PB+PC+PD$ is a minimum, show that the two angles $\angle APB$ and $\angle CPD$ are equal and are bisected by the same straight line. What other pair of angles must be equal?

2013 Sharygin Geometry Olympiad, 5

Tags: rhombus , geometry

Four segments drawn from a given point inside a convex quadrilateral to its vertices, split the quadrilateral into four equal triangles. Can we assert that this quadrilateral is a rhombus?

LMT Team Rounds 2010-20, A4 B14

Tags:

Let $\triangle ABC$ with $AB=AC$ and $BC=14$ be inscribed in a circle $\omega$. Let $D$ be the point on ray $BC$ such that $CD=6$. Let the intersection of $AD$ and $\omega$ be $E$. Given that $AE=7$, find $AC^2$. [i]Proposed by Ephram Chun and Euhan Kim[/i]

2019 Jozsef Wildt International Math Competition, W. 65

Tags: inequalities

If $a$, $b$, $c \geq 1$; $y \geq x \geq 1$; $p$, $q$, $r > 0$ then$$\left(\frac{1+y\left(a^pb^qc^r\right)^{\frac{1}{p+q+r}}}{1+x\left(a^pb^qc^r\right)^{\frac{1}{p+q+r}}}\right)^{\frac{p+q+r}{\left(a^pb^qc^r\right)^{\frac{1}{p+q+r}}}}\left(\frac{1+xa}{1+ya}\right)^{\frac{p}{a}}\left(\frac{1+xb}{1+yb}\right)^{\frac{q}{b}}\left(\frac{1+xc}{1+yc}\right)^{\frac{r}{c}}$$ $$\geq \prod \limits_{cyc}\left(\frac{1+y\left(a^pb^q\right)^{\frac{1}{p+q}}}{1+x\left(a^pb^q\right)^{\frac{1}{p+q}}}\right)^{\frac{p+q}{\left(a^pb^q\right)^{\frac{1}{p+q}}}}$$

2013 Tournament of Towns, 7

Tags: combinatorics , game

Two teams $A$ and $B$ play a school ping pong tournament. The team $A$ consists of $m$ students, and the team $B$ consists of $n$ students where $m \ne n$. There is only one ping pong table to play and the tournament is organized as follows: Two students from different teams start to play while other players form a line waiting for their turn to play. After each game the first player in the line replaces the member of the same team at the table and plays with the remaining player. The replaced player then goes to the end of the line. Prove that every two players from the opposite teams will eventually play against each other.

2003 Alexandru Myller, 2

Tags: number theory , complex number , algebra , newton s binom

Prove that $$ (n+2)^n=\prod_{k=1}^{n+1} \sum_{l=1}^{n+1} le^{\frac{2i\pi k (n-l+1)}{n+2}} , $$ for any natural number $ n. $ [i]Mihai Piticari[/i]

2014 Taiwan TST Round 1, 1

Tags: geometry , trigonometry , geometric transformation

Let $O_1$, $O_2$ be two circles with radii $R_1$ and $R_2$, and suppose the circles meet at $A$ and $D$. Draw a line $L$ through $D$ intersecting $O_1$, $O_2$ at $B$ and $C$. Now allow the distance between the centers as well as the choice of $L$ to vary. Find the length of $AD$ when the area of $ABC$ is maximized.

2012 Peru IMO TST, 3

Tags: combinatorics , discrete continous

Suppose that $1000$ students are standing in a circle. Prove that there exists an integer $k$ with $100 \leq k \leq 300$ such that in this circle there exists a contiguous group of $2k$ students, for which the first half contains the same number of girls as the second half. [i]Proposed by Gerhard Wöginger, Austria[/i]

Denmark (Mohr) - geometry, 1992.2

Tags: geometry , right triangle , tangent circle

In a right-angled triangle, $a$ and $b$ denote the lengths of the two catheti. A circle with radius $r$ has the center on the hypotenuse and touches both catheti. Show that $\frac{1}{a}+\frac{1}{b}=\frac{1}{r}$.

2001 Austrian-Polish Competition, 2

Tags: limit , system of equations , pigeonhole principle , algebra

Let $n$ be a positive integer greater than $2$. Solve in nonnegative real numbers the following system of equations \[x_{k}+x_{k+1}=x_{k+2}^{2}\quad , \quad k=1,2,\cdots,n\] where $x_{n+1}=x_{1}$ and $x_{n+2}=x_{2}$.

2016 Mathematical Talent Reward Programme, SAQ: P 5

Tags: function , injective function , surjective function

Let $\mathbb{N}$ be the set of all positive integers. $f,g:\mathbb{N} \to \mathbb{N}$ be funtions such that $f$ is onto and $g$ is one-one and $f(n)\geq g(n)$ for all positive integers $n$. Prove that $f=g$.

2024 USA TSTST, 4

Tags: geometry , circumcircle , cyclic quadrilateral

Let $ABCD$ be a quadrilateral inscribed in a circle with center $O$ and $E$ be the intersection of segments $AC$ and $BD$. Let $\omega_1$ be the circumcircle of $ADE$ and $\omega_2$ be the circumcircle of $BCE$. The tangent to $\omega_1$ at $A$ and the tangent to $\omega_2$ at $C$ meet at $P$. The tangent to $\omega_1$ at $D$ and the tangent to $\omega_2$ at $B$ meet at $Q$. Show that $OP=OQ$. [i]Merlijn Staps[/i]

2010 Junior Balkan Team Selection Tests - Romania, 2

Tags: equal angles , geometry , convex quadrilateral , angle

Let $ABCD$ be a convex quadrilateral with $\angle BCD= 120^o, \angle {CBA} = 45^o, \angle {CBD} = 15^o$ and $\angle {CAB} = 90^o$. Show that $AB = AD$.

2015 Bangladesh Mathematical Olympiad, 2

Tags: number theory , contest preparation , integer , solutions

[b][u]BdMO National Higher Secondary Problem 3[/u][/b] Let $N$ be the number if pairs of integers $(m,n)$ that satisfies the equation $m^2+n^2=m^3$ Is $N$ finite or infinite?If $N$ is finite,what is its value?

1953 Kurschak Competition, 2

Tags: number theory , perfect square , divide

$n$ and $d$ are positive integers such that $d$ divides $2n^2$. Prove that $n^2 + d$ cannot be a square.

2010 IFYM, Sozopol, 6

Tags: number theory

Let $A=\{ x\in \mathbb{N},x=a^2+2b^2,a,b\in \mathbb{Z},ab\neq 0 \}$ and $p$ is a prime number. Prove that if $p^2\in A$, then $p\in A$.

2017 USAMTS Problems, 3

Tags:

Do there exist two polygons such that, by putting them together in three different ways (without holes, overlap, or reflections), we can obtain first a triangle, then a convex quadrilateral, and lastly a convex pentagon?

2024 Vietnam Team Selection Test, 5

Tags: geometry , complex number , poles and polars , pascal theorem

Let incircle $(I)$ of triangle $ABC$ touch the sides $BC,CA,AB$ at $D,E,F$ respectively. Let $(O)$ be the circumcircle of $ABC$. Ray $EF$ meets $(O)$ at $M$. Tangents at $M$ and $A$ of $(O)$ meet at $S$. Tangents at $B$ and $C$ of $(O)$ meet at $T$. Line $TI$ meets $OA$ at $J$. Prove that $\angle ASJ=\angle IST$.