Olympiad problems

2017 Iranian Geometry Olympiad, 4

Tags: geometry

In the isosceles triangle $ABC$ ($AB=AC$), let $l$ be a line parallel to $BC$ through $A$. Let $D$ be an arbitrary point on $l$. Let $E,F$ be the feet of perpendiculars through $A$ to $BD,CD$ respectively. Suppose that $P,Q$ are the images of $E,F$ on $l$. Prove that $AP+AQ\le AB$ [i]Proposed by Morteza Saghafian[/i]

2005 CentroAmerican, 5

Tags: perimeter , trigonometry , parallelogram , angle bisector , geometry

Let $ABC$ be a triangle, $H$ the orthocenter and $M$ the midpoint of $AC$. Let $\ell$ be the parallel through $M$ to the bisector of $\angle AHC$. Prove that $\ell$ divides the triangle in two parts of equal perimeters. [i]Pedro Marrone, Panamá[/i]

2019 Latvia Baltic Way TST, 1

Tags: inequalities

Prove that for all positive real numbers $a, b, c$ with $\frac{1}{a}+\frac{1}{b}+\frac{1}{c} =1$ the following inequality holds: $$3(ab+bc+ca)+\frac{9}{a+b+c} \le \frac{9abc}{a+b+c} + 2(a^2+b^2+c^2)+1$$

2013 HMNT, 9

Tags: number theory

For an integer $n \ge 0$, let $f(n)$ be the smallest possible value of $ |x + y|$, where $x$ and $y$ are integers such that $3x - 2y = n$. Evaluate $f(0) + f(1) + f(2) +...+ f(2013)$.

2022 APMO, 3

Tags: algebra , number theory

Find all positive integers $k<202$ for which there exist a positive integers $n$ such that $$\bigg {\{}\frac{n}{202}\bigg {\}}+\bigg {\{}\frac{2n}{202}\bigg {\}}+\cdots +\bigg {\{}\frac{kn}{202}\bigg {\}}=\frac{k}{2}$$

2016 Taiwan TST Round 2, 2

Tags: number theory , divisibility

Let $m$ and $n$ be positive integers such that $m>n$. Define $x_k=\frac{m+k}{n+k}$ for $k=1,2,\ldots,n+1$. Prove that if all the numbers $x_1,x_2,\ldots,x_{n+1}$ are integers, then $x_1x_2\ldots x_{n+1}-1$ is divisible by an odd prime.

2013 Junior Balkan Team Selection Tests - Moldova, 8

Tags: coloring , combinatorial geometry , combinatorics , rectangle

A point $M (x, y)$ of the Cartesian plane of $xOy$ coordinates is called [i]lattice [/i] if it has integer coordinates. Each lattice point is colored red or blue. Prove that in the plan there is at least one rectangle with lattice vertices of the same color.

2020 JHMT, 3

Tags: geometry , ellipse , conic

Consider a right cylinder with height $5\sqrt3$. A plane intersects each of the bases of the cylinder at exactly one point, and the cylindric section (the intersection of the plane and the cylinder) forms an ellipse. Find the product of the sum and the dierence of the lengths of the major and minor axes of this ellipse. [i]Note:[/i] An ellipse is a regular oval shape resulting when a cone is cut by an oblique plane which does not intersect the base. The major axis is the longer diameter and the minor axis the shorter.

2004 Bosnia and Herzegovina Team Selection Test, 6

Tags: geometry , parallelogram , concurrency

It is given triangle $ABC$ and parallelogram $ASCR$ with diagonal $AC$. Let line constructed through point $B$ parallel with $CS$ intersects line $AS$ and $CR$ in $M$ and $P$, respectively. Let line constructed through point $B$ parallel with $AS$ intersects line $AR$ and $CS$ in $N$ and $Q$, respectively. Prove that lines $RS$, $MN$ and $PQ$ are concurrent

Russian TST 2019, P1

Tags: number theory , divisor

A positive integer $n{}$ is called [i]discontinuous[/i] if for all its natural divisors $1 = d_0 < d_1 <\cdots<d_k$, written out in ascending order, there exists $1 \leqslant i \leqslant k$ such that $d_i > d_{i-1}+\cdots+d_1+d_0+1$. Prove that there are infinitely many positive integers $n{}$ such that $n,n+1,\ldots,n+2019$ are all discontinuous.

2021-IMOC qualification, G2

Tags: geometry , perpendicular

Given a triangle $ABC$, $D$ is the reflection from the perpendicular foot from $A$ to $BC$ through the midpoint of $BC$. $E$ is the reflection from the perpendicular foot from $B$ to $CA$ through the midpoint of $CA$. $F$ is the reflection from the perpendicular foot from $C$ to $AB$ through the midpoint of $AB$. Prove: $DE \perp AC$ if and only if $DF \perp AB$

2023 Bundeswettbewerb Mathematik, 3

Tags: geometry

Given two parallelograms $ABCD$ and $AECF$ with common diagonal $AC$, where $E$ and $F$ lie inside parallelogram $ABCD$. Show: The circumcircles of the triangles $AEB$, $BFC$, $CED$ and $DFA$ have one point in common.

STEMS 2023 Math Cat A, 4

Tags: algebra , polynomial

Alice has $n > 1$ one variable quadratic polynomials written on paper she keeps secret from Bob. On each move, Bob announces a real number and Alice tells him the value of one of her polynomials at this number. Prove that there exists a constant $C > 0$ such that after $Cn^5$ questions, Bob can determine one of Alice’s polynomials. [i]Proposed by Rohan Goyal and Anant Mudgal[/i]

2023 Czech-Polish-Slovak Junior Match, 1

Tags: number theory , perfect square , sum of digits

Let $S(n)$ denote the sum of all digits of natural number $n$. Determine all natural numbers $n$ for which both numbers $n + S(n)$ and $n - S(n)$ are square powers of non-zero integers.

1999 Slovenia National Olympiad, Problem 1

Tags: algebra , digit

Two three-digit numbers are given. The hundreds digit of each of them is equal to the units digit of the other. Find these numbers if their difference is $297$ and the sum of digits of the smaller number is $23$.

2012 JBMO TST - Turkey, 1

Tags: geometry , 3d geometry , least common multiple , number theory

Find the greatest positive integer $n$ for which $n$ is divisible by all positive integers whose cube is not greater than $n.$

2000 Polish MO Finals, 3

Tags: number theory

The sequence $p_1, p_2, p_3, ...$ is defined as follows. $p_1$ and $p_2$ are primes. $p_n$ is the greatest prime divisor of $p_{n-1} + p_{n-2} + 2000$. Show that the sequence is bounded.

1997 Croatia National Olympiad, Problem 4

Tags: combinatorics , combinatorial geometry , geometry

Let $k$ be a natural number. Determine the number of non-congruent triangles with the vertices at vertices of a given regular $6k$-gon.

2014 May Olympiad, 3

Tags: combinatorics , game , number theory , multiple

Ana and Luca play the following game. Ana writes a list of $n$ different integer numbers. Luca wins if he can choose four different numbers, $a, b, c$ and $d$, so that the number $a+b-(c+d)$ is multiple of $20$. Determine the minimum value of $n$ for which, whatever Ana's list, Luca can win.

1981 Poland - Second Round, 2

Tags: geometry , angle bisector , circles

Two circles touch internally at point $P$. A line tangent to one of the circles at point $A$ intersects the other circle at points $B$ and $C$. Prove that the line $ PA $ is the bisector of the angle $ BPC $.

2008 AMC 12/AHSME, 23

Tags: geometry , symmetry , geometric transformation , rotation , analytic geometry , trigonometry

The solutions of the equation $ z^4 \plus{} 4z^3i \minus{} 6z^2 \minus{} 4zi \minus{} i \equal{} 0$ are the vertices of a convex polygon in the complex plane. What is the area of the polygon? $ \textbf{(A)}\ 2^{5/8} \qquad \textbf{(B)}\ 2^{3/4} \qquad \textbf{(C)}\ 2 \qquad \textbf{(D)}\ 2^{5/4} \qquad \textbf{(E)}\ 2^{3/2}$

2014 BMT Spring, 6

Tags: fe , functional equation , algebra

Find $f(2)$ given that $f$ is a real-valued function that satisfies the equation $$4f(x)+\left(\frac23\right)(x^2+2)f\left(x-\frac2x\right)=x^3+1.$$

2006 Team Selection Test For CSMO, 4

Tags: combinatorics

All the squares of a board of $(n+1)\times(n-1)$ squares are painted with [b]three colors[/b] such that, for any two different columns and any two different rows, the 4 squares in their intersections they don't have all the same color. Find the greatest possible value of $n$.

2002 USAMTS Problems, 2

Tags: function

The integer 72 is the first of three consecutive integers 72, 73, and 74, that can each be expressed as the sum of the squares of two positive integers. The integers 72, 288, and 800 are the first three members of an infinite increasing sequence of integers with the above property. Find a function that generates the sequence and give the next three members.

1995 AIME Problems, 14

Tags: geometry , trigonometry , asymptote , vector , function , area of a triangle , heron's formula

In a circle of radius 42, two chords of length 78 intersect at a point whose distance from the center is 18. The two chords divide the interior of the circle into four regions. Two of these regions are bordered by segments of unequal lenghts, and the area of either of them can be expressed uniquley in the form $m\pi-n\sqrt{d},$ where $m, n,$ and $d$ are positive integers and $d$ is not divisible by the square of any prime number. Find $m+n+d.$