Olympiad problems

1993 AMC 12/AHSME, 18

Tags:

Al and Barb start their new jobs on the same day. Al's schedule is $3$ work-days followed by $1$ rest-day. Barb's schedule is $7$ work-days followed by $3$ rest-days. On how many of their first $1000$ days do both have rest-days on the same day? $ \textbf{(A)}\ 48 \qquad\textbf{(B)}\ 50 \qquad\textbf{(C)}\ 72 \qquad\textbf{(D)}\ 75 \qquad\textbf{(E)}\ 100 $

2014 Contests, 2

The center of the circumcircle of the acute triangle $ABC$ is $M$, and the circumcircle of $ABM$ meets $BC$ and $AC$ at $P$ and $Q$ ($P\ne B$). Show that the extension of the line segment $CM$ is perpendicular to $PQ$.

1969 All Soviet Union Mathematical Olympiad, 120

Tags: number theory , relatively prime

Given natural $n$. Consider all the fractions $1/(pq)$, where $p$ and $q$ are relatively prime, $0<p<q\le n , p+q>n$. Prove that the sum of all such a fractions equals to $1/2$.

2023 239 Open Mathematical Olympiad, 2

Tags: number theory , divisibility

Let $1 < a_1 < a_2 < \cdots < a_N$ be natural numbers. It is known that for any $1\leqslant i\leqslant N$ the product of all these numbers except $a_i$ increased by one, is a multiple of $a_i$. Prove that $a_1\leqslant N$.

2010 Puerto Rico Team Selection Test, 6

Tags: inequalities , algebra

Find all values of $ r$ such that the inequality $$r (ab + bc + ca) + (3- r) \left( \frac{1}{a}+\frac{1}{b}+\frac{1}{c} \right) \ge 9$$ is true for $a,b,c$ arbitrary positive reals

Kvant 2020, M818

Tags: combinatorics , coloring

Some $k{}$ vertices of a regular $n{}$-gon are colored red. We will call a coloring [i]uniform[/i] if for any $m$ the number of red vertices in any two sets of $m$ consecutive vertices of the $n{}$-gon coincide or differ by 1. Prove that a uniform coloring exists for any $k<n$ and is unique, up to rotations of the $n{}$-gon. [i]Proposed by M. Kontsevich[/i]

2015 BMT Spring, 6

Tags: number theory

An integer-valued function $f$ satisfies $f(2) = 4$ and $f(mn) = f(m)f(n)$ for all integers $m$ and $n$. If $f$ is an increasing function, determine $f(2015)$.

PEN A Problems, 75

Tags: modular arithmetic , divisibility theory

Find all triples $(a,b,c)$ of positive integers such that $2^{c}-1$ divides $2^{a}+2^{b}+1$.

2002 Polish MO Finals, 3

Tags: combinatorics

Three non-negative integers are written on a blackboard. A move is to replace two of the integers $k,m$ by $k+m$ and $|k-m|$. Determine whether we can always end with triplet which has at least two zeros

2023 IFYM, Sozopol, 5

Tags: algebra , polynomial

Let $a$ and $b$ be natural numbers. Prove that the number of polynomials $P(x)$ with integer coefficients such that $|P(n)| \leq a^n$ for every natural number $n \geq b$ is finite.

1988 Nordic, 3

Tags: geometry , 3d geometry , sphere

Two concentric spheres have radii $r$ and $R,r < R$. We try to select points $A, B$ and $C$ on the surface of the larger sphere such that all sides of the triangle $ABC$ would be tangent to the surface of the smaller sphere. Show that the points can be selected if and only if $R \le 2r$.

2022 CCA Math Bonanza, I6

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Let regular tetrahedron $ABCD$ have center $O$. Find $\tan^2(\angle AOB)$. [i]2022 CCA Math Bonanza Individual Round #6[/i]

1989 Flanders Math Olympiad, 2

Tags: ratio , trigonometry , geometry

When drawing all diagonals in a regular pentagon, one gets an smaller pentagon in the middle. What's the ratio of the areas of those pentagons?

2009 Iran Team Selection Test, 5

Tags: angle bisector , geometry

$ ABC$ is a triangle and $ AA'$ , $ BB'$ and $ CC'$ are three altitudes of this triangle . Let $ P$ be the feet of perpendicular from $ C'$ to $ A'B'$ , and $ Q$ is a point on $ A'B'$ such that $ QA \equal{} QB$ . Prove that : $ \angle PBQ \equal{} \angle PAQ \equal{} \angle PC'C$

2014 JBMO TST - Macedonia, 3

Tags: number theory , digit , divisibility

Find all positive integers $n$ which are divisible by 11 and satisfy the following condition: all the numbers which are generated by an arbitrary rearrangement of the digits of $n$, are also divisible by 11.

1999 Croatia National Olympiad, Problem 4

Tags: combinatorics , tournament

In a basketball competition, $n$ teams took part. Each pair of teams played exactly one match, and there were no draws. At the end of the competition the $i$-th team had $x_i$ wins and $y_i$ defeats $(i=1,\ldots,n)$. Prove that $x_1^2+x_2^2+\ldots+x_n^2=y_1^2+y_2^2+\ldots+y_n^2$.

2014 AMC 8, 19

Tags: geometry , 3d geometry , probability

A cube with $3$-inch edges is to be constructed from $27$ smaller cubes with $1$-inch edges. Twenty-one of the cubes are colored red and $6$ are colored white. If the $3$-inch cube is constructed to have the smallest possible white surface area showing, what fraction of the surface area is white? $\textbf{(A) }\frac{5}{54}\qquad\textbf{(B) }\frac{1}{9}\qquad\textbf{(C) }\frac{5}{27}\qquad\textbf{(D) }\frac{2}{9}\qquad \textbf{(E) }\frac{1}{3}$

2011 Romanian Masters In Mathematics, 2

Tags: algebra , polynomial , modular arithmetic , function , number theory , greatest common divisor , system of equations

Determine all positive integers $n$ for which there exists a polynomial $f(x)$ with real coefficients, with the following properties: (1) for each integer $k$, the number $f(k)$ is an integer if and only if $k$ is not divisible by $n$; (2) the degree of $f$ is less than $n$. [i](Hungary) Géza Kós[/i]

I Soros Olympiad 1994-95 (Rus + Ukr), 11.5

Tags: function , inequalities , algebra

Function $f(x)$. which is defined on the set of non-negative real numbers, acquires real values. It is known that $f(0)\le 0$ and the function $f(x)/x$ is increasing for $x>0$. Prove that for arbitrary $x\ge 0$ and $y\ge 0$, holds the inequality $f(x+y)\ge f(x)+ f(y)$ .

2006 MOP Homework, 2

Tags: combinatorics

Mykolka the numismatist possesses $241$ coins, each worth an integer number of turgiks. The total value of the coins is $360$ turgiks. Is it necessarily true that the coins can be divided into three groups of equal total value?

2006 Nordic, 4

Tags: combinatorics

Each square of a $100\times 100$ board is painted with one of $100$ different colours, so that each colour is used exactly $100$ times. Show that there exists a row or column of the chessboard in which at least $10$ colours are used.

2021 New Zealand MO, 6

Tags: combinatorics , consecutive , divisor , number theory

Is it possible to place a positive integer in every cell of a $10 \times 10$ array in such a way that both the following conditions are satisfied? $\bullet$ Each number (not in the top row) is a proper divisor of the number immediately above. $\bullet$ Each row consists of 1$0$ consecutive positive integers (but not necessarily in order).

2024 JHMT HS, 9

Tags: number theory

Let $N \in \{10, 11, \ldots, 99\}$ be a two-digit positive integer. Compute the number of values of $N$ for which the last two digits in the decimal expansion of $N^{21}$ are the digits of $N$ in the same order.

2010 District Olympiad, 2

Tags: superior algebra , abstract algebra , group theory

Let $ G$ be a group such that if $ a,b\in \mathbb{G}$ and $ a^2b\equal{}ba^2$, then $ ab\equal{}ba$. i)If $ G$ has $ 2^n$ elements, prove that $ G$ is abelian. ii) Give an example of a non-abelian group with $ G$'s property from the enounce.

2004 Junior Balkan Team Selection Tests - Moldova, 7

Tags: circumcircle , angle bisector , geometry

Let the triangle $ABC$ have area $1$. The interior bisectors of the angles $\angle BAC,\angle ABC, \angle BCA$ intersect the sides $(BC), (AC), (AB) $ and the circumscribed circle of the respective triangle $ABC$ at the points $L$ and $G, N$ and $F, Q$ and $E$. The lines $EF, FG,GE$ intersect the bisectors $(AL), (CQ) ,(BN)$ respectively at points $P, M, R$. Determine the area of the hexagon $LMNPR$.