Olympiad problems

1989 IMO Longlists, 57

Tags: vector , combinatorics

Let $ v_1, v_2, \ldots, v_{1989}$ be a set of coplanar vectors with $ |v_r| \leq 1$ for $ 1 \leq r \leq 1989.$ Show that it is possible to find $ \epsilon_r$, $1 \leq r \leq 1989,$ each equal to $ \pm 1,$ such that \[ \left | \sum^{1989}_{r\equal{}1} \epsilon_r v_r \right | \leq \sqrt{3}.\]

2002 China Team Selection Test, 2

Tags: inequalities , function , induction , algebra

For any two rational numbers $ p$ and $ q$ in the interval $ (0,1)$ and function $ f$, there is always $ \displaystyle f \left( \frac{p\plus{}q}{2} \right) \leq \frac{f(p) \plus{} f(q)}{2}$. Then prove that for any rational numbers $ \lambda, x_1, x_2 \in (0,1)$, there is always: \[ f( \lambda x_1 \plus{} (1\minus{}\lambda) x_2 ) \leq \lambda f(x_i) \plus{} (1\minus{}\lambda) f(x_2)\]

2023 UMD Math Competition Part I, #20

Tags: geometry

A strip is defined as the region between two parallel lines; the width of the strip is the distance between the two lines. Two strips of width $1$ intersect in a parallelogram whose area is $2.$ What is the angle between the strips? \[ \mathrm a. ~ 15^\circ\qquad \mathrm b.~30^\circ \qquad \mathrm c. ~45^\circ \qquad \mathrm d. ~60^\circ \qquad \mathrm e. ~90^\circ\]

2014 Junior Balkan Team Selection Tests - Romania, 4

Tags: floor function , coloring , combinatorics

Let $n \ge 6$ be an integer. We have at our disposal $n$ colors. We color each of the unit squares of an $n \times n$ board with one of the $n$ colors. a) Prove that, for any such coloring, there exists a path of a chess knight from the bottom-left to the upper-right corner, that does not use all the colors. b) Prove that, if we reduce the number of colors to $\lfloor 2n/3 \rfloor + 2$, then the statement from a) is true for infinitely many values of $n$ and it is false also for infinitely many values of $n$

1987 China National Olympiad, 6

Tags: analytic geometry , number theory

Sum of $m$ pairwise different positive even numbers and $n$ pairwise different positive odd numbers is equal to $1987$. Find, with proof, the maximum value of $3m+4n$.

2015 CCA Math Bonanza, T10

Tags: trigonometry

If $\cos 2^{\circ} - \sin 4^{\circ} -\cos 6^{\circ} + \sin 8^{\circ} \ldots + \sin 88^{\circ}=\sec \theta - \tan \theta$, compute $\theta$ in degrees. [i]2015 CCA Math Bonanza Team Round #10[/i]

2000 AMC 8, 23

Tags:

There is a list of seven numbers. The average of the first four numbers is $5$, and the average of the last four numbers is $8$. If the average of all seven numbers is $6\frac{4}{7}$, then the number common to both sets of four numbers is $\text{(A)}\ 5\frac{3}{7} \qquad \text{(B)}\ 6 \qquad \text{(C)}\ 6\frac{4}{7} \qquad \text{(D)}\ 7 \qquad \text{(E)}\ 7\frac{3}{7}$

2011 Tournament of Towns, 3

Tags: combinatorics , coloring

Along a circle are $100$ white points. An integer $k$ is given, where $2 \le k \le 50$. In each move, we choose a block of $k$ adjacent points such that the first and the last are white, and we paint both of them black. For which values of $k$ is it possible for us to paint all $100$ points black after $50$ moves?

2013 ELMO Shortlist, 7

Tags: algebra , polynomial , modular arithmetic , number theory

Let $p$ be a prime satisfying $p^2\mid 2^{p-1}-1$, and let $n$ be a positive integer. Define \[ f(x) = \frac{(x-1)^{p^n}-(x^{p^n}-1)}{p(x-1)}. \] Find the largest positive integer $N$ such that there exist polynomials $g(x)$, $h(x)$ with integer coefficients and an integer $r$ satisfying $f(x) = (x-r)^N g(x) + p \cdot h(x)$. [i]Proposed by Victor Wang[/i]

1966 IMO Longlists, 57

Tags: geometry , 3d geometry , symmetry , rotation , cube

Is it possible to choose a set of $100$ (or $200$) points on the boundary of a cube such that this set is fixed under each isometry of the cube into itself? Justify your answer.

2000 Brazil National Olympiad, 2

Tags: floor function , number theory

Let $s(n)$ be the sum of all positive divisors of $n$, so $s(6) = 12$. We say $n$ is almost perfect if $s(n) = 2n - 1$. Let $\mod(n, k)$ denote the residue of $n$ modulo $k$ (in other words, the remainder of dividing $n$ by $k$). Put $t(n) = \mod(n, 1) + \mod(n, 2) + \cdots + \mod(n, n)$. Show that $n$ is almost perfect if and only if $t(n) = t(n-1)$.

2008 Mathcenter Contest, 4

Tags: inequalities , algebra

Let $p,q,r \in \mathbb{R}^+$ and for every $n \in \mathbb{N}$ where $pqr=1$ , denote $$ \frac{1}{p^n+q^n+1} + \frac{1}{q^n+r^n+1} + \frac{1}{r^n+p^n+ 1} \leq 1$$ [i](Art-Ninja)[/i]

2016 Purple Comet Problems, 5

Tags:

Julius has a set of ﬁve positive integers whose mean is 100. If Julius removes the median of the set of ﬁve numbers, the mean of the set increases by 5, and the median of the set decreases by 5. Find the maximum possible value of the largest of the ﬁve numbers Julius has.

2023 Tuymaada Olympiad, 4

Tags: combinatorics , game theory , combinatorial game theory

Two players play a game. They have $n > 2$ piles containing $n^{10}+1$ stones each. A move consists of removing all the piles but one and dividing the remaining pile into $n$ nonempty piles. The player that cannot move loses. Who has a winning strategy, the player that moves first or his adversary?

2018 IMAR Test, 1

Tags: locus , geometry

Let $ABC$ be a triangle whose angle at $A$ is right, and let $D$ be the foot of the altitude from $A$. A variable point $M$ traces the interior of the minor arc $AB$ of the circle $ABC$. The internal bisector of the angle $DAM$ crosses $CM$ at $N$. The line through $N$ and perpendicular to $CM$ crosses the line $AD$ at $P$. Determine the locus of the point where the line $BN$ crosses the line $CP$. [i]* * *[/i]

2018 Thailand TSTST, 2

Tags: combinatorics

There are three sticks, each of which has an integer length which is at least $n$; the sum of their lengths is $n(n + 1)/2$. Prove that it is possible to break the sticks (possibly several times) so that the resulting sticks have length $1, 2,\dots, n$. [i]Note: a stick of length $a + b$ can be broken into sticks of lengths $a$ and $b$.[/i]

1991 Tournament Of Towns, (286) 2

Tags: geometry , pentagon , perpendicular

The pentagon $ABCDE$ has an inscribed circle and the diagonals $AD$ and $CE$ intersect in its centre $O$. Prove that the segment $BO$ and the side $DE$ are perpendicular. (Folklore)

2010 Harvard-MIT Mathematics Tournament, 7

Tags: geometry

You are standing in an infinitely long hallway with sides given by the lines $x=0$ and $x=6$. You start at $(3,0)$ and want to get to $(3,6)$. Furthermore, at each instant you want your distance to $(3,6)$ to either decrease or stay the same. What is the area of the set of points that you could pass through on your journey from $(3,0)$ to $(3,6)$?

2011 Canadian Open Math Challenge, 6

Tags:

Integers a, b, c, d, and e satisfy the following three properties: (i) $2 \le a < b <c <d <e <100$ (ii)$ \gcd (a,e) = 1 $ (iii) a, b, c, d, e form a geometric sequence. What is the value of c?

2021 JHMT HS, 5

Tags: general

Terry decides to practice his arithmetic by adding the numbers between $10$ and $99$ inclusive. However, he accidentally swaps the digits of one of the numbers, and thus gets the incorrect sum of $4941.$ What is the largest possible number whose digits Terry could have swapped in the summation?

2011 Indonesia TST, 4

Tags: number theory

Given an arbitrary prime $p>2011$. Prove that there exist positive integers $a, b, c$ not all divisible by $p$ such that for all positive integers $n$ that $p\mid n^4- 2n^2+ 9$, we have $p\mid 24an^2 + 5bn + 2011c$.

2025 Bangladesh Mathematical Olympiad, P2

Tags: equation , algebra , exponential equation

Find all real solutions to the equation $(x^2-9x+19)^{x^2-3x+2} = 1$.

2015 Federal Competition For Advanced Students, 1

Tags: algebra , inequalities

Let $a$, $b$, $c$, $d$ be positive numbers. Prove that $$(a^2 + b^2 + c^2 + d^2)^2 \ge (a+b)(b+c)(c+d)(d+a)$$ When does equality hold? (Georg Anegg)

2024 Harvard-MIT Mathematics Tournament, 17

Tags: guts

The numbers $1, 2, \ldots, 20$ are put into a hat. Claire draws two numbers from the hat uniformly at random, $a<b,$ and then puts them back into the hat. Then, William draws two numbers from the hat uniformly at random, $c<d.$ Let $N$ denote the number of integers $n$ that satisfy exactly one of $a \le n \le b$ and $c \le n \le d.$ Compute the probability that $N$ is even.

2014 Contests, 2

Tags: circumcircle , incenter , cyclic quadrilateral , geometry

Let $ABCD$ be a convex cyclic quadrilateral with $AD=BD$. The diagonals $AC$ and $BD$ intersect in $E$. Let the incenter of triangle $\triangle BCE$ be $I$. The circumcircle of triangle $\triangle BIE$ intersects side $AE$ in $N$. Prove \[ AN \cdot NC = CD \cdot BN. \]