Olympiad problems

1985 AMC 8, 1

Tags:

$ \frac{3 \times 5}{9 \times 11} \times \frac{7 \times 9 \times 11}{3 \times 5 \times 7}\equal{}$ \[ \textbf{(A)}\ 1 \qquad \textbf{(B)}\ 0 \qquad \textbf{(C)}\ 49 \qquad \textbf{(D)}\ \frac{1}{49} \qquad \textbf{(E)}\ 50 \]

2023 Malaysian IMO Team Selection Test, 1

Tags: combinatorics

Let $P$ be a cyclic polygon with circumcenter $O$ that does not lie on any diagonal, and let $S$ be the set of points on 2D plane containing $P$ and $O$. The $\textit{Matcha Sweep Game}$ is a game between two players $A$ and $B$, with $A$ going first, such that each choosing a nonempty subset $T$ of points in $S$ that has not been previously chosen, and such that if $T$ has at least $3$ vertices then $T$ forms a convex polygon. The game ends with all points have been chosen, with the player picking the last point wins. For which polygons $P$ can $A$ guarantee a win? [i]Proposed by Anzo Teh Zhao Yang[/i]

2008 Moldova Team Selection Test, 2

Tags: combinatorics

We say the set $ \{1,2,\ldots,3k\}$ has property $ D$ if it can be partitioned into disjoint triples so that in each of them a number equals the sum of the other two. (a) Prove that $ \{1,2,\ldots,3324\}$ has property $ D$. (b) Prove that $ \{1,2,\ldots,3309\}$ hasn't property $ D$.

2008 IMS, 3

Tags: geometry , parabola , ratio , function , calculus , derivative , conic

Let $ A,B$ be different points on a parabola. Prove that we can find $ P_1,P_2,\dots,P_{n}$ between $ A,B$ on the parabola such that area of the convex polygon $ AP_1P_2\dots P_nB$ is maximum. In this case prove that the ratio of $ S(AP_1P_2\dots P_nB)$ to the sector between $ A$ and $ B$ doesn't depend on $ A$ and $ B$, and only depends on $ n$.

1962 Miklós Schweitzer, 8

Tags: function , complex analysis

Denote by $ M(r,f)$ the maximum modulus on the circle $ |z|\equal{}r$ of the transcendent entire function $ f(z)$, and by $ M_n(r,f)$ that of the $ nth$ partial sum of the power series of $ f(z)$. Prove that the existence of an entire function $ f_0(z)$ and a corresponding sequence of positive numbers $ r_1<r_2<...\rightarrow \plus{}\infty$ such that \[ \limsup_{n\rightarrow\infty} \frac{M_n(r_n,f_0)}{M(r_n,f_0)}\equal{}\plus{}\infty\] [P. Turan]

2017 Sharygin Geometry Olympiad, 2

Tags: geometry , inequalities

If $ABC$ is acute triangle, prove distance from each vertex to corresponding excentre is less than sum of two greatest side of triangle

2018 Online Math Open Problems, 19

Tags:

Let $P(x)$ be a polynomial of degree at most $2018$ such that $P(i)=\binom{2018}i$ for all integer $i$ such that $0\le i\le 2018$. Find the largest nonnegative integer $n$ such that $2^n\mid P(2020)$. [i]Proposed by Michael Ren

1994 Tournament Of Towns, (409) 7

Tags: combinatorics , combinatorial geometry , tiling

In a $10$ by $10$ square grid (which we call “the bay”) you are requested to place ten “ships”: one $1$ by $4$ ship, two $1$ by $3$ ships, three $1$ by $2$ ships and four $1$ by $1$ ships. The ships may not have common points (even corners) but may touch the “shore” of the bay. Prove that (a) by placing the ships one after the other arbitrarily but in the order indicated above, it is always possible to complete the process; (b) by placing the ships in reverse order (beginning with the smaller ones), it is possible to reach a situation where the next ship cannot be placed (give an example). (KN Ignatjev)

2001 Romania Team Selection Test, 3

Tags: circumcircle , geometry

The tangents at $A$ and $B$ to the circumcircle of the acute triangle $ABC$ intersect the tangent at $C$ at the points $D$ and $E$, respectively. The line $AE$ intersects $BC$ at $P$ and the line $BD$ intersects $AC$ at $R$. Let $Q$ and $S$ be the midpoints of the segments $AP$ and $BR$ respectively. Prove that $\angle ABQ=\angle BAS$.

1996 AMC 12/AHSME, 26

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An urn contains marbles of four colors: red, white, blue, and green. When four marbles are drawn without replacement, the following events are equally likely: (a) the selection of four red marbles; (b) the selection of one white and three red marbles; (c) the selection of one white, one blue, and two red marbles; and (d) the selection of one marble of each color. What is the smallest number of marbles satisfying the given condition? $\text{(A)}\ 19 \qquad \text{(B)}\ 21 \qquad \text{(C)}\ 46 \qquad \text{(D)}\ 69\qquad \text{(E)}\ \text{more than 69}$

2013 Harvard-MIT Mathematics Tournament, 10

Tags: hmmt , geometry

Wesyu is a farmer, and she's building a cao (a relative of the cow) pasture. Shw starts with a triangle $A_0A_1A_2$ where angle $A_0$ is $90^\circ$, angle $A_1$ is $60^\circ$, and $A_0A_1$ is $1$. She then extends the pasture. FIrst, she extends $A_2A_0$ to $A_3$ such that $A_3A_0=\dfrac12A_2A_0$ and the new pasture is triangle $A_1A_2A_3$. Next, she extends $A_3A_1$ to $A_4$ such that $A_4A_1=\dfrac16A_3A_1$. She continues, each time extending $A_nA_{n-2}$ to $A_{n+1}$ such that $A_{n+1}A_{n-2}=\dfrac1{2^n-2}A_nA_{n-2}$. What is the smallest $K$ such that her pasture never exceeds an area of $K$?

2003 Federal Math Competition of S&M, Problem 2

Tags: geometry , combinatorial geometry , combinatorics

Given a segment $AB$ of length $2003$ in a coordinate plane, determine the maximal number of unit squares with vertices in the lattice points whose intersection with the given segment is non-empty.

1984 Bundeswettbewerb Mathematik, 3

Tags: algebra , recurrence relation , sequence

The sequences $a_1, a_2, a_3,...$ and $b_1, b_2, b_3,... $suffices for all positive integers $n$ of the following recursion: $a_{n+1} = a_n - b_n$ and $b_{n+1} = 2b_n$, if $a_n \ge b_n$, $a_{n+1} = 2a_n$ and $b_{n+1} = b_n - a_n$, if $a_n < b_n$. For which pairs $(a_1, b_1)$ of positive real initial terms is there an index $k$ with $a_k = 0$?

1998 Baltic Way, 1

Tags: function , number theory

Find all functions $f$ of two variables, whose arguments $x,y$ and values $f(x,y)$ are positive integers, satisfying the following conditions (for all positive integers $x$ and $y$): \begin{align*} f(x,x)& =x,\\ f(x,y)& =f(y,x),\\ (x+y)f(x,y)& =yf(x,x+y).\end{align*}

1967 IMO Longlists, 41

Tags: circumcircle , reflection , triangle , concurrency , geometry

A line $l$ is drawn through the intersection point $H$ of altitudes of acute-angle triangles. Prove that symmetric images $l_a, l_b, l_c$ of $l$ with respect to the sides $BC,CA,AB$ have one point in common, which lies on the circumcircle of $ABC.$

2000 Slovenia National Olympiad, Problem 3

Tags: geometry

The diagonals of a cyclic quadrilateral $ABCD$ intersect at $E$. Let $F$ and $G$ be the midpoints of $AB$ and $CD$ respectively. Prove that the lines through $E,F$ and $G$ perpendicular to $AD,BD$ and $AC$, respectively, intersect in a single point.

2018 China Northern MO, 7

Tags: inequalities , algebra

If $a$,$b$,$c$ are positive reals, prove that $$\frac{a+bc}{a+a^2}+\frac{b+ca}{b+b^2}+\frac{c+ab}{c+c^2} \geq 3$$

2022 Bulgaria EGMO TST, 5

Tags: algebra

Let $n$ be a positive integer. Given is a subset $A$ of $\{0,1,...,5^n\}$ with $4n+2$ elements. Prove that there exist three elements $a<b<c$ from $A$ such that $c+2a>3b$. [i]Proposed by Dominik Burek and Tomasz Ciesla, Poland[/i]

2021 IMO Shortlist, C6

Tags: combinatorics , game

A hunter and an invisible rabbit play a game on an infinite square grid. First the hunter fixes a colouring of the cells with finitely many colours. The rabbit then secretly chooses a cell to start in. Every minute, the rabbit reports the colour of its current cell to the hunter, and then secretly moves to an adjacent cell that it has not visited before (two cells are adjacent if they share an edge). The hunter wins if after some finite time either:[list][*]the rabbit cannot move; or [*]the hunter can determine the cell in which the rabbit started.[/list]Decide whether there exists a winning strategy for the hunter. [i]Proposed by Aron Thomas[/i]

2005 All-Russian Olympiad Regional Round, 10.7

Tags: number theory

10.7 Find all pairs $(a,b)$ of natural numbers s.t. $a^n+b^n$ is a perfect $n+1$th power for all $n\in\mathbb{N}$. ([i]V. Senderov[/i])

2003 AIME Problems, 4

Tags: trigonometry , logarithm

Given that $\log_{10} \sin x + \log_{10} \cos x = -1$ and that $\log_{10} (\sin x + \cos x) = \textstyle \frac{1}{2} (\log_{10} n - 1)$, find $n$.

2018-2019 SDML (High School), 9

Tags: geometry

Triangle $ABC$ is isosceles with $AB + AC$ and $BC = 65$ cm. $P$ is a point on $\overline{BC}$ such that the perpendicular distances from $P$ to $\overline{AB}$ and $\overline{AC}$ are $24$ cm and $36$ cm, respectively. The area of $\triangle ABC$, in cm$^2$, is $ \mathrm{(A) \ } 1254 \qquad \mathrm{(B) \ } 1640 \qquad \mathrm {(C) \ } 1950 \qquad \mathrm{(D) \ } 2535 \qquad \mathrm{(E) \ } 2942$

1991 Tournament Of Towns, (287) 3

Tags: number theory , digit

We are looking for numbers ending with the digit $5$ such that in their decimal expansion each digit beginning with the second digit is no less than the previous one. Moreover the squares of these numbers must also possess the same property. (a) Find four such numbers. (b) Prove that there are infinitely many. (A. Andjans, Riga)

2015 IFYM, Sozopol, 4

Tags: combinatorics , coloring , table , cell

A plane is cut into unit squares, which are then colored in $n$ colors. A polygon $P$ is created from $n$ unit squares that are connected by their sides. It is known that any cell polygon created by $P$ with translation, covers $n$ unit squares in different colors. Prove that the plane can be covered with copies of $P$ so that each cell is covered exactly once.

2024 Silk Road, 4

Tags: algebra

Let $a_1, a_2, \ldots$ be a strictly increasing sequence of positive integers, such that for any positive integer $n$, $a_n$ is not representable in the for $\sum_{i=1}^{n-1}c_ia_i$ for $c_i \in \{0, 1\}$. For every positive integer $m$, let $f(m)$ denote the number of $a_i$ that are at most $m$. Show that for any positive integers $m, k$, we have that $$f(m) \leq a_k+\frac{m} {k+1}.$$