Olympiad problems

2014 IPhOO, 4

A rock is dropped off a cliff of height $ h $ As it falls, a camera takes several photographs, at random intervals. At each picture, I measure the distance the rock has fallen. Let the average (expected value) of all of these distances be $ kh $. If the number of photographs taken is huge, find $ k $. That is: what is the time-average of the distance traveled divided by $ h $, dividing by $h$? $ \textbf {(A) } \dfrac{1}{4} \qquad \textbf {(B) } \dfrac{1}{3} \qquad \textbf {(C) } \dfrac{1}{\sqrt{2}} \qquad \textbf {(D) } \dfrac{1}{2} \qquad \textbf {(E) } \dfrac{1}{\sqrt{3}} $ [i]Problem proposed by Ahaan Rungta[/i]

2015 AMC 12/AHSME, 17

Tags: probability

An unfair coin lands on heads with a probability of $\tfrac{1}{4}$. When tossed $n$ times, the probability of exactly two heads is the same as the probability of exactly three heads. What is the value of $n$? $ \textbf {(A) } 5 \qquad \textbf {(B) } 8 \qquad \textbf {(C) } 10 \qquad \textbf {(D) } 11 \qquad \textbf {(E) } 13 $

2008 National Olympiad First Round, 12

Tags: symmetry , geometry , 3d geometry , rotation

In how many ways a cube can be painted using seven different colors in such a way that no two faces are in same color? $ \textbf{(A)}\ 154 \qquad\textbf{(B)}\ 203 \qquad\textbf{(C)}\ 210 \qquad\textbf{(D)}\ 240 \qquad\textbf{(E)}\ \text{None of the above} $

2019 BMT Spring, Tie 2

Tags: number theory

Find the sum of first two integers $n > 1$ such that $3^n$ is divisible by $n$ and $3^n - 1$ is divisible by $n - 1$.

2009 Kazakhstan National Olympiad, 3

Tags: floor function , graph theory , combinatorics

In chess tournament participates $n$ participants ($n >1$). In tournament each of participants plays with each other exactly $1$ game. For each game participant have $1$ point if he wins game, $0,5$ point if game is drow and $0$ points if he lose game. If after ending of tournament participant have at least $ 75 % $ of maximum possible points he called $winner$ $of$ $tournament$. Find maximum possible numbers of $winners$ $of$ $tournament$.

2018 Adygea Teachers' Geometry Olympiad, 3

Tags: concyclic , circles , geometry

Two circles intersect at points $A$ and $B$. Through point $B$, a straight line intersects the circles at points $C$ and $D$, and then tangents to the circles are drawn through points $C$ and $D$. Prove that the points $A, D, C$ and $P$ - the intersection point of the tangents - lie on the same circle.

2016 Brazil Team Selection Test, 2

Tags: functional equation

Determine all functions $f:\mathbb{Z}\rightarrow\mathbb{Z}$ with the property that \[f(x-f(y))=f(f(x))-f(y)-1\] holds for all $x,y\in\mathbb{Z}$.

2012 Moldova Team Selection Test, 8

Tags: number theory

Let $p\geq5$ be a prime and $S_k=1^k+2^k+...+(p-1)^k,\forall k\in\mathbb{N}.$ Prove that there is an infinity of numbers $n\in\mathbb{N}$ such that $p^3$ divides $S_n$ and $ p $ divides $S_{n-1}$ and $S_{n-2}.$

2005 Kazakhstan National Olympiad, 2

Tags: inequalities

Prove that \[ab+bc+ca\ge 2(a+b+c)\] where $a,b,c$ are positive reals such that $a+b+c+2=abc$.

2004 Baltic Way, 18

Tags: circumcircle , geometry , inequalities

A ray emanating from the vertex $A$ of the triangle $ABC$ intersects the side $BC$ at $X$ and the circumcircle of triangle $ABC$ at $Y$. Prove that $\frac{1}{AX}+\frac{1}{XY}\geq \frac{4}{BC}$.

2008 Iran MO (3rd Round), 3

Tags: combinatorics , function , induction

Prove that for each $ n$: \[ \sum_{k\equal{}1}^n\binom{n\plus{}k\minus{}1}{2k\minus{}1}\equal{}F_{2n}\]

Bangladesh Mathematical Olympiad 2020 Final, #3

Tags: conditional probability , probability

[u]Prottasha[/u] has a 10 sided dice. She throws the dice two times and sum the numbers she gets. Which number has the most probability to come out?

2002 Tournament Of Towns, 1

Tags: number theory , rectangle , combinatorics , geometry , greatest common divisor

There are many $a\times b$ rectangular cardboard pieces ($a,b\in\mathbb{N}$ such that $a<b$). It is given that by putting such pieces together without overlapping one can make $49\times 51$ rectangle, and $99\times 101$ rectangle. Can one uniquely determine $a,b$ from this?

1989 Federal Competition For Advanced Students, P2, 6

Tags: polynomial , algebra , function

Determine all functions $ f: \mathbb{N}_0 \rightarrow \mathbb{N}_0$ such that $ f(f(n))\plus{}f(n)\equal{}2n\plus{}6$ for all $ n \in \mathbb{N}_0$.

2023 Puerto Rico Team Selection Test, 8

Tags: geometry

Inside a quadrilateral $ABCD$ there exists a point $P$ such that $AP$ is perpendicular to $AD$ and the line $BP$ is perpendicular to $DC$. Besides, $AB = 7$, $AP = 3$, $BP = 6$, $AD = 5 $ and $CD = 10$. Calculate the area of the triangle $ABC$.

2020 Azerbaijan Senior NMO, 4

Tags: combinatorics

A regular 2021-gon is divided into 2019 triangles,such that no diagonals intersect. Prove that at least 3 of the 2019 triangles are isoscoles

2022 Kyiv City MO Round 1, Problem 3

Tags: geometry

Diagonals of a cyclic quadrilateral $ABCD$ intersect at point $P$. The circumscribed circles of triangles $APD$ and $BPC$ intersect the line $AB$ at points $E, F$ correspondingly. $Q$ and $R$ are the projections of $P$ onto the lines $FC, DE$ correspondingly. Show that $AB \parallel QR$. [i](Proposed by Mykhailo Shtandenko)[/i]

2005 Today's Calculation Of Integral, 9

Tags: logarithm , calculus , absolute value , integration , trigonometry , calculus computations

Calculate the following indefinite integrals. [1] $\int (x^2+4x-3)^2(x+2)dx$ [2] $\int \frac{\ln x}{x(\ln x+1)}dx$ [3] $\int \frac{\sin \ (\pi \log _2 x)}{x}dx$ [4] $\int \frac{dx}{\sin x\cos ^ 2 x}$ [5] $\int \sqrt{1-3x}\ dx$

1966 Swedish Mathematical Competition, 1

Tags: limit , algebra , sequence

Let $\{x\}$ denote the fractional part of $x$, $x - [x]$. The sequences $x_1, x_2, x_3, ...$ and $y_1, y_2, y_3, ...$ are such that $\lim \{x_n\} = \lim \{y_n\} = 0$. Is it true that $\lim \{x_n + y_n\} = 0$? $\lim \{x_n - y_n\} = 0$?

MMPC Part II 1996 - 2019, 2016.3

Tags:

This problem is about pairs of consecutive whole numbers satisfying the property that one of the numbers is a perfect square and the other one is the double of a perfect square. (a) The smallest such pairs are $(0,1)$ and $(8,9)$, Indeed $0=2 \cdot 0^2$ and $1=1^2$; $8=2 \cdot 2^2$ and $9=3^2$. Show that there are infinitely many pairs of the form $(2a^2,b^2)$ where the smaller number is the double of a perfect square satisfying the given property. (b) Find a pair of integers satisfying the property that is not in the form given in the first part, that is, find a pair of integers such that the smaller one is a perfect square and the larger one is the double of a perfect square.

2006 ISI B.Stat Entrance Exam, 7

Tags: algebra , inequalities , induction

for any positive integer $n$ greater than $1$, show that \[2^n<\binom{2n}{n}<\frac{2^n}{\prod\limits_{i=0}^{n-1} \left(1-\frac{i}{n}\right)}\]

2023 JBMO Shortlist, A1

Tags: algebra , inequality

Prove that for all positive real numbers $a,b,c,d$, $$\frac{2}{(a+b)(c+d)+(b+c)(a+d)} \leq \frac{1}{(a+c)(b+d)+4ac}+\frac{1}{(a+c)(b+d)+4bd}$$ and determine when equality occurs.

2019 Regional Competition For Advanced Students, 2

Tags: parallel , geometry , cyclic , equal segments

The convex pentagon $ABCDE$ is cyclic and $AB = BD$. Let point $P$ be the intersection of the diagonals $AC$ and $BE$. Let the straight lines $BC$ and $DE$ intersect at point $Q$. Prove that the straight line $PQ$ is parallel to the diagonal $AD$.

2009 Stanford Mathematics Tournament, 14

Tags:

Let $a$ and $b$ be integer solutions to $17a+6b=13$. What is the smallest possible positive value for $a-b$?

2014 Saudi Arabia IMO TST, 3

Tags: combinatorics , modular arithmetic , analytic geometry , vector , induction

We are given a lattice and two pebbles $A$ and $B$ that are placed at two lattice points. At each step we are allowed to relocate one of the pebbles to another lattice point with the condition that the distance between pebbles is preserved. Is it possible after finite number of steps to switch positions of the pebbles?