Olympiad problems

2013 Saudi Arabia GMO TST, 4

Let $F_0 = 0, F_1 = 1$ and $F_{n+1} = F_n + F_{n-1}$, for all positive integer $n$, be the Fibonacci sequence. Prove that for any positive integer $m$ there exist infinitely many positive integers $n$ such that $F_n + 2 \equiv F_{n+1} + 1 \equiv F_{n+2}$ mod $m$ .

1957 Putnam, B7

Tags: combinatorial geometry , polygon , geometry

Let $C$ consist of a regular polygon and its interior. Show that for each positive integer $n$, there exists a set of points $S(n)$ in the plane such that every $n$ points can be covered by $C$, but $S(n)$ cannot be covered by $C.$

2003 JBMO Shortlist, 7

Tags: geometry , circumcircle , angle bisector

Let $D$, $E$, $F$ be the midpoints of the arcs $BC$, $CA$, $AB$ on the circumcircle of a triangle $ABC$ not containing the points $A$, $B$, $C$, respectively. Let the line $DE$ meets $BC$ and $CA$ at $G$ and $H$, and let $M$ be the midpoint of the segment $GH$. Let the line $FD$ meet $BC$ and $AB$ at $K$ and $J$, and let $N$ be the midpoint of the segment $KJ$. a) Find the angles of triangle $DMN$; b) Prove that if $P$ is the point of intersection of the lines $AD$ and $EF$, then the circumcenter of triangle $DMN$ lies on the circumcircle of triangle $PMN$.

1997 AIME Problems, 14

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Let $v$ and $w$ be distinct, randomly chosen roots of the equation $z^{1997}-1=0.$ Let $m/n$ be the probability that $\sqrt{2+\sqrt{3}}\le |v+w|,$ where $m$ and $n$ are relatively prime positive integers. Find $m+n.$

KoMaL A Problems 2022/2023, A. 840

Tags: geometry , incenter

The incircle of triangle $ABC$ touches the sides in $X$, $Y$ and $Z$. In triangle $XYZ$ the feet of the altitude from $X$ and $Y$ are $X'$ and $Y'$, respectively. Let line $X'Y'$ intersect the circumcircle of triangle $ABC$ at $P$ and $Q$. Prove that points $X$, $Y$, $P$ and $Q$ are concyclic. Proposed by [i]László Simon[/i], Budapest

2015 China Team Selection Test, 1

Tags: perpendicular bisector , angle bisector , geometry

$\triangle{ABC}$ is isosceles with $AB = AC >BC$. Let $D$ be a point in its interior such that $DA = DB+DC$. Suppose that the perpendicular bisector of $AB$ meets the external angle bisector of $\angle{ADB}$ at $P$, and let $Q$ be the intersection of the perpendicular bisector of $AC$ and the external angle bisector of $\angle{ADC}$. Prove that $B,C,P,Q$ are concyclic.

2013 NIMO Problems, 7

Tags: probability , number theory , relatively prime

Dragon selects three positive real numbers with sum $100$, uniformly at random. He asks Cat to copy them down, but Cat gets lazy and rounds them all to the nearest tenth during transcription. If the probability the three new numbers still sum to $100$ is $\tfrac{m}{n}$, where $m$ and $n$ are relatively prime positive integers, compute $100m+n$. [i]Proposed by Aaron Lin[/i]

2005 Tournament of Towns, 6

Tags: game , game strategy , winning strategy , combinatorics

John and James wish to divide $25$ coins, of denominations $1, 2, 3, \ldots , 25$ kopeks. In each move, one of them chooses a coin, and the other player decides who must take this coin. John makes the initial choice of a coin, and in subsequent moves, the choice is made by the player having more kopeks at the time. In the event that there is a tie, the choice is made by the same player in the preceding move. After all the coins have been taken, the player with more kokeps wins. Which player has a winning strategy? [i](6 points)[/i]

2018 Brazil Undergrad MO, 25

Tags: topology , group theory

Consider the $ \mathbb {Z} / (10) $ additive group automorphism group of integers module $10$, that is, $ A = \left \{\phi: \mathbb {Z} / (10) \to \mathbb {Z} / (10) | \phi-automorphism \right \}$

1995 Singapore MO Open, 4

Tags: number theory , divisible , divide

Let $a, b$ and $c$ be positive integers such that $1 < a < b < c$. Suppose that $(ab-l)(bc-1 )(ca-1)$ is divisible by $abc$. Find the values of $a, b$ and $c$. Justify your answer.

1998 Singapore MO Open, 3

Tags: diophantine , diophantine equation , number theory

Do there exist integers $x$ and $y$ such that $19^{19} = x^3 +y^4$ ? Justify your answer.

2023 Romania Team Selection Test, P4

Tags: combinatorics

Consider a $4\times 4$ array of pairwise distinct positive integers such that on each column, respectively row, one of the numbers is equal to the sum of the other three. Determine the least possible value of the largest number such an array may contain. [i]The Problem Selection Committee[/i]

2024 ELMO Shortlist, G6

Tags: geometry , water balloon

In triangle $ABC$ with $AB<AC$ and $AB+AC=2BC$, let $M$ be the midpoint of $\overline{BC}$. Choose point $P$ on the extension of $\overline{BA}$ past $A$ and point $Q$ on segment $\overline{AC}$ such that $M$ lies on $\overline{PQ}$. Let $X$ be on the opposite side of $\overline{AB}$ from $C$ such that $\overline{AX} \parallel \overline{BC}$ and $AX=AP=AQ$. Let $\overline{BX}$ intersect the circumcircle of $BMQ$ again at $Y \neq B$, and let $\overline{CX}$ intersect the circumcircle of $CMP$ again at $Z \neq C$. Prove that $A$, $Y$, and $Z$ are collinear. [i]Tiger Zhang[/i]

2006 Estonia Team Selection Test, 5

Tags: inequalities , mean , algebra

Let $a_1, a_2, a_3, ...$ be a sequence of positive real numbers. Prove that for any positive integer $n$ the inequality holds $\sum_{i=1}^n b_i^2 \le 4 \sum_{i=1}^n a_i^2$ where $b_i$ is the arithmetic mean of the numbers $a_1, a_2, ..., a_n$

2009 Today's Calculation Of Integral, 444

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

Evaluate $ \int_0^{\frac {\pi}{6}} \frac {\sin x \plus{} \cos x}{1 \minus{} \sin 2x}\ln\ (2 \plus{} \sin 2x)\ dx.$

2013 China Northern MO, 3

Tags: geometry , fixed

As shown in figure , $A,B$ are two fixed points of circle $\odot O$, $C$ is the midpoint of the major arc $AB$, $D$ is any point of the minor arc $AB$. Tangent at $D$ intersects tangents at $A,B$ at points $E,F$ respectively. Segments $CE$ and $CF$ intersect chord $AB$ at points $G$ and $H$ respectively. Prove that the length of line segment $GH$ has a fixed value. [img]https://cdn.artofproblemsolving.com/attachments/9/2/85227f169193f61e313293e9128f6ece2ff1f7.png[/img]

2016 Canada National Olympiad, 2

Tags: algebra , system of equations

Consider the following system of $10$ equations in $10$ real variables $v_1, \ldots, v_{10}$: \[v_i = 1 + \frac{6v_i^2}{v_1^2 + v_2^2 + \cdots + v_{10}^2} \qquad (i = 1, \ldots, 10).\] Find all $10$-tuples $(v_1, v_2, \ldots , v_{10})$ that are solutions of this system.

2002 Romania Team Selection Test, 3

Tags: pigeonhole principle , number theory

Let $a,b$ be positive real numbers. For any positive integer $n$, denote by $x_n$ the sum of digits of the number $[an+b]$ in it's decimal representation. Show that the sequence $(x_n)_{n\ge 1}$ contains a constant subsequence. [i]Laurentiu Panaitopol[/i]

2011 India National Olympiad, 5

Tags: cyclic quadrilateral , geometry

Let $ABCD$ be a cyclic quadrilateral inscribed in a circle $\Gamma.$ Let $E,F,G,H$ be the midpoints of arcs $AB,BC,CD,AD$ of $\Gamma,$ respectively. Suppose that $AC\cdot BD=EG\cdot FH.$ Show that $AC,BD,EG,FH$ are all concurrent.

2009 Balkan MO Shortlist, A1

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Let $N \in \mathbb{N}$ and $x_k \in [-1,1]$, $1 \le k \le N$ such that $\sum_{k=1}^N x_k =s$. Find all possible values of $\sum_{k=1}^N |x_k|$

1978 IMO Longlists, 22

Tags: special factorizations , number theory

Let $x$ and $y$ be two integers not equal to $0$ such that $x+y$ is a divisor of $x^2+y^2$. And let $\frac{x^2+y^2}{x+y}$ be a divisor of $1978$. Prove that $x = y$. [i]German IMO Selection Test 1979, problem 2[/i]

2008 Puerto Rico Team Selection Test, 2

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Using digits $ 1, 2, 3, 4, 5, 6$, without repetition, $ 3$ two-digit numbers are formed. The numbers are then added together. Through this procedure, how many different sums may be obtained?

1985 IMO Longlists, 15

Tags: combinatorics

[i]Superchess[/i] is played on on a $12 \times 12$ board, and it uses [i]superknights[/i], which move between opposite corner cells of any $3\times4$ subboard. Is it possible for a [i]superknight[/i] to visit every other cell of a superchessboard exactly once and return to its starting cell ?

2018 Malaysia National Olympiad, A6

Tags: number theory , prime

How many integers $n$ are there such that $n^4 + 2n^3 + 2n^2 + 2n + 1$ is a prime number?

2016 AMC 10, 13

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Five friends sat in a movie theater in a row containing $5$ seats, numbered $1$ to $5$ from left to right. (The directions "left" and "right" are from the point of view of the people as they sit in the seats.) During the movie Ada went to the lobby to get some popcorn. When she returned, she found that Bea had moved two seats to the right, Ceci had moved one seat to the left, and Dee and Edie had switched seats, leaving an end seat for Ada. In which seat had Ada been sitting before she got up? $\textbf{(A) }1 \qquad \textbf{(B) } 2 \qquad \textbf{(C) } 3 \qquad \textbf{(D) } 4\qquad \textbf{(E) } 5$