Olympiad problems

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

Tags:

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

Tags:

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

Tags:

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? $\textbf{(A) }1 \qquad \textbf{(B) } 2 \qquad \textbf{(C) } 3 \qquad \textbf{(D) } 4\qquad \textbf{(E) } 5$

Brazil L2 Finals (OBM) - geometry, 1999.1

Tags: geometry

Let $ABCDE$ be a regular pentagon. The star $ACEBD$ has area 1. $AC$ and $BE$ meet at $P$, while $BD$ and $CE$ meet at $Q$. Find the area of $APQD$.

2011 Math Prize For Girls Problems, 8

Tags: geometry

In the figure below, points $A$, $B$, and $C$ are distance 6 from each other. Say that a point $X$ is [i]reachable[/i] if there is a path (not necessarily straight) connecting $A$ and $X$ of length at most 8 that does not intersect the interior of $\overline{BC}$. (Both $X$ and the path must lie on the plane containing $A$, $B$, and $C$.) Let $R$ be the set of reachable points. What is the area of $R$? [asy] unitsize(40); pair A = dir(90); pair B = dir(210); pair C = dir(330); dot(A); dot(B); dot(C); draw(B -- C); label("$A$", A, N); label("$B$", B, W); label("$C$", C, E); [/asy]

2007 China Team Selection Test, 2

Tags: floor function , number theory

A rational number $ x$ is called [i]good[/i] if it satisfies: $ x\equal{}\frac{p}{q}>1$ with $ p$, $ q$ being positive integers, $ \gcd (p,q)\equal{}1$ and there exists constant numbers $ \alpha$, $ N$ such that for any integer $ n\geq N$, \[ |\{x^n\}\minus{}\alpha|\leq\dfrac{1}{2(p\plus{}q)}\] Find all the good numbers.

2019 PUMaC Combinatorics B, 4

Tags: combinatorics , binary

Keith has $10$ coins labeled $1$ through $10$, where the $i$th coin has weight $2^i$. The coins are all fair, so the probability of flipping heads on any of the coins is $\tfrac{1}{2}$. After flipping all of the coins, Keith takes all of the coins which land heads and measures their total weight, $W$. If the probability that $137\le W\le 1061$ is $\tfrac{m}{n}$ for coprime positive integers $m,n$, determine $m+n$.

2010 Tournament Of Towns, 3

Tags: rectangle , exterior angle , geometry

An angle is given in a plane. Using only a compass, one must find out $(a)$ if this angle is acute. Find the minimal number of circles one must draw to be sure. $(b)$ if this angle equals $31^{\circ}$.(One may draw as many circles as one needs).

1962 IMO, 6

Tags: geometry , circumcircle , trigonometry , circles , isosceles

Consider an isosceles triangle. let $R$ be the radius of its circumscribed circle and $r$ be the radius of its inscribed circle. Prove that the distance $d$ between the centers of these two circle is \[ d=\sqrt{R(R-2r)} \]

Ukrainian TYM Qualifying - geometry, I.17

Tags: geometry , 3d geometry , cone , right triangle

A right triangle when rotating around a large leg forms a cone with a volume of $100\pi$. Calculate the length of the path that passes through each vertex of the triangle at rotation of $180^o$ around the point of intersection of its bisectors, if the sum of the diameters of the circles, inscribed in the triangle and circumscribed around it, are equal to $17$.

2019 Yasinsky Geometry Olympiad, p1

Tags: geometry , inradius , angle chasing , angle

It is known that in the triangle $ABC$ the distance from the intersection point of the angle bisector to each of the vertices of the triangle does not exceed the diameter of the circle inscribed in this triangle. Find the angles of the triangle $ABC$. (Grigory Filippovsky)

LMT Team Rounds 2010-20, B5

Tags: algebra

Given the following system of equations $a_1 + a_2 + a_3 = 1$ $a_2 + a_3 + a_4 = 2$ $a_3 + a_4 + a_5 = 3$ $...$ $a_{12} + a_{13} + a_{14} = 12$ $a_{13} + a_{14} + a_1 = 13$ $a_{14 }+ a_1 + a_2 = 14$ find the value of $a_{14}$.

1952 Putnam, A3

Tags:

Develop necessary and sufficient conditions which ensure that $r_1, r_2, r_3$ and $r_1^2, r_2^2, r_3^2$ are simultaneously roots of the equation $x^3 + ax^2 + bx + c = 0.$

2018-2019 Fall SDPC, 2

Tags: number theory

Find all pairs of positive integers $(m,n)$ such that $2^m-n^2$ is the square of an integer.

2004 Bundeswettbewerb Mathematik, 2

Tags: geometry , perimeter , geometric transformation , reflection , parameterization , function , integration

Let $k$ be a positive integer. In a circle with radius $1$, finitely many chords are drawn. You know that every diameter of the circle intersects at most $k$ of these chords. Prove that the sum of the lengths of all these chords is less than $k \cdot \pi$.

2017 Saint Petersburg Mathematical Olympiad, 6

Tags: geometry , circumcircle

In acute-angled triangle $ABC$, the height $AH$ and median $BM$ were drawn. Point $D$ lies on the circumcircle of triangle $BHM$ such that $AD \parallel BM$ and $B, D$ are on opposite sides of line $AC$. Prove that $BC=BD$.

Indonesia Regional MO OSP SMA - geometry, 2003.3

Tags: geometry , rectangle , 3d geometry , cube

The points $P$ and $Q$ are the midpoints of the edges $AE$ and $CG$ on the cube $ABCD.EFGH$ respectively. If the length of the cube edges is $1$ unit, determine the area of the quadrilateral $DPFQ$ .

1978 IMO Longlists, 50

Tags: 3d geometry , tetrahedron , sphere , geometry

A variable tetrahedron $ABCD$ has the following properties: Its edge lengths can change as well as its vertices, but the opposite edges remain equal $(BC = DA, CA = DB, AB = DC)$; and the vertices $A,B,C$ lie respectively on three fixed spheres with the same center $P$ and radii $3, 4, 12$. What is the maximal length of $PD$?