Olympiad problems

2005 Postal Coaching, 2

Tags: geometry

Let $< \Gamma _j >$ be a sequnce of concentric circles such that the sequence $< R_j >$ , where $R_j$ denotes the radius of $\Gamma_j$, is increasing and $R_j \longrightarrow \infty$ as $j \longrightarrow \infty$. Let $A_1 B_1 C_1$ be a triangle inscribed in $\Gamma _1$. extend the rays $\vec{A_i B_1} , \vec{B_1 C_1 }, \vec{C_1 A_1}$ to meet $\Gamma_2$ in $B_2, C_2$and $A_2$ respectively and form the triangle $A_2 B_2 C_2$. Continue this process. Show that the sequence of triangles $< A_n B_n C_n >$ tends to an equilateral triangle as $n \longrightarrow \infty$

2010 Portugal MO, 2

Tags: geometry

On a circumference, points $A$ and $B$ are on opposite arcs of diameter $CD$. Line segments $CE$ and $DF$ are perpendicular to $AB$ such that $A-E-F-B$ (i.e., $A$, $E$, $F$ and $B$ are collinear on this order). Knowing $AE=1$, find the length of $BF$.

1998 Turkey Team Selection Test, 3

Tags: algebra , polynomial , number theory , combinatorial nullstellensatz

Let $f(x_{1}, x_{2}, . . . , x_{n})$ be a polynomial with integer coeﬃcients of degree less than $n$. Prove that if $N$ is the number of $n$-tuples $(x_{1}, . . . , x_{n})$ with $0 \leq x_{i} < 13$ and $f(x_{1}, . . . , x_{n}) = 0 (mod 13)$, then $N$ is divisible by 13.

2005 Gheorghe Vranceanu, 3

Tags: arithmetic sequence , combinatorics , algebra , counting

Within an arithmetic progression of length $ 2005, $ find the number of arithmetic subprogressions of length $ 501 $ that don't contain the $ \text{1000-th} $ term of the progression.

2006 Thailand Mathematical Olympiad, 6

Tags: algebra , functional equation , functional , trigonometry

A function $f : R \to R$ has $f(1) < 0$, and satisfy the functional equation $$f(\cos (x + y)) = (\cos x)f(\cos y) + 2f(\sin x)f(\sin y)$$ for all reals $x, y$. Compute $f \left(\frac{2006}{2549 }\right)$

2015 Silk Road, 1

Tags: algebra

Prove that there is no positive real numbers $a,b,c,d$ such that both of the following equations hold.$$\frac{a}{b}+\frac{b}{c}+\frac{c}{d}+\frac{d}{a}=6 , \frac{b}{a}+\frac{c}{b}+\frac{d}{c}+\frac{a}{d}=32$$.

1995 Denmark MO - Mohr Contest, 2

Tags: algebra , sum of squares

Find all sets of five consecutive integers with that property that the sum of the squares of the first three numbers is equal to the sum of the squares on the last two.

2018 Harvard-MIT Mathematics Tournament, 10

Tags: probability

Real numbers $x,y,$ and $z$ are chosen from the interval $[-1,1]$ independently and uniformly at random. What is the probability that $$\vert{x}\vert+\vert{y}\vert+\vert{z}\vert+\vert{x+y+z}\vert=\vert{x+y}\vert+\vert{y+z}\vert+\vert{z+x}\vert?$$

2014 Romania Team Selection Test, 1

Tags: circumcircle , trigonometry , angle bisector , projective geometry , geometry

Let $\triangle ABC$ be an acute triangle of circumcentre $O$. Let the tangents to the circumcircle of $\triangle ABC$ in points $B$ and $C$ meet at point $P$. The circle of centre $P$ and radius $PB=PC$ meets the internal angle bisector of $\angle BAC$ inside $\triangle ABC$ at point $S$, and $OS \cap BC = D$. The projections of $S$ on $AC$ and $AB$ respectively are $E$ and $F$. Prove that $AD$, $BE$ and $CF$ are concurrent. [i]Author: Cosmin Pohoata[/i]

2006 China Team Selection Test, 1

Tags: circumcircle , geometry

$H$ is the orthocentre of $\triangle{ABC}$. $D$, $E$, $F$ are on the circumcircle of $\triangle{ABC}$ such that $AD \parallel BE \parallel CF$. $S$, $T$, $U$ are the semetrical points of $D$, $E$, $F$ with respect to $BC$, $CA$, $AB$. Show that $S, T, U, H$ lie on the same circle.

MBMT Guts Rounds, 2015.25

Tags:

Three real numbers $a$, $b$, and $c$ between $0$ and $1$ are chosen independently and at random. What is the probability that $a + 2b + 3c > 5$?

1967 Swedish Mathematical Competition, 4

Tags: algebra , sum , limit

The sequence $a_1, a_2, a_3, ...$ of positive reals is such that $\sum a_i$ diverges. Show that there is a sequence $b_1, b_2, b_3, ...$ of positive reals such that $\lim b_n = 0$ and $\sum a_ib_i$ diverges.

2017 Princeton University Math Competition, A3/B5

Tags:

There is a box containing $100$ balls, each of which is either orange or black. The box is equally likely to contain any number of black balls between $0$ and $100$, inclusive. A random black ball rolls out of the box. The probability that the next ball to roll out of the box is also black can be written in the form $\tfrac{p}{q}$ where $p$ and $q$ are relatively prime positive integers. Find $p + q$.

2017 Princeton University Math Competition, 7

Tags: combinatorics

$2017$ voters vote by submitting a ranking of the integers $\{1, 2, ..., 38\}$ from favorite (a vote for that value in $1$st place) to least favorite (a vote for that value in $38$th/last place). Let $a_k$ be the integer that received the most $k$th place votes (the smallest such integer if there is a tie). Find the maximum possible value of $\Sigma_{k=1}^{38} a_k$.

1998 IMO Shortlist, 3

Tags: modular arithmetic , pigeonhole principle , number theory , divisibility , combinatorics

Determine the smallest integer $n\geq 4$ for which one can choose four different numbers $a,b,c$ and $d$ from any $n$ distinct integers such that $a+b-c-d$ is divisible by $20$.

1992 Iran MO (2nd round), 1

Tags: modular arithmetic , number theory

Prove that for any positive integer $t,$ \[1+2^t+3^t+\cdots+9^t - 3(1 + 6^t +8^t )\] is divisible by $18.$

2010 Contests, 2

Tags: number theory

Consider the sequence $x_n>0$ defined with the following recurrence relation: \[x_1 = 0\] and for $n>1$ \[(n+1)^2x_{n+1}^2 + (2^n+4)(n+1)x_{n+1}+ 2^{n+1}+2^{2n-2} = 9n^2x_n^2+36nx_n+32.\] Show that if $n$ is a prime number larger or equal to $5$, then $x_n$ is an integer.

PEN M Problems, 30

Tags: recursive sequences

Let $k$ be a positive integer. Prove that there exists an infinite monotone increasing sequence of integers $\{a_{n}\}_{n \ge 1}$ such that \[a_{n}\; \text{divides}\; a_{n+1}^{2}+k \;\; \text{and}\;\; a_{n+1}\; \text{divides}\; a_{n}^{2}+k\] for all $n \in \mathbb{N}$.

2024 Singapore Senior Math Olympiad, Q1

Tags: geometry

In an acute triangle $ABC$, $AC>AB$, $D$ is the point on $BC$ such that $AD=AB$. Let $\omega_1$ be the circle through $C$ tangent to $AD$ at $D$, and $\omega_2$ the circle through $C$ tangent to $AB$ at $B$. Let $F(\ne C)$ be the second intersection of $\omega_1$ and $\omega_2$. Prove that $F$ lies on $AC$.

2020 Abels Math Contest (Norwegian MO) Final, 2a

Tags: diophantine equation , diophantine , number theory

Find all natural numbers $k$ such that there exist natural numbers $a_1,a_2,...,a_{k+1}$ with $ a_1!+a_2!+... +a_{k+1}!=k!$ Note that we do not consider $0$ to be a natural number.

Russian TST 2016, P1

Tags: geometry , cyclic quadrilateral

In the cyclic quadrilateral $ABCD$, the diagonal $BD$ is divided in half by the diagonal $AC$. The points $E, F, G$ and $H{}$ are the midpoints of the sides $AB, BC, CD{}$ and $DA$ respectively. Let $P = AD \cap BC$ and $Q = AB \cap CD{}$. The bisectors of the angles $APC$ and $AQC$ intersect the segments $EG$ and $FH$ at the points $X{}$ and $Y{}$ respectively. Prove that $XY \parallel BD$.

2012 India IMO Training Camp, 2

Tags: quadratic

Let $a\ge b$ and $c\ge d$ be real numbers. Prove that the equation \[(x+a)(x+d)+(x+b)(x+c)=0\] has real roots.

2019 Azerbaijan BMO TST, 2

Tags: geometry , perpendicular bisector , orthocenter , perpendicular , perpendicularity , tangent , circumcircle

Let $ABC$ be a triangle inscribed in circle $\Gamma$ with center $O$. Let $H$ be the orthocenter of triangle $ABC$ and let $K$ be the midpoint of $OH$. Tangent of $\Gamma$ at $B$ intersects the perpendicular bisector of $AC$ at $L$. Tangent of $\Gamma$ at $C$ intersects the perpendicular bisector of $AB$ at $M$. Prove that $AK$ and $LM$ are perpendicular. by Michael Sarantis, Greece

1991 Baltic Way, 10

Tags: trigonometry

Express the value of $\sin 3^\circ$ in radicals.

2020 Azerbaijan Senior NMO, 5

Tags: algebra , polynomial

Find all nonzero polynomials $P(x)$ with real coefficents, that satisfies $$P(x)^3+3P(x)^2=P(x^3)-3P(-x)$$ for all real numbers $x$