Found problems: 85335
2005 National Olympiad First Round, 22
For which $k$, there is no integer pair $(x,y)$ such that $x^2 - y^2 = k$?
$
\textbf{(A)}\ 2005
\qquad\textbf{(B)}\ 2006
\qquad\textbf{(C)}\ 2007
\qquad\textbf{(D)}\ 2008
\qquad\textbf{(E)}\ 2009
$
2011 Today's Calculation Of Integral, 698
For a positive integer $n$, let denote $C_n$ the figure formed by the inside and perimeter of the circle with center the origin, radius $n$ on the $x$-$y$ plane.
Denote by $N(n)$ the number of a unit square such that all of unit square, whose $x,\ y$ coordinates of 4 vertices are integers, and the vertices are included in $C_n$.
Prove that $\lim_{n\to\infty} \frac{N(n)}{n^2}=\pi$.
PEN E Problems, 29
Let $s_n$ denote the sum of the first $n$ primes. Prove that for each $n$ there exists an integer whose square lies between $s_n$ and $s_{n+1}$.
2016 PAMO, 5
Let $ABCD$ be a trapezium such that the sides $AB$ and $CD$ are parallel and the side $AB$ is longer than the side $CD$. Let $M$ and $N$ be on the segments $AB$ and $BC$ respectively, such that each of the segments $CM$ and $AN$ divides the trapezium in two parts of equal area.
Prove that the segment $MN$ intersects the segment $BD$ at its midpoint.
LMT Team Rounds 2021+, A19
Let $S$ be the sum of all possible values of $a \cdot c$ such that $$a^3+3ab^2-72ab+432a=4c^3$$ if $a$, $b$, and $c$ are positive integers, $a+b > 11$, $a > b-13$, and $c \le 1000$. Find the sum of all distinct prime factors of $S$.
[i]Proposed by Kevin Zhao[/i]
2017 Baltic Way, 3
Positive integers $x_1,...,x_m$ (not necessarily distinct) are written on a blackboard. It is known that each of the numbers $F_1,...,F_{2018}$ can be represented as a sum of one or more of the numbers on the blackboard. What is the smallest possible value of $m$?
(Here $F_1,...,F_{2018}$ are the first $2018$ Fibonacci numbers: $F_1=F_2=1, F_{k+1}=F_k+F_{k-1}$ for $k>1$.)
KoMaL A Problems 2019/2020, A. 778
Find all square-free integers $d$ for which there exist positive integers $x, y$ and $n$ satisfying
$x^2+dy^2=2^n$
Submitted by Kada Williams, Cambridge
2020 Romanian Masters In Mathematics, 4
Let $\mathbb N$ be the set of all positive integers. A subset $A$ of $\mathbb N$ is [i]sum-free[/i] if, whenever $x$ and $y$ are (not necessarily distinct) members of $A$, their sum $x+y$ does not belong to $A$. Determine all surjective functions $f:\mathbb N\to\mathbb N$ such that, for each sum-free subset $A$ of $\mathbb N$, the image $\{f(a):a\in A\}$ is also sum-free.
[i]Note: a function $f:\mathbb N\to\mathbb N$ is surjective if, for every positive integer $n$, there exists a positive integer $m$ such that $f(m)=n$.[/i]
1998 Austrian-Polish Competition, 7
Consider all pairs $(a, b)$ of natural numbers such that the product $a^ab^b$ written in decimal system ends with exactly $98$ zeros. Find the pair $(a, b)$ for which the product $ab$ is the smallest.
2017 Junior Balkan Team Selection Tests - Moldova, Problem 4
Find the maximum positive integer $k$ such that there exist $k$ positive integers which do not exceed $2017$ and have the property that every number among them cannot be a power of any of the remaining $k-1$ numbers.
1953 Moscow Mathematical Olympiad, 233
Prove that the sum of angles at the longer base of a trapezoid is less than the sum of angles at the shorter base.
2009 Sharygin Geometry Olympiad, 6
Can four equal polygons be placed on the plane in such a way that any two of them don't have common interior points, but have a common boundary segment?
(S.Markelov)
2007 Harvard-MIT Mathematics Tournament, 3
Circles $\omega_1$, $\omega_2$, and $\omega_3$ are centered at $M$, $N$, and $O$, respectively. The points of tangency between $\omega_2$ and $\omega_3$, $\omega_3$ and $\omega_1$, and $\omega_1$ and $\omega_2$ are tangent at $A$, $B$, and $C$, respectively. Line $MO$ intersects $\omega_3$ and $\omega_1$ again at $P$ and $Q$ respectively, and line $AP$ intersects $\omega_2$ again at $R$. Given that $ABC$ is an equilateral triangle of side length $1$, compute the area of $PQR$.
KoMaL A Problems 2018/2019, A. 750
Let $k_1,k_2,\ldots,k_5$ be five circles in the lane such that $k_1$ and $k_2$ are externally tangent to each other at point $T,$ $k_3$ and $k_4$ are exetrnally tangent to both $k_1$ and $k_2,$ $k_5$ is externally tangent to $k_3$ and $k_4$ at points $U$ and $V,$ respectively, and $k_5$ intersects $k_1$ at $P$ and $Q,$ like shown in the figure. Prove that \[\frac{PU}{QU}\cdot\frac{PV}{QV}=\frac{PT^2}{QT^2}.\]
2015 Spain Mathematical Olympiad, 3
Let $ABC$ be a triangle. $M$, and $N$ points on $BC$, such that $BM=CN$, with $M$ in the interior of $BN$. Let $P$ and $Q$ be points in $AN$ and $AM$ respectively such that $\angle PMC= \angle MAB$, and $\angle QNB= \angle NAC$. Prove that $ \angle QBC= \angle PCB$.
2020 Balkan MO Shortlist, A3
Let $P(x), Q(x)$ be distinct polynomials of degree $2020$ with non-zero coefficients. Suppose that they have $r$ common real roots counting multiplicity and $s$ common coefficients. Determine the maximum possible value of $r + s$.
[i]Demetres Christofides, Cyprus[/i]
2005 iTest, 4
How many multiples of $2005$ are factors of $(2005)^2$?
2020 Thailand Mathematical Olympiad, 9
Let $n,k$ be positive integers such that $n>k$. There is a square-shaped plot of land, which is divided into $n\times n$ grid so that each cell has the same size. The land needs to be plowed by $k$ tractors; each tractor will begin on the lower-left corner cell and keep moving to the cell sharing a common side until it reaches the upper-right corner cell. In addition, each tractor can only move in two directions: up and right. Determine the minimum possible number of unplowed cells.
Kyiv City MO Juniors 2003+ geometry, 2021.8.4
Let $BM$ be the median of the triangle $ABC$, in which $AB> BC$. Point $P$ is chosen so that $AB \parallel PC$ and $PM \perp BM$. Prove that $\angle ABM = \angle MBP$.
(Mikhail Standenko)
2020 USEMO, 6
Prove that for every odd integer $n > 1$, there exist integers $a, b > 0$ such that, if we let $Q(x) = (x + a)^
2 + b$, then the following conditions hold:
$\bullet$ we have $\gcd(a, n) = gcd(b, n) = 1$;
$\bullet$ the number $Q(0)$ is divisible by $n$; and
$\bullet$ the numbers $Q(1), Q(2), Q(3), \dots$ each have a prime factor not dividing $n$.
1972 Bundeswettbewerb Mathematik, 2
Prove: out of $ 79$ consecutive positive integers, one can find at least one whose sum of digits is divisible by $ 13$. Show that this isn't true for $ 78$ consecutive integers.
2016 CCA Math Bonanza, I13
Let $P(x)$ be a polynomial with integer coefficients, leading coefficient 1, and $P(0) = 3$. If the polynomial $P(x)^2 + 1$ can be factored as a product of two non-constant polynomials with integer coefficients, and the degree of $P$ is as small as possible, compute the largest possible value of $P(10)$.
[i]2016 CCA Math Bonanza Individual #13[/i]
2010 Kurschak Competition, 3
For what positive integers $n$ and $k$ do there exits integers $a_1,a_2,\dots,a_n$ and $b_1,b_2,\dots,b_k$ such that the products $a_ib_j$ ($1\le i\le n,1\le j\le k$) give pairwise different residues modulo $nk$?
2006 Turkey Team Selection Test, 1
For all integers $n\geq 1$ we define $x_{n+1}=x_1^2+x_2^2+\cdots +x_n^2$, where $x_1$ is a positive integer. Find the least $x_1$ such that 2006 divides $x_{2006}$.
2014 Contests, 1
Let $ABC$ be a triangle, let ${A}'$, ${B}'$, ${C}'$ be the orthogonal projections of the vertices $A$ ,$B$ ,$C$ on the lines $BC$, $CA$ and $AB$, respectively, and let $X$ be a point on the line $A{A}'$.Let $\gamma_{B}$ be the circle through $B$ and $X$, centred on the line $BC$, and let $\gamma_{C}$ be the circle through $C$ and $X$, centred on the line $BC$.The circle $\gamma_{B}$ meets the lines $AB$ and $B{B}'$ again at $M$ and ${M}'$, respectively, and the circle $\gamma_{C}$ meets the lines $AC$ and $C{C}'$ again at $N$ and ${N}'$, respectively.Show that the points $M$, ${M}'$, $N$ and ${N}'$ are collinear.