Found problems: 85335
2024 SG Originals, Q1
Find all permutations $(a_1, a_2, \cdots, a_{2024})$ of $(1, 2, \cdots, 2024)$ such that there exists a polynomial $P$ with integer coefficients satisfying $P(i) = a_i$ for each $i = 1, 2, \cdots, 2024$.
2008 Hanoi Open Mathematics Competitions, 8
Consider a convex quadrilateral $ABCD$. Let $O$ be the intersection of $AC$ and $BD$; $M, N$ be the centroid of $\Delta AOB$ and $\Delta COD$ and $P, Q$ be orthocenter of $\Delta BOC$ and $\Delta DOA$, respectively.
Prove that $MN\bot PQ$.
1997 Tournament Of Towns, (524) 1
How many integers from $1$ to $1997$ have the sum of their digits divisible by $5$?
(AI Galochkin)
2004 Regional Olympiad - Republic of Srpska, 3
Determine all pairs of positive integers $(a,b)$, such that the roots of the equations \[x^2-ax+a+b-3=0,\]
\[x^2-bx+a+b-3=0,\] are also positive integers.
1981 AMC 12/AHSME, 5
In trapezoid $ABCD$, sides $AB$ and $CD$ are parallel, and diagonal $BD$ and side $AD$ have equal length. If $m\angle DBC=110^\circ$ and $m\angle CBD =30^\circ$, then $m \angle ADB=$
$\text{(A)}\ 80^\circ \qquad \text{(B)}\ 90^\circ \qquad \text{(C)}\ 100^\circ \qquad \text{(D)}\ 110^\circ \qquad \text{(E)}\ 120^\circ$
2001 Bulgaria National Olympiad, 2
Find all real values $t$ for which there exist real numbers $x$, $y$, $z$ satisfying :
$3x^2 + 3xz + z^2 = 1$ ,
$3y^2 + 3yz + z^2 = 4$,
$x^2 - xy + y^2 = t$.
2001 CentroAmerican, 3
Find all the real numbers $ N$ that satisfy these requirements:
1. Only two of the digits of $ N$ are distinct from $ 0$, and one of them is $ 3$.
2. $ N$ is a perfect square.
2016 Japan Mathematical Olympiad Preliminary, 1
Calculate the value of $\sqrt{\dfrac{11^4+100^4+111^4}{2}}$ and answer in the form of an integer.
2012 IFYM, Sozopol, 7
A quadrilateral $ABCD$ is inscribed in a circle with center $O$. Let $A_1 B_1 C_1 D_1$ be the image of $ABCD$ after rotation with center $O$ and angle $\alpha \in (0,90^\circ)$. The points $P,Q,R$ and $S$ are intersections of $AB$ and $A_1 B_1$, $BC$ and $B_1 C_1$, $CD$ and $C_1 D_1$, and $DA$ and $D_1 A_1$. Prove that $PQRS$ is a parallelogram.
2023 AMC 12/AHSME, 3
A $3-4-5$ right triangle is inscribed in circle $A$, and a $5-12-13$ right triangle is inscribed in circle $B$. What is the ratio of the area of circle $A$ to the area of circle $B$?
$\textbf{(A)}~\frac{9}{25}\qquad\textbf{(B)}~\frac{1}{9}\qquad\textbf{(C)}~\frac{1}{5}\qquad\textbf{(D)}~\frac{25}{169}\qquad\textbf{(E)}~\frac{4}{25}$
2014 Contests, 2
The $100$ vertices of a prism, whose base is a $50$-gon, are labeled with numbers $1, 2, 3, \ldots, 100$ in any order. Prove that there are two vertices, which are connected by an edge of the prism, with labels differing by not more than $48$.
Note: In all the triangles the three vertices do not lie on a straight line.
2012 Vietnam National Olympiad, 3
Find all $f:\mathbb{R} \to \mathbb{R}$ such that:
(a) For every real number $a$ there exist real number $b$:$f(b)=a$
(b) If $x>y$ then $f(x)>f(y)$
(c) $f(f(x))=f(x)+12x.$
Fractal Edition 2, P4
Show that:
$$
1+\frac{1}{4}+\frac{1}{9}+\dots+\frac{1}{2023^2}+\frac{1}{2024^2} < 2.
$$
2014 PUMaC Team, 6
Find the sum of positive integer solutions of $x$ for $\dfrac{x^2}{1716-x}=p$, where $p$ is a prime. (If there are no solutions, answer $0$.)
2017 China Girls Math Olympiad, 3
Given $a_i\ge 0,x_i\in\mathbb{R},(i=1,2,\ldots,n)$. Prove that
$$((1-\sum_{i=1}^n a_i\cos x_i)^2+(1-\sum_{i=1}^n a_i\sin x_i)^2)^2\ge 4(1-\sum_{i=1}^n a_i)^3$$
2006 Stanford Mathematics Tournament, 2
In a given sequence $\{S_1,S_2,...,S_k\}$, for terms $n\ge3$, $S_n=\sum_{i=1}^{n-1} i\cdot S_{n-i}$. For example, if the first two elements are 2 and 3, respectively, the third entry would be $1\cdot3+2\cdot2=7$, and the fourth would be $1\cdot7+2\cdot3+3\cdot2=19$, and so on. Given that a sequence of integers having this form starts with 2, and the 7th element is 68, what is the second element?
2021 Baltic Way, 8
We are given a collection of $2^{2^k}$ coins, where $k$ is a non-negative integer. Exactly one coin is fake.
We have an unlimited number of service dogs. One dog is sick but we do not know which one.
A test consists of three steps: select some coins from the collection of all coins; choose a service dog; the dog smells all of the selected coins at once.
A healthy dog will bark if and only if the fake coin is amongst them. Whether the sick dog will bark or not is random. \\
Devise a strategy to find the fake coin, using at most $2^k+k+2$ tests, and prove that it works.
2010 HMNT, 5
Circle $O$ has chord $AB$. A circle is tangent to $O$ at $T$ and tangent to$ AB$ at $X$ such that $AX = 2XB$. What is $\frac{AT}{BT}$ ?
2023 MIG, 13
Five cards numbered $1,2,3,4,$ and $5$ are given to Paige, Quincy, Ronald, Selena, and Terrence. Paige, Quincy, and Ronald have the following conversation:
[list=disc]
[*]Paige: My number is between is between Selena's number and Quincy's number.
[*]Quincy: My number is between Ronald's number and Terrence's number.
[*]Ronald: My number is between Paige's number and Quincy's number.
[/list]
Who received the card numbered $3$?
$\textbf{(A) } \text{Paige}\qquad\textbf{(B) } \text{Quincy}\qquad\textbf{(C) } \text{Ronald}\qquad\textbf{(D) } \text{Selena}\qquad\textbf{(E) } \text{Terrence}$
2010 Korea - Final Round, 3
There are $ n$ websites $ 1,2,\ldots,n$ ($ n \geq 2$). If there is a link from website $ i$ to $ j$, we can use this link so we can move website $ i$ to $ j$.
For all $ i \in \left\{1,2,\ldots,n - 1 \right\}$, there is a link from website $ i$ to $ i+1$.
Prove that we can add less or equal than $ 3(n - 1)\log_{2}(\log_{2} n)$ links so that for all integers $ 1 \leq i < j \leq n$, starting with website $ i$, and using at most three links to website $ j$. (If we use a link, website's number should increase. For example, No.7 to 4 is impossible).
Sorry for my bad English.
2023 Estonia Team Selection Test, 2
Let $n$ be a positive integer. Find all polynomials $P$ with real coefficients such that $$P(x^2+x-n^2)=P(x)^2+P(x)$$ for all real numbers $x$.
1961 IMO, 5
Construct a triangle $ABC$ if $AC=b$, $AB=c$ and $\angle AMB=w$, where $M$ is the midpoint of the segment $BC$ and $w<90$. Prove that a solution exists if and only if \[ b \tan{\dfrac{w}{2}} \leq c <b \] In what case does the equality hold?
2016 Irish Math Olympiad, 9
Show that the number $a^3$ where $a=\frac{251}{ \frac{1}{\sqrt[3]{252}-5\sqrt[3]{2}}-10\sqrt[3]{63}}+\frac{1}{\frac{251}{\sqrt[3]{252}+5\sqrt[3]{2}}+10\sqrt[3]{63}}$
is an integer and find its value
1985 Swedish Mathematical Competition, 6
X-wich has a vibrant club-life. For every pair of inhabitants there is exactly one club to which they both belong. For every pair of clubs there is exactly one person who is a member of both. No club has fewer than $3$ members, and at least one club has $17$ members. How many people live in X-wich?
2014 Federal Competition For Advanced Students, P2, 3
(i) For which triangles with side lengths $a, b$ and $c$ apply besides the triangle inequalities $a + b> c, b + c> a$ and $c + a> b$ also the inequalities $a^2 + b^2> c^2, b^2 + c^2> a^2$ and $a^2 + c^2> b^2$ ?
(ii) For which triangles with side lengths $a, b$ and $c$ apply besides the triangle inequalities $a + b> c, b + c> a$ and $c + a> b$ also for all positive natural $n$ the inequalities $a^n + b^n> c^n, b^n + c^n> a^n$ and $a^n + c^n> b^n$ ?