Found problems: 85335
2021 Indonesia TST, G
let $ w_1 $ and $ w_2 $ two circles such that $ w_1 \cap w_2 = \{ A , B \} $
let $ X $ a point on $ w_2 $ and $ Y $ on $ w_1 $ such that $ BY \bot BX $
suppose that $ O $ is the center of $ w_1 $ and $ X' = w_2 \cap OX $
now if $ K = w_2 \cap X'Y $ prove $ X $ is the midpoint of arc $ AK $
2017 Regional Olympiad of Mexico Northeast, 6
Find all triples of real numbers $(a, b, c)$ that satisfy the system of equations $$\begin{cases} b^2 = 4a(\sqrt{c} - 1) \\ c^2 = 4b (\sqrt{a} - 1) \\ a^2 = 4c(\sqrt{b} - 1) \end{cases}$$
1996 Turkey Team Selection Test, 3
Determine all ordered pairs of positive real numbers $(a, b)$ such that every sequence $(x_{n})$ satisfying $\lim_{n \rightarrow \infty}{(ax_{n+1} - bx_{n})} = 0$ must have $\lim_{n \rightarrow \infty} x_n = 0$.
2021 China Team Selection Test, 6
Find the smallest positive real constant $a$, such that for any three points $A,B,C$ on the unit circle, there exists an equilateral triangle $PQR$ with side length $a$ such that all of $A,B,C$ lie on the interior or boundary of $\triangle PQR$.
1971 IMO Longlists, 19
In a triangle $P_1P_2P_3$ let $P_iQ_i$ be the altitude from $P_i$ for $i = 1, 2,3$ ($Q_i$ being the foot of the altitude). The circle with diameter $P_iQ_i$ meets the two corresponding sides at two points different from $P_i.$ Denote the length of the segment whose endpoints are these two points by $l_i.$ Prove that $l_1 = l_2 = l_3.$
1992 IMO Longlists, 33
Let $a, b, c$ be positive real numbers and $p, q, r$ complex numbers. Let $S$ be the set of all solutions $(x, y, z)$ in $\mathbb C$ of the system of simultaneous equations
\[ax + by + cz = p,\]\[ax2 + by2 + cz2 = q,\]\[ax3 + bx3 + cx3 = r.\]
Prove that $S$ has at most six elements.
2000 IMO Shortlist, 6
Let $ ABCD$ be a convex quadrilateral. The perpendicular bisectors of its sides $ AB$ and $ CD$ meet at $ Y$. Denote by $ X$ a point inside the quadrilateral $ ABCD$ such that $ \measuredangle ADX \equal{} \measuredangle BCX < 90^{\circ}$ and $ \measuredangle DAX \equal{} \measuredangle CBX < 90^{\circ}$. Show that $ \measuredangle AYB \equal{} 2\cdot\measuredangle ADX$.
2009 QEDMO 6th, 10
Let $n \in N$. The land of Draconis has more than $2^n$ dungeons. Between two different Dungeons have exactly one path, but each path is a one-way street. Total $n$ Dragon cults share the territory; each path is controlled by exactly one cult. It is said that a dragon cult $K$ has established itself in a dungeon $D$ if there is both one a path beginning in $D$ and one a path ending in $D$, both of which are controlled by $K$ . Prove that there is a cult $K$, which has at least one dungeon controlled.
1995 AMC 8, 18
The area of each of the four congruent L-shaped regions of this 100-inch by 100-inch square is 3/16 of the total area. How many inches long is the side of the center square?
[asy]
draw((2,2)--(2,-2)--(-2,-2)--(-2,2)--cycle);
draw((1,1)--(1,-1)--(-1,-1)--(-1,1)--cycle);
draw((0,1)--(0,2));
draw((1,0)--(2,0));
draw((0,-1)--(0,-2));
draw((-1,0)--(-2,0));
[/asy]
$\text{(A)}\ 25 \qquad \text{(B)}\ 44 \qquad \text{(C)}\ 50 \qquad \text{(D)}\ 62 \qquad \text{(E)}\ 75$
2023 Indonesia TST, N
Find all triplets natural numbers $(a, b, c)$ satisfied
\[GCD(a, b) + LCM(a,b) = 2021^c\]
with $|a - b|$ and $(a+b)^2 + 4$ are both prime number
2024 AMC 8 -, 4
When Yunji added all the integers from $1$ to $9$, she mistakenly left out a number. Her incorrect sum turned out to be a square number. Which number did Yunji leave out?
$\textbf{(A) } 5\qquad\textbf{(B) } 6\qquad\textbf{(C) } 7\qquad\textbf{(D) } 8\qquad\textbf{(E) } 9$
2016 Harvard-MIT Mathematics Tournament, 10
Quadrilateral $ABCD$ satisfies $AB = 8, BC = 5, CD = 17, DA = 10$. Let $E$ be the intersection of $AC$ and $BD$. Suppose $BE : ED = 1 : 2$. Find the area of $ABCD$.
2013 India IMO Training Camp, 2
In a triangle $ABC$, let $I$ denote its incenter. Points $D, E, F$ are chosen on the segments $BC, CA, AB$, respectively, such that $BD + BF = AC$ and $CD + CE = AB$. The circumcircles of triangles $AEF, BFD, CDE$ intersect lines $AI, BI, CI$, respectively, at points $K, L, M$ (different from $A, B, C$), respectively. Prove that $K, L, M, I$ are concyclic.
2019 Irish Math Olympiad, 1
De fine the [i]quasi-primes[/i] as follows.
$\bullet$ The first quasi-prime is $q_1 = 2$
$\bullet$ For $n \ge 2$, the $n^{th}$ quasi-prime $q_n$ is the smallest integer greater than $q_{n_1}$ and not of the form $q_iq_j$ for some $1 \le i \le j \le n - 1$.
Determine, with proof, whether or not $1000$ is a quasi-prime.
2006 Nordic, 2
Real numbers $x,y,z$ are not all equal and satisfy $x+\frac{1}{y} = y + \frac{1}{z} = z + \frac{1}{x}=k$. Find all possible values of $k$.
2007 Nicolae Coculescu, 3
Let $ F:\mathbb{R}\longrightarrow\mathbb{R} $ be a primitive with $ F(0)=0 $ of the function $ f:\mathbb{R}\longrightarrow\mathbb{R} $ defined as $ f(x)=\sin (x^2) , $ and let be a sequence $ \left( a_n \right)_{n\ge 0} $ with $ a_0\in (0,1) $ and defined as $ a_{n}=a_{n-1}-F\left( a_{n-1} \right) . $
Calculate $ \lim_{n\to\infty } a_n\sqrt{n} . $
[i]Florian Dumitrel[/i]
2023 USAMTS Problems, 2
Grogg takes an a × b × c rectangular block (where a, b, c are positive integers), paints
the outside of it purple, and cuts it into abc small 1 × 1 × 1 cubes. He then places all the
small cubes into a bag, and Winnie reaches in and randomly picks one of the small cubes.
If the probability that Winnie picks a totally unpainted cube is 20%, determine all possible
values of the number of cubes in the bag.
1978 AMC 12/AHSME, 30
In a tennis tournament, $n$ women and $2n$ men play, and each player plays exactly one match with every other player. If there are no ties and the ratio of the number of matches won by women to the number of matches won by men is $7/5$, then $n$ equals
$\textbf{(A) }2\qquad\textbf{(B) }4\qquad\textbf{(C) }6\qquad\textbf{(D) }7\qquad \textbf{(E) }\text{none of these}$
1989 IMO Shortlist, 5
Find the roots $ r_i \in \mathbb{R}$ of the polynomial \[ p(x) \equal{} x^n \plus{} n \cdot x^{n\minus{}1} \plus{} a_2 \cdot x^{n\minus{}2} \plus{} \ldots \plus{} a_n\] satisfying \[ \sum^{16}_{k\equal{}1} r^{16}_k \equal{} n.\]
2016 PUMaC Number Theory B, 2
For a positive integer $n$, let $s(n)$ be the sum of the digits of $n$. If $n$ is a two-digit positive integer such that $\frac{n}{s(n)}$ is a multiple of $3$, compute the sum of all possible values of $n$.
2023 Oral Moscow Geometry Olympiad, 4
Given isosceles tetrahedron $PABC$ (faces are equal triangles). Let $A_0$, $B_0$ and $C_0$ be the touchpoints of the circle inscribed in the triangle $ABC$ with sides $BC$, $AC$ and $AB$ respectively, $A_1$, $B_1$ and $C_1$ are the touchpoints of the excircles of triangles $PCA$, $PAB$ and $PBC$ with extensions of sides $PA$, $PB$ and $PC$, respectively (beyond points $A$, $B$, $C$). Prove that the lines $A_0A_1$, $B_0B_1$ and $C_0C_1$ intersect at one point.
2006 Greece Junior Math Olympiad, 2
Find all positive integers $x , y$ which are roots of the equation
$2 x^y-y= 2005$
[u] Babis[/u]
2017 India PRMO, 29
For each positive integer $n$, consider the highest common factor $h_n$ of the two numbers $n!+1$ and $(n+1)!$. For $n<100$, find the largest value of $h_n$.
2011 Tuymaada Olympiad, 2
Circles $\omega_1$ and $\omega_2$ intersect at points $A$ and $B$, and $M$ is the midpoint of $AB$. Points $S_1$ and $S_2$ lie on the line $AB$ (but not between $A$ and $B$). The tangents drawn from $S_1$ to $\omega_1$ touch it at $X_1$ and $Y_1$, and the tangents drawn from $S_2$ to $\omega_2$ touch it at $X_2$ and $Y_2$. Prove that if the line $X_1X_2$ passes through $M$, then line $Y_1Y_2$ also passes through $M$.
1984 Czech And Slovak Olympiad IIIA, 4
Let $r$ be a natural number greater than $1$. Then there exist positive irrational numbers $x, y$ such that $x^y = r$ . Prove it.