Found problems: 85335
2020 China Team Selection Test, 4
Show that the following equation has finitely many solutions $(t,A,x,y,z)$ in positive integers
$$\sqrt{t(1-A^{-2})(1-x^{-2})(1-y^{-2})(1-z^{-2})}=(1+x^{-1})(1+y^{-1})(1+z^{-1})$$
LMT Team Rounds 2010-20, 2020.S19
Let $ABC$ be a triangle such that such that $AB=14, BC=13$, and $AC=15$. Let $X$ be a point inside triangle $ABC$. Compute the minimum possible value of $(\sqrt{2}AX+BX+CX)^2$.
2016 Harvard-MIT Mathematics Tournament, 5
Nine pairwise noncongruent circles are drawn in the plane such that any two circles intersect twice. For each pair of circles, we draw the line through these two points, for a total of $\binom 92 = 36$ lines. Assume that all $36$ lines drawn are distinct. What is the maximum possible number of points which lie on at least two of the drawn lines?
1996 All-Russian Olympiad, 6
Three sergeants and several solders serve in a platoon. The sergeants take turns on duty. The commander has given the following orders:
(a) Each day, at least one task must be issued to a soldier.
(b) No soldier may have more than two task or receive more than one tasks in a single day.
(c) The lists of soldiers receiving tasks for two different days must not be the same.
(d) The first sergeant violating any of these orders will be jailed.
Can at least one of the sergeants, without conspiring with the others, give tasks according to these rules and avoid being jailed?
[i]M. Kulikov[/i]
1947 Moscow Mathematical Olympiad, 131
Calculate (without calculators, tables, etc.) with accuracy to $0.00001$ the product $\left(1-\frac{1}{10}\right)\left(1-\frac{1}{10^2}\right)...\left(1-\frac{1}{10^{99}}\right)$
2010 Harvard-MIT Mathematics Tournament, 8
Let $f(n)=\displaystyle\sum_{k=2}^\infty \dfrac{1}{k^n\cdot k!}.$ Calculate $\displaystyle\sum_{n=2}^\infty f(n)$.
1988 National High School Mathematics League, 5
In rectangular coordinate system, define that if and only if both $x$-axis and $y$-axis of a point are integers, we call it itegral point. $I$ is set of all lines, $M$ is set of lines that pass exactly one intengral point, $N$ is set of lines that pass no itengral point, $P$ is set of lines that pass infinitely many itengral points. Then, how many conclusions are right?
(1)$M\cup N\cup P=I$.
(2)$N\neq\varnothing$.
(3)$M\neq\varnothing$.
(4)$P\neq\varnothing$.
$\text{(A)}1\qquad\text{(B)}2\qquad\text{(C)}3\qquad\text{(D)}4\qquad$
1957 AMC 12/AHSME, 44
In triangle $ ABC$, $ AC \equal{} CD$ and $ \angle CAB \minus{} \angle ABC \equal{} 30^\circ$. Then $ \angle BAD$ is:
[asy]defaultpen(linewidth(.8pt));
unitsize(2.5cm);
pair A = origin;
pair B = (2,0);
pair C = (0.5,0.75);
pair D = midpoint(C--B);
draw(A--B--C--cycle);
draw(A--D);
label("$A$",A,SW);
label("$B$",B,SE);
label("$C$",C,N);
label("$D$",D,NE);[/asy]$ \textbf{(A)}\ 30^\circ\qquad \textbf{(B)}\ 20^\circ\qquad \textbf{(C)}\ 22\frac {1}{2}^\circ\qquad \textbf{(D)}\ 10^\circ\qquad \textbf{(E)}\ 15^\circ$
2015 IMO Shortlist, A4
Let $\mathbb R$ be the set of real numbers. Determine all functions $f:\mathbb R\to\mathbb R$ that satisfy the equation\[f(x+f(x+y))+f(xy)=x+f(x+y)+yf(x)\]for all real numbers $x$ and $y$.
[i]Proposed by Dorlir Ahmeti, Albania[/i]
2007 Thailand Mathematical Olympiad, 13
Let $S = \{1, 2,..., 8\}$. How many ways are there to select two disjoint subsets of $S$?
2015 Peru Cono Sur TST, P4
In a small city there are $n$ bus routes, with $n > 1$, and each route has exactly $4$ stops. If any two routes have exactly one common stop, and each pair of stops belongs to exactly one route, find all possible values of $n$.
2010 Sharygin Geometry Olympiad, 2
Two points $A$ and $B$ are given. Find the locus of points $C$ such that triangle $ABC$ can be covered by a circle with radius $1$.
(Arseny Akopyan)
2019 Peru EGMO TST, 5
Define the sequence sequence $a_0,a_1, a_2,....,a_{2018}, a_{2019}$ of real numbers as follows:
$\bullet$ $a_0 = 1$.
$\bullet$ $a_{n + 1} = a_n - \frac{a_n^2}{2019}$ for $n = 0, 1, ...,2018$.
Prove that $a_{2019} < \frac12 <a_{2018}$.
1979 All Soviet Union Mathematical Olympiad, 283
Given $n$ points (in sequence)$ A_1, A_2, ... , A_n$ on a line. All the segments $A_1A_2$, $A_2A_3$,$ ...$, $A_{n-1}A_n$ are shorter than $1$. We need to mark $(k-1)$ points so that the difference of every two segments, with the ends in the marked points, is shorter than $1$. Prove that it is possible
a) for $k=3$,
b) for every $k$ less than $(n-1)$.
2024 USEMO, 3
Let $ABC$ be a triangle with incenter $I$. Two distinct points $P$ and $Q$ are chosen on the circumcircle of $ABC$ such that
\[ \angle API = \angle AQI = 45^\circ. \]
Lines $PQ$ and $BC$ meet at $S$. Let $H$ denote the foot of the altitude from $A$ to $BC$. Prove that $\angle AHI = \angle ISH$.
[i]Matsvei Zorka[/i]
1999 Estonia National Olympiad, 1
Prove that if $p$ is an odd prime, then $p^2(p^2 -1999)$ is divisible by $6$ but not by $12$.
2006 Princeton University Math Competition, 5
In the diagram shown, how many pathways are there from point $A$ to point $B$ if you are only allowed to travel due East, Southeast, or Southwest?
[img]https://cdn.artofproblemsolving.com/attachments/9/1/0a1219fb430c402fef4b7555ddff7c88fec47e.jpg[/img]
2019 Balkan MO Shortlist, G5
Let $ABC$ ($BC > AC$) be an acute triangle with circumcircle $k$ centered at $O$. The tangent to $k$ at $C$ intersects the line $AB$ at the point $D$. The circumcircles of triangles $BCD, OCD$ and $AOB$ intersect the ray $CA$ (beyond $A$) at the points $Q, P$ and $K$, respectively, such that $P \in (AK)$ and $K \in (PQ)$. The line $PD$ intersects the circumcircle of triangle $BKQ$ at the point $T$, so that $P$ and $T$ are in different halfplanes with respect to $BQ$. Prove that $TB = TQ$.
2011 Croatia Team Selection Test, 4
Find all pairs of integers $x,y$ for which
\[x^3+x^2+x=y^2+y.\]
2022 Princeton University Math Competition, A7
For a positive integer $n \ge 1,$ let $a_n=\lfloor \sqrt[3]{n}+\tfrac{1}{2}\rfloor.$ Given a positive integer $N \ge 1,$ let $\mathcal{F}_N$ denote the set of positive integers $n \ge 1$ such that $a_n \le N.$ Let $S_N = \sum_{n \in \mathcal{F}_N} \tfrac{1}{a_n^2}.$ As $N$ goes to infinity, the quantity $S_N - 3N$ tends to $\tfrac{a\pi^2}{b}$ for relatifvely prime positive integers $a,b.$ Given that $\sum_{k=1}^{\infty} \tfrac{1}{k^2} = \tfrac{\pi^2}{6},$ find $a+b.$
1997 ITAMO, 1
An infinite rectangular stripe of width $3$ cm is folded along a line. What is the minimum possible area of the region of overlapping?
2020 Princeton University Math Competition, A4/B6
Given two positive integers $a \ne b$, let $f(a, b)$ be the smallest integer that divides exactly one of $a, b$, but not both. Determine the number of pairs of positive integers $(x, y)$, where $x \ne y$, $1\le x, y, \le 100$ and $\gcd(f(x, y), \gcd(x, y)) = 2$.
1990 IMO Longlists, 8
Let $a, b, c$ be the side lengths and $P$ be area of a triangle, respectively. Prove that
\[(a^2+b^2+c^2-4\sqrt 3 P) (a^2+b^2+c^2) \geq 2 \left(a^2(b - c)^2 + b^2(c - a)^2 + c^2(a - b)^2\right).\]
2024 German National Olympiad, 4
Let $k>2$ be a positive integer such that the $k$-digit number $n_k=133\dots 3$, consisting of a digit $1$ followed by $k-1$ digits $3$ is prime. Show that $24 \mid k(k+2)$.
2019 Simon Marais Mathematical Competition, A2
Consider the operation $\ast$ that takes pair of integers and returns an integer according to the rule
$$a\ast b=a\times (b+1).$$
[list=a]
[*]For each positive integer $n$, determine all permutations $a_1,a_2,\dotsc , a_n$ of the set $\{ 1,2,\dotsc ,n\}$ that maximise the value of $$(\cdots ((a_1\ast a_2)\ast a_3) \ast \cdots \ast a_{n-1})\ast a_n.$$[/*]
[*]For each positive integer $n$, determine all permutations $b_1,b_2,\dotsc , b_n$ of the set $\{ 1,2,\dotsc ,n\}$ that maximise the value of $$b_1\ast (b_2\ast (b_3\ast \cdots \ast (b_{n-1}\ast b_n)\cdots )).$$[/*]
[/list]