Found problems: 85335
2012 Romania National Olympiad, 3
[color=darkred]Prove that if $n\ge 2$ is a natural number and $x_1,x_2,\ldots,x_n$ are positive real numbers, then:
\[4\left(\frac {x_1^3-x_2^3}{x_1+x_2}+\frac {x_2^3-x_3^3}{x_2+x_3}+\ldots+\frac {x_{n-1}^3-x_n^3}{x_{n-1}+x_n}+\frac {x_n^3-x_1^3}{x_n+x_1}\right)\le \\ \\
\le(x_1-x_2)^2+(x_2-x_3)^2+\ldots+(x_{n-1}-x_n)^2+(x_n-x_1)^2\, .\][/color]
2008 South africa National Olympiad, 3
Let $a,b,c$ be positive real numbers. Prove that
\[(a+b)(b+c)(c+a)\ge 8(a+b-c)(b+c-a)(c+a-b)\]
and determine when equality occurs.
2022 LMT Fall, 3
Billiam is distributing his ample supply of balls among an ample supply of boxes. He distributes the balls as follows: he places a ball in the first empty box, and then for the greatest positive integer n such that all $n$ boxes from box $1$ to box $n$ have at least one ball, he takes all of the balls in those $n$ boxes and puts them into box $n +1$. He then repeats this process indefinitely. Find the number of repetitions of this process it takes for one box to have at least $2022$ balls.
2024 BAMO, 4
Find all polynomials $f$ that satisfy the equation
\[\frac{f(3x)}{f(x)} = \frac{729 (x-3)}{x-243}\]
for infinitely many real values of $x$.
MOAA Gunga Bowls, 2023.22
Harry the knight is positioned at the origin of the Cartesian plane. In a "knight hop", Harry can move from the point $(i,j)$ to a point with integer coordinates that is a distance of $\sqrt{5}$ away from $(i,j)$. What is the number of ways that Harry can return to the origin after 6 knight hops?
[i]Proposed by Harry Kim[/i]
1989 ITAMO, 5
A fair coin is repeatedly tossed. We receive one marker for every ”head” and two markers for every ”tail”. We win the game if, at some moment, we possess exactly $100$ markers. Is the probability of winning the game greater than, equal to, or less than $2/3$?
2020 CHMMC Winter (2020-21), 3
For two base-10 positive integers $a$ and $b$, we say $a \sim b$ if we can rearrange the digits of $a$ in some way to obtain $b$, where the leading digit of both $a$ and $b$ is nonzero. For instance, $463 \sim 463$ and $634 \sim 463$. Find the number of $11$-digit positive integers $K$ such that $K$ is divisible by $2$, $3$, and $5$, and there is some positive integer $K'$ such that $K' \sim K$ and $K'$ is divisible by $7$, $11$, $13$, $17$, $101$, and $9901$.
2004 India IMO Training Camp, 4
Given a permutation $\sigma = (a_1,a_2,a_3,...a_n)$ of $(1,2,3,...n)$ , an ordered pair $(a_j,a_k)$ is called an inversion of $\sigma$ if $a \leq j < k \leq n$ and $a_j > a_k$. Let $m(\sigma)$ denote the no. of inversions of the permutation $\sigma$. Find the average of $m(\sigma)$ as $\sigma$ varies over all permutations.
2003 AMC 10, 15
There are $ 100$ players in a singles tennis tournament. The tournament is single elimination, meaning that a player who loses a match is eliminated. In the first round, the strongest $ 28$ players are given a bye, and the remaining $ 72$ players are paired off to play. After each round, the remaining players play in the next round. The match continues until only one player remains unbeaten. The total number of matches played is
$ \textbf{(A)}\ \text{a prime number} \qquad \textbf{(B)}\ \text{divisible by 2} \qquad \textbf{(C)}\ \text{divisible by 5}$
$ \textbf{(D)}\ \text{divisible by 7} \qquad \textbf{(E)}\ \text{divisible by 11}$
2014 Taiwan TST Round 1, 1
Let $f(x) = x^n + a_{n-2} x^{n-2} + a_{n-3}x^{n-3} + \dots + a_1x + a_0$ be a polynomial with real coefficients $(n \ge 2)$. Suppose all roots of $f$ are real. Prove that the absolute value of each root is at most $\sqrt{\frac{2(1-n)}n a_{n-2}}$.
2011 Bulgaria National Olympiad, 3
Triangle $ABC$ and a function $f:\mathbb{R}^+\to\mathbb{R}$ have the following property: for every line segment $DE$ from the interior of the triangle with midpoint $M$, the inequality $f(d(D))+f(d(E))\le 2f(d(M))$, where $d(X)$ is the distance from point $X$ to the nearest side of the triangle ($X$ is in the interior of $\triangle ABC$). Prove that for each line segment $PQ$ and each point interior point $N$ the inequality $|QN|f(d(P))+|PN|f(d(Q))\le |PQ|f(d(N))$ holds.
2017 Princeton University Math Competition, A3/B5
Define the [i]bigness [/i]of a rectangular prism to be the sum of its volume, its surface area, and the lengths of all of its edges. Find the least integer $N$ for which there exists a rectangular prism with integer side lengths and [i]bigness [/i]$N$ and another one with integer side lengths and [i]bigness [/i]$N + 1$.
2016 Saudi Arabia IMO TST, 1
Let $ABC$ be a triangle whose incircle $(I)$ touches $BC, CA, AB$ at $D, E, F$, respectively. The line passing through $A$ and parallel to $BC$ cuts $DE, DF$ at $M, N$, respectively. The circumcircle of triangle $DMN$ cuts $(I)$ again at $L$.
a) Let $K$ be the intersection of $N E$ and $M F$. Prove that $K$ is the orthocenter of the triangle $DMN$.
b) Prove that $A, K, L$ are collinear.
1987 AMC 12/AHSME, 2
A triangular corner with side lengths $DB=EB=1$ is cut from equilateral triangle $ABC$ of side length $3$. The perimeter of the remaining quadrilateral is
[asy]
draw((0,0)--(2,0)--(2.5,.87)--(1.5,2.6)--cycle, linewidth(1));
draw((2,0)--(3,0)--(2.5,.87));
label("3", (0.75,1.3), NW);
label("1", (2.5, 0), S);
label("1", (2.75,.44), NE);
label("A", (1.5,2.6), N);
label("B", (3,0), S);
label("C", (0,0), W);
label("D", (2.5,.87), NE);
label("E", (2,0), S);[/asy]
$\text{(A)} \ 6 \qquad \text{(B)} \ 6\frac12 \qquad \text{(C)} \ 7 \qquad \text{(D)} \ 7\frac12 \qquad \text{(E)} \ 8$
1992 AMC 12/AHSME, 17
The two digit integers from $19$ to $92$ are written consecutively to form the larger integer $N = 19202122\ldots909192$. If $3^{k}$ is the highest power of $3$ that is a factor of $N$, then $k =$
$ \textbf{(A)}\ 0\qquad\textbf{(B)}\ 1\qquad\textbf{(C)}\ 2\qquad\textbf{(D)}\ 3\qquad\textbf{(E)}\ \text{more than 3} $
2010 Germany Team Selection Test, 2
Prove or disprove that $\forall a,b,c,d \in \mathbb{R}^+$ we have the following inequality:
\[3 \leq \frac{4a+b}{a+4b} + \frac{4b+c}{b+4c} + \frac{4c+a}{c+4a} < \frac{33}{4}\]
2017 Benelux, 3
In the convex quadrilateral $ABCD$ we have $\angle B = \angle C$ and $\angle D = 90^{\circ}.$ Suppose that $|AB| = 2|CD|.$ Prove that the angle bisector of $\angle ACB$ is perpendicular to $CD.$
2000 Vietnam Team Selection Test, 2
Let $k$ be a given positive integer. Define $x_{1}= 1$ and, for each $n > 1$, set $x_{n+1}$ to be the smallest positive integer not belonging to the set $\{x_{i}, x_{i}+ik | i = 1, . . . , n\}$. Prove that there is a real number $a$ such that $x_{n}= [an]$ for all $n \in\mathbb{ N}$.
2006 Stanford Mathematics Tournament, 3
After a typist has written ten letters and had addressed the ten corresponding envelopes, a careless mailing clerk inserted the letters in the envelopes at random, one letter per envelope. What is the probability that [b]exactly[/b] nine letters were inserted in the proper envelopes?
2015 Azerbaijan JBMO TST, 4
Find all integer solutions to the equation $x^2=y^2(x+y^4+2y^2)$ .
2000 AIME Problems, 13
The equation $2000x^6+100x^5+10x^3+x-2=0$ has exactly two real roots, one of which is $\frac{m+\sqrt{n}}r,$ where $m, n$ and $r$ are integers, $m$ and $r$ are relatively prime, and $r>0.$ Find $m+n+r.$
2023 Belarusian National Olympiad, 10.5
On hyperbola $y=\frac{1}{x}$ points $A_1,\ldots,A_{10}$ are chosen such that $(A_i)_x=2^{i-1}a$, where $a$ is some positive constant.
Find the area of $A_1A_2 \ldots A_{10}$
2015 Macedonia National Olympiad, Problem 2
Let $a,b,c \in \mathbb{R}^{+}$ such that $abc=1$. Prove that:
$$a^2b + b^2c + c^2a \ge \sqrt{(a+b+c)(ab + bc +ca)}$$
2004 Croatia National Olympiad, Problem 1
Find all real solutions of the system of equations
$$x^2-y^2=2(xz+yz+x+y),$$$$y^2-z^2=2(yx+zx+y+z),$$$$z^2-x^2=2(zy+xy+z+x).$$
2016 China Team Selection Test, 4
Let $a,b,b',c,m,q$ be positive integers, where $m>1,q>1,|b-b'|\ge a$. It is given that there exist a positive integer $M$ such that
$$S_q(an+b)\equiv S_q(an+b')+c\pmod{m}$$
holds for all integers $n\ge M$. Prove that the above equation is true for all positive integers $n$. (Here $S_q(x)$ is the sum of digits of $x$ taken in base $q$).