Found problems: 85335
2018 China Northern MO, 4
For $n(n\geq3)$ positive intengers $a_1,a_2,\cdots,a_n$. Put the numbers on a circle. In each operation, calculate difference between two adjacent numbers and take its absolute value. Put the $n$ numbers we get on another ciecle (do not change their order). Find all $n$, satisfying that no matter how $a_1,a_2,\cdots,a_n$ are given, all numbers on the circle are equal after limited operations.
2008 Brazil Team Selection Test, 3
If $a, b, c$ and $d$ are positive real numbers such that $a + b + c + d = 2$, prove that
$$\frac{a^2}{(a^2+1)^2}+\frac{b^2}{(b^2+1)^2}+\frac{c^2}{(c^2+1)^2}+\frac{d^2}{(d^2+1)^2} \le \frac{16}{25}$$
2016 China Northern MO, 5
Let $\theta_{i}\in(0,\frac{\pi}{2})(i=1,2,\cdots,n)$. Prove:
$$(\sum_{i=1}^n\tan\theta_{i})(\sum_{i=1}^n\cot\theta_{i})\geq(\sum_{i=1}^n\sin\theta_{i})^2+(\sum_{i=1}^n\cos\theta_{i})^2.$$
2007 AMC 10, 1
Isabella's house has $ 3$ bedrooms. Each bedroom is $ 12$ feet long, $ 10$ feet wide, and $ 8$ feet high. Isabella must paint the walls of all the bedrooms. Doorways and windows, which will not be painted, occupy $ 60$ square feet in each bedroom. How many square feet of walls must be painted?
$ \textbf{(A)}\ 678 \qquad \textbf{(B)}\ 768 \qquad \textbf{(C)}\ 786 \qquad \textbf{(D)}\ 867 \qquad \textbf{(E)}\ 876$
2019 Thailand TSTST, 1
Find all primes $p$ such that $(p-3)^p+p^2$ is a perfect square.
1982 Brazil National Olympiad, 6
Five spheres of radius $r$ are inside a right circular cone. Four of the spheres lie on the base of the cone. Each touches two of the others and the sloping sides of the cone. The fifth sphere touches each of the other four and also the sloping sides of the cone. Find the volume of the cone.
2004 AIME Problems, 11
A right circular cone has a base with radius 600 and height $200\sqrt{7}$. A fly starts at a point on the surface of the cone whose distance from the vertex of the cone is 125, and crawls along the surface of the cone to a point on the exact opposite side of the cone whose distance from the vertex is $375\sqrt{2}$. Find the least distance that the fly could have crawled.
1995 North Macedonia National Olympiad, 3
Prove that the product of $8$ consecutive natural numbers can never be a fourth power of natural number.
2015 JBMO Shortlist, 2
The point ${P}$ is outside the circle ${\Omega}$. Two tangent lines, passing from the point ${P}$ touch the circle ${\Omega}$ at the points ${A}$ and ${B}$. The median${AM \left(M\in BP\right)}$ intersects the circle ${\Omega}$ at the point ${C}$ and the line ${PC}$ intersects again the circle ${\Omega}$ at the point ${D}$. Prove that the lines ${AD}$ and ${BP}$ are parallel.
(Moldova)
1978 Czech and Slovak Olympiad III A, 3
Let $\alpha,\beta,\gamma$ be angles of a triangle. Determine all real triplets $x,y,z$ satisfying the system
\begin{align*}
x\cos\beta+\frac1z\cos\alpha &=1, \\
y\cos\gamma+\frac1x\cos\beta &=1, \\
z\cos\alpha+\frac1y\cos\gamma &=1.
\end{align*}
2022 Stanford Mathematics Tournament, 3
Let $\triangle ABC$ be a triangle with $BA<AC$, $BC=10$, and $BA=8$. Let $H$ be the orthocenter of $\triangle ABC$. Let $F$ be the point on segment $AC$ such that $BF=8$. Let $T$ be the point of intersection of $FH$ and the extension of line $BC$. Suppose that $BT=8$. Find the area of $\triangle ABC$.
1982 IMO Shortlist, 10
A box contains $p$ white balls and $q$ black balls. Beside the box there is a pile of black balls. Two balls are taken out of the box. If they have the same color, a black ball from the pile is put into the box. If they have different colors, the white ball is put back into the box. This procedure is repeated until the last two balls are removed from the box and one last ball is put in. What is the probability that this last ball is white?
2006 Moldova National Olympiad, 12.5
Let $ a_{1},a_{2},...,a_{n} $ be real positive numbers and $ k>m, k,m $ natural numbers. Prove that
$(n-1)(a_{1}^m +a_{2}^m+...+a_{n}^m)\leq\frac{a_{2}^k+a_{3}^k+...+a_{n}^k}{a_{1}^{k-m}}+\frac{a_{1}^k+a_{3}^k+...+a_{n}^k}{a_2^{k-m}}+...+\frac{a_{1}^k+a_{2}^k+...+a_{n-1}^k}{a_{n}^{k-m}} $
1966 Dutch Mathematical Olympiad, 4
A rectangular piece of paper is divided into square cells by lines parallel to the sides of the rectangle. $n$ (horizontal) rows of $m$ cells have emerged and $m$ (vertical) columns of $n$ cells have also been formed. There is a number in each cell. Find the largest number in each of the $n$ rows. The smallest maxima of those $n$ rows is called $A$. We also look for the smallest number in each of the $m$ columns. The largest minima of those $m$ columns is called $B$.
Prove that $A$ is greater than or equal to $B$. Can you give a simple example where $A = B$?
1994 Niels Henrik Abels Math Contest (Norwegian Math Olympiad) Round 2, 8
Let $ f$ be a function defined on $ \text{N}_0 \equal{} \{ 0,1,2,3,...\}$ and with values in $ \text{N}_0$, such that for $ n,m \in \text{N}_0$ and $ m \leq 9, f(10n \plus{} m) \equal{} f(n) \plus{} 11m$ and $ f(0) \equal{} 0.$ How many solutions are there to the equation $ f(x) \equal{} 1995$?
A. None
B. 1
C. 2
D. 11
E. Infinitely many
2006 Singapore Junior Math Olympiad, 5
You have a large number of congruent equilateral triangular tiles on a table and you want to fit $n$ of them together to make a convex equiangular hexagon (i.e. one whose interior angles are $120^o$) . Obviously, $n$ cannot be any positive integer. The first three feasible $n$ are $6, 10$ and $13$. Show that $12$ is not feasible but $14$ is.
2011 Ukraine Team Selection Test, 12
Let $ n $ be a natural number. Consider all permutations $ ({{a} _ {1}}, \ \ldots, \ {{a} _ {2n}}) $ of the first $ 2n $ natural numbers such that the numbers $ | {{a} _ {i +1}} - {{a} _ {i}} |, \ i = 1, \ \ldots, \ 2n-1, $ are pairwise different. Prove that $ {{a} _ {1}} - {{a} _ {2n}} = n $ if and only if $ 1 \le {{a} _ {2k}} \le n $ for all $ k = 1, \ \ldots, \ n $.
2019 CMIMC, 6
Let $a, b$ and $c$ be the distinct solutions to the equation $x^3-2x^2+3x-4=0$. Find the value of
$$\frac{1}{a(b^2+c^2-a^2)}+\frac{1}{b(c^2+a^2-b^2)}+\frac{1}{c(a^2+b^2-c^2)}.$$
2002 Tuymaada Olympiad, 3
The points $D$ and $E$ on the circumcircle of an acute triangle $ABC$ are such that $AD=AE = BC$. Let $H$ be the common point of the altitudes of triangle $ABC$.
It is known that $AH^{2}=BH^{2}+CH^{2}$.
Prove that $H$ lies on the segment $DE$.
[i]Proposed by D. Shiryaev[/i]
1990 Tournament Of Towns, (279) 4
There are $20$ points in the plane and no three of them are collinear. Of these points $10$ are red while the other $10$ are blue. Prove that there exists a straight line such that there are $5$ red points and $5$ blue points on either side of this line.
(A Kushnirenko, Moscow)
2016 AMC 8, 10
Suppose that $a * b$ means $3a-b.$ What is the value of $x$ if
$$2 * (5 * x)=1?$$
$\textbf{(A) }\frac{1}{10} \qquad\textbf{(B) }2\qquad\textbf{(C) }\frac{10}{3} \qquad\textbf{(D) }10\qquad \textbf{(E) }14$
2009 Grand Duchy of Lithuania, 3
Solve the equation $x^2+ 2 = 4\sqrt{x^3+1}$
2022 Harvard-MIT Mathematics Tournament, 7
Let $(x_1, y_1)$, $(x_2, y_2)$, $(x_3, y_3)$, $(x_4, y_4)$, and $(x_5, y_5)$ be the vertices of a regular pentagon centered at $(0, 0)$. Compute the product of all positive integers k such that the equality $x_1^k+x_2^k+x_3^k+x_4^k+x_5^k=y_1^k+y_2^k+y_3^k+y_4^k+y_5^k$ must hold for all possible choices of the pentagon.
2016 CMIMC, 9
Compute the number of positive integers $n \leq 50$ such that there exist distinct positive integers $a,b$ satisfying
\[
\frac{a}{b} +\frac{b}{a} = n \left(\frac{1}{a} + \frac{1}{b}\right).
\]
2021 Taiwan TST Round 3, 5
Let $p$ be an odd prime, and put $N=\frac{1}{4} (p^3 -p) -1.$ The numbers $1,2, \dots, N$ are painted arbitrarily in two colors, red and blue. For any positive integer $n \leqslant N,$ denote $r(n)$ the fraction of integers $\{ 1,2, \dots, n \}$ that are red.
Prove that there exists a positive integer $a \in \{ 1,2, \dots, p-1\}$ such that $r(n) \neq a/p$ for all $n = 1,2, \dots , N.$
[I]Netherlands[/i]