Find all the positive integers $a{}$ and $b{}$ such that $(7^a-5^b)/8$ is a prime number. Cosmin Manea and Dragoș Petrică

Let $M$ be the midpoint of the side $AD$ of the square $ABCD.$ Consider the equilateral triangles $DFM{}$ and $BFE{}$ such that $F$ lies in the interior of $ABCD$ and the lines $EF$ and $BC$ are concurrent. Denote by $P{}$ the midpoint of $ME.$ Prove that" The point $P$ lies on the line $AC.$ The halfline $PM$ is the bisector of the angle $APF.$ Adrian Bud

Determine all positive integers $a,b,c,d,e,f$ satisfying the following condition: for any two of them, $x{}$ and $y{},$ two of the remaining numbers, $z{}$ and $t{},$ satisfy $x/y=z/t.$ Cristi Săvescu

Let $ABC$ be a triangle. An arbitrary circle which passes through the points $B,C$ intersects the sides $AC,AB$ for the second time in $D,E$ respectively. The line $BD$ intersects the circumcircle of the triangle $AEC$ at $P{}$ and $Q{}$ and the line $CE$ intersects the circumcircle of the triangle $ABD$ at $R{}$ and $S{}$ such that $P{}$ is situated on the segment $BD{}$ and $R{}$ lies on the segment $CE.$ Prove that: The points $P,Q,R$ and $S{}$ are concyclic. The triangle $APQ$ is isosceles. Petru Braica

An $n$-type triangle where $n\geqslant 2$ is formed by the cells of a $(2n+1)\times(2n+1)$ board, situated under both main diagonals. For instance, a $3$-type triangle looks like this:Determine the maximal length of a sequence with pairwise distinct cells in an $n$-type triangle, such that, beggining with the second one, any cell of the sequence has a common side with the previous one. Cristi Săvescu

The integers from 1 to 49 are written in a $7\times 7$ table, such that for any $k\in\{1,2,\ldots,7\}$ the product of the numbers in the $k$-th row equals the product of the numbers in the $(8-k)$-th row. Prove that there exists a row such that the sum of the numbers written on it is a prime number. Give an example of such a table. Cristi Săvescu

For any positive integer $n{}$ define $a_n=\{n/s(n)\}$ where $s(\cdot)$ denotes the sum of the digits and $\{\cdot\}$ denotes the fractional part. Prove that there exist infinitely many positive integers $n$ such that $a_n=1/2.$ Determine the smallest positive integer $n$ such that $a_n=1/6.$ Marius Burtea

In the exterior of the acute-angles triangle $ABC$ we construct the isosceles triangles $DAB$ and $EAC$ with bases $AB{}$ and $AC{}$ respectively such that $\angle DBC=\angle ECB=90^\circ.$ Let $M$ and $N$ be the reflections of $A$ with respect to $D$ and $E$ respectively. Prove that the line $MN$ passes through the orthocentre of the triangle $ABC.$ Florin Bojor

Let $n\geqslant 2$ be an integer. A Welsh darts board is a disc divided into $2n$ equal sectors, half of them being red and the other half being white. Two Welsh darts boards are matched if they have the same radius and they are superimposed so that each sector of the first board comes exactly over a sector of the second board. Suppose that two given Welsh darts boards can be matched so that more than half of the paurs of superimposed sectors have different colours. Prove that these Welsh darts boards can be matched so that at least $2\lfloor n/2\rfloor +2$ pairs of superimposed sectors have the same colour.

For positive real numbers $x,y,z$ with $xy+yz+zx=1$, prove that $$\frac{2}{xyz}+9xyz \geq 7(x+y+z)$$

Let $ABC$ be a triangle with $AB<AC$ and $\omega$ be its circumcircle. The tangent line to $\omega$ at $A$ intersects line $BC$ at $D$ and let $E$ be a point on $\omega$ such that $BE$ is parallel to $AD$. $DE$ intersects segment $AB$ and $\omega$ at $F$ and $G$, respectively. The circumcircle of $BGF$ intersects $BE$ at $N$. The line $NF$ intersects lines $AD$ and $EA$ at $S$ and $T$, respectively. Prove that $DGST$ is cyclic.

Let $\sigma(\cdot)$ denote the divisor sum function and $d(\cdot)$ denote the divisor counting function. Find all positve integers $n$ such that $\sigma(d(n))=n.$ Andrei Bâra

Let $n\geqslant 3$ be a positive integer and $N=\{1,2,\ldots,n\}$ and let $k>0$ be a real number. Let's associate each non-empty of $N{}$ with a point in the plane, such that any two distinct subsets correspond to different points. If the absolute value of the difference between the arithmetic means of the elements of two distinct non-empty subsets of $N{}$ is at most $k{}$ we connect the points associated with these subsets with a segment. Determine the minimum value of $k{}$ such that the points associated with any two distinct non-empty subsets of $N{}$ are connected by a segment or a broken line. Cristi Săvescu

Let $n\geqslant 3$ be an integer and $a_1,a_2,\ldots,a_n$ be pairwise distinct positive real numbers with the property that there exists a permutation $b_1,b_2,\ldots,b_n$ of these numbers such that\[\frac{a_1}{b_1}=\frac{a_2}{b_2}=\cdots=\frac{a_{n-1}}{b_{n-1}}\neq 1.\]Prove that there exist $a,b>0$ such that $\{a_1,a_2,\ldots,a_n\}=\{ab,ab^2,\ldots,ab^n\}.$ Cristi Săvescu

Let $ABC$ be a scalene triangle, with circumcircle $\omega$ and incentre $I.{}$ The tangent line at $C$ to $\omega$ intersects the line $AB$ at $D.{}$ The angle bisector of $BDC$ meets $BI$ at $P{}$ and $AI{}$ at $Q{}.$ Let $M{}$ be the midpoint of the segment $PQ.$ Prove that the line $IM$ passes through the middle of the arc $ACB$ of $\omega.$ Dana Heuberger

Version 1. Find all primes $p$ satisfying the following conditions: (i) $\frac{p+1}{2}$ is a prime number. (ii) There are at least three distinct positive integers $n$ for which $\frac{p^2+n}{p+n^2}$ is an integer. Version 2. Let $p \neq 5$ be a prime number such that $\frac{p+1}{2}$ is also a prime. Suppose there exist positive integers $a <b$ such that $\frac{p^2+a}{p+a^2}$ and $\frac{p^2+b}{p+b^2}$ are integers. Show that $b=(a-1)^2+1$.

Let $n\geqslant 2$ be an integer and $A{}$ a set of $n$ points in the plane. Find all integers $1\leqslant k\leqslant n-1$ with the following property: any two circles $C_1$ and $C_2$ in the plane such that $A\cap\text{Int}(C_1)\neq A\cap\text{Int}(C_2)$ and $|A\cap\text{Int}(C_1)|=|A\cap\text{Int}(C_2)|=k$ have at least one common point. Cristi Săvescu