You have an $n \times n$ grid of empty squares. You place a cross in all the squares, one at a time. When you place a cross in an empty square, you receive $i+j$ points if there were $i$ crosses in the same row and $j$ crosses in the same column before you placed the new cross. Which are the possible total scores you can get?

Let $f$ be a function over the domain of all positive real numbers such that $$f(x)=\frac{1-\sqrt{x}}{1+\sqrt{x}}$$ Now, let $g(x)$ be a function given by $$g(x)=f(x)^{\tfrac{2f\left(\tfrac{1}{x}\right)}{f(x)}}$$ $g(100)$ can be expressed as a fraction $\tfrac{a}{b}$ where $a$ and $b$ are relatively prime integers. What is the sum of $a$ and $b$?

We are given three non-negative numbers $A , B$ and $C$ about which it is known that $$A^4 + B^4 + C^4 \le 2(A^2B^2 + B^2C^2 + C^2A^2)$$ (a) Prove that each of $A, B$ and $C$ is not greater than the sum of the others. (b) Prove that $A^2 + B^2 + C^2 \le 2(AB + BC + CA)$ . (c) Does the original inequality follow from the one in (b)? (V.A. Senderov , Moscow)

Determine all functions $f:\mathbb{R} \to \mathbb{R}$ such that \[ xf(y) + f(xf(y)) - xf(f(y)) - f(xy) = 2x + f(y) - f(x+y)\] holds for all $x,y \in \mathbb{R}$.

Find all functions $f: R\to R$, such that $f(xy + x + y) + f(xy-x-y)=2f (x) + 2f (y)$ for all $x, y \in R$.

Problem 2. Prove that for every n≥3, there exists a convex polygon with n sides, such that one can divide it into n-2 triangles that are all similar, but pairwise non-congruent. [color=#00f]Moved to HSO. ~ oVlad[/color]

Prove that for the arbitrary numbers $x_1, x_2, ... , x_n$ from the $[0,1]$ segment $$(x_1 + x_2 + ...+ x_n + 1)^2 \ge 4(x_1^2 + x_2^2 + ... + x_n^2)$$

A semicircle is inscribed in another semicircle if the smaller semicircle’s diameter is a chord of the larger semicircle, and the smaller semicircle’s arc is tangent to the diameter of the larger semicircle. Semicircle $S_1$ is inscribed in a semicircle $S_2,$ which is inscribed in another semicircle $S_3.$ The radii of $S_1$ and $S_3$ are $1$ and $10,$ respectively, and the diameters of $S_1$ and $S_3$ are parallel. The endpoints of the diameter of of $S_3$ are $A$ and $B,$ and $S_2$'s arc is tangent to $AB$ at $C.$ Compute $AC \cdot CB.$ [center] [img] https://cdn.artofproblemsolving.com/attachments/9/6/ad8c82afe131103793cb2684b45c6d20b00ef0.png [/img] [/center]

On a circumference at some points sit $12$ grasshoppers. The points divide the circumference into $12$ arcs. By a signal each grasshopper jumps from its point to the midpoint of its arc (in clockwise direction). In such way new arcs are created. The process repeats for a number of times. Can it happen that at least one of the grasshoppers returns to its initial point after a) $12$ jumps? (4) a) $13$ jumps? (3)

Determine all pairs $ (a,b)$ of positive integers, each integer having two decimal digits, such that $ 100a\plus{}b$ and $ 201a\plus{}b$ are both perfect squares.

If the positive integers $x$ and $y$ are such that $3x + 4y$ and $4x + 3y$ are both perfect squares, prove that both $x$ and $y$ are both divisible with $7$.

Let $a,b,c$ be positive integers such that the numbers $k=b^c+a, l=a^b+c, m=c^a+b$ to be prime numbers. Prove that at least two of the numbers $k,l,m$ are equal.

Find all positive integer $n$ such that the equation $x^3+y^3+z^3=n \cdot x^2 \cdot y^2 \cdot z^2$ has positive integer solutions.

Let $ABC$ be an acute triangle with orthocenter $H$, circumcenter $O$, and circumcircle $\Omega$. Points $E$ and $F$ are the feet of the altitudes from $B$ to $AC$, and from $C$ to $AB$, respectively. Let line $AH$ intersect $\Omega$ again at $D$. The circumcircle of $DEF$ intersects $\Omega$ again at $X$, and $AX$ intersects $BC$ at $I$. The circumcircle of $IEF$ intersects $BC$ again at $G$. If $M$ is the midpoint of $BC$, prove that lines $MX$ and $OG$ intersect on $\Omega$.

In a game, a player can level up to 16 levels. In each level, the player can upgrade an ability spending that level on it. There are three kinds of abilities, however, one ability can not be upgraded before level 6 for the first time. And that special ability can not be upgraded before level 11. Other abilities can be upgraded at any level, any times (possibly 0), but the special ability needs to be upgraded exactly twice. In how many ways can these abilities be upgraded?

The decimal representation of an integer uses only two different digits. The number is at least $10$ digits long, and any two neighbouring digits are distinct. What is the greatest power of two that can divide this number?

Let $a$ and $b$ be real numbers. It is known that the graph of the parabola $y =ax^2 +b$ cuts the graph of the curve $y = x+1/x$ in exactly three points. Prove that $3ab < 1$.

Find the number of $4$-digit numbers (in base $10$) having non-zero digits and which are divisible by $4$ but not by $8$.

Let $a,b,c,d$ are lengths of the sides of a convex quadrangle with the area equal to $S$, set $S =\{x_1, x_2,x_3,x_4\}$ consists of permutations $x_i$ of $(a, b, c, d)$. Prove that \[S \leq \frac{1}{2}(x_1x_2+x_3x_4).\]

Let $ ABC$ be a triangle and $ P$ an arbitrary point in the plane. Let $ \alpha, \beta, \gamma$ be interior angles of the triangle and its area is denoted by $ F.$ Prove: \[ \ov{AP}^2 \cdot \sin 2\alpha + \ov{BP}^2 \cdot \sin 2\beta + \ov{CP}^2 \cdot \sin 2\gamma \geq 2F \] When does equality occur?

Let $k$ be a given circle and $A$ is a fixed point outside $k$. $BC$ is a diameter of $k$. Find the locus of the orthocentre of $\triangle ABC$ when $BC$ varies. [i]Proposed by T. Vitanov, E. Kolev[/i]

[i]Each problem in this section will depend on the previous one! The values $A, B, C$, and $D$ refer to the answers to problems $1, 2, 3$, and $4$, respectively.[/i] [b]TR1.[/b] The number $2020$ has three different prime factors. What is their sum? [b]TR2.[/b] Let $A$ be the answer to the previous problem. Suppose$ ABC$ is a triangle with $AB = 81$, $BC = A$, and $\angle ABC = 90^o$. Let $D$ be the midpoint of $BC$. The perimeter of $\vartriangle CAD$ can be written as $x + y\sqrt{z}$, where $x, y$, and $z$ are positive integers and $z$ is not divisible by the square of any prime. What is $x + y$? [b]TR3.[/b] Let $B$ the answer to the previous problem. What is the unique real value of $k$ such that the parabola $y = Bx^2 + k$ and the line $y = kx + B$ are tangent? [b]TR4.[/b] Let $C$ be the answer to the previous problem. How many ordered triples of positive integers $(a, b, c)$ are there such that $gcd(a, b) = gcd(b, c) = 1$ and $abc = C$? [b]TR5.[/b] Let $D$ be the answer to the previous problem. Let $ABCD$ be a square with side length $D$ and circumcircle $\omega$. Denote points $C'$ and $D'$ as the reflections over line $AB$ of $C$ and $D$ respectively. Let $P$ and $Q$ be the points on $\omega$, with$ A$ and $P$ on opposite sides of line $BC$ and $B$ and $Q$ on opposite sides of line $AD$, such that lines $C'P$ and $D'Q$ are both tangent to $\omega$. If the lines $AP$ and $BQ$ intersect at $T$, what is the area of $\vartriangle CDT$? PS. You had better use hide for answers.

Let $a_1,a_2,\ldots$ be an infinite sequence of real numbers, for which there exists a real number $c$ with $0\leq a_i\leq c$ for all $i$, such that \[\left\lvert a_i-a_j \right\rvert\geq \frac{1}{i+j} \quad \text{for all }i,\ j \text{ with } i \neq j. \] Prove that $c\geq1$.

Find all polinomials $ P(x)$ with real coefficients, such that $ P(\sqrt {3}(a \minus{} b)) \plus{} P(\sqrt {3}(b \minus{} c)) \plus{} P(\sqrt {3}(c \minus{} a)) \equal{} P(2a \minus{} b \minus{} c) \plus{} P( \minus{} a \plus{} 2b \minus{} c) \plus{} P( \minus{} a \minus{} b \plus{} 2c)$ for any $ a$,$ b$ and $ c$ real numbers

A triangle with perimeter $1$ has side lengths $a, b, c$. Show that $a^2 + b^2 + c^2 + 4abc <\frac 12$.