BdMO National 2018 Higher Secondary P2 $AB$ is a diameter of a circle and $AD$ & $BC$ are two tangents of that circle.$AC$ & $BD$ intersect on a point of the circle.$AD=a$ & $BC=b$.If $a\neq b$ then $AB=?$

For arbitrary natural number $n$, prove that $\sum^n_{k=0}C^k_n2^kC^{[(n-k)/2]}_{n-k}=C^n_{2n+1}$, where $C^0_0=1$ and $[\dfrac{n-k}{2}]$ denotes the integer part of $\dfrac{n-k}{2}$.

An $n\times n$ square grid ($n\geqslant 3$) is rolled into a cylinder. Some of the cells are then colored black. Show that there exist two parallel lines (horizontal, vertical or diagonal) of cells containing the same number of black cells. [i]E. Poroshenko[/i]

Let $ABCD$ be a square and let $S$ be the point of intersection of its diagonals $AC$ and $BD$. Two circles $k,k'$ go through $A,C$ and $B,D$; respectively. Furthermore, $k$ and $k'$ intersect in exactly two different points $P$ and $Q$. Prove that $S$ lies on $PQ$.

Determine all the pairs $(a,b)$ of positive integers, such that all the following three conditions are satisfied: 1- $b>a$ and $b-a$ is a prime number 2- The last digit of the number $a+b$ is $3$ 3- The number $ab$ is a square of an integer.

There are $ n$ points ($ n \geq 4$) on a sphere with radius $ R$, and not all of them lie on the same semi-sphere. Prove that among all the angles formed by any two of the $ n$ points and the sphere centre $ O$ ($ O$ is the vertex of the angle), there is at least one that is not less than $ \displaystyle 2 \arcsin{\frac{\sqrt{6}}{3}}$.

Draw a $2004 \times 2004$ array of points. What is the largest integer $n$ for which it is possible to draw a convex $n$-gon whose vertices are chosen from the points in the array?

4. Let $v$ and $w$ be two randomly chosen roots of the equation $z^{1997} -1 = 0$ (all roots are equiprobable). Find the probability that $\sqrt{2+\sqrt{3}}\le |u+w|$

Let $x_1,x_2,...,x_n$ be real numbers with $1 \ge x_1 \ge x_2\ge ... \ge x_n \ge 0$ and $x_1^2 +x_2^2+...+x_n^2= 1$. If $[x_1 +x_2 +...+x_n] = m$, prove that $x_1 +x_2 +...+x_m \ge 1$.

Let $f_1$, $f_2$, $f_3$, ..., be a sequence of numbers such that \[ f_n = f_{n - 1} + f_{n - 2} \] for every integer $n \ge 3$. If $f_7 = 83$, what is the sum of the first 10 terms of the sequence?

On the board is written in decimal the integer positive number $N$. If it is not a single digit number, wipe its last digit $c$ and replace the number $m$ that remains on the board with a number $m -3c$. (For example, if $N = 1,204$ on the board, $120 - 3 \cdot 4 = 108$.) Find all the natural numbers $N$, by repeating the adjustment described eventually we get the number $0$.

Consider all polynomials $P(x)$ with real coefficients that have the following property: for any two real numbers $x$ and $y$ one has \[|y^2-P(x)|\le 2|x|\quad\text{if and only if}\quad |x^2-P(y)|\le 2|y|.\] Determine all possible values of $P(0)$. [i]Proposed by Belgium[/i]

Let $OX, OY$ and $OZ$ be three rays in the space, and $G$ a point "[i]between these rays[/i]" (i. e. in the interior of the part of the space bordered by the angles $Y OZ, ZOX$ and $XOY$). Consider a plane passing through $G$ and meeting the rays $OX, OY$ and $OZ$ in the points $A, B, C$, respectively. There are infinitely many such planes; construct the one which minimizes the volume of the tetrahedron $OABC$.

Find all pairs of positive integers $m$ and $n$ such that $$m! + n! = m^n.$$ .

Let $S$ be a string of $99$ characters, $66$ of which are $A$ and $33$ are $B$. We call $S$ [i]good[/i] if, for each $n$ such that $1\le n \le 99$, the sub-string made from the first $n$ characters of $S$ has an odd number of distinct permutations. How many good strings are there? Which strings are good?

Consider the configurations of integers $a_{1,1}$ $a_{2,1} \quad a_{2,2}$ $a_{3,1} \quad a_{3,2} \quad a_{3,3}$ $\dots \quad \dots \quad \dots$ $a_{2017,1} \quad a_{2017,2} \quad a_{2017,3} \quad \dots \quad a_{2017,2017}$ Where $a_{i,j} = a_{i+1,j} + a_{i+1,j+1}$ for all $i,j$ such that $1 \leq j \leq i \leq 2016$. Determine the maximum amount of odd integers that such configuration can contain.

Let $N$ be the set of natural numbers, and let $f: N \to N$ be a function satisfying $f(x) + f(x + 2) < 2 f(x + 1)$ for any $x \in N$. Prove that there exists a straight line in the $xy$-plane which contains infinitely many points with coordinates $(n,f(n))$.

A set $X=\{ x_1,x_2,\ldots,x_n \}$ consisting of $n$ positive integers is given. It is known that the greatest common divisor of any four different elements of $X$ is $1$. For every number $x_i$ let $m_i$ be the number of elements of $X$, which are divisible by $x_i$ For every $n \geq 4$, find the maximal possible value of the sum $m_1+\ldots+m_n$ [i]A. Vaidzelevich[/i]

Let $f(x) = 1 + \frac{2}{x}$. Put $f_1(x) = f(x)$, $f_2(x) = f(f_1(x))$, $f_3(x) = f(f_2(x))$, $... $. Find the solutions to $x = f_n(x)$ for $n > 0$.

Denote the dot product of two sequences $\langle x_1,\dots,x_n\rangle$ and $\langle y_1,\dots,y_n\rangle$ to be $$x_1y_1+x_2y_2+\dots+x_ny_n.$$ Let $\langle a_1,\dots,a_n\rangle$ and $\langle b_1,\dots,b_n\rangle$ be two sequences of consecutive integers (i.e. for $1\leq i,i+1\leq n$, $a_i+1=a_{i+1}$ and similarly for $b_i$). Minnie permutes the two sequences so that their dot product, $m$, is minimized. Maximilian permutes the two sequences so that their dot product, $M$, is maximized. Given that $m=4410$ and $M=4865$, compute $n$, the number of terms in each sequence.

Given two distinct points $A,B$ in the plane, for each point $C$ not on the line $AB$, we denote by $G$ and $I$ the centroid and incenter of the triangle $ABC$, respectively. (a) For $0<\alpha<\pi$, let $\Gamma$ be the set of points $C$ in the plane such that $\angle\left(\overrightarrow{CA},\overrightarrow{CB}\right)=\alpha+2k\pi$ as an oriented angle, where $k\in\mathbb Z$. If $C$ describes $\Gamma$, show that points $G$ and $I$ also descibre arcs of circles, and determine these circles. (b) Suppose that in addition $\frac\pi3<\alpha<\pi$. For which positions of $C$ in $\Gamma$ is $GI$ minimal? (c) Let $f(\alpha)$ denote the minimal $GI$ from the part (b). Give $f(\alpha)$ explicitly in terms of $a=AB$ and $\alpha$. Find the minimum value of $f(\alpha)$ for $\alpha\in\left(\frac\pi3,\pi\right)$.

In a triangle $ABC$, the perpendicular bisectors of sides $AB$ and $AC$ intersect $BC$ at $X$ and $Y$. Prove that $BC = XY$ if and only if $\tan B\tan C = 3$ or $\tan B\tan C = -1$.

Initially, on the blackboard are written all natural numbers from $1$ to $20$. A move consists of selecting $2$ numbers $a<b$ written on the blackboard such that their difference is at least $2$, erasing these numbers and writting $a+1$ and $b-1$ instead. What is the maximum numbers of moves one can perform?

Let $PROBLEMZ$ be a regular octagon inscribed in a circle of unit radius. Diagonals $MR$, $OZ$ meet at $I$. Compute $LI$.

Carlos took $70\%$ of a whole pie. Maria took one third of the remainder. What portion of the whole pie was left? $\textbf{(A)}\ 10\%\qquad\textbf{(B)}\ 15\%\qquad\textbf{(C)}\ 20\%\qquad\textbf{(D)}\ 30\%\qquad\textbf{(E)}\ 35\%$