Let $D$ be the foot of the internal angle bisector of the angle $A$ of a triangle $ABC$. Let $E$ be a point on side $[AC]$ such that $|CE|= |CD|$ and $|AE|=6\sqrt 5$; let $F$ be a point on the ray $[AB$ such that $|DB|=|BF|$ and $|AB|<|AF| = 8\sqrt 5$. What is $|AD|$? $ \textbf{(A)}\ 10\sqrt 5 \qquad\textbf{(B)}\ 8 \qquad\textbf{(C)}\ 4\sqrt{15} \qquad\textbf{(D)}\ 7\sqrt 5 \qquad\textbf{(E)}\ \text{None of above} $

For positive integers $a, b, c$ (not necessarily distinct), suppose that $a+bc, b+ac, c+ab$ are all perfect squares. Show that $$a^2(b+c)+b^2(a+c)+c^2(a+b)+2abc$$ can be written as sum of two squares.

[hide=E stands for Euclid , L stands for Lobachevsky]they had two problem sets under those two names[/hide] [b]E1.[/b] How many positive divisors does $72$ have? [b]E2 / L2.[/b] Raymond wants to travel in a car with $3$ other (distinguishable) people. The car has $5$ seats: a driver’s seat, a passenger seat, and a row of $3$ seats behind them. If Raymond’s cello must be in a seat next to him, and he can’t drive, but every other person can, how many ways can everyone sit in the car? [b]E3 / L3.[/b] Peter wants to make fruit punch. He has orange juice ($100\%$ orange juice), tropical mix ($25\%$ orange juice, $75\%$ pineapple juice), and cherry juice ($100\%$ cherry juice). If he wants his final mix to have $50\%$ orange juice, $10\%$ cherry juice, and $40\%$ pineapple juice, in what ratios should he mix the $3$ juices? Please write your answer in the form (orange):(tropical):(cherry), where the three integers are relatively prime. [b]E4 / L4.[/b] Points $A, B, C$, and $D$ are chosen on a circle such that $m \angle ACD = 85^o$, $m\angle ADC = 40^o$,and $m\angle BCD = 60^o$. What is $m\angle CBD$? [b]E5.[/b] $a, b$, and $c$ are positive real numbers. If $abc = 6$ and $a + b = 2$, what is the minimum possible value of $a + b + c$? [b]E6 / L5.[/b] Circles $A$ and $B$ are drawn on a plane such that they intersect at two points. The centers of the two circles and the two intersection points lie on another circle, circle $C$. If the distance between the centers of circles $A$ and $B$ is $20$ and the radius of circle $A$ is $16$, what is the radius of circle $B$? [b]E7.[/b] Point $P$ is inside rectangle $ABCD$. If $AP = 5$, $BP = 6$, and $CP = 7$, what is the length of $DP$? [b]E8 / L6.[/b] For how many integers $n$ is $n^2 + 4$ divisible by $n + 2$? [b]E9. [/b] How many of the perfect squares between $1$ and $10000$, inclusive, can be written as the sum of two triangular numbers? We define the $n$th triangular number to be $1 + 2 + 3 + ... + n$, where $n$ is a positive integer. [b]E10 / L7.[/b] A small sphere of radius $1$ is sitting on the ground externally tangent to a larger sphere, also sitting on the ground. If the line connecting the spheres’ centers makes a $60^o$ angle with the ground, what is the radius of the larger sphere? [b]E11 / L8.[/b] A classroom has $12$ chairs in a row and $5$ distinguishable students. The teacher wants to position the students in the seats in such a way that there is at least one empty chair between any two students. In how many ways can the teacher do this? [b]E12 / L9.[/b] Let there be real numbers $a$ and $b$ such that $a/b^2 + b/a^2 = 72$ and $ab = 3$. Find the value of $a^2 + b^2$. [b]E13 / L10.[/b] Find the number of ordered pairs of positive integers $(x, y)$ such that $gcd \, (x, y)+lcm \, (x, y) =x + y + 8$. [b]E14 / L11.[/b] Evaluate $\sum_{i=1}^{\infty}\frac{i}{4^i}=\frac{1}{4} +\frac{2}{16} +\frac{3}{64} +...$ [b]E15 / L12.[/b] Xavier and Olivia are playing tic-tac-toe. Xavier goes first. How many ways can the game play out such that Olivia wins on her third move? The order of the moves matters. [b]L1.[/b] What is the sum of the positive divisors of $100$? [b]L13.[/b] Let $ABCD$ be a convex quadrilateral with $AC = 20$. Furthermore, let $M, N, P$, and $Q$ be the midpoints of $DA, AB, BC$, and $CD$, respectively. Let $X$ be the intersection of the diagonals of quadrilateral $MNPQ$. Given that $NX = 12$ and $XP = 10$, compute the area of $ABCD$. [b]L14.[/b] Evaluate $(\sqrt3 + \sqrt5)^6$ to the nearest integer. [b]L15.[/b] In Hatland, each citizen wears either a green hat or a blue hat. Furthermore, each citizen belongs to exactly one neighborhood. On average, a green-hatted citizen has $65\%$ of his neighbors wearing green hats, and a blue-hatted citizen has $80\%$ of his neighbors wearing blue hats. Each neighborhood has a different number of total citizens. What is the ratio of green-hatted to blue-hatted citizens in Hatland? (A citizen is his own neighbor.) PS. You should use hide for answers. Collected [url=https://artofproblemsolving.com/community/c5h2760506p24143309]here[/url].

Prove that all positive real $x, y, z$ satisfy the inequality $x^y + y^z + z^x > 1$. (D. Bazylev)

Given a quadrilateral \(ABCD\), point \(M\) is the midpoint of the side \(CD\). It turns out that \(\angle BMA = 90^{\circ}\) and \(\angle MAB = \angle CBD\). Prove that \(AC = AB\). [i]Proposed by Anton Trygub[/i]

Show that \[ \min \{ |PA|, |PB|, |PC| \} + |PA| + |PB| + |PC| < |AB|+|BC|+|CA| \] if $P$ is a point inside $\triangle ABC$.

[b]a)[/b] Solve in $ \mathbb{R} $ the equation $ 2^x=x+1. $ [b]b)[/b] If a function $ f:\mathbb{R}\longrightarrow\mathbb{R} $ has the property that $$ (f\circ f)(x)=2^x-1,\quad\forall x\in\mathbb{R} , $$ then $ f(0)+f(1)=1. $

For breakfast, Milan is eating a piece of toast shaped like an equilateral triangle. On the piece of toast rests a single sesame seed that is one inch away from one side, two inches away from another side, and four inches away from the third side. He places a circular piece of cheese on top of the toast that is tangent to each side of the triangle. What is the area of this piece of cheese?

Given a triangle $OAB$ with the vetices $O(0,\ 0,\ 0),\ A(1,\ 0,\ 0),\ B(1,\ 1,\ 0)$ in the $xyz$ space. Let $V$ be the cone obtained by rotating the triangle around the $x$-axis. Find the volume of the solid obtained by rotating the cone $V$ around the $y$-axis.

Are there integers $a$ and $b$ such that $a^5b+3$ and $ab^5+3$ are both perfect cubes of integers?

Given a triangle $ABC$, let $D$ be a point on segment $BC$. Construct the circumcircle $\omega$ of triangle $ABD$ and point $E$ on $\omega$ such that $CE$ is tangent to $\omega$ and $A, E$ are on opposite sides of $BC$ (as shown in diagram). If $\angle CAD = \angle ECD$ and $AC = 12$, $AB = 7$, find $AE$.

Given is a convex $ABCD$, which is $ |\angle ABC| = |\angle ADC|= 135^\circ $. On the $AB, AD$ are also selected points $M, N$ such that $ |\angle MCD| = |\angle NCB| = 90^ \circ $. The circumcircles of the triangles $AMN$ and $ABD$ intersect for the second time at point $K \ne A$. Prove that lines $AK $ and $KC$ are perpendicular. (Irán)

We are given an $n \times n$ board. Rows are labeled with numbers $1$ to $n$ downwards and columns are labeled with numbers $1$ to $n$ from left to right. On each field we write the number $x^2 + y^2$ where $(x, y)$ are its coordinates. We are given a figure and can initially place it on any field. In every step we can move the figure from one field to another if the other field has not already been visited and if at least one of the following conditions is satisfied:[list] [*] the numbers in those $2$ fields give the same remainders when divided by $n$, [*] those fields are point reflected with respect to the center of the board.[/list]Can all the fields be visited in case: [list=a][*] $n = 4$, [*] $n = 5$?[/list] [i]Josip Pupić[/i]

Let $ABC$ be a triangle with $AB=4$, $BC=5$, and $CA=6$. Suppose $X$ and $Y$ are points such that [list] [*] $BC$ and $XY$ are parallel [*] $BX$ and $CY$ intersect at a point $P$ on the circumcircle of $\triangle{ABC}$ [*] the circumcircles of $\triangle{BCX}$ and $\triangle{BCY}$ are tangent to $AB$ and $AC$, respectively. [/list] Then $AP^2$ can be written in the form $\frac{p}{q}$ for relatively prime positive integers $p$ and $q$. Compute $100p+q$. [i]Proposed by Tristan Shin[/i]

Solve the following in equation in $\mathbb{R}^3$: \[4x^4-x^2(4y^4+4z^4-1)-2xyz+y^8+2y^4z^4+y^2z^2+z^8=0.\]

Find all subsets of $\mathbb N$ like $S$ such that \[\forall m,n \in S \implies \dfrac{m+n}{\gcd(m,n)} \in S \]

In a lottery there are $14$ balls, numbered from $1$ to $14$. Four of these balls are drawn at random. D'Angelo wins the lottery if he can split the four balls into two disjoint pairs, where the two balls in each pair have difference at least $5$. The probability that D'Angelo wins the lottery can be expressed as $\frac{m}{n}$, with $m,n$ relatively prime. Find $m+n$. [i]Proposed by Richard Chen[/i]

Prove that none of the numbers $11, 111, 1111, ...$ is a square number, cube number or higher power of a natural number.

Ten positive integers are arranged around a circle. Each number is one more than the greatest common divisor of its two neighbors. What is the sum of the ten numbers?

Every day, some pairs of families living in a city may choose to exchange their apartments. A family may only participate in one exchange in a day. Prove that any complex exchange of apartments between several families can be carried out in two days. [i]Proposed by N. Konstantinov and A. Shnirelman[/i]

Prove that there are no integers $x, y, z$ satisfying the equation $$x^2+y^2-z^2=xyz-2.$$ [i]Proposed by Navid Safaei[/i]

Let $n\geq 4$ be an integer. Find the maximum possible area of an $n-gon$ inscribed in a unit cicle and having two perpendicular diagonals.

Find all sequences $a_{0}, a_{1},\ldots, a_{n}$ of real numbers such that $a_{n}\neq 0$, for which the following statement is true: If $f: \mathbb{R}\to\mathbb{R}$ is an $n$ times differentiable function and $x_{0}<x_{1}<\ldots <x_{n}$ are real numbers such that $f(x_{0})=f(x_{1})=\ldots =f(x_{n})=0$ then there is $h\in (x_{0}, x_{n})$ for which \[a_{0}f(h)+a_{1}f'(h)+\ldots+a_{n}f^{(n)}(h)=0.\]

Let $I$ and $J$ be intervals. Let $\varphi,\psi:I\to\mathbb{R}$ be strictly increasing continuous functions and let $\Phi,\Psi:J\to\mathbb{R}$ be continuous functions. Suppose that $\varphi(x)+\psi(x)=x$ and $\Phi(u)+\Psi(u)=u$ holds for all $x\in I$ and $u\in J$. Show that if $f:I\to J$ is a continuous solution of the functional inequality $$f\big(\varphi(x)+\psi(y)\big)\le \Phi\big(f(x)\big)+\Psi\big(f(y)\big)\qquad (x,y\in I),$$then $\Phi\circ f\circ \varphi^{-1}$ and $\Psi\circ f\circ \psi^{-1}$ are convex functions.

On the line which contains diameter $PQ$ of circle $k(S,r)$, point $A$ is chosen outside the circle such that tangent $t$ from point $A$ touches the circle in point $T$. Tangents on circle $k$ in points $P$ and $Q$ are $p$ and $q$, respectively. If $PT \cap q={N}$ and $QT \cap p={M}$, prove that points $A$, $M$ and $N$ are collinear.