Found problems: 85335
2004 Iran MO (2nd round), 5
The interior bisector of $\angle A$ from $\triangle ABC$ intersects the side $BC$ and the circumcircle of $\Delta ABC$ at $D,M$, respectively. Let $\omega$ be a circle with center $M$ and radius $MB$. A line passing through $D$, intersects $\omega$ at $X,Y$. Prove that $AD$ bisects $\angle XAY$.
1986 Czech And Slovak Olympiad IIIA, 3
Prove that the entire space can be partitioned into “crosses” made of seven unit cubes as shown in the picture.
[img]https://cdn.artofproblemsolving.com/attachments/2/b/77c4a4309170e8303af321daceccc4010da334.png[/img]
2014 May Olympiad, 2
In a convex quadrilateral $ABCD$, let $M$, $N$, $P$, and $Q$ be the midpoints of $AB$, $BC$, $CD$, and $DA$ respectively. If $MP$ and $NQ$ divide $ABCD$ in four quadrilaterals with the same area, prove that $ABCD$ is a parallelogram.
2007 France Team Selection Test, 1
Do there exist $5$ points in the space, such that for all $n\in\{1,2,\ldots,10\}$ there exist two of them at distance between them $n$?
2014 NZMOC Camp Selection Problems, 9
Let $AB$ be a line segment with midpoint $I$. A circle, centred at $I$ has diameter less than the length of the segment. A triangle $ABC$ is tangent to the circle on sides $AC$ and $BC$. On $AC$ a point $X$ is given, and on $BC$ a point $Y$ is given such that $XY$ is also tangent to the circle (in particular $X$ lies between the point of tangency of the circle with $AC$ and $C$, and similarly $Y$ lies between the point of tangency of the circle with $BC$ and $C$. Prove that $AX \cdot BY = AI \cdot BI$.
Estonia Open Senior - geometry, 2009.1.3
Three circles in a plane have the sides of a triangle as their diameters. Prove that there is a point that is in the interior of all three circles.
2008 Bulgaria Team Selection Test, 2
In the triangle $ABC$, $AM$ is median, $M \in BC$, $BB_{1}$ and $CC_{1}$ are altitudes, $C_{1} \in AB$, $B_{1} \in AC$. The line through $A$ which is perpendicular to $AM$ cuts the lines $BB_{1}$ and $CC_{1}$ at points $E$ and $F$, respectively. Let $k$ be the circumcircle of $\triangle EFM$. Suppose also that $k_{1}$ and $k_{2}$ are circles touching both $EF$ and the arc $EF$ of $k$ which does not contain $M$. If $P$ and $Q$ are the points at which $k_{1}$ intersects $k_{2}$, prove that $P$, $Q$, and $M$ are collinear.
2007 Today's Calculation Of Integral, 253
Evaluate $ \int_0^1 (1 \plus{} x \plus{} x^2 \plus{} \cdots \plus{} x^{n \minus{} 1})\{1 \plus{} 3x \plus{} 5x^2 \plus{} \cdots \plus{} (2n \minus{} 3)x^{n \minus{} 2} \plus{} (2n \minus{} 1)x^{n \minus{} 1}\}\ dx.$
2000 Iran MO (3rd Round), 1
A sequence of natural numbers $c_1, c_2,\dots$ is called [i]perfect[/i] if every natural
number $m$ with $1\le m \le c_1 +\dots+ c_n$ can be represented as
$m =\frac{c_1}{a_1}+\frac{c_2}{a_2}+\dots+\frac{c_n}{a_n}$
Given $n$, find the maximum possible value of $c_n$ in a perfect sequence $(c_i)$.
2008 Germany Team Selection Test, 2
[b](i)[/b] Determine the smallest number of edges which a graph of $ n$ nodes may have given that adding an arbitrary new edge would give rise to a 3-clique (3 nodes joined pairwise by edges).
[b](ii)[/b] Determine the smallest number of edges which a graph of $ n$ nodes may have given that adding an arbitrary new edge would give rise to a 4-clique (4 nodes joined pairwise by edges).
2023 Junior Balkan Mathematical Olympiad, 4
Let $ABC$ be an acute triangle with circumcenter $O$. Let $D$ be the foot of the altitude from $A$ to $BC$ and let $M$ be the midpoint of $OD$. The points $O_b$ and $O_c$ are the circumcenters of triangles $AOC$ and $AOB$, respectively. If $AO=AD$, prove that points $A$, $O_b$, $M$ and $O_c$ are concyclic.
[i]Marin Hristov and Bozhidar Dimitrov, Bulgaria[/i]
2013 Harvard-MIT Mathematics Tournament, 27
Let $W$ be the hypercube $\{(x_1,x_2,x_3,x_4)\,|\,0\leq x_1,x_2,x_3,x_4\leq 1\}$. The intersection of $W$ and a hyperplane parallel to $x_1+x_2+x_3+x_4=0$ is a non-degenerate $3$-dimensional polyhedron. What is the maximum number of faces of this polyhedron?
I Soros Olympiad 1994-95 (Rus + Ukr), 9.6
A circle can be drawn around the quadrilateral $ABCD$. $K$ is a point on the diagonal $BD$ . The straight line $CK$ intersects the side $AD$ at the point $M$. Prove that the circles circumscribed around the triangles $BCK$ and $ACM$ are tangent.
LMT Speed Rounds, 2011.17
Let $ABC$ be a triangle with $AB = 15$, $AC = 20$, and right angle at $A$. Let $D$ be the point on $\overline{BC}$ such that $\overline{AD}$ is perpendicular to $\overline{BC}$, and let $E$ be the midpoint of $\overline{AC}$. If $F$ is the point on $\overline{BC}$ such that $\overline{AD} \parallel \overline{EF}$, what is the area of quadrilateral $ADFE$?
2016 Azerbaijan JBMO TST, 3
Find all the pime numbers $(p,q)$ such that :
$p^{3}+p=q^{2}+q$
2016-2017 SDML (Middle School), 1
What is the integer value of $\left(\sqrt{3}^{\sqrt{2}}\right)^{\sqrt{8}}$?
2007 Silk Road, 4
The set of polynomials $f_1, f_2, \ldots, f_n$ with real coefficients is called [i]special [/i], if for any different $i,j,k \in \{ 1,2, \ldots, n\}$ polynomial $\dfrac{2}{3}f_i + f_j + f_k$ has no real roots, but for any different $p,q,r,s \in \{ 1,2, \ldots, n\}$ of a polynomial $f_p + f_q + f_r + f_s$ there is a real root.
a) Give an example of a [i]special [/i] set of four polynomials whose sum is not a zero polynomial.
b) Is there a [i]special [/i] set of five polynomials?
2021 Portugal MO, 3
All sequences of $k$ elements $(a_1,a_2,...,a_k)$ are considered, where each $a_i$ belongs to the set $\{1,2,... ,2021\}$. What is the sum of the smallest elements of all these sequences?
2023 South East Mathematical Olympiad, 7
The positive integer number $S$ is called a "[i]line number[/i]". if there is a positive integer $n$ and $2n$ positive integers $a_1$, $a_2$,...,$a_n$, $b_1$,$b_2$,...,$b_n$, such that $S = \sum^n_{i=1} a_ib_i$, $\sum^n_{i=1} (a_i^2-b_1^2)=1$, and $\sum^n_{i=1} (a_i+b_i)=2023$, find:
(1) The minimum value of [i]line numbers[/i].
(2)The maximum value of [i]line numbers[/i].
1997 Brazil Team Selection Test, Problem 1
Let $ABC$ be a triangle and $L$ its circumscribed circle. The internal bisector of angle $A$ meets $BC$ at point $P$. Let $L_1$ be the circle tangent to $AP,BP$ and $L$. Similarly, let $L_2$ be the circle tangent to $AP,CP$ and $L$. Prove that the tangency points of $L_1$ and $L_2$ with $AP$ coincide.
2018 Federal Competition For Advanced Students, P2, 5
On a circle $2018$ points are marked. Each of these points is labeled with an integer.
Let each number be larger than the sum of the preceding two numbers in clockwise order.
Determine the maximal number of positive integers that can occur in such a configuration of $2018$ integers.
[i](Proposed by Walther Janous)[/i]
2019 CCA Math Bonanza, T7
How many ordered triples $\left(a,b,c\right)$ of positive integers are there such that at least two of $a,b,c$ are prime and $abc=11\left(a+b+c\right)$?
[i]2019 CCA Math Bonanza Team Round #7[/i]
Brazil L2 Finals (OBM) - geometry, 2000.3
A rectangular piece of paper has top edge $AD$. A line $L$ from $A$ to the bottom edge makes an angle $x$ with the line $AD$. We want to trisect $x$. We take $B$ and $C$ on the vertical ege through $A$ such that $AB = BC$. We then fold the paper so that $C$ goes to a point $C'$ on the line $L$ and $A$ goes to a point $A'$ on the horizontal line through $B$. The fold takes $B$ to $B'$. Show that $AA'$ and $AB'$ are the required trisectors.
2017 Dutch IMO TST, 4
Let $ABC$ be a triangle, let $M$ be the midpoint of $AB$, and let $N$ be the midpoint of $CM$. Let $X$ be a point satisfying both $\angle XMC = \angle MBC$ and $\angle XCM = \angle MCB$ such that $X$ and $B$ lie on opposite sides of $CM$. Let $\omega$ be the circumcircle of triangle $AMX$.
$(a)$ Show that $CM$ is tangent to $\omega$.
$(b)$ Show that the lines $NX$ and $AC$ intersect on $\omega$
1993 AMC 12/AHSME, 15
For how many values of $n$ will an $n$-sided regular polygon have interior angles with integral degree measures?
$ \textbf{(A)}\ 16 \qquad\textbf{(B)}\ 18 \qquad\textbf{(C)}\ 20 \qquad\textbf{(D)}\ 22 \qquad\textbf{(E)}\ 24 $