$O$ is a circle with a radius of $1$, with strings $CD$ and $EF$. $CD//EF$, and diameter $AB$ intersects $CD,EF$ at $P,Q$. If $\angle BPD=\frac{\pi}{4}$, prove that $$PC\cdot QE+PD \cdot QF<2.$$

In a triangle, each median forms an angle with the side it is drawn to, which is less than $\alpha$. Prove that one of the angles of the triangle is greater than $180^\circ-\frac{3}{2}\alpha$. [i]Proposed by Sergey Berlov[/i]

Given the distinct points $P(x_1, y_1)$, $Q(x_2, y_2)$ and $R(x_1+x_2, y_1+y_2)$. Line segments are drawn connecting these points to each other and to the origin $0$. Of the three possibilities: (1) parallelogram (2) straight line (3) trapezoid, figure $OPRQ$, depending upon the location of the points $P, Q,$ and $R$, can be: $ \textbf{(A)}\ \text{(1) only}\qquad\textbf{(B)}\ \text{(2) only}\qquad\textbf{(C)}\ \text{(3) only}\qquad\textbf{(D)}\ \text{(1) or (2) only}\qquad\textbf{(E)}\ \text{all three} $

Consider a cube $C$ and two planes $\sigma, \tau$, which divide Euclidean space into several regions. Prove that the interior of at least one of these regions meets at least three faces of the cube.

Two circles $O_1$ and $O_2$ intersect at $A,B$. Diameter $AC$ of $\odot O_1$ intersects $\odot O_2$ at $E$, Diameter $AD$ of $\odot O_2$ intersects $\odot O_1$ at $F$. $CF$ intersects $O_2$ at $H$, $DE$ intersects $O_1$ at $G,H$. $GH\cap O_1=P$. Prove that $PH=PK$.

Let $ABCD$ be a cyclic quadrilateral where $AB = 4$, $BC = 11$, $CD = 8$, and $DA = 5$. If $BC$ and $DA$ intersect at $X$, find the area of $\vartriangle XAB$.

An octahedron (a solid with 8 triangular faces) has a volume of $1040$. Two of the spatial diagonals intersect, and their plane of intersection contains four edges that form a cyclic quadrilateral. The third spatial diagonal is perpendicularly bisected by this plane and intersects the plane at the circumcenter of the cyclic quadrilateral. Given that the side lengths of the cyclic quadrilateral are $7, 15, 24, 20$, in counterclockwise order, the sum of the edge lengths of the entire octahedron can be written in simplest form as $a/b$. Find $a + b$.

Isosceles triangles $T$ and $T'$ are not congruent but have the same area and the same perimeter. The sides of $T$ have lengths $5$, $5$, and $8$, while those of $T'$ have lengths $a$, $a$, and $b$. Which of the following numbers is closest to $b$? $\textbf{(A) }3\qquad\textbf{(B) }4\qquad\textbf{(C) }5\qquad\textbf{(D) }6\qquad\textbf{(E) }8$

Let $ABC$ be an arbitrary triangle,$P$ is the intersection point of the altitude from $C$ and the tangent line from $A$ to the circumcircle. The bisector of angle $A$ intersects $BC$ at $D$ . $PD$ intersects $AB$ at $K$, if $H$ is the orthocenter then prove : $HK\perp AD$

Let \( ABCDE \) be a convex pentagon whose vertices lie on a circle \( \Gamma \). The tangents to \( \Gamma \) at \( C \) and \( E \) intersect at \( X \), and the segments \( CE \) and \( AD \) intersect at \( Y \). Given that \( CE \) is perpendicular to \( BD \), that \( XY \) is parallel to \( BD \), that \( AY = BD \), and that \( \angle BAD = 30^\circ \), what is the measure of the angle \( \angle BDA \)? Proposed by João Gilberti Alves Tavares

The hall in a castle is a regular hexagon where its sides' length is 6 meters. The floor of the hall is to be tiled with equilateral triangular tiles where its sides' length is 50 centimeters. Each tile is divided into three congruent triangles by their altitudes up to its orthocenter (see below). Each of these small triangles are colored such that each tile has different colors and no two tiles have identical colorings. How many colors at least are required? A tile's pattern is: [asy] draw((0,0.000)--(2,0.000)); draw((2,0.000)--(1,1.732)); draw((1,1.732)--(0,0.000)); draw((1,0.577)--(0,0.000)); draw((1,0.577)--(2,0.000)); draw((1,0.577)--(1,1.732)); [/asy]

Let $O$ be the circumcenter of the acute triangle $ABC$ ($AB < AC$). Let $A_1$ and $P$ be the feet of the perpendicular lines drawn from $A$ and $O$ to $BC$, respectively. The lines $BO$ and $CO$ intersect $AA_1$ in $D$ and $E$, respectively. Let $F$ be the second intersection point of $\odot ABD$ and $\odot ACE$. Prove that the angle bisector od $\angle FAP$ passes through the incenter of $\triangle ABC$.

In a triangle $ ABC$, take point $ D$ on $ BC$ such that $ DB \equal{} 14, DA \equal{} 13, DC \equal{} 4$, and the circumcircle of $ ADB$ is congruent to the circumcircle of $ ADC$. What is the area of triangle $ ABC$?

Minor base $BC$ of trapezoid $ABCD$ is equal to side $AB$, and diagonal $AC$ is equal to base $AD$. The line passing through B and parallel to $AC$ intersects line $DC$ in point $M$. Prove that $AM$ is the bisector of angle $\angle BAC$. A.Blinkov, Y.Blinkov

Let $O$ be a point of three-dimensional space and let $l_1, l_2, l_3$ be mutually perpendicular straight lines passing through $O$. Let $S$ denote the sphere with center $O$ and radius $R$, and for every point $M$ of $S$, let $S_M$ denote the sphere with center $M$ and radius $R$. We denote by $P_1, P_2, P_3$ the intersection of $S_M$ with the straight lines $l_1, l_2, l_3$, respectively, where we put $P_i \neq O$ if $l_i$ meets $S_M$ at two distinct points and $P_i = O$ otherwise ($i = 1, 2, 3$). What is the set of centers of gravity of the (possibly degenerate) triangles $P_1P_2P_3$ as $M$ runs through the points of $S$?

Let $ABCD$ be a parallelogram. Suppose that there exists a point $P$ in the interior of the parallelogram which is on the perpendicular bisector of $AB$ and such that $\angle PBA = \angle ADP$ Show that $\angle CPD = 2 \angle BAP$

Three circles $\omega_1,\omega_2$ and $\omega_3$ pass through one point $D{}$. Let $A{}$ be the intersection of $\omega_1$ and $\omega_3$, and $E{}$ be the intersections of $\omega_3$ and $\omega_2$ and $F{}$ be the intersection of $\omega_2$ and $\omega_1$. It is known that $\omega_3$ passes through the center $B{}$ of the circle $\omega_2$. The line $EF$ intersects $\omega_1$ a second time at the point $G{}$. Prove that $\angle GAB=90^\circ$. [i]Proposed by K. Knop[/i]

Let the curve $ C: y\equal{}x\sqrt{9\minus{}x^2}\ (x\geq 0)$. (1) Find the maximum value of $ y$. (2) Find the area of the figure bounded by the curve $ C$ and the $ x$ axis. (3) Find the volume of the solid generated by rotation of the figure about the $ y$ axis.

[b]p1.[/b] What is the last digit of $$1! + 2! + ... + 10!$$ where $n!$ is defined to equal $1 \cdot 2 \cdot ... \cdot n$? [b]p2.[/b] What pair of positive real numbers, $(x, y)$, satisfies $$x^2y^2 = 144$$ $$(x - y)^3 = 64?$$ [b]p3.[/b] Paul rolls a standard $6$-sided die, and records the results. What is the probability that he rolls a $1$ ten times before he rolls a $6$ twice? [b]p4.[/b] A train is approaching a $1$ kilometer long tunnel at a constant $40$ km/hr. It so happens that if Roger, who is inside, runs towards either end of the tunnel at a contant $10$ km/hr, he will reach that end at the exact same time as the train. How far from the center of the tunnel is Roger? [b]p5.[/b] Let $ABC$ be a triangle with $A$ being a right angle. Let $w$ be a circle tangent to $\overline{AB}$ at $A$ and tangent to $\overline{BC}$ at some point $D$. Suppose $w$ intersects $\overline{AC}$ again at $E$ and that $\overline{CE} = 3$, $\overline{CD} = 6$. Compute $\overline{BD}$. [b]p6.[/b] In how many ways can $1000$ be written as a sum of consecutive integers? [b]p7.[/b] Let $ABC$ be an isosceles triangle with $\overline{AB} = \overline{AC} = 10$ and $\overline{BC} = 6$. Let $M$ be the midpoint of $\overline{AB}$, and let $\ell$ be the line through $A$ parallel to $\overline{BC}$. If $\ell$ intersects the circle through $A$, $C$ and $M$ at $D$, then what is the length of $\overline{AD}$? [b]p8.[/b] How many ordered triples of pairwise relatively prime, positive integers, $\{a, b, c\}$, have the property that $a + b$ is a multiple of $c$, $b + c$ is a multiple of $a$, and $a + c$ is a multiple of $b$? [b]p9.[/b] Consider a hexagon inscribed in a circle of radius $r$. If the hexagon has two sides of length $2$, two sides of length $7$, and two sides of length $11$, what is $r$? [b]p10.[/b] Evaluate $$\sum^{\infty}_{i=0} \sum^{\infty}_{j=0} \frac{\left( (-1)^i + (-1)^j\right) \cos (i) \sin (j)}{i!j!} ,$$ where angles are measured in degrees, and $0!$ is defined to equal $1$. PS. You had better use hide for answers. Collected [url=https://artofproblemsolving.com/community/c5h2760506p24143309]here[/url].

Let $ABCD$ be a cyclic quadrilateral such that $AB \cdot BC=2 \cdot AD \cdot DC$. Prove that its diagonals $AC$ and $BD$ satisfy the inequality $8BD^2 \le 9AC^2$. [color=#FF0000] Moderator says: Do not double post [url]http://www.artofproblemsolving.com/Forum/viewtopic.php?f=46&t=590175[/url][/color]

Two parallel lines $r,s$ and two points $P \in r$ and $Q \in s$ are given in a plane. Consider all pairs of circles $(C_P, C_Q)$ in that plane such that $C_P$ touches $r$ at $P$ and $C_Q$ touches $s$ at $Q$ and which touch each other externally at some point $T$. Find the locus of $T$.

Inside the angle $ABC$ of $60^o$, point $O$ is selected, which is located at distances from the sides of the angle $a$ and $b$, respectively. Determine the distance from the top of the angle to this point.

Two squares $ACXE$ and $CBDY$ are constructed in the exterior of an acute-angled triangle $ABC$. Prove that the intersection of the lines $AD$ and $BE$ lies on the altitude of the triangle from $C$.

Let $ a$, $ b$, $ c$ be the sides of a triangle, and $ S$ its area. Prove: \[ a^{2} \plus{} b^{2} \plus{} c^{2}\geq 4S \sqrt {3} \] In what case does equality hold?

Let $O$ be the circumcenter of the isosceles triangle $ABC$ ($AB = AC$). Let $P$ be a point of the segment $AO$ and $Q$ the symmetric of $P$ with respect to the midpoint of $AB$. If $OQ$ cuts $AB$ at $K$ and the circle that passes through $A, K$ and $O$ cuts $AC$ in $L$, show that $\angle ALP = \angle CLO$.