Found problems: 85335
2011 Denmark MO - Mohr Contest, 1
Georg writes the numbers from $1$ to $15$ on different pieces of paper.
He attempts to sort these pieces of paper into two stacks so that none of the stacks contains two numbers whose sum is a square number.Prove that this is impossible.
(The square numbers are the numbers $0 = 0^2$, $1 = 1^2$, $4 = 2^2$, $9 = 3^2$ etc.)
1985 AMC 12/AHSME, 13
Pegs are put in a board $ 1$ unit apart both horizontally and vertically. A reubber band is stretched over $ 4$ pegs as shown in the figure, forming a quadrilateral. Its area in square units is
[asy]
int i,j;
for(i=0; i<5; i=i+1) {
for(j=0; j<4; j=j+1) {
dot((i,j));
}}
draw((0,1)--(1,3)--(4,1)--(3,0)--cycle, linewidth(0.7));
[/asy]
$ \textbf{(A)}\ 4 \qquad \textbf{(B)}\ 4.5 \qquad \textbf{(C)}\ 5 \qquad \textbf{(D)}\ 5.5 \qquad \textbf{(E)}\ 6$
1991 Arnold's Trivium, 5
Calculate the $100$th derivative of the function
\[\frac{1}{x^2+3x+2}\]
at $x=0$ with $10\%$ relative error.
2006 Princeton University Math Competition, 4
There is a circle $c$ centered about the origin of radius $ 1$. There are circles $c_1$,$ . . .$ ,$c_6$, each of radius $r_1$, such that each circle is completely inside c and is tangent to it, and $c_2$ is tangent to $c_1$, $c_3$ is tangent to $c_2$, . . ., and $c_1$ is tangent to $c_6$. There is a circle $d$ which is tangent to $c$, $c_1$, and $c_2$, but does not intersect any of these circles. What is the radius of circle $d$? Express your answer in the form $\frac{a+b\sqrt{c}}{d}$ , where $a,b,c,d$ are integers, $d$ is positive and as small as possible, and $c$ is squarefree.
2002 District Olympiad, 4
Let $ n\ge 2 $ be a natural number. Prove the following propositions:
[b]a)[/b] $ a_1,a_2,\ldots ,a_n\in\mathbb{R}\wedge a_1+\cdots +a_n=a_1^2+\cdots +a_n^2\implies a_1+\cdots +a_n\le a_n. $
[b]b)[/b] $ x\in [1,n]\implies\exists b_1,b_2,\ldots ,b_n\in\mathbb{R}_{\ge 0}\quad x=b_1+\cdots +b_n=b_1^2 +\cdots +b_n^2 . $
2010 ELMO Shortlist, 3
A circle $\omega$ not passing through any vertex of $\triangle ABC$ intersects each of the segments $AB$, $BC$, $CA$ in 2 distinct points. Prove that the incenter of $\triangle ABC$ lies inside $\omega$.
[i]Evan O' Dorney.[/i]
2017 All-Russian Olympiad, 5
$P(x)$ is polynomial with degree $n\geq 2$ and nonnegative coefficients. $a,b,c$ - sides for some triangle. Prove, that $\sqrt[n]{P(a)},\sqrt[n]{P(b)},\sqrt[n]{P(c)}$ are sides for some triangle too.
1983 Swedish Mathematical Competition, 5
Show that a unit square can be covered with three equal disks with radius less than $\frac{1}{\sqrt{2}}$.
What is the smallest possible radius?
2025 Al-Khwarizmi IJMO, 2
Let $ABCD$ be a convex quadrilateral with \[\angle ADC = 90^\circ, \ \ \angle BCD = \angle ABC > 90^\circ, \mbox{ and } AB = 2CD.\] The line through \(C\), parallel to \(AD\), intersects the external angle bisector of \(\angle ABC\) at point \(T\). Prove that the angles $\angle ATB$, $\angle TBC$, $\angle BCD$, $\angle CDA$, $\angle DAT$ can be divided into two groups, so that the angles in each group have a sum of $270^{\circ}$.
[i] Miroslav Marinov, Bulgaria [/i]
1999 Baltic Way, 14
Let $ABC$ be an isosceles triangle with $AB=AC$. Points $D$ and $E$ lie on the sides $AB$ and $AC$, respectively. The line passing through $B$ and parallel to $AC$ meets the line $DE$ at $F$. The line passing through $C$ and parallel to $AB$ meets the line $DE$ at $G$. Prove that
\[\frac{[DBCG]}{[FBCE]}=\frac{AD}{DE} \]
1994 Austrian-Polish Competition, 9
On the plane are given four distinct points $A,B,C,D$ on a line $g$ in this order, at the mutual distances $AB = a, BC = b, CD = c$.
(a) Construct (if possible) a point $P$ outside line $g$ such that $\angle APB =\angle BPC =\angle CPD$.
(b) Prove that such a point $P$ exists if and only if $ (a+b)(b+c) < 4ac$
2007 AMC 12/AHSME, 7
Let $ a,$ $ b,$ $ c,$ $ d,$ and $ e$ be five consecutive terms in an arithmetic sequence, and suppose that $ a \plus{} b \plus{} c \plus{} d \plus{} e \equal{} 30.$ Which of the following can be found?
$ \textbf{(A)}\ a \qquad \textbf{(B)}\ b \qquad \textbf{(C)}\ c \qquad \textbf{(D)}\ d \qquad \textbf{(E)}\ e$
1991 AMC 12/AHSME, 14
If $x$ is the cube of a positive integer and $d$ is the number of positive integers that are divisors of $x$, then $d$ could be
$ \textbf{(A)}\ 200\qquad\textbf{(B)}\ 201\qquad\textbf{(C)}\ 202\qquad\textbf{(D)}\ 203\qquad\textbf{(E)}\ 204 $
1998 Romania National Olympiad, 2
$\textbf{a) }$ Let $p \geq 2$ be a natural number and $G_p = \bigcup\limits_{n \in \mathbb{N}} \lbrace z \in \mathbb{C} \mid z^{p^n}=1 \rbrace.$ Prove that $(G_p, \cdot)$ is a subgroup of $(\mathbb{C}^*, \cdot).$
$\textbf{b) }$ Let $(H, \cdot)$ be an infinite subgroup of $(\mathbb{C}^*, \cdot).$ Prove that all proper subgroups of $H$ are finite if and only if $H=G_p$ for some prime $p.$
1962 All-Soviet Union Olympiad, 10
In a triangle, $AB=BC$ and $M$ is the midpoint of $AC$. $H$ is chosen on $BC$ so that $MH$ is perpendicular to $BC$. $P$ is the midpoint of $MH$. Prove that $AH$ is perpendicular to $BP$.
1961 AMC 12/AHSME, 10
Each side of triangle $ABC$ is $12$ units. $D$ is the foot of the perpendicular dropped from $A$ on $BC$, and $E$ is the midpoint of $AD$. The length of $BE$, in the same unit, is:
${{ \textbf{(A)}\ \sqrt{18} \qquad\textbf{(B)}\ \sqrt{28} \qquad\textbf{(C)}\ 6 \qquad\textbf{(D)}\ \sqrt{63} }\qquad\textbf{(E)}\ \sqrt{98} } $
1949-56 Chisinau City MO, 42
A trapezoid and an isosceles triangle are inscribed in a circle. The larger base of the trapezoid is the diameter of the circle, and the sides of the triangle are parallel to the sides of the trapezoid. Show that the trapezoid and the triangle have equal areas.
2022 IMO Shortlist, N8
Prove that $5^n-3^n$ is not divisible by $2^n+65$ for any positive integer $n$.
2020 Kosovo Team Selection Test, 4
Prove that for all positive integers $m$ and $n$ the following inequality hold:
$$\pi(m)-\pi(n)\leq\frac{(m-1)\varphi(n)}{n}$$
When does equality hold?
[i]Proposed by Shend Zhjeqi and Dorlir Ahmeti, Kosovo[/i]
2023 Moldova EGMO TST, 10
Cirlce $\Omega$ is inscribed in triangle $ABC$ with $\angle BAC=40$. Point $D$ is inside the angle $BAC$ and is the intersection of exterior bisectors of angles $B$ and $C$ with the common side $BC$. Tangent form $D$ touches $\Omega$ in $E$. FInd $\angle BEC$.
2013 Princeton University Math Competition, 3
The area of a circle centered at the origin, which is inscribed in the parabola $y=x^2-25$, can be expressed as $\tfrac ab\pi$, where $a$ and $b$ are coprime positive integers. What is the value of $a+b$?
2006 Switzerland Team Selection Test, 1
The three roots of $P(x) = x^3 - 2x^2 - x + 1$ are $a>b>c \in \mathbb{R}$. Find the value of $a^2b+b^2c+c^2a$. :D
2023 Harvard-MIT Mathematics Tournament, 32
Let $ABC$ be a triangle with $\angle{BAC}>90^\circ.$ Let $D$ be the foot of the perpendicular from $A$ to side $BC.$ Let $M$ and $N$ be the midpoints of segments $BC$ and $BD,$ respectively. Suppose that $AC=2, \angle{BAN}=\angle{MAC},$ and $AB \cdot BC = AM.$ Compute the distance from $B$ to line $AM.$
2025 India STEMS Category C, 6
Let $G$ be a finite abelian group. There is a magic box $T$. At any point, an element of $G$ may be added to the box and all elements belonging to the subgroup (of $G$) generated by the elements currently inside $T$ are moved from outside $T$ to inside (unless they are already inside). Initially $
T$ contains only the group identity, $1_G$. Alice and Bob take turns moving an element from outside $T$ to inside it. Alice moves first. Whoever cannot make a move loses. Find all $G$ for which Bob has a winning strategy.
1975 IMO Shortlist, 5
Let $M$ be the set of all positive integers that do not contain the digit $9$ (base $10$). If $x_1, \ldots , x_n$ are arbitrary but distinct elements in $M$, prove that
\[\sum_{j=1}^n \frac{1}{x_j} < 80 .\]