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Indonesia Juniors 2004 day 1 OSN SMP
parmenides51   9
N 2 minutes ago by Mathelets
p1. Known points $A (-1.2)$, $B (0,2)$, $C (3,0)$, and $D (3, -1)$ as seen in the following picture.
Determine the measure of the angle $AOD$ .
IMAGE


p2. Determine all prime numbers $p> 2$ until $p$ divides $71^2 - 37^2 - 51$.


p3. A ball if dropped perpendicular to the ground from a height then it will bounce back perpendicular along the high third again, down back upright and bouncing back a third of its height, and next. If the distance traveled by the ball when it touches the ground the fourth time is equal to $106$ meters. From what height is the ball was dropped?


p4. The beam $ABCD.EFGH$ is obtained by pasting two unit cubes $ABCD.PQRS$ and $PQRS.EFGH$. The point K is the midpoint of the edge $AB$, while the point $L$ is the midpoint of the edge $SH$. What is the length of the line segment $KL$?


p5. How many integer numbers are no greater than $2004$, with remainder $1$ when divided by $2$, with remainder $2$ when divided by $3$, with remainder $3$ when divided by $4$, and with remainder $4$ when divided by $5$?
9 replies
1 viewing
parmenides51
Oct 30, 2021
Mathelets
2 minutes ago
IMO ShortList 2003, combinatorics problem 4
darij grinberg   39
N 2 hours ago by ThatApollo777
Source: Problem 5 of the German pre-TST 2004, written in December 03
Let $x_1,\ldots, x_n$ and $y_1,\ldots, y_n$ be real numbers. Let $A = (a_{ij})_{1\leq i,j\leq n}$ be the matrix with entries \[a_{ij} = \begin{cases}1,&\text{if }x_i + y_j\geq 0;\\0,&\text{if }x_i + y_j < 0.\end{cases}\]Suppose that $B$ is an $n\times n$ matrix with entries $0$, $1$ such that the sum of the elements in each row and each column of $B$ is equal to the corresponding sum for the matrix $A$. Prove that $A=B$.
39 replies
darij grinberg
May 17, 2004
ThatApollo777
2 hours ago
greatest volume
hzbrl   4
N 2 hours ago by hzbrl
Source: purple comet
A large sphere with radius 7 contains three smaller balls each with radius 3 . The three balls are each externally tangent to the other two balls and internally tangent to the large sphere. There are four right circular cones that can be inscribed in the large sphere in such a way that the bases of the cones are tangent to all three balls. Of these four cones, the one with the greatest volume has volume $n \pi$. Find $n$.
4 replies
hzbrl
May 8, 2025
hzbrl
2 hours ago
Projective geo
drmzjoseph   1
N 2 hours ago by Luis González
Any pure projective solution? I mean no metrics, Menelaus, Ceva, bary, etc
Only pappus, desargues, dit, etc
Btw prove that $X',P,K$ are collinear, and $P,Q$ are arbitrary points
1 reply
drmzjoseph
Mar 6, 2025
Luis González
2 hours ago
2019 Iberoamerican Mathematical Olympiad, P1
jbaca   9
N 2 hours ago by jordiejoh
For each positive integer $n$, let $s(n)$ be the sum of the squares of the digits of $n$. For example, $s(15)=1^2+5^2=26$. Determine all integers $n\geq 1$ such that $s(n)=n$.
9 replies
jbaca
Sep 15, 2019
jordiejoh
2 hours ago
Conditional geo with centroid
a_507_bc   7
N 2 hours ago by Tkn
Source: Singapore Open MO Round 2 2023 P1
In a scalene triangle $ABC$ with centroid $G$ and circumcircle $\omega$ centred at $O$, the extension of $AG$ meets $\omega$ at $M$; lines $AB$ and $CM$ intersect at $P$; and lines $AC$ and $BM$ intersect at $Q$. Suppose the circumcentre $S$ of the triangle $APQ$ lies on $\omega$ and $A, O, S$ are collinear. Prove that $\angle AGO = 90^{o}$.
7 replies
a_507_bc
Jul 1, 2023
Tkn
2 hours ago
People live in Kansas?
jj_ca888   13
N 3 hours ago by Ilikeminecraft
Source: SMO 2020/5
In triangle $\triangle ABC$, let $E$ and $F$ be points on sides $AC$ and $AB$, respectively, such that $BFEC$ is cyclic. Let lines $BE$ and $CF$ intersect at point $P$, and $M$ and $N$ be the midpoints of $\overline{BF}$ and $\overline{CE}$, respectively. If $U$ is the foot of the perpendicular from $P$ to $BC$, and the circumcircles of triangles $\triangle BMU$ and $\triangle CNU$ intersect at second point $V$ different from $U$, prove that $A, P,$ and $V$ are collinear.

Proposed by Andrew Wen and William Yue
13 replies
jj_ca888
Aug 28, 2020
Ilikeminecraft
3 hours ago
Symmetric integer FE
a_507_bc   5
N 3 hours ago by Tkn
Source: Singapore Open MO Round 2 2023 P4
Find all functions $f: \mathbb{Z} \to \mathbb{Z}$, such that $$f(x+y)((f(x) - f(y))^2+f(xy))=f(x^3)+f(y^3)$$for all integers $x, y$.
5 replies
a_507_bc
Jul 1, 2023
Tkn
3 hours ago
Channel name changed
Plane_geometry_youtuber   6
N 3 hours ago by Yiyj
Hi,

Due to the search handle issue in youtube. My channel is renamed to Olympiad Geometry Club. And the new link is as following:

https://www.youtube.com/@OlympiadGeometryClub

Recently I introduced the concept of harmonic bundle. I will move on to the conjugate median soon. In the future, I will discuss more than a thousand theorems on plane geometry and hopefully it can help to the students preparing for the Olympiad competition.

Please share this to the people may need it.

Thank you!
6 replies
Plane_geometry_youtuber
Yesterday at 9:31 PM
Yiyj
3 hours ago
How many cases did you check?
avisioner   18
N 3 hours ago by ezpotd
Source: 2023 ISL N2
Determine all ordered pairs $(a,p)$ of positive integers, with $p$ prime, such that $p^a+a^4$ is a perfect square.

Proposed by Tahjib Hossain Khan, Bangladesh
18 replies
avisioner
Jul 17, 2024
ezpotd
3 hours ago
Beautiful geo but i cant solve this
phonghatemath   3
N 3 hours ago by phonghatemath
Source: homework
Given triangle $ABC$ inscribed in $(O)$. Two points $D, E$ lie on $BC$ such that $AD, AE$ are isogonal in $\widehat{BAC}$. $M$ is the midpoint of $AE$. $K$ lies on $DM$ such that $OK \bot AE$. $AD$ intersects $(O)$ at $P$. Prove that the line through $K$ parallel to $OP$ passes through the Euler center of triangle $ABC$.

Sorry for my English!
3 replies
phonghatemath
Yesterday at 4:48 PM
phonghatemath
3 hours ago
Euler Line in a quadrilateral
Entrepreneur   1
N Nov 18, 2024 by Entrepreneur
Let $ABCD$ be a convex quadrilateral and let $G_a,O_a,H_a\;\&\;N_a$ be the centroid, circumcentre, orthocenter & nine-point centre of $\Delta BCD$ respectively. We define the points $G_b,G_c,G_d,$ $O_b,O_c,O_d,$ $H_b,H_c,H_d$ $N_b,N_c\;\&\;N_d$ analogously. Now, we define the area centroid $\cal G$ of $ABCD$ as the intersection of $G_a,G_c\;\&\;G_bG_d,$ the qausicircumcentre $\cal O$ as the intersection of $ O_aO_c\;\&\;O_bO_d,$ quasiorthocentre $\cal H$ as the intersection of $H_aH_c\;\&\;H_bH_d$ and the quasinine-point centre $\cal N$ as the intersection of $N_aN_c\;\&\;N_bN_d.$ Prove that the points $\mathcal{G,O,H,N}$ are collinear with $\mathcal{HG}=2\mathcal{GO}$ and $\mathcal{HN}=\mathcal{NO}.$
1 reply
Entrepreneur
Nov 9, 2024
Entrepreneur
Nov 18, 2024
Euler Line in a quadrilateral
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Let $ABCD$ be a convex quadrilateral and let $G_a,O_a,H_a\;\&\;N_a$ be the centroid, circumcentre, orthocenter & nine-point centre of $\Delta BCD$ respectively. We define the points $G_b,G_c,G_d,$ $O_b,O_c,O_d,$ $H_b,H_c,H_d$ $N_b,N_c\;\&\;N_d$ analogously. Now, we define the area centroid $\cal G$ of $ABCD$ as the intersection of $G_a,G_c\;\&\;G_bG_d,$ the qausicircumcentre $\cal O$ as the intersection of $ O_aO_c\;\&\;O_bO_d,$ quasiorthocentre $\cal H$ as the intersection of $H_aH_c\;\&\;H_bH_d$ and the quasinine-point centre $\cal N$ as the intersection of $N_aN_c\;\&\;N_bN_d.$ Prove that the points $\mathcal{G,O,H,N}$ are collinear with $\mathcal{HG}=2\mathcal{GO}$ and $\mathcal{HN}=\mathcal{NO}.$
This post has been edited 1 time. Last edited by Entrepreneur, Nov 15, 2024, 8:23 AM
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Bump.....
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