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Circumcenter is equidistant from the midpoints
buratinogigle   0
a minute ago
Source: Own, Entrance Exam for Grade 10 Admission, HSGS 2025
Let $ABC$ be a triangle with points $E$ and $F$ lying on rays $AC$ and $AB$, respectively, such that $AE = AF$. On the line $EF$, let points $M$ and $N$ be chosen so that $CM \perp CA$ and $BN \perp BA$. Prove that the circumcenter of triangle $ABC$ is equidistant from the midpoints of segments $EM$ and $FN$.
0 replies
buratinogigle
a minute ago
0 replies
<AST = 90^o wanted, incircle and altitude related
parmenides51   1
N 2 minutes ago by Gimbrint
Source: 2017 Latvia BW TST P10
In an obtuse triangle $ABC$, for which $AC < AB$, the radius of the inscribed circle is $R$, the midpoint of its arc $BC$ (which does not contain $A$) is $S$. A point $T$ is placed on the extension of altitude $AD$ such that $D$ is between $ A$ and $T$ and $AT = 2R$. Prove that $\angle AST = 90^o$.
1 reply
parmenides51
Dec 16, 2022
Gimbrint
2 minutes ago
Prove that there exists a convex 1990-gon
orl   15
N 14 minutes ago by Kempu33334
Source: IMO 1990, Day 2, Problem 6, IMO ShortList 1990, Problem 16 (NET 1)
Prove that there exists a convex 1990-gon with the following two properties :

a.) All angles are equal.
b.) The lengths of the 1990 sides are the numbers $ 1^2$, $ 2^2$, $ 3^2$, $ \cdots$, $ 1990^2$ in some order.
15 replies
orl
Nov 11, 2005
Kempu33334
14 minutes ago
inequality 11
sadwinter   7
N 32 minutes ago by sadwinter
Not hard?
7 replies
sadwinter
Aug 18, 2021
sadwinter
32 minutes ago
m-n and 2m+2n+1 are perfect squares
AnormalGUY   3
N 34 minutes ago by ilikemath247365
let m,n belongs to natural numbers , such that

2m^2+m=3n^2+n

then prove that m-n and 2m+2n+1 are perfect squares .also find the integral solution of 2m^2+m=3n^2+n
(i am newbie and didnt got the answer to this question in search so i asked .plz correct me if a problem exists)
3 replies
AnormalGUY
an hour ago
ilikemath247365
34 minutes ago
2 years ago
sadwinter   0
37 minutes ago
Source: Vasile Cîrtoaje
I have 2 solutions
:showoff:
0 replies
sadwinter
37 minutes ago
0 replies
IMO Shortlist 2014 N2
hajimbrak   34
N 41 minutes ago by cursed_tangent1434
Determine all pairs $(x, y)$ of positive integers such that \[\sqrt[3]{7x^2-13xy+7y^2}=|x-y|+1.\]
Proposed by Titu Andreescu, USA
34 replies
+1 w
hajimbrak
Jul 11, 2015
cursed_tangent1434
41 minutes ago
Elementary Problems Compilation
Saucepan_man02   33
N 43 minutes ago by lakshya2009
Could anyone send some elementary problems, which have tricky and short elegant methods to solve?

For example like this one:
Solve over reals: $$a^2 + b^2 + c^2 + d^2  -ab-bc-cd-d +2/5=0$$
33 replies
Saucepan_man02
May 26, 2025
lakshya2009
43 minutes ago
classical number theory
vinoth_90_2004   18
N an hour ago by cj13609517288
Source: IMO ShortList 2003, number theory problem 5
An integer $n$ is said to be good if $|n|$ is not the square of an integer. Determine all integers $m$ with the following property: $m$ can be represented, in infinitely many ways, as a sum of three distinct good integers whose product is the square of an odd integer.

Proposed by Hojoo Lee, Korea
18 replies
vinoth_90_2004
May 12, 2004
cj13609517288
an hour ago
GCD relation
Kimchiks926   3
N an hour ago by math-olympiad-clown
Source: Baltic Way 2022, Problem 18
Find all pairs $(a, b)$ of positive integers such that $a \le b$ and
$$ \gcd(x, a) \gcd(x, b) = \gcd(x, 20) \gcd(x, 22) $$holds for every positive integer $x$.
3 replies
Kimchiks926
Nov 12, 2022
math-olympiad-clown
an hour ago
How many friends can sit in that circle at most?
Arytva   5
N an hour ago by Arytva

A group of friends sits in a ring. Each friend picks a different whole number and holds a stone marked with it. Then they pass their stone one seat to the right so everyone ends up with two stones: one they made and one they received. Now they notice something odd: if your original number is $x$, your right-neighbor’s is $y$, and the next person over is $z$, then for every trio in the circle they see

$$
x + z = (2 - x)\,y.
$$
They want as many friends as possible before this breaks (since all stones must stay distinct).

How many friends can sit in that circle at most?
5 replies
Arytva
Yesterday at 10:00 AM
Arytva
an hour ago
Interesting functional equation with polynomial
Iveela   0
an hour ago
Find all functions $f : \mathbb{R}^{+} \to \mathbb{R}^{+}$ and polynomial $P(x) \in \mathbb{R}[x]$ with nonnegative coefficients such that
\[f(x + P(x)f(y)) = (y + 1)f(x)\]for all $x, y \in \mathbb{R^{+}}$.
0 replies
Iveela
an hour ago
0 replies
construction of line knowing newton line direction
PROF65   7
N Jan 7, 2023 by ancamagelqueme
Source: own?
Let $ABC$ be a triangle and $\ell ,d \   $ be two lines .
Construct a line $DE$ s.t. $D\in AB,E\in AC ,DE\parallel d $ and the newton line of $ DBCE$ is parallel to $\ell$.
RH HAS
Best regards.
7 replies
PROF65
Jan 4, 2023
ancamagelqueme
Jan 7, 2023
construction of line knowing newton line direction
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G H BBookmark kLocked kLocked NReply
Source: own?
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PROF65
2016 posts
#1 • 1 Y
Y by GeoKing
Let $ABC$ be a triangle and $\ell ,d \   $ be two lines .
Construct a line $DE$ s.t. $D\in AB,E\in AC ,DE\parallel d $ and the newton line of $ DBCE$ is parallel to $\ell$.
RH HAS
Best regards.
This post has been edited 2 times. Last edited by PROF65, Jan 5, 2023, 1:20 AM
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youthdoo
1312 posts
#2
Y by
What is a construction you mean?
Compasses and straightedge?
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PROF65
2016 posts
#3
Y by
youthdoo wrote:
What is a construction you mean?
Compasses and straightedge?

exactly
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PROF65
2016 posts
#4
Y by
any idea
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youthdoo
1312 posts
#5
Y by
Do we just have to show that it is possible?
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GeoKing
520 posts
#6 • 1 Y
Y by PROF65
Sol:- Construct $H$ the orthocenter of $ABC$. Construct line $\ell'$ through $H$ perpendicular to $\ell$. Construct the line through $A$ perpendicular to $d$ and let it intersect $\ell'$ at $H'$. Let the line though $H'$ perpendicular to $AC,AB$ meet $AB,AC$ respectively at $ D,E$ . These are the required points.
Note that the construction works because steiner line and newton line are perpendicular and using this we constructed $H'$ the orthocenter of $ADE$ and using that we finally constructed $D,E$
This post has been edited 1 time. Last edited by GeoKing, Jan 6, 2023, 4:34 AM
Reason: latex
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PROF65
2016 posts
#7 • 1 Y
Y by GeoKing
another way
the isogonal of $M$ ,the miquel point of $ BCED$ is the infinity point of the newton line so we can construct $ M$ then we draw a line parallel to $d$ through $C$; it hits $(ABC)$ at another point say $G$ let the line $MG$ cuts $AB $ at point that is exactly $D$ . Finally we draw a parallel to $d$ to construct $E$.
RH HAS
Best regards.
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ancamagelqueme
104 posts
#8
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Quote:
Given three lines $a, b, c$ and two vectors $\vec{u}, \vec{v}$, construct a line $d$, with direction $u$, such that Newton line of the quadrilateral formed by the four lines $a, b, c, d$ has direction $\vec{v}$.

The three lines $a, b, c$ form a triangle $ABC$. Let ${\mathcal P}$ be the parabola inscribed in its medial triangle and whose axis has the direction of the vector $\vec{u}$. Let $d$ be the conjugate diameter of the direction of the vector $\vec{v}$. ${\mathcal P}$ and $d$ intersects at a point $T_{uv}$. The tangent at $T_{uv}$ to ${\mathcal P}$ is Newton's line of the quadrilateral $abcd$, which has the direction of the vector $\vec{v}$.

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