Right tetrahedron of fixed volume and min perimeter

by Miquel-point, Apr 6, 2025, 7:57 PM

Determine the lengths of the edges of a right tetrahedron of volume $a^3$ so that the sum of its edges' lengths is minumum.

V \le RS/2 in tetrahderon with equil base

by Miquel-point, Apr 6, 2025, 7:54 PM

Consider a tetrahedron $OABC$ with $ABC$ equilateral. Let $S$ be the area of the triangle of sides $OA$, $OB$ and $OC$. Show that $V\leqslant \dfrac12 RS$ where $R$ is the circumradius and $V$ is the volume of the tetrahedron.

Stere Ianuș

Arithmetic properties of ax^2-x/6

by Miquel-point, Apr 6, 2025, 7:50 PM

Let $P(X)=aX^2-\frac16 X$ where $a\in\mathbb{R}$.
1) Determine $a$ such that for every $\alpha\in\mathbb{Z}$ we have $P(\alpha)\in\mathbb{Z}$.
2) Show that if $a$ is irrational then for every $0<u<v<1$ there exists $n\in\mathbb{Z}$ such that
\[u<P(n)-\lfloor P(n)\rfloor <v.\]Generalize the problem!

Point moving towards vertices and changing plans again and again

by Miquel-point, Apr 6, 2025, 7:47 PM

In the plane of traingle $ABC$ we consider a variable point $M$ which moves on line $MA$ towards $A$. Halfway there, it stops and starts moving in a straight line line towards $B$. Halfway there, it stops and starts moving in a straight line towards $C$, and halfway there it stops and starts moving in a straight line towards $A$, and so on. Show that $M$ will get as close as we want to the vertices of a fixed triangle with area $\text{area}(ABC)/7$.

max(PA,PC) when ABCD square

by Miquel-point, Apr 6, 2025, 6:15 PM

Determine the set of points $P$ in the plane of a square $ABCD$ for which \[\max (PA, PC)=\frac1{\sqrt2}(PB+PD).\]
Titu Andreescu and I.V. Maftei
This post has been edited 1 time. Last edited by Miquel-point, 2 hours ago

functional equation

by hanzo.ei, Apr 6, 2025, 6:08 PM

Find all functions \( f : \mathbb{R} \to \mathbb{R} \) satisfying the equation
\[
(f(x+y))^2= f(x^2) + f(2xf(y) + y^2), \quad \forall x, y \in \mathbb{R}.
\]

Two Functional Inequalities

by Mathdreams, Apr 6, 2025, 1:34 PM

Determine all functions $f : \mathbb{R} \rightarrow \mathbb{R}$ such that $$f(x) \le x^3$$and $$f(x + y) \le f(x) + f(y) + 3xy(x + y)$$for any real numbers $x$ and $y$.

(Miroslav Marinov, Bulgaria)

Beautiful problem

by luutrongphuc, Apr 4, 2025, 5:35 AM

Let triangle $ABC$ be circumscribed about circle $(I)$, and let $H$ be the orthocenter of $\triangle ABC$. The circle $(I)$ touches line $BC$ at $D$. The tangent to the circle $(BHC)$ at $H$ meets $BC$ at $S$. Let $J$ be the midpoint of $HI$, and let the line $DJ$ meet $(I)$ again at $X$. The tangent to $(I)$ parallel to $BC$ meets the line $AX$ at $T$. Prove that $ST$ is tangent to $(I)$.

Incircle-excircle config geo

by a_507_bc, Apr 4, 2024, 4:26 PM

Let $ABC$ be a triangle with incenter and $A$-excenter $I, I_a$, whose incircle touches $BC, CA, AB$ at $D, E, F$. The line $EF$ meets $BC$ at $P$ and $X$ is the midpoint of $PD$. Show that $XI \perp DI_a$.

Every subset of size k has sum at most N/2

by orl, Apr 20, 2006, 5:58 PM

For a given positive integer $k$ find, in terms of $k$, the minimum value of $N$ for which there is a set of $2k + 1$ distinct positive integers that has sum greater than $N$ but every subset of size $k$ has sum at most $\tfrac{N}{2}.$

Stay insane,Coz it's your will, labour and pain,which takes you to the top of the mountain.

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utkarshgupta
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  • Here goes first post of 2025! Great blog.

    by math_holmes15, Jan 14, 2025, 8:53 AM

  • First post of 2024

    by Yiyj1, Feb 8, 2024, 5:40 AM

  • First post of 2023

    by HoRI_DA_GRe8, Jul 22, 2023, 7:45 AM

  • Nice blog ! Your isogonality lemma is really powerful !

    by 554183, Oct 14, 2021, 8:55 AM

  • Post plss....

    by samrocksnature, Apr 11, 2021, 10:12 PM

  • alas,this is ded

    by Hamroldt, Mar 18, 2021, 4:13 PM

  • Thanks for the nice blog.

    by Feridimo, Mar 6, 2020, 4:17 PM

  • I think this might be silly but ... when should we expect to have another post ?? I am very keen to see it :D

    by gamerrk1004, Nov 4, 2019, 4:54 PM

  • Let's all echo what's written in the blog description - Stay Insane / 'Cause it's your labor, will and pain/ That takes you to the top of soda fountain :D

    by Kayak, Oct 2, 2017, 7:18 PM

  • hey utkarsh jee is over now ... continue your elementary blog pleaseeeeeee!

    by kk108, Jun 17, 2017, 11:19 AM

  • Congrats on becoming a contest moderator!

    by Ankoganit, Mar 9, 2017, 5:22 AM

  • INTERSTING BLOG

    by kk108, Feb 19, 2017, 2:04 PM

  • I have no plans for this blog right now....
    No time here people !
    But lets see....
    I may try some combinatorics :P

    by utkarshgupta, Feb 15, 2017, 12:47 PM

  • Thanks for the nice blog!

    by Orkhan-Ashraf_2002, Feb 13, 2017, 6:34 PM

  • Revive it!!!
    Best blog out there, for sure!

    by rmtf1111, Jan 12, 2017, 6:02 PM

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