Help me please

by sealight2107, Apr 23, 2025, 2:40 PM

Let $m,n,p,q$ be positive reals such that $m+n+p+q+\frac{1}{mnpq} = 18$. Find the minimum and maximum value of $m,n,p,q$

Interesting combinatoric problem on rectangles

by jaydenkaka, Apr 23, 2025, 2:22 PM

Define act <Castle> as following:
For rectangle with dimensions i * j, doing <Castle> means to change its dimensions to (i+p) * (j+q) where p,q is a natural number smaller than 3.

Define 1*1 rectangle as "C0" rectangle, and define "Cn" ("n" is a natural number) as a rectangle that can be created with "n" <Castle>s.
Plus, there is a constraint for "Cn" rectangle. The constraint is that "Cn" rectangle's area must be bigger than n^2 and be same or smaller than (n+1)^2. (n^2 < Area =< (n+1)^2)

Let all "C20" rectangle's area's sum be A, and let all "C20" rectangles perimeter's sum be B.
What is A-B?
This post has been edited 1 time. Last edited by jaydenkaka, an hour ago

(help urgent) Classic Geo Problem / Angle Chasing?

by orangesyrup, Apr 23, 2025, 1:51 PM

In the given figure, ABC is an isosceles triangle with AB = AC and ∠BAC = 78°. Point D is chosen inside the triangle such that AD=DC. Find the measure of angle X (∠BDC).

ps: see the attachment for figure
Attachments:

Collect ...

by luutrongphuc, Apr 21, 2025, 12:59 PM

Find all functions $f: \mathbb{R^+} \rightarrow \mathbb{R^+}$ such that:
$$f\left(f(xy)+1\right)=xf\left(x+f(y)\right)$$

Rectangular line segments in russia

by egxa, Apr 18, 2025, 10:00 AM

Several line segments parallel to the sides of a rectangular sheet of paper were drawn on it. These segments divided the sheet into several rectangles, inside of which there are no drawn lines. Petya wants to draw one diagonal in each of the rectangles, dividing it into two triangles, and color each triangle either black or white. Is it always possible to do this in such a way that no two triangles of the same color share a segment of their boundary?

A game optimization on a graph

by Assassino9931, Apr 8, 2025, 1:59 PM

Let \( X_0, X_1, \dots, X_{n-1} \) be \( n \geq 2 \) given points in the plane, and let \( r > 0 \) be a real number. Alice and Bob play the following game. Firstly, Alice constructs a connected graph with vertices at the points \( X_0, X_1, \dots, X_{n-1} \), i.e., she connects some of the points with edges so that from any point you can reach any other point by moving along the edges.Then, Alice assigns to each vertex \( X_i \) a non-negative real number \( r_i \), for \( i = 0, 1, \dots, n-1 \), such that $\sum_{i=0}^{n-1} r_i = 1$. Bob then selects a sequence of distinct vertices \( X_{i_0} = X_0, X_{i_1}, \dots, X_{i_k} \) such that \( X_{i_j} \) and \( X_{i_{j+1}} \) are connected by an edge for every \( j = 0, 1, \dots, k-1 \). (Note that the length $k \geq 0$ is not fixed and the first selected vertex always has to be $X_0$.) Bob wins if
\[
  \frac{1}{k+1} \sum_{j=0}^{k} r_{i_j} \geq r;
  \]otherwise, Alice wins. Depending on \( n \), determine the largest possible value of \( r \) for which Bobby has a winning strategy.

hard problem

by Cobedangiu, Apr 2, 2025, 6:11 PM

Let $x,y,z>0$ and $xy+yz+zx=3$ : Prove that :
$\sum  \ \frac{x}{y+z}\ge\sum  \frac{1}{\sqrt{x+3}}$

A Hard Geometry

by utkarshgupta, Apr 16, 2016, 5:54 AM

It took me so long to solve. Only if I knew a few projective theorems.

Problem : (RMM 2013 Problem 3)
Let $ABCD$ be a quadrilateral inscribed in a circle $\omega$. The lines $AB$ and $CD$ meet at $P$, the lines $AD$ and $BC$ meet at $Q$, and the diagonals $AC$ and $BD$ meet at $R$. Let $M$ be the midpoint of the segment $PQ$, and let $K$ be the common point of the segment $MR$ and the circle $\omega$. Prove that the circumcircle of the triangle $KPQ$ and $\omega$ are tangent to one another.

Solution :

Let $O$ be the center of $\odot ABCD$.
Let $Q'=\odot PAD \cap PBC$ and $R' = \odot RCD \cap \odot RAB$.
Let $M'$ be the reflection of $P$ in $Q'$

Lemma : $\angle OQ'P = \angle OR'P = 90$
Proof :
Well known

Obviously, $PRR'$ is a straight line (radical axis are concurrent).
Let $OR \cap PQ = Q_1$
Then, since $PQ$ is the polar od $R$, $OQ_1 \perp PQ$
$\implies Q' = Q_1$

Thus $PRR'$ and $ORQ'$ are straight lines..

We know that $OL'Q'K'$ and $OR'Q'P$ are cyclic quadrilaterals.
$\implies RK' \cdot RL' = RO \cdot RQ' = RR' \cdot RP$

Hence $\boxed{R' \in \odot PK'L'}$

It is well known that $Q' \in PQ$
Since $Q'$ lies on the polar of $R$,
$R \in K'L'$
Also since $ORQ'$ is a straight line; $R$ is the midpoint of $K'L'$ and $K'L' || PQ$

Since $M'$ is the reflection of $P$ about $Q'$, $M'$ is the reflection of $P$ about $OQ'$
Hence $L'P = K'M'$
That is $\boxed{M ' \in \odot PK'L'}$


Using the above results,
$PL'R'K'M'$ is a cyclic pentagon.

Now invert with radius $\sqrt{PB \cdot PA}$ and centre $P$.

Obviously $Q' \to Q, M' \to M , R' \to R, \odot(ABCD) \to \odot{ABCD},K' \to K'', L' \to L'' $
The cyclic pentagon implies, $K'',L'' \in MR$ and $K'',L'' \in \odot(ABCD)$

Thus one of $K'',L''$ maps to $K$.
Since $QK',QL'$ are tangent to $\odot (ABCD)$;
$\odot K''PQ$ and $\odot L''PQ$ are tangent to $\odot(ABCD)$.

QED.

Problem 1

by SpectralS, Jul 10, 2012, 5:24 PM

Given triangle $ABC$ the point $J$ is the centre of the excircle opposite the vertex $A.$ This excircle is tangent to the side $BC$ at $M$, and to the lines $AB$ and $AC$ at $K$ and $L$, respectively. The lines $LM$ and $BJ$ meet at $F$, and the lines $KM$ and $CJ$ meet at $G.$ Let $S$ be the point of intersection of the lines $AF$ and $BC$, and let $T$ be the point of intersection of the lines $AG$ and $BC.$ Prove that $M$ is the midpoint of $ST.$

(The excircle of $ABC$ opposite the vertex $A$ is the circle that is tangent to the line segment $BC$, to the ray $AB$ beyond $B$, and to the ray $AC$ beyond $C$.)

Proposed by Evangelos Psychas, Greece

Factor of P(x)

by Brut3Forc3, Apr 4, 2010, 2:45 AM

If $ P(x),Q(x),R(x)$, and $ S(x)$ are all polynomials such that \[ P(x^5)+xQ(x^5)+x^2R(x^5)=(x^4+x^3+x^2+x+1)S(x),\] prove that $ x-1$ is a factor of $ P(x)$.

Composite sum

by rohitsingh0812, Jun 3, 2006, 5:39 AM

Let $ a$, $ b$, $ c$, $ d$, $ e$, $ f$ be positive integers and let $ S = a+b+c+d+e+f$.
Suppose that the number $ S$ divides $ abc+def$ and $ ab+bc+ca-de-ef-df$. Prove that $ S$ is composite.

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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