Geometry

by Lukariman, May 7, 2025, 4:02 PM

Given acute triangle ABC ,AB=b,AC=c . M is a variable point on side AB. The circle circumscribing triangle BCM intersects AC at N.

a)Let I be the center of the circle circumscribing triangle AMN. Prove that I always lies on a fixed line.

b)Let J be the center of the circle circumscribing triangle MBC. Prove that line segment IJ has a constant length.
This post has been edited 2 times. Last edited by Lukariman, Yesterday at 4:24 PM

Geometry

by Lukariman, May 6, 2025, 12:43 PM

Given circle (O) and point P outside (O). From P draw tangents PA and PB to (O) with contact points A, B. On the opposite ray of ray BP, take point M. The circle circumscribing triangle APM intersects (O) at the second point D. Let H be the projection of B on AM. Prove that $\angle HDM$ = 2∠AMP.
Attachments:
This post has been edited 2 times. Last edited by Lukariman, Yesterday at 6:47 AM

Tangent to two circles

by Mamadi, May 2, 2025, 7:01 AM

Two circles \( w_1 \) and \( w_2 \) intersect each other at \( M \) and \( N \). The common tangent to two circles nearer to \( M \) touch \( w_1 \) and \( w_2 \) at \( A \) and \( B \) respectively. Let \( C \) and \( D \) be the reflection of \( A \) and \( B \) respectively with respect to \( M \). The circumcircle of the triangle \( DCM \) intersect circles \( w_1 \) and \( w_2 \) respectively at points \( E \) and \( F \) (both distinct from \( M \)). Show that the line \( EF \) is the second tangent to \( w_1 \) and \( w_2 \).

Isosceles Triangle Geo

by oVlad, Apr 12, 2025, 9:38 AM

Consider the isosceles triangle $ABC$ with $\angle A>90^\circ$ and the circle $\omega$ of radius $AC$ centered at $A.$ Let $M$ be the midpoint of $AC.$ The line $BM$ intersects $\omega$ a second time at $D.$ Let $E$ be a point on $\omega$ such that $BE\perp AC.$ Let $N$ be the intersection of $DE$ and $AC.$ Prove that $AN=2\cdot AB.$

Kingdom of Anisotropy

by v_Enhance, Jul 12, 2022, 1:41 PM

The kingdom of Anisotropy consists of $n$ cities. For every two cities there exists exactly one direct one-way road between them. We say that a path from $X$ to $Y$ is a sequence of roads such that one can move from $X$ to $Y$ along this sequence without returning to an already visited city. A collection of paths is called diverse if no road belongs to two or more paths in the collection.

Let $A$ and $B$ be two distinct cities in Anisotropy. Let $N_{AB}$ denote the maximal number of paths in a diverse collection of paths from $A$ to $B$. Similarly, let $N_{BA}$ denote the maximal number of paths in a diverse collection of paths from $B$ to $A$. Prove that the equality $N_{AB} = N_{BA}$ holds if and only if the number of roads going out from $A$ is the same as the number of roads going out from $B$.

Proposed by Warut Suksompong, Thailand
This post has been edited 1 time. Last edited by v_Enhance, Jul 12, 2022, 1:59 PM
Reason: add authorship

Line passes through fixed point, as point varies

by Jalil_Huseynov, May 17, 2022, 6:48 PM

Let $ABC$ be a right triangle with $\angle B=90^{\circ}$. Point $D$ lies on the line $CB$ such that $B$ is between $D$ and $C$. Let $E$ be the midpoint of $AD$ and let $F$ be the seconf intersection point of the circumcircle of $\triangle ACD$ and the circumcircle of $\triangle BDE$. Prove that as $D$ varies, the line $EF$ passes through a fixed point.

Deduction card battle

by anantmudgal09, Mar 7, 2021, 10:32 AM

A Magician and a Detective play a game. The Magician lays down cards numbered from $1$ to $52$ face-down on a table. On each move, the Detective can point to two cards and inquire if the numbers on them are consecutive. The Magician replies truthfully. After a finite number of moves, the Detective points to two cards. She wins if the numbers on these two cards are consecutive, and loses otherwise.

Prove that the Detective can guarantee a win if and only if she is allowed to ask at least $50$ questions.

Proposed by Anant Mudgal

IMO 2018 Problem 5

by orthocentre, Jul 10, 2018, 11:19 AM

Let $a_1$, $a_2$, $\ldots$ be an infinite sequence of positive integers. Suppose that there is an integer $N > 1$ such that, for each $n \geq N$, the number
$$\frac{a_1}{a_2} + \frac{a_2}{a_3} + \cdots + \frac{a_{n-1}}{a_n} + \frac{a_n}{a_1}$$is an integer. Prove that there is a positive integer $M$ such that $a_m = a_{m+1}$ for all $m \geq M$.

Proposed by Bayarmagnai Gombodorj, Mongolia
This post has been edited 3 times. Last edited by djmathman, Jun 16, 2020, 4:03 AM
Reason: problem author

perpendicularity involving ex and incenter

by Erken, Dec 24, 2008, 2:56 PM

Suppose that $ B_1$ is the midpoint of the arc $ AC$, containing $ B$, in the circumcircle of $ \triangle ABC$, and let $ I_b$ be the $ B$-excircle's center. Assume that the external angle bisector of $ \angle ABC$ intersects $ AC$ at $ B_2$. Prove that $ B_2I$ is perpendicular to $ B_1I_B$, where $ I$ is the incenter of $ \triangle ABC$.

q(x) to be the product of all primes less than p(x)

by orl, Aug 10, 2008, 5:19 PM

For an integer $x \geq 1$, let $p(x)$ be the least prime that does not divide $x$, and define $q(x)$ to be the product of all primes less than $p(x)$. In particular, $p(1) = 2.$ For $x$ having $p(x) = 2$, define $q(x) = 1$. Consider the sequence $x_0, x_1, x_2, \ldots$ defined by $x_0 = 1$ and \[ x_{n+1} = \frac{x_n p(x_n)}{q(x_n)} \] for $n \geq 0$. Find all $n$ such that $x_n = 1995$.
This post has been edited 1 time. Last edited by v_Enhance, Apr 5, 2015, 12:49 PM
Reason: tex cleanup

A guide to the science of secrecy

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  • Good website!

    by bluegoose101, Aug 5, 2021, 6:28 PM

  • uh-huh, a great place here

    by fenchelfen, Sep 1, 2019, 11:30 AM

  • uh, yeah he is o_O

    by SonyWii, Oct 8, 2010, 2:11 PM

  • dude i think you're my roommate from camp :O

    by themorninglighttt, Aug 29, 2010, 10:06 PM

  • what i'm still not a contrib D:

    by SonyWii, Aug 6, 2010, 2:20 PM

  • I see what you did there

    by Jongy, Aug 1, 2010, 11:52 PM

  • omg, apparently you like cryptography; and apparently I'm not a contribb D:

    by SonyWii, Jul 26, 2010, 9:48 PM

  • Thank You

    by fortenforge, Jan 17, 2010, 6:35 PM

  • Wow this is a really cool blog

    by alkjash, Jan 16, 2010, 7:04 PM

  • Hi :)

    by fortenforge, Jan 7, 2010, 12:12 AM

  • Hi :)

    by Richard_Min, Jan 5, 2010, 9:29 PM

  • Hi :) :)

    by fortenforge, Jan 3, 2010, 10:14 PM

  • HELLO FORTENFORGE I AM THE PERSON SITTING NEXT TO YOU IN IDEAMATH

    by ButteredButNotEaten, Dec 24, 2009, 4:19 AM

  • @dragon96 Not if you celebrate Christmas with neon lights
    @batteredbutnotdefeated Sure, You are now a contributer

    by fortenforge, Dec 20, 2009, 4:39 AM

  • I too share a love for cryptography and cryptanalysis, may I be a contrib?

    by batteredbutnotdefeated, Dec 20, 2009, 2:38 AM

  • The green is too bright for Christmas. :P

    by dragon96, Dec 20, 2009, 2:12 AM

  • I thought I'd change the colors for the Holidays :lol:

    by fortenforge, Dec 13, 2009, 10:53 PM

  • hi, some "simple" cryptography here: http://www.artofproblemsolving.com/Forum/weblog_entry.php?t=317795

    by phiReKaLk6781, Dec 12, 2009, 3:46 AM

  • Yeah, that is binary, for modern cryptography, most text is converted to binary first and then algorithm's for encryption are preformed on the binary rather than the English letters. The text is converted using the ASCII table or UNICODE.

    by fortenforge, Oct 13, 2009, 10:33 PM

  • Whoa, I love your background! Is that binary?

    by pianogirl, Oct 13, 2009, 8:34 PM

  • Sure, I'll add you as a contributer...

    by fortenforge, Oct 2, 2009, 4:44 AM

  • May I make a post on one cipher I made up? (It's a good code for science people! *hint hint*)

    by dragon96, Oct 2, 2009, 4:04 AM

  • Nice blog, this is interesting... :lol:

    and guess who i am :ninja:

    by Yoshi, Sep 21, 2009, 4:02 AM

  • Thanks :lol:

    by fortenforge, Sep 17, 2009, 1:33 AM

  • Very interesting blog. Nice!

    by AIME15, Sep 16, 2009, 5:21 PM

  • When you mean 'write' do you mean like programming? Much of cryptography has to do with programming and most modern cryptographers are excellent programmers because modern complex ciphers are difficult to implement by hand.

    See if you can write a program for the substitution cipher. The user should be able to enter the key and the message. I know it is possible to do it in pretty much any language because I was able to do it in c.

    by fortenforge, Aug 7, 2009, 8:17 PM

  • Hello. I don't know much about advanced cryptography but I did write a Caeser Chipher encrypter and decrypter!

    by Poincare, Jul 31, 2009, 8:55 PM

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