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Arbitrary point on BC and its relation with orthocenter
falantrng   33
N an hour ago by Thapakazi
Source: Balkan MO 2025 P2
In an acute-angled triangle \(ABC\), \(H\) be the orthocenter of it and \(D\) be any point on the side \(BC\). The points \(E, F\) are on the segments \(AB, AC\), respectively, such that the points \(A, B, D, F\) and \(A, C, D, E\) are cyclic. The segments \(BF\) and \(CE\) intersect at \(P.\) \(L\) is a point on \(HA\) such that \(LC\) is tangent to the circumcircle of triangle \(PBC\) at \(C.\) \(BH\) and \(CP\) intersect at \(X\). Prove that the points \(D, X, \) and \(L\) lie on the same line.

Proposed by Theoklitos Parayiou, Cyprus
33 replies
falantrng
Apr 27, 2025
Thapakazi
an hour ago
Rubber bands
v_Enhance   5
N 2 hours ago by lpieleanu
Source: OTIS Mock AIME 2024 #12
Let $\mathcal G_n$ denote a triangular grid of side length $n$ consisting of $\frac{(n+1)(n+2)}{2}$ pegs. Charles the Otter wishes to place some rubber bands along the pegs of $\mathcal G_n$ such that every edge of the grid is covered by exactly one rubber band (and no rubber band traverses an edge twice). He considers two placements to be different if the sets of edges covered by the rubber bands are different or if any rubber band traverses its edges in a different order. The ordering of which bands are over and under does not matter.
For example, Charles finds there are exactly $10$ different ways to cover $\mathcal G_2$ using exactly two rubber bands; the full list is shown below, with one rubber band in orange and the other in blue.
IMAGE
Let $N$ denote the total number of ways to cover $\mathcal G_4$ with any number of rubber bands. Compute the remainder when $N$ is divided by $1000$.

Ethan Lee
5 replies
v_Enhance
Jan 16, 2024
lpieleanu
2 hours ago
Geometry with orthocenter config
thdnder   6
N 2 hours ago by ohhh
Source: Own
Let $ABC$ be a triangle, and let $AD, BE, CF$ be its altitudes. Let $H$ be its orthocenter, and let $O_B$ and $O_C$ be the circumcenters of triangles $AHC$ and $AHB$. Let $G$ be the second intersection of the circumcircles of triangles $FDO_B$ and $EDO_C$. Prove that the lines $DG$, $EF$, and $A$-median of $\triangle ABC$ are concurrent.
6 replies
thdnder
Apr 29, 2025
ohhh
2 hours ago
Strange Inequality
anantmudgal09   40
N 2 hours ago by starchan
Source: INMO 2020 P4
Let $n \geqslant 2$ be an integer and let $1<a_1 \le a_2 \le \dots \le a_n$ be $n$ real numbers such that $a_1+a_2+\dots+a_n=2n$. Prove that$$a_1a_2\dots a_{n-1}+a_1a_2\dots a_{n-2}+\dots+a_1a_2+a_1+2 \leqslant a_1a_2\dots a_n.$$
Proposed by Kapil Pause
40 replies
anantmudgal09
Jan 19, 2020
starchan
2 hours ago
Finding Solutions
MathStudent2002   22
N 2 hours ago by ihategeo_1969
Source: Shortlist 2016, Number Theory 5
Let $a$ be a positive integer which is not a perfect square, and consider the equation \[k = \frac{x^2-a}{x^2-y^2}.\]Let $A$ be the set of positive integers $k$ for which the equation admits a solution in $\mathbb Z^2$ with $x>\sqrt{a}$, and let $B$ be the set of positive integers for which the equation admits a solution in $\mathbb Z^2$ with $0\leq x<\sqrt{a}$. Show that $A=B$.
22 replies
MathStudent2002
Jul 19, 2017
ihategeo_1969
2 hours ago
USAMO 2000 Problem 3
MithsApprentice   10
N 2 hours ago by HamstPan38825
A game of solitaire is played with $R$ red cards, $W$ white cards, and $B$ blue cards. A player plays all the cards one at a time. With each play he accumulates a penalty. If he plays a blue card, then he is charged a penalty which is the number of white cards still in his hand. If he plays a white card, then he is charged a penalty which is twice the number of red cards still in his hand. If he plays a red card, then he is charged a penalty which is three times the number of blue cards still in his hand. Find, as a function of $R, W,$ and $B,$ the minimal total penalty a player can amass and all the ways in which this minimum can be achieved.
10 replies
MithsApprentice
Oct 1, 2005
HamstPan38825
2 hours ago
Hard limits
Snoop76   7
N 2 hours ago by MihaiT
$a_n$ and $b_n$ satisfies the following recursion formulas: $a_{0}=1, $ $b_{0}=1$, $ a_{n+1}=a_{n}+b_{n}$$ $ and $ $$ b_{n+1}=(2n+3)b_{n}+a_{n}$. Find $ \lim_{n \to \infty} \frac{a_n}{(2n-1)!!}$ $ $ and $ $ $\lim_{n \to \infty} \frac{b_n}{(2n+1)!!}.$
7 replies
Snoop76
Mar 25, 2025
MihaiT
2 hours ago
Additive combinatorics (re Cauchy-Davenport)
mavropnevma   3
N 3 hours ago by Orzify
Source: Romania TST 3 2010, Problem 4
Let $X$ and $Y$ be two finite subsets of the half-open interval $[0, 1)$ such that $0 \in X \cap Y$ and $x + y = 1$ for no $x \in X$ and no $y \in Y$. Prove that the set $\{x + y - \lfloor x + y \rfloor : x \in X \textrm{ and } y \in Y\}$ has at least $|X| + |Y| - 1$ elements.

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3 replies
mavropnevma
Aug 25, 2012
Orzify
3 hours ago
Ducks can play games now apparently
MortemEtInteritum   34
N 3 hours ago by HamstPan38825
Source: USA TST(ST) 2020 #1
Let $a$, $b$, $c$ be fixed positive integers. There are $a+b+c$ ducks sitting in a
circle, one behind the other. Each duck picks either rock, paper, or scissors, with $a$ ducks
picking rock, $b$ ducks picking paper, and $c$ ducks picking scissors.
A move consists of an operation of one of the following three forms:

[list]
[*] If a duck picking rock sits behind a duck picking scissors, they switch places.
[*] If a duck picking paper sits behind a duck picking rock, they switch places.
[*] If a duck picking scissors sits behind a duck picking paper, they switch places.
[/list]
Determine, in terms of $a$, $b$, and $c$, the maximum number of moves which could take
place, over all possible initial configurations.
34 replies
MortemEtInteritum
Nov 16, 2020
HamstPan38825
3 hours ago
Floor sequence
va2010   87
N 3 hours ago by Mathgloggers
Source: 2015 ISL N1
Determine all positive integers $M$ such that the sequence $a_0, a_1, a_2, \cdots$ defined by \[ a_0 = M + \frac{1}{2}   \qquad  \textrm{and} \qquad    a_{k+1} = a_k\lfloor a_k \rfloor   \quad \textrm{for} \, k = 0, 1, 2, \cdots \]contains at least one integer term.
87 replies
va2010
Jul 7, 2016
Mathgloggers
3 hours ago
INMO 2019 P3
div5252   45
N 3 hours ago by anudeep
Let $m,n$ be distinct positive integers. Prove that
$$gcd(m,n) + gcd(m+1,n+1) + gcd(m+2,n+2) \le 2|m-n| + 1. $$Further, determine when equality holds.
45 replies
div5252
Jan 20, 2019
anudeep
3 hours ago
complex bashing in angles??
megahertz13   2
N Apr 18, 2025 by ali123456
Source: 2013 PUMAC FA2
Let $\gamma$ and $I$ be the incircle and incenter of triangle $ABC$. Let $D$, $E$, $F$ be the tangency points of $\gamma$ to $\overline{BC}$, $\overline{CA}$, $\overline{AB}$ and let $D'$ be the reflection of $D$ about $I$. Assume $EF$ intersects the tangents to $\gamma$ at $D$ and $D'$ at points $P$ and $Q$. Show that $\angle DAD' + \angle PIQ = 180^\circ$.
2 replies
megahertz13
Nov 5, 2024
ali123456
Apr 18, 2025
complex bashing in angles??
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G H BBookmark kLocked kLocked NReply
Source: 2013 PUMAC FA2
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megahertz13
3183 posts
#1 • 1 Y
Y by Rayanelba
Let $\gamma$ and $I$ be the incircle and incenter of triangle $ABC$. Let $D$, $E$, $F$ be the tangency points of $\gamma$ to $\overline{BC}$, $\overline{CA}$, $\overline{AB}$ and let $D'$ be the reflection of $D$ about $I$. Assume $EF$ intersects the tangents to $\gamma$ at $D$ and $D'$ at points $P$ and $Q$. Show that $\angle DAD' + \angle PIQ = 180^\circ$.
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TestX01
341 posts
#2
Y by
Polars or Lahire gives that $IQ\perp AD'$, $AD\perp IP$. Let $PI\cap DQ=X$. We claim $\triangle AD'D\sim\triangle IQX$. This follows from $\angle IDA=\angle DPI=\angle QXI$, and $180^\circ-\angle AD'I=180^\circ-\angle IQX$ from the perpendiculars.

The similarity easily finishes.
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ali123456
52 posts
#3 • 1 Y
Y by Rayanelba
My solution
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