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k a April Highlights and 2025 AoPS Online Class Information
jlacosta   0
Apr 2, 2025
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0 replies
jlacosta
Apr 2, 2025
0 replies
Vasc = 1?
Li4   6
N 2 minutes ago by IceyCold
Source: 2025 Taiwan TST Round 3 Independent Study 1-N
Find all integer tuples $(a, b, c)$ such that
\[(a^2 + b^2 + c^2)^2 = 3(a^3b + b^3c + c^3a) + 1. \]
Proposed by Li4, Untro368, usjl and YaWNeeT.
6 replies
Li4
Apr 26, 2025
IceyCold
2 minutes ago
Determine all the sets of six consecutive positive integers
sqing   7
N 7 minutes ago by Adventure1000
Source: JBMO 2017, Q1
Determine all the sets of six consecutive positive integers such that the product of some two of them . added to the product of some other two of them is equal to the product of the remaining two numbers.
7 replies
sqing
Jun 26, 2017
Adventure1000
7 minutes ago
Polygon with minimum internal angle 120^\circ
Kunihiko_Chikaya   1
N 10 minutes ago by Mathzeus1024
How many sides can have the polygon with minimum internal angle of $ 120^\circ$ by adding every $ 5^\circ$?
Sorry for my poor English.
1 reply
Kunihiko_Chikaya
Aug 9, 2007
Mathzeus1024
10 minutes ago
One more problem defined only with lines
Assassino9931   2
N 13 minutes ago by sami1618
Source: Balkan MO 2024 Shortlist G6
Let $ABC$ be a triangle and the points $K$ and $L$ on $AB$, $M$ and $N$ on $BC$, and $P$ and $Q$ on $AC$ be such that $AK = LB < \frac{1}{2}AB, BM = NC < \frac{1}{2}BC$ and $CP = QA < \frac{1}{2}AC$. The intersections of $KN$ with $MQ$ and $LP$ are $R$ and $T$ respectively, and the intersections of $NP$ with $LM$ and $KQ$ are $D$ and $E$, respectively. Prove that the lines $DR, BE$ and $CT$ are concurrent.
2 replies
Assassino9931
Yesterday at 10:31 PM
sami1618
13 minutes ago
Inequality
blamm01   1
N 33 minutes ago by tom-nowy
Given $a, b, c$ are different real numbers. Prove that:
$$\frac{ab}{(a-b)^2} + \frac{bc}{(b-c)^2} + \frac{ca}{(c-a)^2} \geq \frac{-1}{4}$$
1 reply
blamm01
3 hours ago
tom-nowy
33 minutes ago
2011 Japan Mathematical Olympiad Preliminary Problem 6
Kunihiko_Chikaya   1
N 2 hours ago by Mathzeus1024
Given a right-angled triangle with $\angle{ABC}=90^\circ$. Three points $P,\ Q,\ R$ are on the sides $BC,\ CA,\ AB$ respectively such that $AQ:QC=2:1,\ AR=AQ,\ QP=QR,\ \angle{PQR}=90^\circ$. If $CP=1$, then find the length of $AR$?
1 reply
Kunihiko_Chikaya
Jan 10, 2011
Mathzeus1024
2 hours ago
A Hard Function Problem
Saucepan_man02   2
N 3 hours ago by alwaystogether
If $f(x)=\frac{(\sqrt{5}+1)x-1}{(10-2 \sqrt{5})x+(\sqrt{5}+1)}$, find the value of$\underbrace{f(f(....f(\sqrt{5})..)}_{61 \text{times}}$.
2 replies
Saucepan_man02
Today at 1:39 AM
alwaystogether
3 hours ago
2009 PUMaC NT B2: sum of factorials is perfect square
PUMACuploader   2
N 5 hours ago by P162008
Suppose you are given that for some positive integer $n$, $1! + 2! + \ldots + n!$ is a perfect square. Find the sum of all possible values of $n$.
2 replies
PUMACuploader
Aug 31, 2011
P162008
5 hours ago
Inequalities
sqing   2
N 6 hours ago by sqing
Let \( x, y \geq  \frac{3}{2}  \). Prove that
$$ \frac{2}{1+xy} + x + y \geq  \frac{x}{y} + \frac{y}{x}+ \frac{21}{13}$$Let \( x, y \geq 2  \). Prove that
$$ \frac{2}{1+xy} + x + y \geq  \frac{x}{y} + \frac{y}{x}+ \frac{12}{5}$$Let \(0< x, y \leq \frac{3}{2}   \). Prove that
$$ \frac{2}{1+xy} + x + y \leq  \frac{x}{y} + \frac{y}{x}+\frac{21}{13}$$Let \(0< x, y \leq 2   \). Prove that
$$ \frac{2}{1+xy} + x + y \leq  \frac{x}{y} + \frac{y}{x}+\frac{12}{5}$$
2 replies
sqing
Yesterday at 2:59 PM
sqing
6 hours ago
BrUMO 2025 Team Round Problem 4
lpieleanu   1
N Today at 8:06 AM by alexheinis
What is the smallest positive integer $n$ such that $z^n-1$ and $(z-\sqrt{3})^n-1$ share a common complex root?
1 reply
lpieleanu
Yesterday at 11:06 PM
alexheinis
Today at 8:06 AM
2009 PUMaC NT A1/B3: decimal representation of 17!
PUMACuploader   2
N Today at 7:50 AM by P162008
You are given that \[17! = 355687ab8096000\] for some digits $a$ and $b$. Find the two-digit number $\overline{ab}$ that is missing above.
2 replies
PUMACuploader
Aug 31, 2011
P162008
Today at 7:50 AM
2009 PUMaC Algebra B2: polynomial of nested roots
PUMACuploader   2
N Today at 7:46 AM by P162008
Let $p(x)$ be the polynomial with leading coefficent 1 and rational coefficents, such that \[p\left(\sqrt{3 + \sqrt{3 + \sqrt{3 + \ldots}}}\right) = 0,\] and with the least degree among all such polynomials. Find $p(5)$.
2 replies
PUMACuploader
Aug 31, 2011
P162008
Today at 7:46 AM
hmmt quadratic power of a prime
martianrunner   2
N Today at 6:20 AM by lgx57
I was practicing problems and came across one as such:

"Find all integers $x$ such that $2x^2 + x-6$ is a positive integral power of a prime positive integer."

I mean after factoring I don't really know where to go...

A hint would be appreciated, and if you want to solve it, please hide your solutions!

Thanks :)
2 replies
martianrunner
Today at 5:11 AM
lgx57
Today at 6:20 AM
n! \prod f(i)^{-1.5} is divisible by 4 but not 8 2024 TMC AIME Mock #10
parmenides51   2
N Today at 5:11 AM by Amkan2022
Let $f(k)$ for positive integral values of $k$ be the largest perfect square that divides $k$. Find the number of positive integers $n$ less than $1024$ such that $$n! \cdot  \prod^n_{i=1}f(i)^{-1.5}$$is divisible by $4$ but not $8$.
2 replies
parmenides51
Saturday at 8:30 PM
Amkan2022
Today at 5:11 AM
point on the altitude
Pitagar   11
N Dec 24, 2023 by soryn
Source: Bulgaria NMO 2021P2
A point $T$ is given on the altitude through point $C$ in the acute triangle $ABC$ with circumcenter $O$, such that $\measuredangle TBA=\measuredangle ACB$. If the line $CO$ intersects side $AB$ at point $K$, prove that the perpendicular bisector of $AB$, the altitude through $A$ and the segment $KT$ are concurrent.
11 replies
Pitagar
May 16, 2021
soryn
Dec 24, 2023
point on the altitude
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G H BBookmark kLocked kLocked NReply
Source: Bulgaria NMO 2021P2
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Pitagar
67 posts
#1
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A point $T$ is given on the altitude through point $C$ in the acute triangle $ABC$ with circumcenter $O$, such that $\measuredangle TBA=\measuredangle ACB$. If the line $CO$ intersects side $AB$ at point $K$, prove that the perpendicular bisector of $AB$, the altitude through $A$ and the segment $KT$ are concurrent.
This post has been edited 2 times. Last edited by Pitagar, May 16, 2021, 2:32 PM
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hakN
429 posts
#2
Y by
Bad Solution:)
Does anybody have synthetic though?
This post has been edited 2 times. Last edited by hakN, May 17, 2021, 9:15 AM
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Marinchoo
407 posts
#3 • 3 Y
Y by Mango247, Mango247, Mango247
Yay, trig bash: Let the feet of the altitude from points $A$ and $C$ to $BC$ and $AB$ respectively are $H_{a}$ and $H_{c}$. Also let $H$ be the orthocenter of $\triangle ABC$ and $M$ be the midpoint of $AB$. Now let's call $O_{1}=AH_{a}\cap OM$ (I chose this name because if $B_{1}=BT\cap AC$, then the intersection point of the three lines is the circumcenter of $\triangle ABB_{1}$). We will prove that the point $O_{1}$ is the intersection point. This is equivalent to showing that $O_{1}\in KT$. However, using the Menelaus theorem for $\triangle AH_{c}H$ and the points $T\in HH_{c}$ , $K\in AH_{c}$ , $O_{1}\in AH$ we know that:
$$O_{1}\in KT\iff\left(\frac{HT}{TH_{c}}\right)\left(\frac{H_{c}K}{KA}\right)\left(\frac{AO_{1}}{O_{1}H}\right)=1$$Now $\frac{TH_{c}}{BH_{c}}=\tan{\gamma}=\frac{\sin{\gamma}}{\cos{\gamma}}$ and $\frac{HH_{c}}{BH_{c}}=\tan(90^{\circ}-\alpha)=\frac{\sin(90^{\circ}-\alpha)}{\cos(90^{\circ}-\alpha)}=\frac{\cos\alpha}{\sin\alpha}$, so:
$$\frac{HT}{TH_{c}}=1-\frac{HH_{c}}{TH_{c}}=1-\frac{\left(\frac{HH_{c}}{BH_{c}}\right)}{\left(\frac{TH_{c}}{BH_{c}}\right)}=1-\frac{(\cos\alpha)(\cos\gamma)}{(\sin\alpha)(\sin\gamma)}=\frac{(\sin\alpha)(\sin\gamma)-(\cos\alpha)(\cos\gamma)}{(\sin\alpha)(\sin\gamma)}$$$$\Longrightarrow \boxed{\frac{HT}{TH_{c}}=\frac{(\sin\alpha)(\sin\gamma)-(\cos\alpha)(\cos\gamma)}{(\sin\alpha)(\sin\gamma)}}$$Now $\frac{KH_{c}}{CH_{c}}=\tan(\beta-\alpha)=\frac{\sin(\beta-\alpha)}{\cos(\beta-\alpha)}=\frac{\sin\beta\cos\alpha-\sin\alpha\cos\beta}{\cos\alpha\cos\beta+\sin\alpha\sin\beta}$ and $\frac{AH_{c}}{CH_{c}}=\tan(90^{\circ}-\alpha)=\frac{\sin(90^{\circ}-\alpha)}{\cos(90^{\circ}-\alpha)}=\frac{\cos\alpha}{\sin\alpha}$, so:
$$\frac{KH_{c}}{AK}=\frac{KH_{c}}{AH_{c}-KH_{c}}=\frac{\left(\frac{\sin\beta\cos\alpha-\sin\alpha\cos\beta}{\cos\alpha\cos\beta+\sin\alpha\sin\beta}\right)}{\left(\frac{\cos\alpha}{\sin\alpha}\right)-\left(\frac{\sin\beta\cos\alpha-\sin\alpha\cos\beta}{\cos\alpha\cos\beta+\sin\alpha\sin\beta}\right)}=\frac{(\sin\alpha)(\sin\beta\cos\alpha-\sin\alpha\cos\beta)}{(\cos\alpha)(\cos\alpha\cos\beta+\sin\alpha\sin\beta)-(\sin\alpha)(\sin\beta\cos\alpha-\sin\alpha\cos\beta)}=$$$$=\frac{\sin\alpha(\sin\beta\cos\alpha-\sin\alpha\cos\beta)}{\cos\beta(\sin^{2}\alpha+\cos^{2}\alpha)}=\frac{\sin\alpha(\sin\beta\cos\alpha-\sin\alpha\cos\beta)}{\cos\beta}\Longrightarrow \boxed{\frac{KH_{c}}{AK}=\frac{\sin\alpha(\sin\beta\cos\alpha-\sin\alpha\cos\beta)}{\cos\beta}}$$Finally $\frac{AO_{1}}{O_{1}H}=\frac{AM}{MH_{c}}$, because $O_{1}M\perp AB\perp HH_{c}$. Now $BH_{c}=a\sin(90^{\circ}-\beta)=a\cos\beta$, so:
$$\frac{AO_{1}}{O_{1}H}=\frac{AM}{MH_{c}}=\frac{\left(\frac{AB}{2}\right)}{\left(\frac{AB}{2}\right)-BH_{c}}=\frac{\left(\frac{c}{2}\right)}{\left(\frac{c}{2}\right)-a\cos\beta}=\frac{2R\sin\gamma}{2R\sin\gamma-4R\sin\alpha\cos\beta}=\frac{\sin{\gamma}}{\sin\gamma-2\sin\alpha\cos\beta}\Longrightarrow \boxed{\frac{AO_{1}}{O_{1}H}=\frac{\sin{\gamma}}{\sin\gamma-2\sin\alpha\cos\beta}}$$$$\Longrightarrow \left(\frac{HT}{TH_{c}}\right)\left(\frac{H_{c}K}{KA}\right)\left(\frac{AO_{1}}{O_{1}H}\right)=\left(\frac{(\sin\alpha)(\sin\gamma)-(\cos\alpha)(\cos\gamma)}{(\sin\alpha)(\sin\gamma)}\right)\left(\frac{\sin\alpha(\sin\beta\cos\alpha-\sin\alpha\cos\beta)}{\cos\beta}\right)\left(\frac{\sin{\gamma}}{\sin\gamma-2\sin\alpha\cos\beta}\right)=$$$$=\left(\frac{(\sin\alpha)(\sin\gamma)-(\cos\alpha)(\cos\gamma)}{\cos\beta}\right)\left(\frac{\sin\beta\cos\alpha-\sin\alpha\cos\beta}{\sin\gamma-2\sin\alpha\cos\beta}\right)$$Now it's easy, because $\cos\gamma=\cos(180^{\circ}-\alpha-\beta)=\cos(180^{\circ}-\alpha)\cos(\beta)-\sin(180^{\circ}-\alpha)\sin\beta=\sin\alpha\sin\beta-\cos\alpha\cos\beta$, so $\frac{(\sin\alpha)(\sin\gamma)-(\cos\alpha)(\cos\gamma)}{\cos\beta}=1$ and $\sin\gamma=\sin\beta\cos\alpha+\sin\alpha\cos\beta$, so $\sin\gamma-2\sin\alpha\cos\beta=(\sin\beta\cos\alpha+\sin\alpha\cos\beta)-2\sin\alpha\cos\beta=\sin\beta\cos\alpha-\sin\alpha\cos\beta$, so the second fraction is also $1$, so we've solved the problem.
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Olympedias
504 posts
#4 • 1 Y
Y by Mango247
that looks like 2 hours and a lot of pain
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KST2003
173 posts
#5
Y by
I doubt there is a nice synthetic solution for this problem. (I might be very wrong.)

Let $E$ and $F$ be the foots of perpendiculars from $C$ and $A$. Let $M$ be the midpoint of $AB$, and let $\overline{OM}$ intersect $\overline{AF}$ at $X$. It suffices to show that $CT:TE = OX:XM$. Scale the diagram so that $(ABC)$ becomes the unit circle. Then by the extended law of sines, it is evident that $AM=\sin C$. Now some easy angle chasing shows that
\[\angle CBT = \angle B - \angle C = (90^\circ-\angle C)-(90^\circ-\angle B) = \angle OCF.\]By the ratio lemma, we have
\[\frac{CT}{TE}=\frac{\sin\angle BEC}{\sin\angle BCE}\cdot\frac{\sin\angle CBT}{\sin\angle TBE}=\frac{1}{\sin\angle XAM}\cdot\frac{\sin\angle OAM}{\sin C} = \frac{OA}{AM}\cdot\frac{\sin\angle OAM}{\sin\angle XAM}=\frac{OX}{XM},\]and we're done!
This post has been edited 2 times. Last edited by KST2003, May 16, 2021, 9:31 PM
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Marinchoo
407 posts
#6
Y by
Olympedias wrote:
that looks like 2 hours and a lot of pain

It did indeed take me about 2 hours in the competition, but the idea is straightforward, so it wasn't that bad, it just looks painful. Also, it was really clean: first, the denominator of the second ratio clears nicely into just $\cos\beta$ and after that, if you get the idea to turn the sides into trig functions using the sine law you are pretty much set as two cosines cancel and you finally get two fractions which are 1s, so I really enjoyed bashing this one actually.
This post has been edited 4 times. Last edited by Marinchoo, May 17, 2021, 4:45 AM
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Nofancyname
102 posts
#7
Y by
Bruh, I also agree that it looks rather ugly , I tried to take a point $B'$ on $BT$ such that $AB=AB'$ as then $B'\in (ABC)$ even though I'm not sure if $(ABC)$ is involved at all !
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VicKmath7
1389 posts
#8
Y by
Solution 1, length bash
This post has been edited 4 times. Last edited by VicKmath7, Sep 18, 2023, 2:24 PM
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VicKmath7
1389 posts
#9
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Let me bump this nice config geo with the completely synthetic solution I found today.
Solution 2, synthetic
This post has been edited 5 times. Last edited by VicKmath7, Sep 18, 2023, 4:27 PM
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gvole
201 posts
#10
Y by
DDIT on XOCT from B.
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Assassino9931
1280 posts
#11
Y by
Here is a trig free solution, pretty much as VicKmath7's first solution.

Let $M$ be the midpoint of $AB$ and let the altitude through $A$ intersect the perpendicular bisector $OM$ of $AB$ at $P$ - it suffices to show that $K$, $P$, $T$ are collinear. An efficient way to get rid of $K$ is as follows - we have $\triangle OKM \sim \triangle CKD$ and (as $OM \parallel CD$) the desired collinearity holds if and only if $P$ and $T$ are corresponding points in this similarity -- that is, we have reduced to showing $\displaystyle \frac{PM}{OM} = \frac{TD}{CD}$.

Since $\angle APM = 90^{\circ} - \angle PAM = \angle ABC = \beta$, we have $\triangle APM \sim \triangle CBD$ and so $\frac{PM}{AM} = \frac{BD}{CD}$. On the other hand, $\angle AOM = \frac{1}{2}\angle AOB = \angle ACB = \gamma = \angle TBD$ and so $\triangle AOM \sim \triangle TBD$, which gives $\frac{OM}{AM} = \frac{DT}{CD}$. Dividing the latter two equalities of ratios gives the result.
This post has been edited 2 times. Last edited by Assassino9931, Dec 24, 2023, 8:54 PM
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soryn
5337 posts
#12
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Thanks,guys!
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