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k a June Highlights and 2025 AoPS Online Class Information
jlacosta   0
Jun 2, 2025
Congratulations to all the mathletes who competed at National MATHCOUNTS! If you missed the exciting Countdown Round, you can watch the video at this link. Are you interested in training for MATHCOUNTS or AMC 10 contests? How would you like to train for these math competitions in half the time? We have accelerated sections which meet twice per week instead of once starting on July 8th (7:30pm ET). These sections fill quickly so enroll today!

[list][*]MATHCOUNTS/AMC 8 Basics
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0 replies
jlacosta
Jun 2, 2025
0 replies
One of the lines is tangent
Rijul saini   8
N 18 minutes ago by ihategeo_1969
Source: LMAO 2025 Day 2 Problem 2
Let $ABC$ be a scalene triangle with incircle $\omega$. Denote by $N$ the midpoint of arc $BAC$ in the circumcircle of $ABC$, and by $D$ the point where the $A$-excircle touches $BC$. Suppose the circumcircle of $AND$ meets $BC$ again at $P \neq D$ and intersects $\omega$ at two points $X$, $Y$.

Prove that either $PX$ or $PY$ is tangent to $\omega$.

Proposed by Sanjana Philo Chacko
8 replies
Rijul saini
Wednesday at 7:02 PM
ihategeo_1969
18 minutes ago
Tricky coloured subgraphs
bomberdoodles   2
N 23 minutes ago by bomberdoodles
Consider a graph with nine vertices, with the vertices labelled 1 through 9. An
edge is drawn between each pair of vertices.

Sally picks any edge of her choice, and colours that edge either red or blue. She keeps repeating
this process, choosing any uncoloured edge, and colouring that edge either red or blue.
The only rule is that she is never allowed to colour an edge either red or blue so that one
of these scenarios occurs:

(i) There exist three numbers $a, b, c$, with $1 \le a < b < c \le 9$, for which the edges $ab, bc, ac$ are
all coloured red.

(ii) There exist four numbers $p, q, r, s,$ with $1 \le p < q < r < s \le 9$, for which the edges $pq, pr,
ps, qr, qs, rs$ are all coloured blue.

For example, suppose Sally starts by choosing edges 14 and 34, and colouring both of these
edges red. Then if she picks edge 13, she must colour this edge blue, because she cannot colour
it red.

What is the maximum number of edges that Sally can colour?
2 replies
bomberdoodles
6 hours ago
bomberdoodles
23 minutes ago
x^2+6x+33 is perfect square
Demetres   6
N 26 minutes ago by thdwlgh1229
Source: Cyprus 2022 Junior TST-1 Problem 1
Find all integer values of $x$ for which the value of the expression
\[x^2+6x+33\]is a perfect square.
6 replies
Demetres
Feb 21, 2022
thdwlgh1229
26 minutes ago
IMO ShortList 2002, number theory problem 6
orl   33
N 28 minutes ago by lksb
Source: IMO ShortList 2002, number theory problem 6
Find all pairs of positive integers $m,n\geq3$ for which there exist infinitely many positive integers $a$ such that \[ \frac{a^m+a-1}{a^n+a^2-1}  \] is itself an integer.

Laurentiu Panaitopol, Romania
33 replies
orl
Sep 28, 2004
lksb
28 minutes ago
Problem 43: Balkan MO Shortlist 2003
henderson   0
Jul 29, 2016
$$\color{red}\bf{Problem \ 43}$$Two circles $\Gamma_{1}$ and $\Gamma_2$ with radii $r_1$ and $r_2$ $(r_2>r_1),$ respectively are externally tangent. The straight line $t_1$ is tangent to the circles $\Gamma_1$ and $\Gamma_2$ at points $A$ and $D,$ respectively.The parallel line $t_2$ to the line $t_1$ is tangent to the circle $\Gamma_1$ and intersects $\Gamma_2$ at points $E$ and $F.$ The line $t_3$ through $D$ intersects the line $ t_2$ and the circle $\Gamma_2$ at points $B$ and $C,$ respectively, different from $E$ and $F.$ Prove that the circumcircle of triangle $ABC$ is tangent to the line $t_1.$
(Balkan MO Shortlist, 2003)
0 replies
henderson
Jul 29, 2016
0 replies
No more topics!
Easy geo
oVlad   3
N Apr 21, 2025 by Primeniyazidayi
Source: Romania EGMO TST 2019 Day 1 P1
A line through the vertex $A{}$ of the triangle $ABC{}$ which doesn't coincide with $AB{}$ or $AC{}$ intersectes the altitudes from $B{}$ and $C{}$ at $D{}$ and $E{}$ respectively. Let $F{}$ be the reflection of $D{}$ in $AB{}$ and $G{}$ be the reflection of $E{}$ in $AC{}.$ Prove that the circles $ABF{}$ and $ACG{}$ are tangent.
3 replies
oVlad
Apr 21, 2025
Primeniyazidayi
Apr 21, 2025
Easy geo
G H J
G H BBookmark kLocked kLocked NReply
Source: Romania EGMO TST 2019 Day 1 P1
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oVlad
1746 posts
#1
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A line through the vertex $A{}$ of the triangle $ABC{}$ which doesn't coincide with $AB{}$ or $AC{}$ intersectes the altitudes from $B{}$ and $C{}$ at $D{}$ and $E{}$ respectively. Let $F{}$ be the reflection of $D{}$ in $AB{}$ and $G{}$ be the reflection of $E{}$ in $AC{}.$ Prove that the circles $ABF{}$ and $ACG{}$ are tangent.
Z K Y
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kokcio
71 posts
#2
Y by
Let $X$ be intersection of $AF$ and $CE$. Then we have that $\angle AXC = \angle XEA =\pi - \angle AEC = \pi - \angle AGC$. Hence, $AXCG$ is cyclic. We can also see that $\angle ACX = \angle ABD = \angle ABF$, so by theorem about angle between chord and tangent, we know that circles on $AXC$, $ABF$ have common tangent.
Z K Y
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Gggvds1
10 posts
#3
Y by
Let the line passing through the vertex A intersect BC at M and angleBAM is greater than angleCAM.
Let angle BAM be x ,then angleFAB become x and angle CAM becomes A-x. It is easy to see that angle FBA is equals to angle GCA are 90-A. Then angle AFB is 90+A-x and angleAGC is 90+x.
Let l be a line tangent to circumcircle of triangle AGC. Let P be a point on l ahead of A in direction of G. Then angleGAF becomes 90 -A. Also angle BAF is angle BAC +angle GAC+angle GAF. Therefore angle BAF becomes 90+A-x that is equal to angle BFA.

Hence the line l is also tangent to circumcircle of triangle BFA. This implies that these two circle are tangent to each other.
This post has been edited 1 time. Last edited by Gggvds1, Apr 21, 2025, 3:38 PM
Z K Y
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Primeniyazidayi
119 posts
#4
Y by
Let H be the intersection of BD and AG. Note that H is the reflection of D on AC. Then by angle chasing it can be proven that (AFBH) exists(A, F, B, H are concyclic). Let X be the intersection point of BD and the line which is tangent to (AFB)and $H_C$ be foot of perpendicular line of C. Then $$\angle XAH=\angle ABH=\angle ACH_C=\angle ACG$$done.
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