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Geometry
youochange   2
N a few seconds ago by youochange
m:}
Let $\triangle ABC$ be a triangle inscribed in a circle, where the tangents to the circle at points $B$ and $C$ intersect at the point $P$. Let $M$ be a point on the arc $AC$ (not containing $B$) such that $M \neq A$ and $M \neq C$. Let the lines $BC$ and $AM$ intersect at point $K$. Let $P'$ be the reflection of $P$ with respect to the line $AM$. The lines $AP'$ and $PM$ intersect at point $Q$, and $PM$ intersects the circumcircle of $\triangle ABC$ again at point $N$.

Prove that the point $Q$ lies on the circumcircle of $\triangle ANK$.
2 replies
youochange
3 hours ago
youochange
a few seconds ago
Beautiful problem
luutrongphuc   20
N 4 minutes ago by r7di048hd3wwd3o3w58q
Let triangle $ABC$ be circumscribed about circle $(I)$, and let $H$ be the orthocenter of $\triangle ABC$. The circle $(I)$ touches line $BC$ at $D$. The tangent to the circle $(BHC)$ at $H$ meets $BC$ at $S$. Let $J$ be the midpoint of $HI$, and let the line $DJ$ meet $(I)$ again at $X$. The tangent to $(I)$ parallel to $BC$ meets the line $AX$ at $T$. Prove that $ST$ is tangent to $(I)$.
20 replies
+5 w
luutrongphuc
Apr 4, 2025
r7di048hd3wwd3o3w58q
4 minutes ago
Common tangent to diameter circles
Stuttgarden   1
N 25 minutes ago by jrpartty
Source: Spain MO 2025 P2
The cyclic quadrilateral $ABCD$, inscribed in the circle $\Gamma$, satisfies $AB=BC$ and $CD=DA$, and $E$ is the intersection point of the diagonals $AC$ and $BD$. The circle with center $A$ and radius $AE$ intersects $\Gamma$ in two points $F$ and $G$. Prove that the line $FG$ is tangent to the circles with diameters $BE$ and $DE$.
1 reply
Stuttgarden
Mar 31, 2025
jrpartty
25 minutes ago
Two Functional Inequalities
Mathdreams   2
N 25 minutes ago by kokcio
Source: 2025 Nepal Mock TST Day 2 Problem 2
Determine all functions $f : \mathbb{R} \rightarrow \mathbb{R}$ such that $$f(x) \le x^3$$and $$f(x + y) \le f(x) + f(y) + 3xy(x + y)$$for any real numbers $x$ and $y$.

(Miroslav Marinov, Bulgaria)
2 replies
Mathdreams
37 minutes ago
kokcio
25 minutes ago
FE with a lot of terms
MrHeccMcHecc   0
27 minutes ago
Determine all functions $f : \mathbb{R} \rightarrow \mathbb{R}$ such that for all $x,y \in \mathbb{R}$ $$f(x)f(y)+f(x+y)=xf(y)+yf(x)+f(xy)+x+y+1$$
0 replies
+2 w
MrHeccMcHecc
27 minutes ago
0 replies
Sum of Squares of Digits is Periodic
Mathdreams   1
N 35 minutes ago by kokcio
Source: 2025 Nepal Mock TST Day 1 Problem 2
For any positive integer $n$, let $f(n)$ denote the sum of squares of digits of $n$. Prove that the sequence $$f(n), f(f(n)), f(f(f(n))), \cdots$$is eventually periodic.

(Kritesh Dhakal, Nepal)
1 reply
Mathdreams
43 minutes ago
kokcio
35 minutes ago
Set Combo <-> Grid Combo
Mathdreams   0
35 minutes ago
Source: 2025 Nepal Mock TST Day 2 Problem 3
Consider an $n \times n$ grid, where $n$ is a composite integer.

The $n^2$ unit squares are divided up into $a$ disjoint sets of $b$ unit squares arbitrarily such that $ab = n^2$. Denote this family of sets as $S$.

The $n^2$ unit squares are again divided up into $c$ disjoint sets of $d$ unit squares arbitrarily such that $cd = n^2$. Denote this family of sets as $T$.

Is it necessarily possible to choose $\min(a,c)$ unit squares such that no two unit squares are in the same set of $S$ or the same set of $T$?

(Shining Sun, USA)
0 replies
Mathdreams
35 minutes ago
0 replies
Two Orthocenters and an Invariant Point
Mathdreams   0
41 minutes ago
Source: 2025 Nepal Mock TST Day 1 Problem 3
Let $\triangle{ABC}$ be a triangle, and let $P$ be an arbitrary point on line $AO$, where $O$ is the circumcenter of $\triangle{ABC}$. Define $H_1$ and $H_2$ as the orthocenters of triangles $\triangle{APB}$ and $\triangle{APC}$. Prove that $H_1H_2$ passes through a fixed point which is independent of the choice of $P$.

(Kritesh Dhakal, Nepal)
0 replies
Mathdreams
41 minutes ago
0 replies
Inspired by 2012 Romania and 2021 BH
sqing   0
43 minutes ago
Source: Own
Let $ a, b, c, d\geq 0 , bc + d + a = 5, cd + a + b = 2 $ and $ da + b + c = 6. $ Prove that
$$3\leq  ab + c + d\leq 2\sqrt{13}-1 $$$$5\leq a+ b+ c +d  \leq\frac{1}{2}(11+\sqrt{13})$$$$ \sqrt{13}+1  \leq   a b +bc+ c d+d a \leq 6$$
0 replies
sqing
43 minutes ago
0 replies
Ratios in a right triangle
PNT   1
N an hour ago by Mathzeus1024
Source: Own.
Let $ABC$ be a right triangle in $A$ with $AB<AC$. Let $M$ be the midpoint of $AB$ and $D$ a point on $AC$ such that $DC=DB$. Let $X=(BDC)\cap MD$.
Compute in terms of $AB,BC$ and $AC$ the ratio $\frac{BX}{DX}$.
1 reply
PNT
Jun 9, 2023
Mathzeus1024
an hour ago
3 var inquality
sqing   0
an hour ago
Source: Own
Let $ a,b,c>0 $ and $ \dfrac{a}{bc}+\dfrac{2b}{ca}+\dfrac{5c}{ab}\leq 12.$ Prove that$$ a^2+b^2+c^2\geq 1$$
0 replies
sqing
an hour ago
0 replies
inequality
pennypc123456789   6
N an hour ago by sqing
Let \( x, y \) be positive real numbers satisfying \( x + y = 2 \). Prove that

\[
3(x^{\frac{2}{3}} + y^{\frac{2}{3}}) \geq 4 + 2x^{\frac{1}{3}}y^{\frac{1}{3}}.
\]
6 replies
1 viewing
pennypc123456789
Mar 24, 2025
sqing
an hour ago
Geometry with known distances
nAalniaOMliO   1
N Mar 29, 2025 by RagvaloD
Source: Belarusian National Olympiad 2025
In a rectangle $ABCD$ two not intersecting circles $\omega_1$ and $\omega_2$ are drawn such that $\omega_1$ is tangent to $AB$ and $AD$ at points $P$ and $S$ respectively, and $\omega_2$ is tangent to $CB$ and $CD$ at $T$ and $Q$ respectively. It is known that $PQ=11, ST=10, BD=14$.
Find the distance between centers of circles $\omega_1$ and $\omega_2$.
1 reply
nAalniaOMliO
Mar 28, 2025
RagvaloD
Mar 29, 2025
Geometry with known distances
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G H BBookmark kLocked kLocked NReply
Source: Belarusian National Olympiad 2025
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nAalniaOMliO
295 posts
#1
Y by
In a rectangle $ABCD$ two not intersecting circles $\omega_1$ and $\omega_2$ are drawn such that $\omega_1$ is tangent to $AB$ and $AD$ at points $P$ and $S$ respectively, and $\omega_2$ is tangent to $CB$ and $CD$ at $T$ and $Q$ respectively. It is known that $PQ=11, ST=10, BD=14$.
Find the distance between centers of circles $\omega_1$ and $\omega_2$.
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RagvaloD
4894 posts
#2
Y by
Easy to see that $d^2=PQ^2+ST^2-BD^2=25 \to d=5$
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