Inspired by SunnyEvan

by sqing, May 17, 2025, 2:59 PM

Let $ a,b,c   $ be reals such that $ a^2+b^2+c^2=2(ab+bc+ca). $ Prove that$$ \frac{1}{12} \leq \frac{a^2b+b^2c+c^2a}{(a+b+c)^3} \leq \frac{5}{36} $$Let $ a,b,c   $ be reals such that $ a^2+b^2+c^2=5(ab+bc+ca). $ Prove that$$ -\frac{1}{25} \leq \frac{a^3b+b^3c+c^3a}{(a^2+b^2+c^2)^2} \leq \frac{197}{675} $$
This post has been edited 3 times. Last edited by sqing, a few seconds ago

A circle tangent to the circumcircle, excircles related

by kosmonauten3114, May 17, 2025, 1:12 PM

Let $ABC$ be a scalene triangle with excircles $\odot(I_A)$, $\odot(I_B)$, $\odot(I_C)$. Let $\odot(A')$ be the circle which touches $\odot(I_B)$ and $\odot(I_C)$ and passes through $A$, and whose center $A'$ lies outside of the excentral triangle of $\triangle{ABC}$. Define $\odot(B')$ and $\odot(C')$ cyclically. Let $\odot(O')$ be the circle externally tangent to $\odot(A')$, $\odot(B')$, $\odot(C')$.

Prove that $\odot(O')$ is tangent to the circumcircle of $\triangle{ABC}$ at the anticomplement of the Feuerbach point of $\triangle{ABC}$.
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3 var inequality

by SunnyEvan, May 17, 2025, 1:05 PM

Let $ a,b,c \in R $ ,such that $ a^2+b^2+c^2=4(ab+bc+ca)$Prove that :$$ \frac{53}{2}-9\sqrt{14} \leq \frac{8(a^3b+b^3c+c^3a)}{27(a^2+b^2+c^2)^2} \leq \frac{53}{2}+9\sqrt{14} $$
This post has been edited 2 times. Last edited by SunnyEvan, 2 hours ago

Insspired by Shandong 2025

by sqing, May 17, 2025, 9:23 AM

Let $ a,b,c>0,abc>1$. Prove that$$ \frac {abc(a+b+c+ab+bc+ca+3)}{  abc-1}\geq \frac {81}{4}$$$$  \frac {abc(a+b+c+ab+bc+ca+abc+2)}{  abc-1}\geq  12+8\sqrt{2}$$
This post has been edited 1 time. Last edited by sqing, 6 hours ago

Find the minimum

by sqing, May 17, 2025, 9:12 AM

Let $ a,b,c>0,abc>1$. Find the minimum value of $ \frac {abc(a+b+c+8)}{abc-1}. $

A point on BC

by jayme, May 17, 2025, 6:08 AM

Dear Mathlinkers,

1. ABC a triangle
2. 0 the circumcircle
3. D the pole of BC wrt 0
4. B', C' the symmetrics of B, C wrt AC, AB
5. 1b, 1c the circumcircles of the triangles BB'D, CC'D
6. T the second point of intersection of the tangent to 1c at D with 1b.

Prove : B, C and T are collinear.

Sincerely
Jean-Louis

Prove that the triangle is isosceles.

by TUAN2k8, May 16, 2025, 9:55 AM

Given acute triangle $ABC$ with two altitudes $CF$ and $BE$.Let $D$ be the point on the line $CF$ such that $DB \perp BC$.The lines $AD$ and $EF$ intersect at point $X$, and $Y$ is the point on segment $BX$ such that $CY \perp BY$.Suppose that $CF$ bisects $BE$.Prove that triangle $ACY$ is isosceles.

Bound of number of connected components

by a_507_bc, Aug 12, 2023, 12:44 PM

Let $G$ be a connected graph and let $X, Y$ be two disjoint subsets of its vertices, such that there are no edges between them. Given that $G/X$ has $m$ connected components and $G/Y$ has $n$ connected components, what is the minimal number of connected components of the graph $G/(X \cup Y)$?
This post has been edited 3 times. Last edited by a_507_bc, Aug 13, 2023, 7:00 AM

Israel Number Theory

by mathisreaI, Jul 13, 2022, 2:55 AM

Find all triples $(a,b,p)$ of positive integers with $p$ prime and \[ a^p=b!+p. \]
This post has been edited 2 times. Last edited by v_Enhance, Jul 13, 2023, 6:28 PM
Reason: missing full stop

Bounds on degree of polynomials

by Phorphyrion, Jun 11, 2022, 11:34 AM

For each positive integer $n$, define $f(n)$ to be the least positive integer for which the following holds:

For any partition of $\{1,2,\dots, n\}$ into $k>1$ disjoint subsets $A_1, \dots, A_k$, all of the same size, let $P_i(x)=\prod_{a\in A_i}(x-a)$. Then there exist $i\neq j$ for which
\[\deg(P_i(x)-P_j(x))\geq \frac{n}{k}-f(n)\]
a) Prove that there is a constant $c$ so that $f(n)\le c\cdot \sqrt{n}$ for all $n$.

b) Prove that for infinitely many $n$, one has $f(n)\ge \ln(n)$.

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