3D geometry theorem

by KAME06, Apr 21, 2025, 10:18 PM

Let $M$ a point in the space and $G$ the centroid of a tetrahedron $ABCD$. Prove that:
$$\frac{1}{4}(AB^2+AC^2+AD^2+BC^2+BD^2+CD^2)+4MG^2=MA^2+MB^2+MC^2+MD^2$$

domino question

by kjhgyuio, Apr 21, 2025, 10:02 PM

demonic monic polynomial problem

by iStud, Apr 21, 2025, 9:51 PM

(a) Let $P(x)$ be a monic polynomial so that there exists another real coefficients $Q(x)$ that satisfy
\[P(x^2-2)=P(x)Q(x)\]Determine all complex roots that are possible from $P(x)$
(b) For arbitrary polynomial $P(x)$ that satisfies (a), determine whether $P(x)$ should have real coefficients or not.

fun set problem

by iStud, Apr 21, 2025, 9:47 PM

Given a set $S$ with exactly 9 elements that is subset of $\{1,2,\dots,72\}$. Prove that there exist two subsets $A$ and $B$ that satisfy the following:
- $A$ and $B$ are non-empty subsets from $S$,
- the sum of all elements in each of $A$ and $B$ are equal, and
- $A\cap B$ is an empty subset.

Funny easy transcendental geo

by qwerty123456asdfgzxcvb, Apr 21, 2025, 7:23 PM

Let $\mathcal{S}$ be a logarithmic spiral centered at the origin (ie curve satisfying for any point $X$ on it, line $OX$ makes a fixed angle with the tangent to $\mathcal{S}$ at $X$). Let $\mathcal{H}$ be a rectangular hyperbola centered at the origin, scaled such that it is tangent to the logarithmic spiral at some point.

Prove that for a point $P$ on the spiral, the polar of $P$ wrt. $\mathcal{H}$ is tangent to the spiral.
This post has been edited 3 times. Last edited by qwerty123456asdfgzxcvb, 5 hours ago

Inequality with three conditions

by oVlad, Apr 21, 2025, 1:48 PM

Let $a,b,c$ be non-negative real numbers such that \[b+c\leqslant a+1,\quad c+a\leqslant b+1,\quad a+b\leqslant c+1.\]Prove that $a^2+b^2+c^2\leqslant 2abc+1.$

An easy FE

by oVlad, Apr 21, 2025, 1:36 PM

Determine all functions $f:\mathbb R\to\mathbb R$ such that \[f(xy-1)+f(x)f(y)=2xy-1,\]for any real numbers $x{}$ and $y{}.$

Interesting F.E

by Jackson0423, Apr 18, 2025, 4:12 PM

Show that there does not exist a function
\[
f : \mathbb{R}^+ \to \mathbb{R}
\]satisfying the condition that for all \( x, y \in \mathbb{R}^+ \),
\[
f(x + y^2) \geq f(x) + y.
\]

~Korea 2017 P7
This post has been edited 3 times. Last edited by Jackson0423, Yesterday at 3:23 PM
Reason: Sorry guys..

GCD Functional Equation

by pinetree1, Jun 25, 2019, 5:36 PM

Let $f: \mathbb Z\to \{1, 2, \dots, 10^{100}\}$ be a function satisfying
$$\gcd(f(x), f(y)) = \gcd(f(x), x-y)$$for all integers $x$ and $y$. Show that there exist positive integers $m$ and $n$ such that $f(x) = \gcd(m+x, n)$ for all integers $x$.

Ankan Bhattacharya

1. Algebra and sigma algebra on a set

by adityaguharoy, Feb 28, 2018, 3:28 PM

Definitions of algebra on a set and sigma algebra on a set

Let $X$ be an infinite set. And let $\mathcal{A}$ be the collection of all subsets $A$ of $X$ such that either $A$ or $A^{c} ( = X \setminus A)$ is finite. Prove that $\mathcal{A}$ is an algebra on $X$, but not a sigma algebra on $X$.

Proof



A related exercise :
Let $X$ be an uncountable set. Let $\mathcal{A}$ be the collection of all subsets $A$ of $X$ such that either $A$ is countable or $A^{c} ( = X \setminus A )$ is countable.
Is $\mathcal{A}$ a $\sigma$ algebra on $X$ ?

Answer
Hint to sketch
This post has been edited 2 times. Last edited by adityaguharoy, Feb 28, 2018, 4:16 PM

p^3 divides (a + b)^p - a^p - b^p

by 62861, Feb 23, 2017, 5:14 PM

Prove that there are infinitely many triples $(a, b, p)$ of positive integers with $p$ prime, $a < p$, and $b < p$, such that $(a + b)^p - a^p - b^p$ is a multiple of $p^3$.

Noam Elkies
This post has been edited 1 time. Last edited by 62861, May 18, 2018, 11:58 PM

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