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k a May Highlights and 2025 AoPS Online Class Information
jlacosta   0
May 1, 2025
May is an exciting month! National MATHCOUNTS is the second week of May in Washington D.C. and our Founder, Richard Rusczyk will be presenting a seminar, Preparing Strong Math Students for College and Careers, on May 11th.

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[*]May 21st, 4:00pm PT/7:00pm ET, Mathcamp 2025 Qualifying Quiz Part 2 Math Jam, Problems 5 and 6, Canada/USA Mathcamp staff will discuss solutions to Problems 5 and 6 of the 2025 Mathcamp Qualifying Quiz![/list]
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0 replies
jlacosta
May 1, 2025
0 replies
Floor double summation
CyclicISLscelesTrapezoid   53
N 21 minutes ago by ezpotd
Source: ISL 2021 A2
Which positive integers $n$ make the equation \[\sum_{i=1}^n \sum_{j=1}^n \left\lfloor \frac{ij}{n+1} \right\rfloor=\frac{n^2(n-1)}{4}\]true?
53 replies
CyclicISLscelesTrapezoid
Jul 12, 2022
ezpotd
21 minutes ago
Sets with Polynomials
insertionsort   27
N 36 minutes ago by ezpotd
Source: ISL 2020 A2
Let $\mathcal{A}$ denote the set of all polynomials in three variables $x, y, z$ with integer coefficients. Let $\mathcal{B}$ denote the subset of $\mathcal{A}$ formed by all polynomials which can be expressed as
\begin{align*}
(x + y + z)P(x, y, z) + (xy + yz + zx)Q(x, y, z) + xyzR(x, y, z)
\end{align*}with $P, Q, R \in \mathcal{A}$. Find the smallest non-negative integer $n$ such that $x^i y^j z^k \in \mathcal{B}$ for all non-negative integers $i, j, k$ satisfying $i + j + k \geq n$.
27 replies
insertionsort
Jul 20, 2021
ezpotd
36 minutes ago
Constructing two sets from conditions on their intersection, union and product
jbaca   17
N 36 minutes ago by MathLuis
Source: 2021 Iberoamerican Mathematical Olympiad, P5
For a finite set $C$ of integer numbers, we define $S(C)$ as the sum of the elements of $C$. Find two non-empty sets $A$ and $B$ whose intersection is empty, whose union is the set $\{1,2,\ldots, 2021\}$ and such that the product $S(A)S(B)$ is a perfect square.
17 replies
jbaca
Oct 20, 2021
MathLuis
36 minutes ago
Functional Inequality Implies Uniform Sign
peace09   34
N 37 minutes ago by MathIQ.
Source: 2023 ISL A2
Let $\mathbb{R}$ be the set of real numbers. Let $f:\mathbb{R}\rightarrow\mathbb{R}$ be a function such that \[f(x+y)f(x-y)\geqslant f(x)^2-f(y)^2\]for every $x,y\in\mathbb{R}$. Assume that the inequality is strict for some $x_0,y_0\in\mathbb{R}$.

Prove that either $f(x)\geqslant 0$ for every $x\in\mathbb{R}$ or $f(x)\leqslant 0$ for every $x\in\mathbb{R}$.
34 replies
peace09
Jul 17, 2024
MathIQ.
37 minutes ago
Ez comb proposed by ME
IEatProblemsForBreakfast   1
N Today at 3:09 PM by n1g3r14n
A and B play a game on two table:
1.At first one table got $n$ different coloured marbles on it and another one is empty
2.At each move player choose set of marbles that hadn't choose either players before and all chosen marbles from same table, and move all the marbles in that set to another table
3.Player who can not move lose
If A starts and they move alternatily who got the winning strategy?
1 reply
IEatProblemsForBreakfast
Today at 9:02 AM
n1g3r14n
Today at 3:09 PM
geometry
luckvoltia.112   0
Today at 3:04 PM
ChGiven an acute triangle ABC inscribed in circle $(O)$ The altitudes $BE, CF$ , intersect
each other at $H$. The tangents at $B$ and $C $of $(O)$ intersect at $S$. Let $M $be the midpoint of $BC$. $EM$ intersects $SC$
at $I$, $FM$ intersects $SB$ at $J.$
a) Prove that the points $I, S, M, J$ lie on the same circle.
b) The circle with diameter $AH$ intersects the circle $(O)$ at the second point $T.$ The line $AH$ intersects
$(O)$ at the second point $K$. Prove that $S,K,T$ are collinear.
0 replies
luckvoltia.112
Today at 3:04 PM
0 replies
Exponents of integer question
Dheckob   4
N Today at 2:45 PM by LeYohan
Find the smallest positive integer $m$ such that $5m$ is an exact 5th power, $6m$ is an exact 6th power, and $7m$ is an exact 7th power.
4 replies
Dheckob
Apr 12, 2017
LeYohan
Today at 2:45 PM
ISI 2025
Zeroin   0
Today at 2:29 PM
Let $\mathbb{N}$ denote the set of natural numbers and let $(a_i,b_i),1 \leq i \leq 9$ denote $9$ ordered pairs in $\mathbb{N} \times \mathbb{N}$. Prove that there exist $3$ distinct elements in the set $2^{a_i}3^{b_i}$ for $1 \leq i \leq 9$ whose product is a perfect cube.
0 replies
Zeroin
Today at 2:29 PM
0 replies
Inequalities
sqing   3
N Today at 1:49 PM by sqing
Let $ a,b>0   $ . Prove that
$$ \frac{a}{a^2+a +2b+1}+ \frac{b}{b^2+2a +b+1}  \leq  \frac{2}{5} $$$$ \frac{a}{a^2+2a +b+1}+ \frac{b}{b^2+a +2b+1}  \leq  \frac{2}{5} $$
3 replies
sqing
May 13, 2025
sqing
Today at 1:49 PM
Pertenacious Polynomial Problem
BadAtCompetitionMath21420   5
N Today at 1:17 PM by BadAtCompetitionMath21420
Let the polynomial $P(x) = x^3-x^2+px-q$ have real roots and real coefficients with $q>0$. What is the maximum value of $p+q$?

This is a problem I made for my math competition, and I wanted to see if someone would double-check my work (No Mike allowed):

solution
Is this solution good?
5 replies
BadAtCompetitionMath21420
Yesterday at 3:13 AM
BadAtCompetitionMath21420
Today at 1:17 PM
Max and min of ab+bc+ca-abc
Tiira   5
N Today at 1:01 PM by sqing
a, b and c are three non-negative reel numbers such that a+b+c=1.
What are the extremums of
ab+bc+ca-abc
?
5 replies
Tiira
Jan 29, 2021
sqing
Today at 1:01 PM
2017 DMI Individual Round - Downtown Mathematics Invitational
parmenides51   14
N Today at 11:39 AM by SomeonecoolLovesMaths
p1. Compute the smallest positive integer $x$ such that $351x$ is a perfect cube.


p2. A four digit integer is chosen at random. What is the probability all $4$ digits are distinct?


p3. If $$\frac{\sqrt{x + 1}}{\sqrt{x}}+ \frac{\sqrt{x}}{\sqrt{x + 1}} =\frac52.$$Solve for $x$.


p4. In $\vartriangle ABC$, $AB = 13$, $BC = 14$, and $AC = 15$. Let $D$ be the point on $BC$ such that $AD \perp BC$, and let $E$ be the midpoint of $AD$. If $F$ is a point such that $CDEF$ is a rectangle, compute the area of $\vartriangle AEF$.


p5. Square $ABCD$ has a sidelength of $4$. Points $P$, $Q$, $R$, and $S$ are chosen on $AB$, $BC$, $CD$, and $AD$ respectively, such that $AP$, $BQ$, $CR$, and $DS$ are length $1$. Compute the area of quadrilateral $P QRS$.


p6. A sequence $a_n$ satisfies for all integers $n$, $$a_{n+1} = 3a_n - 2a_{n-1}.$$If $a_0 = -30$ and $a_1 = -29$, compute $a_{11}$.


p7. In a class, every child has either red hair, blond hair, or black hair. All but $20$ children have black hair, all but $17$ have red hair, and all but $5$ have blond hair. How many children are there in the class?


p8. An Akash set is a set of integers that does not contain two integers such that one divides the other. Compute the minimum positive integer $n$ such that the set $\{1, 2, 3, ..., 2017\}$ can be partitioned into n Akash subsets.


PS. You should use hide for answers. Collected here.
14 replies
parmenides51
Oct 2, 2023
SomeonecoolLovesMaths
Today at 11:39 AM
Range of a function
Pscgylotti   1
N Today at 9:24 AM by Mathzeus1024
Try to get the range of function $f(x)=cosx+\sqrt{cos^{2}x-4\sqrt{2}cosx+4sinx+9}$ :
1 reply
Pscgylotti
Jul 22, 2019
Mathzeus1024
Today at 9:24 AM
Inequalities
sqing   17
N Today at 9:05 AM by sqing
Let $ a,b,c>0 , a+b+c +abc=4$. Prove that
$$ \frac {a}{a^2+2}+\frac {b}{b^2+2}+\frac {c}{c^2+2} \leq 1$$Let $ a,b,c>0 , ab+bc+ca+abc=4$. Prove that
$$ \frac {a}{a^2+2}+\frac {b}{b^2+2}+\frac {c}{c^2+2} \leq 1$$
17 replies
sqing
May 15, 2025
sqing
Today at 9:05 AM
Polynomial approximation and intersections
egxa   2
N Apr 29, 2025 by iliya8788
Source: All Russian 2025 10.6
What is the smallest value of \( k \) such that for any polynomial \( f(x) \) of degree $100$ with real coefficients, there exists a polynomial \( g(x) \) of degree at most \( k \) with real coefficients such that the graphs of \( y = f(x) \) and \( y = g(x) \) intersect at exactly $100$ points?
2 replies
egxa
Apr 18, 2025
iliya8788
Apr 29, 2025
Polynomial approximation and intersections
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Source: All Russian 2025 10.6
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egxa
211 posts
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What is the smallest value of \( k \) such that for any polynomial \( f(x) \) of degree $100$ with real coefficients, there exists a polynomial \( g(x) \) of degree at most \( k \) with real coefficients such that the graphs of \( y = f(x) \) and \( y = g(x) \) intersect at exactly $100$ points?
This post has been edited 1 time. Last edited by egxa, Apr 18, 2025, 5:21 PM
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MathLuis
1539 posts
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Recall the well known analysis fact that if a polynomial has exactly $n$ real roots counting multiplicity then its derivative has exactly $n-1$ real roots counting multiplicity. So if $k \le 97$ the considering the $98$-th derivative of $f-g$ gives that for any three given reals $a,b,c$ we must have that $ax^2+bx+c$ always has two real root if $f-g$ had exactly $100$ different roots, thus a contradiction by picking suitable $a,b,c$ (my example would just be $a,c>0$ and $b=0$ lol).
Now we will show $k=98$ works, first clean all the polynomial of degree $98$ inside $f$ using $g$ and from there many a suitable pick, in fact $100$ does not matter so we can prove a slightly stronger claim using induction.
We show that it is possible to construct a polynomial of degree $n$ where only coefficients $x^{n}, x^{n-1}$ are not in our control, so that the polynomial has all real roots with no multiplicity and none of them are zero.
For $n=2$ we can obviously shift the parabola by a constant acording to what is needed due to discriminant criteria so we should be fine here, due to continuity in fact you can just avoid a root being zero or the resulting parabola being tangent to the $x$ axis, for $n=3$ you have something unfree of the form $x^2(ax+b)$ and so you pick some $c(ax+b)+d$ on the free part where $c,d$ have freedom acording to needs like if $b=0$ then $d \ne 0$ and could just be suficiently small given we put $c<0$ to have two roots and that in order to not have repeated roots or the cubic being tangent to the $x$ axis at any moment (continuity yay).
For $n=4$ you basically have control over a depressed cubic upon doing a similar setup as seen above just shift everything by a small constant at the end to satisfy all the given conditions needed and now we will show this is the kind of work you have to do.
Suppose it was true for $n=\ell$ then we prove it for $n=\ell+1$, notice the not free term is $x^{\ell}(ax+b)$ so we just use the free term to focusing on a polynomial of the kind $x^{\ell}+q(x)$ for $\deg q \le \ell-2$, notice here it might happen that $-b \cdot a^{-1}$ is a root twice then we just shift by a very small constant as needed whether is above or below, one of them should work by continuity and IVT, since have $0$ as a root is only a one thing thing on a small interval we can also use continuity to avoid this and still satisfy the no double roots or more condition, hence by using the inductive hypothesis this is also completed and thus our claim is proven.
Just throw the claim and use a suitable $g$ to win, and thus we are done :cool:.
This post has been edited 1 time. Last edited by MathLuis, Apr 19, 2025, 3:49 AM
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iliya8788
8 posts
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We claim that the answer is $\boxed{k=98}$.
Claim 1: $k>97$
Assume the contrary. there exists a polynomial $g(x)$ with degree less than $98$ such that $f(x)-g(x)$ has 100 roots for $f(x)=x^100$. Define $k$ as the degree of $g(x)$
Define $a_{1},...,a_{k}$ such that $f(x)-g(x)=x^{100}+a_{k}x^k+...+a_{0}$. Define $r_{1},...,r_{100}$ as the real roots of $f(x)-g(x)$.
By vieta's formula since the coefficient of $x^{99}$ and $x^{98}$ is equal to $0$: $\sum_{i=1}^{100}r_{i} = 0$ and $\sum_{i<j}^{}r_{i}r_{j} = 0 \implies \sum_{i=1}^{100}r_{i}^2 = 0 \implies r_{i}=0 \implies f(x)-g(x)=0$ which is a contradiction. It follows that $k>97$.
Claim 2: $k=98$
We can basically alter all the coefficients of $f(x)-g(x)$ except the coefficients of $x^{100}$ and $x^{99}$ so by vieta's formula our only restriction is the sum of the roots of the polynomial. So we just need to pick 100 arbitrary pairwise different real numbers such that their sum is equal to $-\frac{a_{99}}{a_{100}}$ with $a_{99}$ being the coefficient of $x^{99}$ and $a_{100}$ being the leading coefficient. Obviously this is possible. From here we just alter the other coefficients so that each one of the coefficients becomes equal to the coefficients of the polynomial that has all these $100$ numbers as roots multiplied by $a_{n}$ and so we are done. $\blacksquare$.
This post has been edited 2 times. Last edited by iliya8788, May 14, 2025, 9:54 AM
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