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k a April Highlights and 2025 AoPS Online Class Information
jlacosta   0
Apr 2, 2025
Spring is in full swing and summer is right around the corner, what are your plans? At AoPS Online our schedule has new classes starting now through July, so be sure to keep your skills sharp and be prepared for the Fall school year! Check out the schedule of upcoming classes below.

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[*]April 22nd (Webinar), 4:00pm PT/7:00pm ET, Competitive Programming at AoPS (USACO).[/list]
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0 replies
jlacosta
Apr 2, 2025
0 replies
k i Adding contests to the Contest Collections
dcouchman   1
N Apr 5, 2023 by v_Enhance
Want to help AoPS remain a valuable Olympiad resource? Help us add contests to AoPS's Contest Collections.

Find instructions and a list of contests to add here: https://artofproblemsolving.com/community/c40244h1064480_contests_to_add
1 reply
dcouchman
Sep 9, 2019
v_Enhance
Apr 5, 2023
k i Zero tolerance
ZetaX   49
N May 4, 2019 by NoDealsHere
Source: Use your common sense! (enough is enough)
Some users don't want to learn, some other simply ignore advises.
But please follow the following guideline:


To make it short: ALWAYS USE YOUR COMMON SENSE IF POSTING!
If you don't have common sense, don't post.


More specifically:

For new threads:


a) Good, meaningful title:
The title has to say what the problem is about in best way possible.
If that title occured already, it's definitely bad. And contest names aren't good either.
That's in fact a requirement for being able to search old problems.

Examples:
Bad titles:
- "Hard"/"Medium"/"Easy" (if you find it so cool how hard/easy it is, tell it in the post and use a title that tells us the problem)
- "Number Theory" (hey guy, guess why this forum's named that way¿ and is it the only such problem on earth¿)
- "Fibonacci" (there are millions of Fibonacci problems out there, all posted and named the same...)
- "Chinese TST 2003" (does this say anything about the problem¿)
Good titles:
- "On divisors of a³+2b³+4c³-6abc"
- "Number of solutions to x²+y²=6z²"
- "Fibonacci numbers are never squares"


b) Use search function:
Before posting a "new" problem spend at least two, better five, minutes to look if this problem was posted before. If it was, don't repost it. If you have anything important to say on topic, post it in one of the older threads.
If the thread is locked cause of this, use search function.

Update (by Amir Hossein). The best way to search for two keywords in AoPS is to input
[code]+"first keyword" +"second keyword"[/code]
so that any post containing both strings "first word" and "second form".


c) Good problem statement:
Some recent really bad post was:
[quote]$lim_{n\to 1}^{+\infty}\frac{1}{n}-lnn$[/quote]
It contains no question and no answer.
If you do this, too, you are on the best way to get your thread deleted. Write everything clearly, define where your variables come from (and define the "natural" numbers if used). Additionally read your post at least twice before submitting. After you sent it, read it again and use the Edit-Button if necessary to correct errors.


For answers to already existing threads:


d) Of any interest and with content:
Don't post things that are more trivial than completely obvious. For example, if the question is to solve $x^{3}+y^{3}=z^{3}$, do not answer with "$x=y=z=0$ is a solution" only. Either you post any kind of proof or at least something unexpected (like "$x=1337, y=481, z=42$ is the smallest solution). Someone that does not see that $x=y=z=0$ is a solution of the above without your post is completely wrong here, this is an IMO-level forum.
Similar, posting "I have solved this problem" but not posting anything else is not welcome; it even looks that you just want to show off what a genius you are.

e) Well written and checked answers:
Like c) for new threads, check your solutions at least twice for mistakes. And after sending, read it again and use the Edit-Button if necessary to correct errors.



To repeat it: ALWAYS USE YOUR COMMON SENSE IF POSTING!


Everything definitely out of range of common sense will be locked or deleted (exept for new users having less than about 42 posts, they are newbies and need/get some time to learn).

The above rules will be applied from next monday (5. march of 2007).
Feel free to discuss on this here.
49 replies
ZetaX
Feb 27, 2007
NoDealsHere
May 4, 2019
Number of lucky numbers
NO_SQUARES   0
12 minutes ago
Source: Kvant 2025 no. 2 M2830
There are coins in denominations of $a$ and $b$ doubloons, where $a$ and $b$ are given mutually prime natural numbers, with $a < b < 100$. A non-negative integer $n$ is called lucky if the sum in $n$ doubloons can be scored with using no more than $1000$ coins. Find the number of lucky numbers.
From the folklore
0 replies
NO_SQUARES
12 minutes ago
0 replies
Woaah a lot of external tangents
egxa   3
N an hour ago by NO_SQUARES
Source: All Russian 2025 11.7
A quadrilateral \( ABCD \) with no parallel sides is inscribed in a circle \( \Omega \). Circles \( \omega_a, \omega_b, \omega_c, \omega_d \) are inscribed in triangles \( DAB, ABC, BCD, CDA \), respectively. Common external tangents are drawn between \( \omega_a \) and \( \omega_b \), \( \omega_b \) and \( \omega_c \), \( \omega_c \) and \( \omega_d \), and \( \omega_d \) and \( \omega_a \), not containing any sides of quadrilateral \( ABCD \). A quadrilateral whose consecutive sides lie on these four lines is inscribed in a circle \( \Gamma \). Prove that the lines joining the centers of \( \omega_a \) and \( \omega_c \), \( \omega_b \) and \( \omega_d \), and the centers of \( \Omega \) and \( \Gamma \) all intersect at one point.
3 replies
egxa
Apr 18, 2025
NO_SQUARES
an hour ago
2023 Hong Kong TST 3 (CHKMO) Problem 4
PikaNiko   3
N an hour ago by lightsynth123
Source: 2023 Hong Kong TST 3 (CHKMO)
Let $ABCD$ be a quadrilateral inscribed in a circle $\Gamma$ such that $AB=BC=CD$. Let $M$ and $N$ be the midpoints of $AD$ and $AB$ respectively. The line $CM$ meets $\Gamma$ again at $E$. Prove that the tangent at $E$ to $\Gamma$, the line $AD$ and the line $CN$ are concurrent.
3 replies
PikaNiko
Dec 3, 2022
lightsynth123
an hour ago
Continuity of function and line segment of integer length
egxa   2
N an hour ago by NO_SQUARES
Source: All Russian 2025 11.8
Let \( f: \mathbb{R} \to \mathbb{R} \) be a continuous function. A chord is defined as a segment of integer length, parallel to the x-axis, whose endpoints lie on the graph of \( f \). It is known that the graph of \( f \) contains exactly \( N \) chords, one of which has length 2025. Find the minimum possible value of \( N \).
2 replies
egxa
Apr 18, 2025
NO_SQUARES
an hour ago
Disjoint Pairs
MithsApprentice   41
N an hour ago by NerdyNashville
Source: USAMO 1998
Suppose that the set $\{1,2,\cdots, 1998\}$ has been partitioned into disjoint pairs $\{a_i,b_i\}$ ($1\leq i\leq 999$) so that for all $i$, $|a_i-b_i|$ equals $1$ or $6$. Prove that the sum \[ |a_1-b_1|+|a_2-b_2|+\cdots +|a_{999}-b_{999}|  \] ends in the digit $9$.
41 replies
MithsApprentice
Oct 9, 2005
NerdyNashville
an hour ago
5 Ways to Reach Expedia Customer Service by Phone, Chat, and Email Methods
RichaSheoran   0
an hour ago
To reach a live person at Expedia customer service for support, you can call their 24/7 Expedia Phone number hotline at 1 = 860 = 540 = 0471 . OTA (Live Person) or 1-800-Expedia 1 = 860 = 540 = 0471 . You can also use the live chat feature on their website or reach out to them via email. Speaking with a live representative at Expedia is straightforward . Whether you're dealing with booking issues, need to make changes to your travel plans, or have specific inquiries, reaching out to a live agent can quickly resolve your concerns. This guide explains the steps to contact Expedia customer service via phone and provides tips on the best times to call to minimize wait times.
0 replies
RichaSheoran
an hour ago
0 replies
Help my diagram has too many points
MarkBcc168   27
N an hour ago by Om245
Source: IMO Shortlist 2023 G6
Let $ABC$ be an acute-angled triangle with circumcircle $\omega$. A circle $\Gamma$ is internally tangent to $\omega$ at $A$ and also tangent to $BC$ at $D$. Let $AB$ and $AC$ intersect $\Gamma$ at $P$ and $Q$ respectively. Let $M$ and $N$ be points on line $BC$ such that $B$ is the midpoint of $DM$ and $C$ is the midpoint of $DN$. Lines $MP$ and $NQ$ meet at $K$ and intersect $\Gamma$ again at $I$ and $J$ respectively. The ray $KA$ meets the circumcircle of triangle $IJK$ again at $X\neq K$.

Prove that $\angle BXP = \angle CXQ$.

Kian Moshiri, United Kingdom
27 replies
MarkBcc168
Jul 17, 2024
Om245
an hour ago
Discuss the Stanford Math Tournament Here
Aaronjudgeisgoat   299
N 2 hours ago by techb
I believe discussion is allowed after yesterday at midnight, correct?
If so, I will put tentative answers on this thread.
By the way, does anyone know the answer to Geometry Problem 5? I was wondering if I got that one right
Also, if you put answers, please put it in a hide tag

Answers for the Algebra Subject Test
Estimated Algebra Cutoffs
Answers for the Geometry Subject Test
Estimated Geo Cutoffs
Answers for the Discrete Subject Test
Estimated Cutoffs for Discrete
Answers for the Team Round
Guts Answers
299 replies
Aaronjudgeisgoat
Apr 14, 2025
techb
2 hours ago
Geometry, SMO 2016, not easy
Zoom   18
N 2 hours ago by SimplisticFormulas
Source: Serbia National Olympiad 2016, day 1, P3
Let $ABC$ be a triangle and $O$ its circumcentre. A line tangent to the circumcircle of the triangle $BOC$ intersects sides $AB$ at $D$ and $AC$ at $E$. Let $A'$ be the image of $A$ under $DE$. Prove that the circumcircle of the triangle $A'DE$ is tangent to the circumcircle of triangle $ABC$.
18 replies
Zoom
Apr 1, 2016
SimplisticFormulas
2 hours ago
A touching question on perpendicular lines
Tintarn   2
N 2 hours ago by pi_quadrat_sechstel
Source: Bundeswettbewerb Mathematik 2025, Round 1 - Problem 3
Let $k$ be a semicircle with diameter $AB$ and midpoint $M$. Let $P$ be a point on $k$ different from $A$ and $B$.

The circle $k_A$ touches $k$ in a point $C$, the segment $MA$ in a point $D$, and additionally the segment $MP$. The circle $k_B$ touches $k$ in a point $E$ and additionally the segments $MB$ and $MP$.

Show that the lines $AE$ and $CD$ are perpendicular.
2 replies
Tintarn
Mar 17, 2025
pi_quadrat_sechstel
2 hours ago
RIP BS2012
gavinhaominwang   12
N 3 hours ago by KevinYang2.71
Rip BS2012, I hope you come back next year stronger and prove everyone wrong.
12 replies
gavinhaominwang
Today at 12:32 AM
KevinYang2.71
3 hours ago
Some nice summations
amitwa.exe   31
N 3 hours ago by soryn
Problem 1: $\Omega=\left(\sum_{0\le i\le j\le k}^{\infty} \frac{1}{3^i\cdot4^j\cdot5^k}\right)\left(\mathop{{\sum_{i=0}^{\infty}\sum_{j=0}^{\infty}\sum_{k=0}^{\infty}}}_{i\neq j\neq k}\frac{1}{3^i\cdot3^j\cdot3^k}\right)=?$
31 replies
amitwa.exe
May 24, 2024
soryn
3 hours ago
Interesting inequalities
sqing   0
3 hours ago
Source: Own
Let $ a,b,c\geq 0 ,b+c-ca=1 $ and $ c+a-ab=3.$ Prove that
$$a+\frac{19}{10}b-bc\leq 2-\sqrt 2$$$$a+\frac{17}{10}b+c-bc\leq  3$$$$ a^2+\frac{9}{5}b-bc\leq 6-4\sqrt 2$$$$ a^2+\frac{8}{5}b^2-bc\leq 6-4\sqrt 2$$$$a+1.974873b-bc\leq 2-\sqrt 2$$$$a+1.775917b+c-bc\leq  3$$

0 replies
1 viewing
sqing
3 hours ago
0 replies
How many people get waitlisted st promys?
dragoon   26
N 4 hours ago by wuwang2002
Asking for a friend here
26 replies
dragoon
Apr 18, 2025
wuwang2002
4 hours ago
Prove a polynomial has a nonreal root
KevinYang2.71   46
N 6 hours ago by megarnie
Source: USAMO 2025/2
Let $n$ and $k$ be positive integers with $k<n$. Let $P(x)$ be a polynomial of degree $n$ with real coefficients, nonzero constant term, and no repeated roots. Suppose that for any real numbers $a_0,\,a_1,\,\ldots,\,a_k$ such that the polynomial $a_kx^k+\cdots+a_1x+a_0$ divides $P(x)$, the product $a_0a_1\cdots a_k$ is zero. Prove that $P(x)$ has a nonreal root.
46 replies
KevinYang2.71
Mar 20, 2025
megarnie
6 hours ago
Prove a polynomial has a nonreal root
G H J
G H BBookmark kLocked kLocked NReply
Source: USAMO 2025/2
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golue3120
57 posts
#36 • 2 Y
Y by Ilikeminecraft, Math4Life2020
Here, we have a solution without WLOG $k=n-1$, without Descartes rule of signs, without Rolle, but is somehow not terrible?

Let $e_k$ denote the degree $k$ elementary symmetric polynomial.

Suppose for the sake of contradiction that all roots of $P(x)$ were real. Let the roots be $r_1,r_2,\dotsc,r_n$. Let $R$ be the set of roots. Then for every subset $S$ of $k$ roots, $e_i(S)=0$ for some $1\le i\le k-1$.

Define two subsets of $k$ elements to be adjacent if they share $k-1$ elements.
Lemma. If $\mathcal F$ is a family of $k$-element subsets of $R$ such that no two are adjacent, then $|\mathcal F|\le\frac{1}{k+1}\binom nk$.
Proof. Every set $T$ in $\mathcal F$ is adjacent to exactly $k(n-k)$ sets outside of $\mathcal F$ because we may pick any element of $T$ to remove and any root not in $T$ to add. Meanwhile, any set $U$ not in $\mathcal F$ is adjacent to at most $n-k$ sets in $\mathcal F$ because when we remove an element of $U$ and add a root not in $U$, the additional roots for the adjacent sets in $\mathcal F$ must be distinct. Counting then gives the desired result.

Now define $\mathcal F_i$ to be the family of $k$-element subsets of roots such that $e_i$ vanishes. Then there are $k-1$ such families, and every size $k$ subset of the roots is in at least one family, so one family must have size at least $\frac{1}{k-1}\binom nk$. Therefore, at least one family must contain two adjacent subsets. This implies that there are roots $r_1,r_2,\dotsc,r_k,r_k'$ such that
\[e_i(r_1,r_2,\dotsc,r_k)=e_i(r_1,r_2,\dotsc,r_{k-1},r_k')=0.\]Now we expand these by writing
\[e_i(r_1,r_2,\dotsc,r_{k-1})+r_ke_{i-1}(r_1,r_2,\dotsc,r_{k-1})=e_i(r_1,r_2,\dotsc,r_{k-1})+r_k'e_{i-1}(r_1,r_2,\dotsc,r_{k-1})=0,\]and since $r_k\neq r_k'$, we have
\[e_i(r_1,r_2,\dotsc,r_{k-1})=e_{i-1}(r_1,r_2,\dotsc,r_{k-1})=0.\]
Suppose that $\{r_1,r_2,\dotsc,r_{k-1}\}$ is a set of real numbers such that $e_i(r_1,r_2,\dotsc,r_{k-1})=e_{i-1}(r_1,r_2,\dotsc,r_{k-1})=0$ for some $i$, but no strict subset has this property. It suffices to show that such a set cannot exist.

Now we perform a bit of further expansion
\[e_i(r_1,r_2,\dotsc,r_{k-2})+r_{k-1}e_{i-1}(r_1,r_2,\dotsc,r_{k-2})=e_{i-1}(r_1,r_2,\dotsc,r_{k-2})+r_{k-1}e_{i-2}(r_1,r_2,\dotsc,r_{k-2})=0,\]from which we may deduce that
\[e_i(r_1,r_2,\dotsc,r_{k-2})e_{i-2}(r_1,r_2,\dotsc,r_{k-2})=e_{i-1}(r_1,r_2,\dotsc,r_{k-2})^2.\]
By minimality, none of $e_i$, $e_{i-2}$, or $e_{i-1}$ can vanish, for then two consecutive elementary symmetric polynomials must vanish. By "On the nonnegativity of generalized discriminants of quadratics",
\[e_{i-1}(r_1,r_2,\dotsc,r_{k-2})^2\ge \frac{i(k-i)}{(i-1)(k-i-1)}e_i(r_1,r_2,\dotsc,r_{k-2})e_{i-2}(r_1,r_2,\dotsc,r_{k-2}).\]Since $e_{i-1}(r_1,r_2,\dotsc,r_{k-2})^2$ is strictly positive, we have a contradiction.
This post has been edited 1 time. Last edited by golue3120, Mar 21, 2025, 1:32 AM
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MathLuis
1501 posts
#37 • 2 Y
Y by KevinYang2.71, Pengu14
This problem is kinda funny, I guess giving a rating is hard.
So FTSOC $P$ has $n$ real roots and WLOG it is monic and WLOG $k=n-1$ by simply shifting with the other monomials then define $P(x)$ as $(x-r_1) \cdots (x-r_n)$ then it happens that all of $\frac{P(x)}{x-r_i}$ for $1 \le i \le n$ happen to have one coefficient equal to $0$, since there is at most $n-2$ choices for these so by pigeonhole there exists $i \ne j$ such that $\frac{P(x)}{x-r_i}$ and $\frac{P(x)}{x-r_j}$ are missing the same coefficient $x^{\ell}$ for $\ell \ge 1$ (this is because none of the roots are equal and none can be zero), let these be called $A(x), B(x)$ respectively and let $i,j$ be $n,n-1$ (it doesn't matter anyway lmao), then we have that $A(x)=(x-r_1) \cdots (x-r_{n-1})$ and $B(x)=(x-r_1) \cdots (x-r_{n-2})(x-r_n)$ which the means that $A(x)-B(x)=(r_n-r_{n-1})(x-r_1) \cdots (x-r_{n-2})$ is missing the $x^{\ell}$ coefficient but also notice that $r_nA(x)-r_{n-1}B(x)=(r_n-r_{n-1})x(x-r_1) \cdots (x-r_{n-2})$ is missing the $x^{\ell}$ coefficient but since $r_n \ne r_{n-1}$ then we do in fact have that $A(x)-B(x)$ is missing both the $x^{\ell}$ and $x^{\ell-1}$ coefficient. (Now we can wlog say $\ell$ is minimal).
Call it $C(x)$ then it has $n-2$ real roots, is $\deg C=n-2$ but also it has two consecutive missing coefficients so write it as $x^2D(x)+E(x)$ (essy to check that if $n=2$ this is trivial and if $n=3$ then you have cuadratics of the form $x^2+b$ which shows pairwise sum of roots is zero and thus all are zero, a contradiction!), so for $n \ge 4$ notice that $n-4=\deg D$ and that you can set it so that least degree term on $D$ is greater than $\deg E$.
Now from derivative analytical definition and propeties we can check graphically that counting multiplicity a polynomial has all its roots real if and only if the derivative does as well, so now take the $\deg E+1$-th derivative of $Q$, then $Q^{(\deg E+1)}(0)=0$ but also $Q^{(\deg E+2)}(0)=0$ and these polynomials must have all of their roots real by chaining the fact that $Q$ does follow this so because of minimality of $\ell$, it happens that $E$ has a non-zero leading coefficient and thus if it were the zero polynomial we are done by the same argument as if it weren't which we will see now.
So let $R(x)=Q^{(\deg E)}(x)=c(x-q_1) \cdots (x-q_m)$, now focus on the product of monomials and call it $R_1(x)$, then it happens that $\frac{R_1'(x)}{R_1(x)}=\sum_{i=1}^{m} \frac{1}{x-q_i}$ and thus by taking derivative w.r.t. $x$ it happens that $\frac{R_1'(x)^2-R_1''(x)R_1(x)}{R_1(x)^2}=-\sum_{i=1}^{m} \frac{1}{(x-q_i)^2}$ for all $x \ne q_j$ for $1 \le j \le m$, however clearly none of the roots are zero once again but replacing $x=0$ we have LHS is zero while RHS isn't, contradiction!.
Therefore $P$ must have a non-real root thus we are done :cool:.
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Pomansq
12 posts
#38
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How many points for proving it suffices to prove k=n-1? I made a mistake for the part where I prove k=n-1 works, so I don’t think I’m getting any credit there.
This post has been edited 2 times. Last edited by Pomansq, Mar 21, 2025, 6:56 PM
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atdaotlohbh
185 posts
#39
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This problem is certainly based on the following lemma:
Lemma: A polynomial with only real distinct roots can't have two consecutive zero coefficients
Proof: Let the polynomial be $P$ and the coefficients be for $x^k,x^{k+1}$. Then $P^{(k)}$ is a polynomial which must have only real distinct roots, but it is divisible by $x^2$, a contradiction.

Now to the problem. We consider the contrary. Take any $k+1$ of this $n$ roots, say they are $r_1,\ldots,r_{k+1}$. Consider the polynomials of the form $\frac{(x-r_1)\ldots (x-r_{k+1})}{x-r_i}$. $P$ is divisible by it, hence it has a zero coefficient. But its coefficients are elementary symmetric polynomials in variables $r_1,r_2,\ldots,r_{k+1}$. Also, it is not the product since none of the roots is zero. So there are only $k-1$ options for the size of the elementary polynomial, and $k+1$ polynomials, by Pigeonhole there is a repetition. So $\sigma_i(r_1,\ldots,r_{k-1},r_k)=\sigma_i(r_1,\ldots,r_{k-1},r_{k+1})=0$. But $\sigma_i(r_1,\ldots,r_{k-1},x)=x\sigma_{i-1}(r_1,\ldots,r_{k-1})+\sigma_i(r_1,\ldots,r_{k-1})$, and as it is a linear function with two distinct roots, we must have $\sigma_{i-1}(r_1,\ldots,r_{k-1})=\sigma_i(r_1,\ldots,r_{k-1})=0$. This contradicts the Lemma. Thus, $P$ must have a nonreal root.
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laniakea0
33 posts
#40
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How much do I get if I proved everything but assumed n=k+1 without justifying? I was a bit confused by the wording
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awesomeming327.
1699 posts
#41
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Assume on the contrary. We prove the result for $k=n-1$. This is equivalent to the original problem because we can simply remove some of the roots of $P$ to reduce to this new problem anyway.

Let $P(x)=(x-r_1)(x-r_2)\dots(x-r_n)$ for some distinct nonzero roots $r_1$, $r_2$, $\dots$, $r_n$. Let $P_i(x)$ be $(x-r_1)\dots(x-r_{i-1})(x-r_{i+1})\dots(x-r_n)$, which must have a zero coefficient. Note that there are $n$ polynomials $P_1$, $P_2$, $\dots$, $P_n$ and $n-2$ coefficients at which this zero coefficient can appear. Therefore, there exists two polynomials (WLOG let them be $P_n$ and $P_{n-1}$) such that $P_n$ and $P_{n-1}$ both have a zero as an $x^i$ coefficient.

Let $(x-r_1)(x-r_2)\dots(x-r_{n-2})$ be $Q(x)$. We have
\begin{align*}P_n-P_{n-1} &= (x-r_{n-1})Q(x)-(x-r_n)Q(x)=(r_n-r_{n-1})Q(x) \\
r_nP_n-r_{n-1}P_{n-1} &= (r_nx-r_{n-1}r_n)Q(x)-(r_{n-1}x-r_{n-1}r_n)Q(x)=(r_n-r_{n-1})xQ(x)\end{align*}both of which have zero as an $x^i$ coefficient, which means that $Q(x)$ has a zero in both its $x^i$ and $x^{i-1}$ coefficients. We now prove the following claim, which finishes the problem:

Claim 1: Let $Q(x)=(x-r_1)(x-r_2)\dots(x-r_n)$ be a polynomial with distinct nonzero real roots. Then $Q(x)$ may not have two consecutive zero coefficients.
Let $R(x)$ be a polynomial with $r$ distinct roots. Then, $R'(x)$ has $r-1$ distinct roots because between each two roots of $R(x)$, there will be a relative extrema. Therefore, if we keep taking derivatives of $Q$, there will always be $\operatorname{deg} Q$ distinct roots. But since there are two consecutive zero coefficients, eventually some multiple derivative of $Q$ will have both its constant and linear coefficients zero, which means that it has a double root at $0$, impossible.
This post has been edited 2 times. Last edited by awesomeming327., Mar 22, 2025, 2:58 AM
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Mysteriouxxx
7 posts
#42
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...........
This post has been edited 1 time. Last edited by Mysteriouxxx, Apr 8, 2025, 8:38 AM
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yanling
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#43
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KevinYang2.71 wrote:
scannose wrote:
tfw you wrote up a fakesolve that turned out to be correct
KevinYang2.71 wrote:
Claim. $A(x)$ cannot have $n-2$ distinct real roots.
how many points for claiming this but not proving it

2 for everything before using derivative maybe

if only prove the claim,including Rolle,but havent solved others,how many point can i get,thanks
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dno1467
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#44
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this problem has so much aura

argue by contradiction.
consider $k+1$ real roots $a_1, a_2, .., a_k$ and the $k+1$ factors of degree $k$. Clearly two of these factors have a 0 in the same place. This means that the gcd of these two factors have two consecutive zeroes.

We can prove by induction that no polynomial with all real roots has two consecutive zeroes. If $Q$ has all real roots with single multiplicity, then $Q'$ does too, as each root of $Q'$ lies in between two roots of $Q$. But if $Q$ has two consecutive zeroes, so does $Q'$, contradiction.
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sixoneeight
1138 posts
#45 • 1 Y
Y by Ilikeminecraft
Trivial by Advanced Academic Course Precalculus (PNI Chart)
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Math4Life2020
2963 posts
#46
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golue3120 wrote:
Here, we have a solution without WLOG $k=n-1$, without Descartes rule of signs, without Rolle, but is somehow not terrible?

Let $e_k$ denote the degree $k$ elementary symmetric polynomial.

Suppose for the sake of contradiction that all roots of $P(x)$ were real. Let the roots be $r_1,r_2,\dotsc,r_n$. Let $R$ be the set of roots. Then for every subset $S$ of $k$ roots, $e_i(S)=0$ for some $1\le i\le k-1$.

Define two subsets of $k$ elements to be adjacent if they share $k-1$ elements.
Lemma. If $\mathcal F$ is a family of $k$-element subsets of $R$ such that no two are adjacent, then $|\mathcal F|\le\frac{1}{k+1}\binom nk$.
Proof. Every set $T$ in $\mathcal F$ is adjacent to exactly $k(n-k)$ sets outside of $\mathcal F$ because we may pick any element of $T$ to remove and any root not in $T$ to add. Meanwhile, any set $U$ not in $\mathcal F$ is adjacent to at most $n-k$ sets in $\mathcal F$ because when we remove an element of $U$ and add a root not in $U$, the additional roots for the adjacent sets in $\mathcal F$ must be distinct. Counting then gives the desired result.

Now define $\mathcal F_i$ to be the family of $k$-element subsets of roots such that $e_i$ vanishes. Then there are $k-1$ such families, and every size $k$ subset of the roots is in at least one family, so one family must have size at least $\frac{1}{k-1}\binom nk$. Therefore, at least one family must contain two adjacent subsets. This implies that there are roots $r_1,r_2,\dotsc,r_k,r_k'$ such that
\[e_i(r_1,r_2,\dotsc,r_k)=e_i(r_1,r_2,\dotsc,r_{k-1},r_k')=0.\]Now we expand these by writing
\[e_i(r_1,r_2,\dotsc,r_{k-1})+r_ke_{i-1}(r_1,r_2,\dotsc,r_{k-1})=e_i(r_1,r_2,\dotsc,r_{k-1})+r_k'e_{i-1}(r_1,r_2,\dotsc,r_{k-1})=0,\]and since $r_k\neq r_k'$, we have
\[e_i(r_1,r_2,\dotsc,r_{k-1})=e_{i-1}(r_1,r_2,\dotsc,r_{k-1})=0.\]
Suppose that $\{r_1,r_2,\dotsc,r_{k-1}\}$ is a set of real numbers such that $e_i(r_1,r_2,\dotsc,r_{k-1})=e_{i-1}(r_1,r_2,\dotsc,r_{k-1})=0$ for some $i$, but no strict subset has this property. It suffices to show that such a set cannot exist.

Now we perform a bit of further expansion
\[e_i(r_1,r_2,\dotsc,r_{k-2})+r_{k-1}e_{i-1}(r_1,r_2,\dotsc,r_{k-2})=e_{i-1}(r_1,r_2,\dotsc,r_{k-2})+r_{k-1}e_{i-2}(r_1,r_2,\dotsc,r_{k-2})=0,\]from which we may deduce that
\[e_i(r_1,r_2,\dotsc,r_{k-2})e_{i-2}(r_1,r_2,\dotsc,r_{k-2})=e_{i-1}(r_1,r_2,\dotsc,r_{k-2})^2.\]
By minimality, none of $e_i$, $e_{i-2}$, or $e_{i-1}$ can vanish, for then two consecutive elementary symmetric polynomials must vanish. By "On the nonnegativity of generalized discriminants of quadratics",
\[e_{i-1}(r_1,r_2,\dotsc,r_{k-2})^2\ge \frac{i(k-i)}{(i-1)(k-i-1)}e_i(r_1,r_2,\dotsc,r_{k-2})e_{i-2}(r_1,r_2,\dotsc,r_{k-2}).\]Since $e_{i-1}(r_1,r_2,\dotsc,r_{k-2})^2$ is strictly positive, we have a contradiction.

Wow. I spent around two hours in contest looking for an inequality-based approach to proving this but failed. Honestly, given the premise of the problem, I'm slightly surprised one exists. Is there any clean way to prove the inequality you cite? It looks like the terms could line up with some Cauchy-Schwartz / Sum of Squares approach, but I don't immediately see how to do it.
(I ultimately found the Descartes' Rule of Signs approach, so fortunately I avoided a crisis. :P)
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Mathandski
746 posts
#47 • 1 Y
Y by KevinYang2.71
We show the converse that, if all polynomials dividing $P$ have some zero coefficient, $P(x)$ has a nonzero root. Clearly, it suffices to show $k = n - 1$. WLOG let $a_k = 1$ and WLOG only consider monic degree-$k$ polynomials dividing $P$. Let, $P(x) = (x -  r_1) \dots (x - r_n)$. Its divisors with degree $n - 1$ have,
\[\frac{P(x)}{x - r_1}, \frac{P(x)}{x - r_2}, \dots, \frac{P(x)}{x - r_n}\]
FTSOC, let each of these divisors have a zero coefficient. By Pigeonhole, since they are monic degree $n-1$ polynomials, two of them share a zero coefficient in the same spot. WLOG, let $\frac{P(x)}{x - r_1}, \frac{P(x)}{x - r_2}$ both have a coefficient of zero for $x^{n-1-e}$.

Let $\sigma (S, k)$ be the $k$-th symmetric sum of a set $S$. Let $R = \{r_1, \dots, r_n\}$. By Vieta's,
\[\sigma (R - r_1, e) = \sigma (R - r_2, e) = 0\]\[\iff r_2 \sigma (R - r_1 - r_2, e - 1) + \sigma (R - r_1 - r_2, e) = r_1 \sigma (R - r_1 - r_2, e - 1) + \sigma (R - r_1 - r_2, e) = 0\]
As a result, $r_2 \sigma (R - r_1 - r_2, e - 1) = r_1 \sigma (R - r_1 - r_2, e - 1)$. Since roots are distinct, we must have $\sigma (R - r_1 - r_2, e - 1) = 0$. Plugging this back in, $\sigma (R - r_1 - r_2, e) = 0$ as well.

(Above is the extent to which I wrote up my solution on the test)

By Vieta's, $Q(x) = \frac{P(x)}{(x - r_1)(x - r_2)}$ has two consecutive zero coefficients for $x^m, x^{m-1}$ for some $m$. However, it also has distinct real roots. We show that this is impossible on real polynomials.

(In red is what I couldn't figure out during the test :wallbash_red:)
Whenever we take the derivative of a polynomial with distinct real roots, which we label $r_1 < r_2 < \dots < r_p$, by the Mean value theorem, there exists a point on each of $(r_1, r_2), (r_2, r_3), \dots, (r_{p-1}, r_p)$ where the derivative is zero meaning there are $p-1$ instances where the derivative is zero. Therefore, since there cannot be additional roots by FToA, the derivative of the polynomial also has distinct real roots.

Therefore, by induction, if we take the $m-1$th derivative of $Q(x)$, we still have a polynomial with distinct roots. However, the coefficient of the $x$ term and the constant term is zero so the polynomial has a double root at zero. (!)
This post has been edited 1 time. Last edited by Mathandski, Mar 26, 2025, 4:11 PM
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Tintarn
9038 posts
#48 • 1 Y
Y by ItalianZebra
Math4Life2020 wrote:
Wow. I spent around two hours in contest looking for an inequality-based approach to proving this but failed. Honestly, given the premise of the problem, I'm slightly surprised one exists. Is there any clean way to prove the inequality you cite? It looks like the terms could line up with some Cauchy-Schwartz / Sum of Squares approach, but I don't immediately see how to do it.
Well, the inequality we need is just the famous Newton inequality
\[S_k^2>S_{k-1}S_{k+1}\]for non-zero $x_1,\dots,x_n$, which are not all equal. Here, $S_k=\frac{\sigma_k}{\binom{n}{k}}$ is the normalized version of the elementary symmetric polynomials of $x_1,\dots,x_n$. This of course finishes off the problem immediately, so I am slightly surprised that no one has mentioned it before... :maybe:
This post has been edited 1 time. Last edited by Tintarn, Apr 12, 2025, 12:11 PM
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SimogmH1
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#49
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KevinYang2.71 wrote:
Let $n$ and $k$ be positive integers with $k<n$. Let $P(x)$ be a polynomial of degree $n$ with real coefficients, nonzero constant term, and no repeated roots. Suppose that for any real numbers $a_0,\,a_1,\,\ldots,\,a_k$ such that the polynomial $a_kx^k+\cdots+a_1x+a_0$ divides $P(x)$, the product $a_0a_1\cdots a_k$ is zero. Prove that $P(x)$ has a nonreal root.
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megarnie
5590 posts
#50 • 2 Y
Y by KevinYang2.71, brainfertilzer
Assume FTSOC $P$ has $n$ real roots. WLOG $P$ has leading coefficient $1$ so let $P(x)=:(x-r_1)\cdots(x-r_n)$. WLOG $k=n-1$, as otherwise we can take $Q:=(x-r_1)\cdots(x-r_{k+1})$ and deal with $Q$ instead of $P$.

By the assertion, each $\frac{P(x)}{x-r_i}$ for $1\leq i\leq n$ has a coefficient equal to $0$. Since $r_i\neq 0$ for all $i$, this coefficient is not the leading or constant coefficient, so it has $k-1=n-2$ choices. By pigeonhole, there exists $1\leq i<j\leq n$ such that $Q(x):=\frac{P(x)}{x-r_i}$ and $R(x):=\frac{P(x)}{x-r_j}$ have no $x^\alpha$ term, for some $\alpha\geq 1$. WLOG $i=n-1$ and $j=n$. Hence
\begin{align*}
P(x)&=(x-r_1)\cdots(x-r_{n-2})(x-r_n)\\
Q(x)&=(x-r_1)\cdots(x-r_{n-2})(x-r_{n-1}).
\end{align*}Then $P(x)-Q(x)=(r_{n-1}-r_n)(x-r_1)\cdots(x-r_{n-2})$ has no $x^\alpha$ term, so $A(x):=(x-r_1)\cdots(x-r_{n-2})$ has no $x^\alpha$ term since $r_{n-1}\neq r_n$. Also, $r_{n-1}P(x)-r_nQ(x)=x(r_{n-1}-r_n)(x-r_1)\cdots(x-r_{n-2})$ has no $x^\alpha$ term so $A(x)$ has no $x^{\alpha-1}$ term.

Claim. $A(x)$ cannot have $n-2$ distinct real roots.
Proof. Suppose FTSOC $A(x)$ has $n-2$ distinct real roots. We claim that $A^{(m)}(x)$, the $m$th derivative of $A(x)$, has $n-2-m$ distinct real roots for all $m\leq n-2$. We proceed by induction on $m$ with the base case $m=0$ trivial.

Assume $A^{(m)}(x)$ has $n-2-m$ distinct real roots. Between every $2$ consecutive real roots there exists a local extremum, where $A^{(m+1)}(x)$ is $0$. $A^{(m)}(x)$ has $n-2-m-1$ pairs of consecutive real roots so $A^{(m+1)}(x)$ has $n-2-(m+1)$ distinct real roots, completing the induction step.

$A^{(\alpha-1)}$ has no $x^{(\alpha-1)-(\alpha-1)}=x^0$ or $x^{\alpha-(\alpha-1)}=x$ term, so $0$ is a double root of $A^{(\alpha-1)}$. However, all the roots of $A^{(\alpha-1)}$ are distinct and real, a contradiction. $\square$

This is a contradiction since $r_1,\,\ldots,\,r_{n-2}$ are distinct real numbers. Thus $P$ has a nonreal root. $\square$
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