Algebra #29: Vieta's Expression

by djmathman, Nov 27, 2011, 1:13 AM

Problem wrote:
Let $a$, $b$, and $c$ be the roots of $x^3-9x^2+11x-1=0$, and let $s=\sqrt{a}+\sqrt{b}+\sqrt{c}$. Find $s^4-18s^2-8s$.

(SOURCE: HMMT)

Well, we see square roots, so let's square our expression for $s$: \[s^2=\left(\sqrt{a}+\sqrt{b}+\sqrt{c}\right)^2=a+b+c+2\left(\sqrt{ab}+\sqrt{ac}+\sqrt{bc}\right).\] By Vieta's, we have $a+b+c=9$, so the expression simplifies to $9+2\left(\sqrt{ab}+\sqrt{ac}+\sqrt{bc}\right).$

Hrm. We don't know what the value of $\sqrt{ab}+\sqrt{bc}+\sqrt{ac}$ is. It's certainly not $\sqrt{ab+ac+bc}$, and there's no way we can find the polynomial whose roots are $\sqrt{a}$, $\sqrt{b}$, and $\sqrt{c}$, cause that would be an $6^{\text{th}}$ degree polynomial. Not good.

Let's step away from $s^2$ and focus on the polynomial we're trying to find: $s^4-18s^2-8s$. We can factor an $s$ to get $s\left(s^3-18s-8\right)$, and then we can use Rational Root Theorem to factor even further and get $s(s+4)\left(s^2-4s-2\right)$. However, it's not clear what we're going to do now. We might have to go back to $s^2$ and find what $\sqrt{ab}+\sqrt{ac}+\sqrt{bc}$ is.

But wait. We don't need to evaluate it directly. We just need to find it. Whenever we've encountered expressions we didn't know before, we've set variables equal to them and solved. Let's do that here. Let $k=\sqrt{ab}+\sqrt{ac}+\sqrt{bc}$. This means that \begin{align*}k^2=\left(\sqrt{ab}+\sqrt{ac}+\sqrt{bc}\right)^2\\=ab+ac+bc+2\left(\sqrt{a^2bc}+\sqrt{ab^2c}+\sqrt{abc^2}\right)\\=(ab+ac+bc)+2\left(\sqrt{a(abc)}+\sqrt{b(abc)}+\sqrt{c(abc)}\right)\end{align*} By Vieta's, we have $ab+ac+bc=11$ and $abc=1$, so the expression simplifies to $11+2\left(\sqrt{1a}+\sqrt{1b}+\sqrt{1c}\right)=11+2\left(\sqrt{a}+\sqrt{b}+\sqrt{c}\right)$.

Bingo! The expression inside the parenthesis is just $s$! Now we can set up a system! In particular, we can take the two big expressions we have and substitute all occurrences of $s=\sqrt{a}+\sqrt{b}+\sqrt{c}$ and $k=\sqrt{ab}+\sqrt{ac}+\sqrt{bc}$ to get the system \begin{align*}s^2=9+2k,\\k^2=11+2s.\end{align*} Solving this gives us... uh oh. There's no neat solution. By neither addition nor subtraction do we get a clean equation. Nor can we square either equation, since that still leaves a degree $1$ term that we don't know what to do with. Additionally, taking the square root of both sides won't work, since then we'd have to do casework (positive square root or negative square root?) and that becomes messy and tedious. But as alluded to earlier, the polynomial we're trying to evaluate is a little weird... ah we have an expression for $s^2$, let's plug it in. \begin{align*}s^4-18s^2-8s\\=(9+2k)^2-18(9+2k)-8s\\=(81+36k+4k^2)-18(9+2k)-8s\\=81+36k+4(11+2s)-(162+36k)-8s\\=81+\cancel{36k}+44+\cancel{8s}-162-\cancel{36k}-\cancel{8s}\\=81+44-162\\=125-162\\=\boxed{-37}.\end{align*}
This post has been edited 4 times. Last edited by djmathman, Apr 6, 2015, 2:58 AM
Reason: old AoPS did very weird things

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  • dj so orz :omighty:

    by Yiyj1, Mar 29, 2025, 1:42 AM

  • legendary problem writer

    by Clew28, Jul 29, 2024, 7:20 PM

  • orz $$\,$$

    by balllightning37, Jul 26, 2024, 1:05 AM

  • hi dj $ $ $ $

    by OronSH, Jul 23, 2024, 2:14 AM

  • i wanna submit my own problems lol

    by ethanzhang1001, Jul 20, 2024, 9:54 PM

  • hi dj, may i have the role of contributer? :D

    by lpieleanu, Feb 23, 2024, 1:31 AM

  • This was helpful!

    by YIYI-JP, Nov 23, 2023, 12:42 PM

  • waiting for a recap of your amc proposals for this year :D

    by ihatemath123, Feb 17, 2023, 3:18 PM

  • also happy late bday man! i missed it by 2 days but hope you are enjoyed it

    by ab456, Dec 30, 2022, 10:58 AM

  • Contrib? :D

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  • :love: tfw kakuro appears on amc :love:

    by bissue, Aug 18, 2022, 4:32 PM

  • Hi dj :)

    by 799786, Aug 10, 2022, 1:44 AM

  • Roses are red,
    Wolfram is banned,
    The best problem writer is
    Djmathman

    by ihatemath123, Aug 6, 2022, 12:19 AM

  • hello :)

    by aidan0626, Jul 26, 2022, 5:49 PM

  • Do you have a link to your main blog that you started after graduating from high school, I couldn't find it. @dj I met you IRL at Awesome Math summer Program several years ago.

    by First, Mar 1, 2022, 5:18 PM

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