# Quartic polynomials

#### kuruman

Homework Helper
Gold Member
this is a tough one...still struggling, my latest attempt
$-(1/a +1/b+1/c+1/d)= abcd$..........attempt 1
and
$-(cd(a+b)+ab(c+d)=4$
$cd(-1-c-d)$+$\frac 2 {cd} (c+d)=4$........attempt 2, am i on the right path?
It is not as tough as you think. Take a deep breath, clear your mind then read very carefully the following that I repeat from post #24.
Your goal is to find an expression for $A$, meaning that you need to have $A$ alone on the left hand side and some expression on the right hand side involving $a$, $b$, $c$ and $d$.
Can you achieve this goal by looking at equations that do not contain $A$?

#### chwala

Gold Member
Thanks for your insight, let me look at it again.

#### chwala

Gold Member
Lol still getting stuck...i will post my attempts...came up with equation...
$\frac {-2} {cd}$$(c+d)+cd(a+b)$= -4................................7

another attempt: simultaneous equations,
$1/a+1/b+1/c+1/d =2$............................................................8
$a+b+c+d = -1$

another attempt:simultaneous equation,
$1/a^2+1/b^2+1/c^2+1/d^2 + 2/A = 4$....................................9
$a^2+b^2+c^2+d^2+2A = 1$
kindly advise if i am on the right track.

#### kuruman

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Gold Member
It seems we are talking past each other. Please explain to me, in your own words, what the problem is asking you to find. Once we agree on that, I will guide you to the next step.

#### chwala

Gold Member
The problem requires that we find a numeric value for $A$ or rather the value of $A$ which is numeric ...

#### Ray Vickson

Homework Helper
Dearly Missed
The problem requires that we find a numeric value for $A$ or rather the value of $A$ which is numeric ...
Yes, but don't forget you are pretending that you know the values of the four roots, so you need a formula that, somehow, involves those roots.

#### kuruman

Homework Helper
Gold Member
To illustrate what @Ray Vickson said, suppose I told you that
$\Theta = -5.50427$
$\Psi=0.08167 + 0.27755 i$
$\phi=0.08167 - 0.27755 i$
$\xi=4.34094$
Can you find a numeric value for $A$ given this set of roots? Note that there are infinitely many sets of roots because $A$ can be chosen to have infinitely many values.

On edit (2 days later)
Actually, the numbers above are the actual roots consistent with the problem's statement. In other words
$1/\Theta = -5.50427$
$1/\Psi=0.08167 + 0.27755 i$
$1/\phi=0.08167 - 0.27755 i$
$1/\xi=4.34094$

I apologize for the confusion.

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#### chwala

Gold Member
let me look at it again...

#### chwala

Gold Member
so reading your comments, and from my understanding, the only possibility is to use trial and error in trying to figure out the roots of the problem, this is my latest equation.
$\frac {-2} {cd}$$(c+d) + cd(-1-c-d)=-4$
is the above equation correct? if so, then how do we get the values of $c$ and $d$ ?

#### kuruman

Homework Helper
Gold Member
so reading your comments, and from my understanding, the only possibility is to use trial and error in trying to figure out the roots of the problem, this is my latest equation.
$\frac {-2} {cd}$$(c+d) + cd(-1-c-d)=-4$
is the above equation correct? if so, then how do we get the values of $c$ and $d$ ?
What roots? I gave you the roots for one choice of $A$ in #32. Let me remind you of the definitions
Rather than work with all those Greek letters, I would make these substitutions:
Let $a = \frac 1 \Theta, b = \frac 1 \Psi, c = \frac 1 \xi,d = \frac 1 \phi$.
So you know $\Theta$, $\Psi$, $\xi$ and $\phi$ and you can easily find $a$, $b$, $c$ and $d$ from the definitions.
Can you find $A$???

#### PAllen

Ok, maybe I am totally confused, but it seems to me we have 4 equations in 5 unknowns (a,b,c,d,A), so we can’t get a numeric answer for A without some lucky cancellation.

[edit: oops, the 5th equation is that A can be expressed in terms of any given root directly from the starting equation]

[edit: further oops, this last equation cannot be independent of the others, so A cannot be determined to be a specific number, as others have already said].

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#### PAllen

So, it seems to me it is easy to find a simple expression for A in terms of the 4 roots, but it is also true that specifying a numeric value for any root is sufficient to determine A, and then determine all other roots.

#### kuruman

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Gold Member
So, it seems to me it is easy to find a simple expression for A in terms of the 4 roots, but it is also true that specifying a numeric value for any root is sufficient to determine A, and then determine all other roots.
That is the gist of post #12.

#### chwala

Gold Member
What roots? I gave you the roots for one choice of $A$ in #32. Let me remind you of the definitions

So you know $\Theta$, $\Psi$, $\xi$ and $\phi$ and you can easily find $a$, $b$, $c$ and $d$ from the definitions.
Can you find $A$???
i cannot find them...how did you find your values? i am illiterate here, show me how? What is wrong with equation $34$?

#### chwala

Gold Member
further are you implying that some roots are complex numbers?

#### chwala

Gold Member
Ok, maybe I am totally confused, but it seems to me we have 4 equations in 5 unknowns (a,b,c,d,A), so we can’t get a numeric answer for A without some lucky cancellation.

[edit: oops, the 5th equation is that A can be expressed in terms of any given root directly from the starting equation]

[edit: further oops, this last equation cannot be independent of the others, so A cannot be determined to be a specific number, as others have already said].
i am getting conflicting messages here. can we get the value of $A$ algebraically? or are we using trial and error to fix values for the roots. I still don't understand how the values in post $32$ were found...i agree with you that we have 5 unknowns here and it may not be possible to get a value for $A$.

#### chwala

Gold Member
So, it seems to me it is easy to find a simple expression for A in terms of the 4 roots, but it is also true that specifying a numeric value for any root is sufficient to determine A, and then determine all other roots.
specifying has to involve two values and not specifying just a value for any root....

#### PAllen

specifying has to involve two values and not specifying just a value for any root....
Specifying one root value easily determines A from just the quartic equation itself, as @kuruman was the first on this thread to note. And once you have A, you can find all the other roots.

It is also true that A can be expressed as a pretty simple expression in the 4 roots.

This is not inconsistent. Given a set of valid roots, the latter expression will hold. But given 4 arbitrary choices for roots, and computing A, will have probability zero that these are actually roots of the quartic with the thus computed value of A.

#### chwala

Gold Member
Kindly see attached, a fellow African teacher in my whattsap group was able to give a way forward and a solution to the problem $A$=$-1$

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#### PAllen

You know you must have a mistake because, as several here explained, pick any numeric value of A and there are 4 roots to P(x) over the complex numbers (counting possible multiple roots separately). The reciprocals of these must also satisfy Q(x) as you've defined it, and they will. Thus, there cannot be a unique value for A.

There is an error in your second page that leads to the false conclusion that A can be uniquely determined. See if you can find it.

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#### kuruman

Homework Helper
Gold Member
i am getting conflicting messages here. can we get the value of $A$ algebraically? or are we using trial and error to fix values for the roots. I still don't understand how the values in post $32$ were found...i agree with you that we have 5 unknowns here and it may not be possible to get a value for $A$.
I found the values in post #32 by assuming a numerical value for $A$ and then finding the roots of the quartic for that particular value of $A.$ Note that I originally posted the inverses of the roots due to some confusion on my part and I edited #32 to correct that mistake. The statement of the problem as you posted in #1 says
1. Homework Statement
Given $x^4+x^3+Ax^2+4x-2=0$ and giben that the roots are $1/Φ, 1/Ψ, 1/ξ ,1/φ$
find A
So, just as the problem requires, I gave you specific values for the inverses of the roots and asked you if you can find $A$. I did this because I sensed that you were stuck in a vicious circle with the symbols $a$, $b$, $c$ and $d$ that produced more and more equations without you realizing that they are supposed to be given (i.e. known) quantities. I had hoped that if you had specific numbers to work with, you would see what's going on here - you can still do it.
further are you implying that some roots are complex numbers?
They are for the particular value of $A$ that I chose.
Kindly see attached, a fellow African teacher in my whattsap group was able to give a way forward and a solution to the problem $A$=$-1$
This algebraic manipulation recasts the original polynomial $P(x)$ as another polynomial $Q(x)$ whose roots are the inverses of the roots of $P(x)$. You can always do that as long as $x=0$ is not a root. It is not clear to me how this leads to $A=-1$.
For $A=-1$ the roots are
$1/\Theta =-2.32708$
$1/\Psi=0.406238 - 1.22682 i$
$1/\phi=0.406238 + 1.22682 i$
$1/\xi=0.514603$
That is not given by the problem.

#### chwala

Gold Member
You know you must have a mistake because, as several here explained, pick any numeric value of A and there are 4 roots to P(x) over the complex numbers (counting possible multiple roots separately). The reciprocals of these must also satisfy Q(x) as you've defined it, and they will. Thus, there cannot be a unique value for A.

There is an error in your second page that leads to the false conclusion that A can be uniquely determined. See if you can find it.
yes i can see it....$a^-2+b^-2+c^-2+d^-2 ≠ a^2+b^2+c^2+d^2$ therefore
$4+A ≠1-2A$
.....unless i am missing something.

#### PAllen

yes i can see it....$a^-2+b^-2+c^-2+d^-2 ≠ a^2+b^2+c^2+d^2$ therefore
$4+A ≠1-2A$
.....unless i am missing something.
Bingo!

Gold Member

#### chwala

Gold Member
so to be precise,in this question we may not have a unique value for $A$ because we have not been given values for any of the two roots. If we had those values, then it would have been possible to use Vieta's theorem in finding $A$ right?

"Quartic polynomials"

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