Inverse of polynomial in dy^2 + ey + f form

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Discussion Overview

The discussion revolves around finding the inverse polynomial of the form y = ax^2 + bx + c, specifically seeking an expression for x in terms of y as x = dy^2 + ey + f. Participants explore various methods and interpretations of the problem, including the use of the quadratic formula and polynomial approximations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Steve seeks a more elegant solution for the inverse polynomial than using Excel.
  • One participant suggests isolating x using the quadratic formula as a method to find the inverse.
  • Another participant claims that the inverse of y = ax^2 + bx + c is simply x = ay^2 + by + c, asserting that d = a, e = b, f = c.
  • A different reply provides a link to a series expansion as a potential approach to the problem.
  • Steve acknowledges the responses and indicates that he may need to adjust the terms based on data comparisons.
  • A participant reiterates that the inverse function is not truly one-to-one due to the nature of quadratic functions, suggesting the use of the quadratic formula with consideration of the vertex of the parabola.
  • Another participant challenges the understanding of the question, suggesting that Chebyshev Polynomials and approximation theory might be relevant for minimizing error in the context of the inverse.
  • There is a reiteration of the claim that the inverse is x = ay^2 + by + c, with some participants expressing skepticism about the simplicity of this answer.

Areas of Agreement / Disagreement

Participants express differing interpretations of the problem and its requirements, leading to multiple competing views on the correct approach to finding the inverse polynomial. There is no consensus on a single method or understanding of the question.

Contextual Notes

Some participants note the limitations of the quadratic function in providing a true inverse due to its non-one-to-one nature, and there are references to the need for clarification in the problem statement.

stevec
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Hello,

I'm trying to find the inverse polynomial of y = ax^2 + bx + c in the form of x = dy^2 + ey + f.

I'm able to approximate this using Excel, but would prefer a more elegant solution. Any suggestions?

Steve
 
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Isolate x in terms of y using the quadratic formula should give you the inverse.
 
I don't think I'm understanding your problem properly because the inverse of [tex]y=ax^2+bx+c[/tex] IS [tex]x=ay^2+by+c[/tex] and so d=a, e=b, f=c
 
Thanks for the replies, I apologize for not being more descriptive in my question.

Gerenuk's reply is close to what I am looking for, although I think I may need to increase the terms - a quick set of data against the equation was off.
 
Mentallic is correct: the "inverse function" to [itex]y= ax^2+ bx+ c[/itex] is just [itex]x= ay^2+ by+ c[/itex]. Now use the quadratic formula to solve for y:
[tex]y= \frac{-b\pm\sqrt{b^2- 4ac}}{2a}[/tex]

But notice the "[itex]\pm[/itex]". The quadratic function is not one-to-one and so does not have a true "inverse". You could restrict x to one side or the other of the vertex of the parabolic graph, thus using either the "+" or the "-".
 
HallsofIvy said:
Mentallic is correct
Neither of you is correct, because you don't understand the question. He might not have used the proper wording, but it's not hard to guess what he is really looking for.

@SteveC: Maybe you want to look at Chebychev Polynomials and their Approximation theory. They provide a method to vaguely minimize the maximum total error. Whereas the series expansion I wrote down only aims to be best a y=0.
 
Gerenuk said:
Neither of you is correct, because you don't understand the question.

Yep, I already acknowledged that I might not be understanding it properly.

Mentallic said:
I don't think I'm understanding your problem properly because the inverse of [tex]y=ax^2+bx+c[/tex] IS [tex]x=ay^2+by+c[/tex] and so d=a, e=b, f=c

The way he asked it, my answer is correct. I was only sceptical about my answer because it was too simple.
 

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