How parameter affects the roots of the equation.

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Homework Help Overview

The discussion revolves around the relationship between a parameter \( C \) and the largest root \( r \) of the cubic equation \( x^3 - x + C = 0 \). Participants are exploring how changes in \( C \) affect the roots of the equation, particularly focusing on the largest real root.

Discussion Character

  • Exploratory, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants are attempting to relate the largest root \( r \) to the parameter \( C \) and are questioning how to express \( r \) mathematically. There is also discussion about finding the derivative \( dr/dc \) and the implications of linear approximations or numerical methods like Newton's Method.

Discussion Status

Some participants have provided insights into the nature of the roots based on the value of \( C \) and have attempted to derive \( dr/dc \). There is an ongoing exploration of the correct expression for \( dr/dc \), with differing opinions on the sign of the derivative.

Contextual Notes

Participants are working under the assumption that \( C \) is a variable rather than a constant, which influences their calculations and reasoning. The original poster has noted a specific request for quantitative information regarding \( r \) and its derivative.

tysonk
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I have trouble figuring out this problem. If someone can help me out that would be appreciated.
Suppose we have x^3 -x + C
let r denote the largest root of the equation.
What's a mathematical way of relating r and C?
(quantitative).
Also what's dr/dc

Thanks in advance.
 
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tysonk said:
I have trouble figuring out this problem. If someone can help me out that would be appreciated.
Suppose we have x^3 -x + C
let r denote the largest root of the equation.
What you have above is not an equation.
Did you mean x^3 - x + C = 0? That's an equation.
tysonk said:
What's a mathematical way of relating r and C?
Just as a guess, you would have to find the solutions to the cubic equation x^3 - x + C = 0 (assuming that's the equation here).

Depending on the value of C, there will be:
  • 3 real solutions
  • 2 real solutions, with one that is repeated
  • 1 real solution, and two complex solutions

When you say that r is the largest solution, do you mean largest real solution?
tysonk said:
(quantitative).
Also what's dr/dc

Thanks in advance.
 
Thanks for the reply.
Yes, largest real root. The equation you have is correct. So how would I got about finding r (or expressing it). And then also finding the derivative. I suspect that it has something to do with linear approximations or Newton's Method but am not sure how to go about doing it. Any help is much appreciated.
 
Thanks for your reply.
Hmm looking back at the question. It just says "give some quantitative information about r."
"Calculate dr/dc"

For dr/dc I did,

y=x^3 -x + c =0

r^3 -r +c = 0

finding the derivative of that i got
dr/dc = 1/ (3r^2 -1)
I used the fact that c is not a constant yet it is a function of r... I'm not sure if I'm right.
 
It should be dr/dc = -1/(3r^2 - 1)
 

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