Find Eigen Values: Solve Char. Equation |A-#I| = 0 of Degree 3

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Discussion Overview

The discussion revolves around finding the eigenvalues of a characteristic equation of degree 3, specifically through the expression |A-#I| = 0. Participants explore various methods for solving cubic equations, including numerical methods and algebraic techniques.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Homework-related

Main Points Raised

  • One participant seeks guidance on finding the roots of the characteristic equation for degree 3 polynomials, expressing familiarity with numerical methods but requesting algebraic approaches.
  • Another participant suggests using Cardano's cubic formula as a method to solve the equation if factoring is not feasible.
  • A participant mentions discovering methods such as the factor and remainder theorem and asks for insights on their advantages and disadvantages.
  • One participant explains that the factor theorem is commonly used for solving cubic equations, particularly when an easy root can be identified, allowing for reduction to a quadratic equation.
  • Another participant clarifies the remainder theorem, stating that it relates to the remainder of a polynomial after division and its implications for factoring.
  • A later reply emphasizes the importance of understanding polynomial equations by factoring, questioning the approach of discussing eigenvalues without this knowledge.

Areas of Agreement / Disagreement

Participants express differing levels of familiarity with polynomial solving techniques, and while some methods are discussed, there is no consensus on a single approach to finding eigenvalues for degree 3 equations.

Contextual Notes

Some participants assume familiarity with algebraic methods, while others highlight the limitations of certain techniques, such as the integer root theorem, which may not apply if no integer roots exist.

Who May Find This Useful

Readers interested in algebraic methods for solving cubic equations, particularly in the context of eigenvalue problems in linear algebra.

zeroxff0000ff
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hello everybody,

Consider '#' as lamda.
How to find roots(eigen values) of characteristic equation |A-#I| = 0.
I know how to find it it using numerical methods.
But can anyone please show me how to procced for degree 3 equations.

thanks and regards.
 
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I'm not sure what the remainder theorem is, but the factor theorem is usually what's used to solve cubics... if there's an easy root (like 1, 2, 0, -1) you can reduce the degree of your equation to a quadratic fairly easily, and from there solve for the remaining roots.

You can use the integer root theorem (on the website you posted) to figure out which integers are worth guessing. The advantage is that most problems will have a root that you can find in this way. The disadvantage is that if the polynomial doesn't have an integer root, you can't find any roots using this method. But it doesn't take a lot of time to try, so usually this is the best way to start.
 
the remainder theorem is the theorem from high school algebra that says the remainder of f(x) after dividing it by x-a, is f(a).
 
mathwonk said:
the remainder theorem is the theorem from high school algebra that says the remainder of f(x) after dividing it by x-a, is f(a).
And in particular, if f(a)= 0, the remainder is 0 so x-a is a factor of f(x).

It seems very strange to me that a person would be working with eigenvalues while not knowing how to solve polynomial equations by factoring!
 

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