How can I factor a 3rd order polynomial using the cubic formula?

  • Thread starter Kiwiro0ls
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In summary, the conversation discusses the steps to factoring a 3rd order polynomial and finding its eigenvalues. It suggests trying the factors of the constant term and using the cubic formula, although it may be tedious, or using online utilities for numerical results.
  • #1
Kiwiro0ls
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Homework Statement


What are the steps to factoring 3rd order polynomials like x^3+8x^2-21x+10?
It's to find eigenvalues of a matrix in linear algebra, I completely forgot how to factor and it's killing me.

Homework Equations





The Attempt at a Solution


None, unless its a polynomial that I can factor by grouping, I have no clue how to begin.
 
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  • #2
EDIT: Nevermind the below, that factor trick (the 'rational root test') doesn't work in this case. The discriminant is positive, but it turns out that all three roots are irrational.

For a cubic polynomial with integer coefficients, which is what you have, I believe one of the roots is typically a factor of the constant term. So, try the factors of 10 and see if one of them is a root. Once you've figured out one of the roots, you can factor an (x-root) term out by polynomial division, leaving you with (x-root)*quadratic, and the quadratic you can factor with the quadratic formula.
 
Last edited:
  • #3
:( so the pq thing with synthetic and what-the-other-one's-name division?
I don't know why i didnt think of it (but that sounds really tedious).
Thanks for your help! :)
 
  • #4
Got it, so i happened to pick the only example in the book of a matrix with irrational eigenvalues. I'm still not so sure how to find the irrational roots of a 3rd order polynomial... i should change the name of this thread haha.
 
  • #5
There's always the cubic formula, but using it is a bit... tedious. :eek:
 
  • #6
eumyang said:
There's always the cubic formula, but using it is a bit... tedious. :eek:

To add to what "eumyang"" said, using the Cubic Formula to get an exact algebraic result is, indeed, a bit tedious.

On the other hand, if you are fine with numerical results, there are several free online utilities available; some of them are listed at the bottom of that Wikipedia page.

In addition, I refer you to an old post I made at the bottom of the following thread:

https://www.physicsforums.com/showthread.php?t=580075&page=2

A search of these forums would probably turn up much more information about factoring a cubic polynomial.
 

What is a 3rd order polynomial?

A 3rd order polynomial is an algebraic expression that contains a variable raised to the third power, also known as a cubic polynomial. It can be written in the form of ax^3 + bx^2 + cx + d, where a, b, c, and d are constants and x is the variable.

How do I factor a 3rd order polynomial?

To factor a 3rd order polynomial, you can use the rational root theorem or synthetic division to find the roots of the polynomial. Once the roots are found, you can use long division or the factor theorem to factor the polynomial into its linear and quadratic factors.

Why is factoring a 3rd order polynomial important?

Factoring a 3rd order polynomial is important because it helps to simplify and solve polynomial equations. It also allows us to find the x-intercepts, or roots, of the polynomial, which can provide valuable information about the behavior and graph of the polynomial.

What are the common mistakes when factoring a 3rd order polynomial?

Some common mistakes when factoring a 3rd order polynomial include forgetting to check for common factors, making errors in long division or synthetic division, and misidentifying the correct factors. It is important to double check your work and use multiple methods to ensure the accuracy of your factorization.

Can all 3rd order polynomials be factored?

Yes, all 3rd order polynomials can be factored. However, the factors may not always be rational numbers. In some cases, the factors may be complex numbers or irrational numbers. This is why it is important to use multiple methods and check your work when factoring a 3rd order polynomial.

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