How can I find the roots of a cubic equation?

  • Context: High School 
  • Thread starter Thread starter James...
  • Start date Start date
  • Tags Tags
    Cubic Roots
Click For Summary

Discussion Overview

The discussion centers around finding the roots of a cubic equation, particularly in the context of preparing for a university entrance exam. Participants explore various methods and concepts related to solving cubic equations, including factoring and the use of specific theorems.

Discussion Character

  • Homework-related
  • Mathematical reasoning

Main Points Raised

  • James expresses difficulty in understanding the formulas for finding roots of cubic equations and seeks help.
  • One participant suggests focusing on finding linear binomial factors and non-factorable quadratic factors, emphasizing the importance of understanding the underlying concepts rather than just memorizing formulas.
  • Another participant mentions that for cubic equations, it is often sufficient to factor into linear and possibly a quadratic factor to find the roots.
  • There is a suggestion to use the Rational Roots Theorem and polynomial division as additional methods for finding roots.
  • James reports successfully finding the roots by creating linear and quadratic factors and expresses a willingness to study the suggested concepts further.
  • James indicates a need for an example of the Rational Roots Theorem and polynomial division, depending on his progress with the material.

Areas of Agreement / Disagreement

Participants generally agree on the importance of understanding the methods for finding roots of cubic equations, but there is no consensus on a single approach, as multiple methods are discussed.

Contextual Notes

James's understanding of the material is still developing, and he acknowledges that he has not previously encountered these concepts in his studies. The discussion reflects a range of approaches without resolving which is the most effective for all situations.

Who May Find This Useful

Students preparing for exams in algebra or those seeking to understand methods for solving cubic equations may find this discussion beneficial.

James...
Messages
25
Reaction score
0
I've been looking at some practise exams for the University I would like to apply to, I have to sit the exam on 4th November.

We have never done finding the roots of a cubic equation before and I cannot figure it out from looking on the internet, the formulas are all horrible to understand.

Any help would be appreciated.

James
 
Mathematics news on Phys.org
Knowing many formulas does not help. What helps is finding the linear binomial factors and any nonfactorable quadratic factors; and equate the whole expression to zero. You are interested in where the function equals zero, or where the function shares a point with the x-axis.

Your wish for knowing "formulas which are horrible to understand" reflects ones first-through study of College Algebra, in which you learn to deal with Descartes Law of Signs, Rational Roots Theorem, the Factor and Remainder Theorems. You could spend a good 2 months studying this stuff from a College Algebra textbook and become very well knowledgeable about these things. You really want to KNOW this stuff and you are not really concerned with a course grade for credit. Those laws and theorems really CAN be well learned if you spend the time seriously studying them; keeping the knowledge several weeks later is another thing entirely.
 
(CONTINUED)...
Actually for just a cubic equation, you don't need those fancy formulas. Usually, you would be expected to factor into linear and possibly a quadratic factor and determine what values of the variable make the product equal to zero.
 
(one more thought)
You could resort to Rational Roots Theorem and polynomial division, but not much else would be needed. Do you need an example?
 
Many many thanks for your replies, I think I will try and have a go at studying the things you mentioned in your first post for definite! Anything that will expand my knowledge and help me at University can only be a good thing!

I managed to get the answer by creating a linear and quadratic factor of the cubic equation. We have done this before many times, but never as part of a polynomial, I should have realized I could have done it as it equals zero and therefore I can split it into factors.

I will have a play about with the Rational Roots theorum and polynomial division tomorrow but I may need an example depending on how i find it.

Thanks again
James
 

Similar threads

  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 6 ·
Replies
6
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 10 ·
Replies
10
Views
3K
  • · Replies 2 ·
Replies
2
Views
6K
  • · Replies 3 ·
Replies
3
Views
9K
  • · Replies 5 ·
Replies
5
Views
3K
Replies
8
Views
2K
  • · Replies 2 ·
Replies
2
Views
2K