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A graph of a cubic polynomial and its zeroes
Three zeroes hath this cubic

Cubic polynomials and their roots

Just as for quadratic functions, knowing the zeroes of a cubic makes graphing it much simpler. Typically a cubic function will have three zeroes or one zero, at least approximately, depending on the position of the curve. Finding these zeroes, however, is much more of a challenge. In fact this challenge was a historical highlight of 16th century mathematics.

In this step we will

  • see how Descartes’ factor theorem applies to cubic functions

  • review how to find zeroes of a cubic in special situations.

Zeroes of a cubic polynomial

One simple way of cooking up a cubic polynomial is just to take a product of linear factors, for example

We hope that you can all expand the right-hand side to get

The graph of this function is shown here

pic of y=x^3-3x^2-6x+8

Note that it does pass through the points , and , as it should by Descartes’ Factor theorem (which you will find in Factors of quadratic polynomials and zeroes).

Factorising simple cubics

Here is a simple cubic polynomial that has been chosen to have a nice factorisation:

y=x^3-7x+6

Let us note that the curve passes through the points , and . This corresponds to the fact that . We say that , and are the zeroes or roots of .

The zeroes of a polynomial have a particular importance: from Descartes’ theorem if a polynomial has a zero , that is , then it follows that is a factor of .

In our example, since has zeroes , and , we know that has factors , and . And indeed

So in practice trial and error may be used when looking for possible zeroes by hand. It is a good guess to try factors of the constant term if the polynomial is monic (that means that the highest degree coefficient is ). Of course using a computer is much simpler: our electronic friends are particular adept at this kind of work!

Q1 (M): Try to factor .

If you made progress with this factorisation, then you should find a factor of appearing, and you will find it resists further attempts to factor it. Perhaps it doesn’t factor, but how would we know that the quadratic term does not factor further?

One way is to keep trying the various possible factors, but a better way is to use the quadratic formula. The discriminant of this quadratic is which is not a square, so there is no rational factorisation. We say that is irreducible (over the rational numbers) and leave it at that.

Q2 (M): Find zeroes and factorisations of the following cubics:

a)

b)

c)

Knowing the factorisation of a cubic makes it easier to graph, since we know where it crosses the -axis.

Tartaglia, Cardano and factoring general cubics

Suppose someone gave you the cubic function

and asked you to factor it. Not knowing the left hand side of the equation, it might take some work to find the factors. But what if the cubic does not factor nicely into factors? Just by changing the cubic a little to

makes things vastly more complicated! This is just the kind of challenge that 16th century mathematicians like G. Cardano (1501-1576) and N. Tartaglia (1500-1557) gave each other.

During one of these challenges, Tartaglia discovered a general formula for solving cubics which extends the much more familiar quadratic formula. His formula is quite remarkable in that it requires complex numbers in an essential way, and also both square roots and cube roots. However it is not for the faint-hearted, and it is fair to say that these days it is rarely used.

Portrait of Cardano Girolamo Cardano, By Wellcome Library, London, CC BY 4.0, via Wikimedia Commons

The story of how Cardano (pictured) stole the credit for this formula from Tartaglia is one of the most juicy episodes in the history of mathematics. For this story and other historical information on cubics, check out the See Also link at the very bottom of the page.

Tartaglia’s cubic formula is workable if your example has been chosen carefully to have linear factors in the first place. But a random cubic does not have such a property, and so most cubics can in fact only be approximately factored.

Answers

A1. Let’s look at this problem in some detail. To factor one should look for zeroes which are factors of , which are built from and , and we should not forget about negative possible factors. Now with a bit of trial and error you can find that , so by Descartes’ theorem we know that is a factor.

After that we want to actually divide by . This requires polynomial long division. If you have forgotten how to do long division, you will be happy to learn that polynomial long division is generally simpler than that with numbers.

By looking at successive powers of x, we sequentially determine that the other factor must be . Please try it! The final factoring is

A2.

a) We have .

b) We have .

c) We have .

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This article is from the free online course:

Maths for Humans: Inverse Relations and Power Laws

UNSW Sydney