Factoring cubic polynomials

I do not understand how we know a cubic polynomial such as

z^3 -3z^2 + 4z - 2 = 0

can be factored as

(z - 1)(z^2 - 2z +2) = 0

I can, of course, work it backwards but is this actually some kind of identity? and if so what is its derivation?

At this sort of level, it's pretty much a case of trial and error.

Substitute values for z and hope that one of them gets you a zero value for the polynomial.

Start off by trying z = 0,1,2,-1,-2,3 etc. For this particular example you get a hit with z = 1 in which case z - 1 is a factor and you can now divide out to determine the quadratic.

Notice though that if for two of your trial values the polynomial changes sign then there has to be a zero between those two values. So for your example z = 0 and z = 2 gets you polynomial values of -2 and +2 in which case there has to be a value of z between 0 and 2 making the poynomial zero. (In general this need not be an integer value.)

If you get no joy from this method you might consider sketching a graph or graphs (usually graphs is better).
For this example rewrite the equation as and sketch the graphs of and The values of z at which the graphs intersect will be the roots of the equation.
For this example this will tell you that there is a root somewhere between 1/2 and 3.

Hello, alyosha2!

Quote:

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I do not understand how we know a cubic polynomial such as: .

can be factored as: .

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We are given a polynomial equation: .

We are expected to know two theorems.

[1] If , then is a root of the equation
. . .and is a factor of

[2] If has a rational root, it is of the form:
. . .where is a factor of the constant term
. . .and is a factor of the leading coefficient.


Our polynomial is: .
Its constant term is 2; its factors are:
Its leading coefficient is 1; its factor are:

Hence, the only possible rational roots are: .

So, we try each of them until one of them makes equal to zero.


We find is a zero of the polynomial.
. . Hence, is a factor.

Using long division, we have: .