# Are there any characteristics of polynomials with rational roots?

• Dec 15th 2012, 04:32 AM
HenrikEgeland
Are there any characteristics of polynomials with rational roots?
In a few days i´m having a presentation in number theory.

Are there any characteristics of polynomials with rational roots? Both the expression and graphical.

How do I do this?
• Dec 15th 2012, 04:51 AM
jakncoke
Re: Are there any characteristics of polynomials with rational roots?
Like what do you mean.

Like this one? Assume $deg(f(x)) \geq 2$

if $f(x) \in \mathbb{Q}[X] \text{ such that f(x) is irreducible in } \mathbb{Q}[X] \text{ then f has no roots in } \mathbb{Q}$ ?

or

$f(x) \in \mathbb{Z}[X] \text{ if f(x) is reducible in } \mathbb{Q}[X] \text{ then f(x) is reducible in also} \mathbb{Z}[X]$
• Dec 15th 2012, 05:27 AM
HenrikEgeland
Re: Are there any characteristics of polynomials with rational roots?
Sorry, did´nt get that :P

But the teacher gave me a tip
x^2+px+q=0
x=r1 V x=r2

(x-r1)(x-r2)=x2-x(r1+r2)+r2*r1
p=-(r2+r1)
q=r2*r1
• Dec 15th 2012, 06:08 AM
jakncoke
Re: Are there any characteristics of polynomials with rational roots?
Check out the rational root theorem
Rational root theorem - Wikipedia, the free encyclopedia