# factors of a polynomial

• January 13th 2009, 02:46 AM
factors of a polynomial
The polynomial $2x^4-ax^3+19x^2-20x+12=0$ has a factor in the form of (x-k)^2 , where k are natural numbers . Find the values of k and a .
• January 13th 2009, 03:33 AM
Moo
Hello,
Quote:

The polynomial $2x^4-ax^3+19x^2-20x+12=0$ has a factor in the form of (x-k)^2 , where k are natural numbers . Find the values of k and a .

$f(x)=2x^4-ax^3+19x^2-20x+12$
If it has such a factor, then $f(k)=0$ and $f'(k)=0$

This should give you a system of 2 equations, and you can solve for a and k !
• January 15th 2009, 12:27 AM
Quote:

Originally Posted by Moo
Hello,

$f(x)=2x^4-ax^3+19x^2-20x+12$
If it has such a factor, then $f(k)=0$ and $f'(k)=0$

This should give you a system of 2 equations, and you can solve for a and k !

Thanks Moo , just wondering $f(k')$ , does it mean differentiate and why is it equals 0 .
• January 15th 2009, 12:38 AM
Moo
Quote:

Thanks Moo , just wondering $f(k')$ , does it mean differentiate and why is it equals 0 .

It's f'(k), and yes, it means that you differentiate f, and then you take the value x=k.

Okay, let's say (x-k)² is a factor for f.
There exists a polynomial Q such that :
$f(x)=(x-k)^2 Q(x)$
It is obvious that $f(k)=0$
Now if you differentiate, use the product rule and you'll have :
$f'(k)=(x-k)^2 Q'(x)+2(x-k)Q(x)=(x-k)[(x-k)Q'(x)+2Q(x)]$
Is it clear that $f'(k)=0$ ?

In fact, it is true for higher powers : if f has a factor $(x-k)^n$, then $f(k)=f'(k)=f^{(2)}(k)=\dots=f^{(n-1)}(k)=0$
• January 15th 2009, 07:34 AM