• January 3rd 2010, 06:40 AM
Stuck Man
I don't know how to do this question:

Given that alpha is a root of the equation x^2=2x-3 show that alpha^3=alpha-6.

I have a list of equations involving the roots calling them alpha and beta.
• January 3rd 2010, 06:57 AM
Hello Stuck Man
Quote:

Originally Posted by Stuck Man
I don't know how to do this question:

Given that alpha is a root of the equation x^2=2x-3 show that alpha^3=alpha-6.

I have a list of equations involving the roots calling them alpha and beta.

If $\alpha$ is a root, then:
$\alpha^2=2\alpha-3$ (1)
Multiply both sides by $\alpha$:
$\Rightarrow \alpha^3 = (2\alpha-3)\alpha$
$=2\alpha^2-3\alpha$

$= 2(2\alpha -3)-3\alpha$, using (1)
I'm sure you can complete it now.

• January 3rd 2010, 07:12 AM
Stuck Man
I see, thanks.

Is there any particular topic that results such as this are in?:
a^2+b^2=(a+b)^2-2ab
• January 3rd 2010, 07:26 AM