# solving equation

• Apr 15th 2009, 11:59 PM
noob mathematician
solving equation
Hello.. I'm not sure whether this should be posted under here:

What is the standard method to solve for the following equation:
$4x^4-x-9=0$
without using software.

Thanks!
• Apr 16th 2009, 12:09 AM
• Apr 16th 2009, 12:13 AM
Twig
Are you sure itīs not supposed to be $4x^{4} -x^{2} -9 = 0$
That would simplify things..
• Apr 16th 2009, 12:38 AM
noob mathematician
That's the equation.. Thanks for the link too.. but it's so difficult.. wondering anyone with the software can help me with the 4 roots?
• Apr 16th 2009, 01:51 AM
Twig
hi

Did u mean its supposed to be an $x^2$ term?

In this case:

let $x^2 = t$

Giving us: $4t^2 -t -9 = 0$

Which can be written: $(t-\frac{1}{8})^{2} = \frac{145}{8}$

Solving gives: $t_{1} = \frac{1+\sqrt{145}}{8}$
$t_{2} = \frac{1-\sqrt{145}}{8}$

But we have $x^2 = t$, so two solutions are:
$\sqrt{\frac{1+\sqrt{145}}{8}} \mbox{ and } -\sqrt{\frac{1+\sqrt{145}}{8}}$

You will also get two imaginary roots, but I donīt write them out here...

Btw: If you DID mean $4x^4-x-9 = 0$, Mathematica gives me a HORRIBLE answer...