hey can someone remind me how you get the factors of something like this:

-x^3 + 5x^2 - 5x - 24 = 0

thank you for your help, its been so long since idone these i tried googling but could not find a systematic approach,

Printable View

- Oct 13th 2008, 09:51 AMjonbob81factoring 3rd order polynomial...
hey can someone remind me how you get the factors of something like this:

-x^3 + 5x^2 - 5x - 24 = 0

thank you for your help, its been so long since idone these i tried googling but could not find a systematic approach, - Oct 14th 2008, 12:51 AMmr fantastic
As far as I can see -x^3 + 5x^2 - 5x - 24 has no 'simple' factors. A general approach would be to use the cubic formula, but often that's like killing a fly with an elephant gun. Sometimes grouping will work, sometimes simple linear factors can be found by inspection.

- Oct 14th 2008, 01:23 AMjonbob81
i was thinking the same. i was trying to produce the eigenvalues of this matrix

call lambda x

(2-x ) 0 3

0 (6-x) -3

3 -3 (2-x)

and that formula below was what i got - Oct 14th 2008, 02:36 AMCaptainBlack
If this has rational roots (and so nice factors) by the rational root theorem the roots are amoung the factors of 24, so try +/11, +/-2, +/-3, +/-4, +/-6, +/-8, +/-12. If none of these are roots then it has no rational roots and so no linear factors with integer coefficients.

(As it happens none of these are roots and so this cubic has no nice factors, but it does have exactly one real root)

You can find the root algebraicaly by using the Cardano-Tartaglia cubic formula, though you will probably be better off solving for the real root numerically

**CB** - Oct 14th 2008, 05:25 AMmr fantastic
- Oct 14th 2008, 05:32 AMjonbob81