pseudockb,
Go to the following website:
h**p://en.wikipedia.org/wiki/Cubic_equation
Scroll down to equation 4.
Let A = the value of u with the + sign under the radical.
Let B = the value of u with the - sign under the radical.
A, B are the principal cube roots.
It can be shown, e.g.,CF Burrington "Handbook of Mathematical Tables and Formulas", available at any decent library, that the solution for the real part of the root is
-(1/2)(A+B).
So if (A+B) is positive, the real part of the root is negative. If (A+B) is neagtive, then the real part of the root is positive.
.
Incidentally, the imaginary parts of the roots are given by +/-[(jSQRT(3)/2)](A-B)
.
This exercise may not be worth the effort, since by the time you calculate A, B, you almost have the complete solution! This is the best that I can do. Hope it helps.
Regards,
Kral