# [Maxima] solving a cubic eq

Sam Steingold sds at gnu.org
Fri Apr 17 09:12:26 CDT 2009

```Hi,
I am sorry about asking yet another trivial question, but here it goes again.
solve(A^3+3*A^2-13*A-38=0,A);
results is 3 roots (as it should), EACH containing %i.
this is clearly wrong because at least one root is real.
also, the formulas are horrible.
how do I reduce them to something more palatable?
at the very least I should be able to get a float approximation.
alas,
%o1,numer;
does not produce anything simple, as in
A=a+b*%i
A=c+d*%i
A=e
where a,b,c,d,e are floats.
(actually, b=d=0 as plot2d(A^3+3*A^2-13*A-38,[A,-4,4]); clearly shows 3 real
roots -- BTW, how do I get the grid on the plot? [grid,10,10] does not get me
the grid lines! I am, obviously, most interested in the A=0 line).

Fortunately, I found the allroots function which did what I wanted.

Thus, my long rant can be reduced to 2 questions:
1. how do I the float (complex) value of a symbolic expression involving %i, %e &c?
2. how do I get the plot to contain grid lines?

thanks!
Sam

```

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