Re: trivial calculator problem

Just to add to the options, since Macsyma was mentioned too

With Reduce (available via sourceforge) I can try

on rounded;
precision 5;
equation := impedance = 1/(2*pi*frequency*capacitance);
capacitance := 1.0e-6;
if not numberp impedance then first solve(equation, impedance);
if not numberp frequency then first solve(equation, frequency);
if not numberp capacitance then first solve(equation, capacitance);
end;

and my output is then

12


                              0.159155
equation := impedance=-----------------------
                        capacitance*frequency


capacitance := 0.000001


            159155.0
impedance=-----------
            frequency


            159155.0
frequency=-----------
            impedance



=====

I think that from outside the immediate community I would characterise 
Axiom/Fricas as a sensible choice for those concerned with avoiding 
getting wrong answers because of "special cases" or slips that could 
emerge from failing to be aware of the detailed mathematics theories and 
structures that underpin a calculation. Axiom rather than Fricas may 
especially appeal to those who want to ability to inspect its (literate) 
source and based on that check and follow through everything it does so 
that they can formalise a justification of the correctness of its results.

Arthur