Thank you guys for the answers.
> You will need to tell us which version of Maxima you are using.
I use Maxima with wxMaxima:
wxMaxima 0.8.4
Maxima version: 5.22.1
Lisp: CLISP 2.48 (2009-07-28)
> If you want a bigfloat result, omit numer from the example above.
With numer removed I got -7.091962086421661b2. I don't know whether or
not this differs only in representation with Leo's result
-709.1962086421661. But I definitely don't get his other results.
> find_root only works with ordinary floats, not bigfloats.
> find_root tries to convert any other kind of number to ordinary floats.
I thought bfloat(find_root()) would do that. Anyway, so how do you
find the solutions of non-polynomial equations in big float?
>> (%i4) sqrt(1b-1000000);
>> (%o4) 1.0b-500000
> I don't understand what is there error here.
No errors here. I just mentioned this one to say that sometimes big
floats work and sometimes they don't.