I want to solve a set of equations, linear, or sometimes quadratic. I don\'t have a specific problem, but often, I have been in this situation often.
It is simple to
It depends on your needs:
If you want an interactive graphical interface, then sage is probably the best solution.
If you want to avoid using a graphical interface, but you still want to do computer algebra, then sympy or maxima may cover your needs. (sympy looks very promising, but it still have a long way to go before they can replace mathematica).
If you don't really need symbolic algrebra, but you need a way to program with matrices, solve differential equations, and minimize functions, then scipy or octave are excelent starting points.