Restrain fsolve and fzero to return only postivie solutions

I am trying to solve some non linear equations, quite simply I would say, with fzero and fsolve. I get a solution but I would like to aquire only positve solutions, which they do exist. Is there a way to avoid negative solutions ?

I would like to note that I have tried fzero(fun,[xlower, xupper]) but unfortunately it doesn't work.

Answer 1

[Torsten]

In fzero or fsolve, square the x vector with which you are asked to evaluate your equation:

function res = fun(x)
  x = x.^2;
  ...
  res = ...
end

Then, when fzero or fsolve returns to the calling program, x.^2 solves your equations, and x.^2 is non-negative.