@Preben Alsholm Thank you! I am going to try the line you propose. I am absolutely blocked, any help are wellcome

Thank you very much for the reply, but sorry: the example above is only a easier example that I have made up for ilustrate what I am looking for. My true problem is quite more complex:

restart;
eq1 := diff(Delta(xi), xi) = -q(xi);
eq2 := diff(Theta(xi), xi) = 1.25*q(xi)-1.1*Delta(xi);
eq3 := Theta(xi) = .8*q(xi)+0.8e-1*q(xi)*ln((1-xi+sqrt(0.1e-1+(1-xi)^2))*(sqrt(0.1e-1+xi^2)+xi)/((1+xi+sqrt(0.1e-1+(1+xi)^2))*(sqrt(0.1e-1+xi^2)-xi)))-0.8e-1*(int((q(xi)-q(zeta))*(1/sqrt(0.1e-1+(xi-zeta)^2)-1/sqrt(0.1e-1+(xi+zeta)^2)), zeta = 0 .. 1));
cc1 := Delta(0) = 1; cc2 := Delta(1) = 0

Almost certainly it does not have analitical solution, I need a way to get the numerical solution. Could anyone help me?

Thanks again, and sorry again for not to post my real problem from the begining.