> solve(Theta[e] = Theta[1],xi);

                         1., 0., 0.5081451346

> subs(xi = 0.5081451346, Theta[e]);

                             0.573392702

> solve(Theta[e] = Theta[q],xi);

                         1., 0., 0.5081451394

> subs(xi = 0.5081451394, Theta[e]);

                             0.573392698

> evalf(solve(Theta[e] = Theta[s],xi));

                         1., 0., 0.541381265

> subs(xi = 0.541381265, Theta[e]);

                             0.541381265

