ok thank you very much

Just a last question: if I want to express  the variable x3  as an interval and not a fixed value, can I do that with solve??

yes, but this result is not explicit, I can not identify the intervals on the variables.     there is no other way???

As example, the following system of the nonlinear inequalities :

(1+x1)(1+x3)>=0.966;

(1+x1)(1+x3)<=1.026;

(1+x2)(1+x3)>=0.9975;

(1+x2)(1+x3)<=0.9987;