first of all thank you for your useful and quick answers!
The method of the two lines works, and I am happy of that.
The problem is that when I want to extrude, I must delete every time the two lines (my final result is a "clever" geometry that changes with parameters without any other action).
Roland, your solution seems interesting, but being a novel I didn't understand it well: I draw a circle>right clic on the center>more dimensions>horizontal ordinate (right?). And then what should I do? Could you explain me step by step?
Thank you all again guys