Interactive theorem-finding in plane geometry -- an example

This contribution to heuristic geometry proves the efficiency of the new generation of interactive tools for drawing in plane geometry in processes of theorem-finding. The background of such processes should be seen in the general skills and abilities to be acquired by the pupils. The paper concludes with a discussion of the problem of securing knowledge with respect to the propositions gained by computer-assisted conjecturing. 'When you have satisfied yourself that the theorem is true, you start proving it.' Polya [1]