The aim of this research is to advance understanding of how mathematical knowledge functions in the proving in geometry. We focus on the rules whose mobilization is due rather to the mathematical knowledge at stake than to the proof. We observed students who are asked to solve construction problem and proving problem. The problems require to mobilize the "same" rule from the theoretical point of view. The first result is that even if the students can construct correctly a symmetric and are conscious of the necessity of perpendicular and equal distance, it doesn't mean that they can use the rule appropriate in proving. [For complete proceedings, see ED489538.]