The algaebra is a major issue in the teaching of mathematics at the college. Its introduction poses many problems to teachers and students. In this article we report on a 5th class experiment carried out at the time of the introduction of algaebra. The activity uses a calculation program that always gives the starting number, the aim is to have the conjecture established by the pupils and then to demonstrate it. On the basis of this example, we want to analyse the potential of the calculation programmes in the context of algae evidence. We set out our questions in the context of the work on elementary algebra and on the evidence developed in the teaching of mathematics. Following an analysis of the official documents on the current place of evidence in alga, we then show how an evidentiary activity can contribute to the construction of the meaning of the letter in class 5e.