Casyopée is a free open-source software environment for the learning of functions at secondary school level. It aims at making numeric, graphic and formal exploration of mathematical functions easier. A dynamic geometry unit, integrated in Casyopée’s environment, offers possibilities to explore and to model geometrical situations into functions. Casyopée is focused on problem solving about and using functions, modelling and proof. It gives specific help to the student in his/her activities, thanks to symbolic computation. With Casyopée, power means neither complexity nor lack of rigor : choices of representation are in line with mathematical conventions at secondary level ; driving Casyopée is easy, without any command language ; the goal is to help the student, not to restrain him/her.