Class discussion as an opportunity for proportional reasoning

The initial phase of the construction of proportional reasoning with students aged 12 to 13 is described. In the proposed problems the recourse to the constance of ratios originates as a strategy necessary to tackle the situaztions, in opposition to other resolutive strategies which are quite spontaneous but not really suitable. In refernce to costructivist perspectives, the main points of contact are underlined, with a particular enphasis on social constructivis