Zusammenfassung
In this proposal, we present some elements of a didactical engineering concerning an algebraic question from a double algebraic and topological viewpoint. Teachers know that “ the study of sign of polynomials” is difficult in spite of the simplicity to build and use the array of signs. We think that a reason of this difficulty is in the following fact : impossibility for pupils to implement personal knowledge which is not algebraic but which can be pertinent to tackle the question. For instance, the intermediate value principle is a result which is very easily available as an implicit tool to draw graphics and exploit them.According to our analysis, the pupils have to deal with problems where functions, graphics and algebraic expressions are put in stage dialectically.
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