Mental Geometry

Abstract

This article illustrates pedagogy through training in the handling of abstractions. Mental arithmetic is not limited to numerical calculation; one can mentally calculate primitives and simplify analytical expressions. Even if there is software that does this very well, this training retains its pedagogical value. Can we go further and consider geometric mental arithmetic: mentally proceeding with transformations of simple figures allowing the calculation of areas or volumes? It turns out that the intuition that allowed Archimedes to obtain his main geometric results, if we take only the ideas without the old-fashioned style, provides the opportunity for a pleasant and quite rich mental game that I present here in the form of a short narrative dialogue, not a philosophical tale because it does not bring any thesis, simply a story to be classified among the invitations to exercise the mind. It starts with the area of a triangle and ends with Guldin's two theorems.