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In his G\'eom\'etrie (1637) Descartes introduces the algebra of segments. This is a fundamental step in the mathematical treatment of variable quantities before the creation of differential calculus. It is an algebra with symbols but without numbers, in which the covariation between geometric variables, constrained by ruler and compass constructions or with other geometric constructions, can be expressed with symbolic equations. By using algebraic manipulations, it is possible to easily deduce the properties of the corresponding geometric constructions, including those that produce graphs of rational functions. We believe that the study of functions through Descartes's algebra can be didactically effective in teaching and learning the concept of function in secondary school. Firstly, it avoids the reference to real numbers; secondly, the interpretation of formulas as geometric constructions and vice versa facilitates the "transition" from functions understood as processes to functions understood as objects.

In this paper we make some considerations about using a dynamic geometry software for teaching history of mathematica at university level. After a short introduction to the software GeoGebra, we discuss four activities. The first is an exploration of the menu of the tools menu following the first part of the first book of the elements of Euclid. The second is an activity about pentagons and pentagrams which aims at finding a more natural way of discovering their properties than that suggested by Euclid as an illustration of the method of synthesis and analysis. The third is an activity on circle inversion and its connection with polarity and the construction of the fourth harmonic point after three given point on a line. The activity continues with the reading of Poncelet's ideas about ideal secants of conics and how to use GeoGebra to make sense of them. The fourth activity shows how to use the possibility to move objects in GeoGebra to make sense of the geometric intuition behind Cremona and Hirst letters about the Bellavitis-Hirst inversion of a circle. We discuss the reasons why we believe that these kind of activities may help students to read and understand original writings in projective geometry and why we believe that they may be very effective in the training of prospective teachers of mathematics., Comment: 38 pages, in Italian, 44 figures

We test against two different sets of data an apparently new approach to the analysis of the variance of a numerical variable which depends on qualitative characters. We suggest that this approach be used to complement other existing techniques to study the interdependence of the variables involved. According to our method the variance is expressed as a sum of orthogonal components, obtained as differences of conditional means, with respect to the qualitative characters. The resulting expression for the variance depends on the ordering in which the characters are considered. We suggest an algorithm which leads to an ordering which is deemed natural. The first set of data concerns the score achieved by a population of students, on an entrance examination, based on a multiple choice test with 30 questions. In this case the qualitative characters are dyadic and correspond to correct or incorrect answer to each question. The second set of data concerns the delay in obtaining the degree for a population of graduates of Italian universities. The variance in this case is analyzed with respect to a set of seven specific qualitative characters of the population studied (gender, previous education, working condition, parent's educational level, field of study, etc.), Comment: 3 figures