From Geometrical Figures to Definitional Rigour: Teachers' analysis of Teaching Units Mediated Trough van Hiele's Theory.

The present work deals with a curriculum design used in 8 parallel classes of high school (with students aged 14-15) by means of worksheets dealing with the properties of the principal plane geometric figures. Van Hiele's theory was used to facilitate teachers' discussion of pupils' performances on agreed worksheets, in order to enhance their didactical awareness. Our hypothesis is that van Hiele’s theory is a proper means to provide encouragement and support to teachers for rethinking their teaching goals and practices. This environment enables teachers to manage their pupils' transition from the perception of a figure to its definition. In this paper we will recount the teachers’ discussion as it unfolded in a school-university workgroup. The analysis of pupils’ work on worksheets interweaves with an analysis of the teaching actions that foster the process of understanding definitions.