Resumen

One academic semester long pilot teaching-experiment on the learning of geometry was conducted with one pre-service teacher, one lateral entry teacher, and two in-service teachers. The purpose of the pilot teaching experiment was to understand how learners, who already have some knowledge of geometric objects, are able to reorganize and broaden their knowledge when using the Geometer’s Sketchpad (GSP). The guiding principle of the teaching-experiment was to use semi structured tasks especially designed for the GSP to foster the three way-interaction among the student-teacher, the GSP, and the teacher interviewer. The objective of each task was to allow each participating student-teacher the freedom to explore geometric situations, to make conjectures, and to prove them. In this article, we analyze how the lateral entry teacher, here with the pseudonym of Susan, solved a task which purpose was to investigate isosceles trapezoids and some of the properties. The analysis indicates that Susan used the GSP as a technical tool but also constituted it into a psychological-symbolic tool in the process of conceptualizing and proving some of the properties of isosceles trapezoids.