Principles towards justification: A focus on teacher actions

International audienceThe aim of this paper is to understand how a proposed set of design principles regarding tasks and teacher actions provide students with opportunities to justify. We see justification as a reasoning process that relies on mathematical concepts, properties, procedures, ideas, and, in some situations, particular cases. The teaching intervention, part of a design-based research, is carried out in a grade 7 class of an experienced teacher in nine lessons about linear equations. Data is gathered by classroom observations (video and audio recorded) and a researcher logbook. Data analysis takes into account a set of design principles and a framework regarding students' justifications. The results show that paths of teacher's actions that rely on the design principles enable students to present rather complete justifications based on logical coherence and on mathematical aspects of the situation