Discovering "whys" behind the "change side–change sign" rule in algebra

The change side–change sign procedure (CS procedure) for equation solving is often used by students as a memorized rule without justification. This paper reports on findings from the DynaMat project that sees remediation not as repetition of procedures, but as access to mathematical meaning through Digital Exploratory Objects (DEOs). Grounded in Commognition, the study analyzes a case of two grade-10 students to investigate whether and how the remedial activities supported the substantiation of the CS procedure. The findings show that the intervention supported students through a guided process of discourse expansion with the DEOs, followed by a compression in which the CS procedure came to be seen as a substantiated shortcut.