Our study concerns the conceptual mathematical knowledge that emerges during the resolution of tasks on the equivalence of polynomial and rational algebraic expressions, by using CAS and paper-and-pencil techniques. The theoretical framework we adopt is the Anthropological Theory of Didactics ("Chevallard" 19:221-266, 1999), in combination with semiotic aspects from the instrumental approach to tool use. The analysis we present is based on interviews carried out with a 10th grade student who participated in our research. Our findings highlight the mathematical knowledge ("technological discourse") constructed in the process of confronting, differentiating, and articulating the several mathematical techniques and theoretical ideas (pertaining to the "numeric perspective" and the "syntactic perspective" on algebraic equivalence) related to the designed equivalence tasks.