Covariational Reasoning and Instrumented Techniques in the Resolution of an Optimization Problem Mediated by GeoGebra

The aim of this study was to document how different instrumental techniques are related to different mental actions during the work of university students in an optimization activity mediated by GeoGebra. To minimize the length of the fence of a rectangular plot, the students put into play mental actions, which were visible in the manipulations with GeoGebra, associated with different levels of covariational reasoning, and which allowed us to identify different instrumented techniques. It is concluded that the use of instrumented techniques involving concepts related to the derivative, such as the slope of the tangent line or the derivative function, make visible behaviors associated with mental actions of a more sophisticated covariational reasoning.