Student Understanding of Directional Derivatives of Functions of Two Variables.

Action-Process-Object-Schema (APOS) Theory is applied to study student understanding of directional derivatives of functions of two variables. A conjecture of the main mental constructions that students may do in order to come to understand directional derivatives is proposed and is tested by conducting semi-structured interviews with 26 students who had just taken multivariable calculus. The interviews explored the specific constructions of the genetic decomposition that student are able to do and also the ones they have difficulty doing. The conjecture, called a genetic decomposition, is largely based on the elementary notion of slope and on a development of the concept of tangent plane. The results of the empirical study suggest the importance of constructing coordinations of plane, tangent plane, and vertical change processes in order for students to conceptually understand directional derivatives.