Driven by the underlying need to combine face-to-face lectures and online activities for the teaching and learning of mathematics at the university level and assuming a co-disciplinary perspective, we investigate theoretical issues concerning task design suitable for blended environments. The tetrahedron model for didactic systems in e-learning is used as a reflective tool to consider three case examples that are selected to highlight the similarities, differences and main principles in the design of online resources. This analysis results in a set of guidelines for effective task design that responds to the needs of both students’ learning and the teacher’s work.