Students must be able to relate many representations of slope to form an integrated understanding of the concept.
Courtney R. Nagle and Deborah Moore-Russo
S. Asli Özgün-Koca, Michael Todd Edwards, and Michael Meagher
The Spaghetti Sine Curves activity, which uses GeoGebra applets to enhance student learning, illustrates how technology supports effective use of physical materials.
Chalk Talk and Claim-Support-Question are two routines for developing students' ability to use multiple representations and encouraging classroom discussion.
When teaching slopes of parallel and perpendicular lines, I want students to have a visual image of the lines, not just memorize a formula. A simple exercise with parallel lines can get the message across.