In this investigation, students are asked to find the slopes of various lines. They use the slopes to come up with conjectures. Because the lines are graphed for the students, this investigation allows them to focus on the relationships between slopes of parallel and perpendicular lines.
An extension helps students learn the equations of vertical and horizontal lines as they relate to their slopes.
Investigations empower students to take charge of their learning. The challenge of discovering the relationships is much more satisfying than being told them.