Learn how to find equations of parallel lines, work out the gradient of perpendicular lines, and prove that given lines are parallel or perpendicular.

Patricia Wallace-Gomez, Jodie A. Miller, Slopes of Parallel and Perpendicular Lines: A Visual Justification, The Mathematics Teacher, Vol. 104, No. 9 (MAY 2011), p. 728 ...

Horizontal lines run left to right, parallel to the horizon, with slope 0 and equation y = k. Vertical lines run top to bottom, perpendicular to the horizon, with undefined slope and equation x = k.

Horizontal lines run left to right, parallel to the horizon, with slope 0 and equation y = k. Vertical lines run top to bottom, perpendicular to the horizon, with undefined slope and equation x = k.