This lesson uses algebra to prove that given lines are parallel or perpendicular. It also gives students a more precise definition of parallel and perpendicular.
Students will use slope to show that 2 lines are parallel or perpendicular.
a. y = 2x + 3
y = 2x – 6
b. Have students tell if these lines are parallel or perpendicular and explain their answers.
c. Now plot these equations: y = 2x + 3 and y = -1/2x + 6
d. Have students tell if these lines are parallel or perpendicular and explain their answers. If using a paper graph, use a print or braille protractor to measure the angles where the lines intersect.
e. Find the slope for all equations. Have students tell what they notice about the slopes of the parallel lines and then about the perpendicular lines.
f. Add this to the definition of parallel and perpendicular lines. Parallel lines have the same slope. Perpendicular lines have slopes which are negative reciprocals.
By Marilyn Mears