On the left panel, click on the check box "two
lines". Two lines are now displayed and you can change their position by
dragging points A, B, C and D. Change the position of one line so that the two
are parallel ( no point(s) of intersection). Compare the slopes of the lines.
Slopes of Perpendicular lines
Change the position of the lines so that they are
perpendicular ( make an angle of 90 degrees). To make sure that they are
perpendicular, adjust the lines so that they go though the diagonals of the
small squares in the rectangular system. Find the product of the two slopes. Do
this last experiment several times. What can you say about the product of the
slopes of two perpendicular lines?
More references on slopes.
Easy to use calculator to find slope and equation of a line through two points.Two Points Calculator