From the following geogebra applet we can visualise that slope of two parallel lines are same and slope of two perpendicular lise are negetive resiprocals of each other
[[http://tube.geogebra.org/material/show/id/143280 From this geogebra applet we can visualise that slope of two parallel lines are same and slope of two perpendicular lise are negetive resiprocals of each other]]