Slope of Parallel Lines :
Theorem : If two non vertical lines are parallel, then they have the same slope.
If the two lines have the same slope then the lines are parallel and non vertical.
Slope of Perpendicular lines :
Theorem : If two non-vertical lines are perpendicular to each other then their slopes are opposite reciprocals of each other.
If the product of slopes of two lines is -1, then the lines are non-vertical and perpendicular to each other.
If a line l has a slope of 5/4 then (a) find the slope of a line parallel to l (b) find slope of a line perpendicular to l .
a) The slope of line parallel to l is 5/4.
b) Let the slope of line perpendicular to be x.