Parallel & Perpendicular Lines
Perpendicular Lines
By definition, two lines are perpendicular if they intersect at right angles. Two lines with slopes m1 and m2 are perpendicular if 
. In other words the slopes of perpendicular lines are the opposite reciprocals of each other. For example, if the slope of a line is 
, then the slope of a line perpendicular to it is 
. Using this property of the slopes of perpendicular lines, an equation of a line perpendicular to a given line can be written.