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Perpendicular lines are two lines that intersect and create  90 degree angles.  ( right angles )

The slopes of perpendicular lines are negative reciprocals. 

For example the negative reciprocal of 1 /2 = - 2/1
If the slope of line m is m=-4/2  then the perpendicular lines slope equals m = 2/4

Parallel Lines

Perpendicular Lines

Slope of a Line

Parallel lines are two lines in a plane that never intersect.

Parallel lines have the same slope. 

Example Question 1 One line passes through the points (-2,-1) and (1, 2); another through the points ( 1,2 ) and (3,4) are they parallel, perpendicular, or neither?

Step 1. Find the slope of each line using the slope formula:

Step 2. Because the slope of each line is equal the lines are parallel.

Video Answer to Example  Question 1

Additional Information Slope of Parallel Lines