Definition of a perpendicular line

When two lines intersect and form a 90° angle, also called a right angle, then they are said to be perpendicular.

The symbol ⊥ is used to represent perpendicular
 
Line AB⊥CD

Perpendicular lines intersect at right angles  ( 90 degrees )

A straight line has a measure of 180 degrees therefore, all four of the angles will be 90 degrees when the lines are perpendicular.

Examples of perpendicular lines

Parallel lines

Parallel lines never cross each other. 
 
The slopes of parallel lines are equal.  
 
The rise and the run will be the same for both lines. In this example, both lines have a slope of 1 
( Rise=2,Run=2) 
 
The symbol for parallel lines is ||  
 
Parallel lines are the same distance apart from one another at any given poin

Parallel lines

 
A negative reciprocal is the fraction form of a number flipped upside down ( the reciprocal part), and its sign changed. (that’s the negative part). 
When the  slopes are written  side by side it becomes more obvious: Slope of line m  =-4/2  
Slope of line l = 2/4     -4/2   2/4

How do you determine if two lines are parallel or perpendicular?

Given two lines. One line passes through (-3,0) (1.1), the other line passes through (0,3) (2,-5)
 
Are these two lines perpendicular,parallel, or neither?
 
These lines are perpendicular because when you find their slope they are negative reciprocals.  
 
The video gives step by step directions for determining if two lines are perpendicular, parallel or neither.

Transcript

Welcome to MooMooMath. Today we are going to look at perpendicular lines. What are perpendicular lines or what is perpendicular? Well perpendicular are two lines that intersect at 90 degree angles. So let’s look at a couple of examples of where we will see those. So we have two lines and they are intersecting at 90 degree angles, and I will label it with a little box that represents 90 degrees, and once we know this is 90 degrees we know the other three are 90 degrees because these are lines and lines have a degree of 180 so that would make all four of these 90 degrees. Now let’s look at the two line segments. This angle is 90 degrees and you will often see this in a right triangle. So a right triangle has two segments that are perpendicular and measure 90 degrees. So that is perpendicular lines. Hope this helps