Alternate Exterior Angles

Sunday, June 30, 2013
What are alternate exterior angles?

In this video, you will learn:

The rules of alternate exterior angles
How to identify alternate exterior angles.
How to use the rules of alternate exterior angles to find missing angles.

When do we have alternate exterior angles? 

alternate exterior angles

Alternate exterior angles are created when you have two lines and a transversal crosses both of those lines. Sometimes those two lines are parallel and sometimes they aren’t. 

Imagine two lines, line a and line b that are parallel to each other. 

This line intersecting the two parallel lines is the transversal. 

I like to think of this as a hamburger with the meat in between the two lines and the buns are where the alternate exterior angles will be found. 

alt ext angles

Alternate means on opposite sides of the transversal. Then we have an external angle which means it is not between the two buns but on the outside on the top and the bottom. 

The exterior is on the outside. So I will mark these in different colors. You have an alternate exterior angle here so I will call them angle 1 and angle 2 and they are your alternate exterior angles. 

Now I’m going to mark another angle. I’m going to mark 3 here and 4 here and those two angles are alternate exterior angles. Now there is something different about these angles. 

When you have two parallel lines and a transversal these alternate exterior angles are also congruent to each other and angles 1 and 3 are linear pairs so I’m going to throw out some numbers. 

Let’s say angle three is 50 degrees so we know angle 4 will also be 50 degrees because alternate exterior angles are congruent. Now we can also figure one and two since angle 3 is 50 and 1 and 3 are linear pairs and supplementary so I take 50 and subtract from 180 and that gives me 130 degrees. 

So angle 1 will be 130 degrees. So angle 2 down here will also be 130 degrees since they are alternate exterior angles and a linear pair with angle 4.

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