Parallel Lines
A unit in almost every geometry class has parallel lines cut by a transversal.
These three lines create a slew of angles that you will need to know and understand. They include corresponding, alternate interior, alternate exterior, sameside interior and sameside exterior.
GradeA will make it easy for your to learn these vocabulary terms, and also how to solve problems using them!
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Sameside angles are supplementary (notice the s.s.s.), and the other angles are congruent. We will have a lot more to say about this, as well as some GradeA diagrams to make your life easier!
What are Parallel Lines Anyway? 
Two lines that are in the same plane that do not intersect are said to be parallel. (More information about the undefined terms in geometry).
A transversal is a line that intersects two or more lines. When this happens, eight different angles are created, many of which come in special pairs. This becomes particularly interesting whenever the two lines are also parallel because the angles are either congruent or supplementary.
"Interior" vs. "Exterior" Angles  What's the Difference? 
Interior: Between the Parallel Lines 
Exterior: Outside the Parallel Lines 






Corresponding Angles: There are four pairs of corresponding angles that are created by the transversal. The pairs are in the same location in their respective cluster of four. For example, the "topleft" angle corresponds with the other "topleft" angle. See the diagram below.
TopLeft with TopLeft 

TopRight with TopRight 






BottomLeft with BottomLeft 

BottomRight with BottomRight 






Alternate Interior Angles: We already know that interior angles are between the two lines. The word alternate means that the angles are on opposite sides of the transveral. See for yourself.
Alternate Exterior Angles: Almost the same as alternate interior, except now the angles are on the outside. See below.
SameSide Interior Angles: Interior angles that are both on the same side of the transversal. See below.
SameSide Exterior Angles: Exterior angles that are both on the same side of the transversal. See below.
Degree Measures of Special Angle Pairs 


Corresponding and alternate angles are congruent. Remember, congruent means they have the same degree measure (it's the geometry equivalent of equals)
Sameside angles are supplementary (remember, sss).
Take a look back at the diagrams above. You should be able to see visually which angles are supplemenatry and which angles are congruent. 
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