Parallel line

Pairs of Angles

parallel lines angle example

When parallel lines get crossed by another line (which is called aTransversal), you can see that many angles are the same, as in this example:

These angles can be made into pairs of angles which have special names.

Testing for Parallel Lines

Some of those special pairs of angles can be used to test if lines really are parallel:

If Any Pair Of … Example:
Corresponding Angles are equal a = e
or
Alternate Interior Angles are equal c = f
or
Alternate Exterior Angles are equal b = g
or
Consecutive Interior Angles add up to 180° d + f = 180°
… then the lines are Parallel
parallel angle pairs

Examples

These lines are parallel, because a pair ofCorresponding Angles are equal. parallel angle example 110 110
not parallel angle 81 101 These lines are not parallel, because a pair ofConsecutive Interior Anglesdo not add up to 180° (81° + 101° =182°)
These lines are parallel, because a pair of Alternate Interior Angles are equal parallel angle example 70 70

 

 

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