1. Parallel Lines

3. Definition
Two lines are parallel if they lie in the same plane and do not intersect.
If lines m and n are parallel we write p q
p and q are not parallel m n

4. We also say that two line segments, two rays, a line segment and a ray, etc. are parallel if they are parts of parallel lines. D C A B P m Q

5. Transversals
A transversal for lines m and n is a line t that intersects lines m and n at distinct points. We say that t cuts m and n. t m n

6. When a transversal cuts parallel lines, special pairs of angles are formed that are sometimes congruent and sometimes supplementary.

7. Corresponding Angles
A transversal creates two groups of four angles in each group. Corresponding angles are two angles, one in each group, in the same relative position. 1 2 m 3 4 5 6 n 7 8

8. Vertical Angles
When a transversal cuts two lines, vertical angles are angles that are opposite one another and share a common vertex.
If m || n, then vertical angles are congruent. 3 m 1
Angles 1 and 3 are vertical angles.
Angles 2 and 4 are vertical angles. 2 n 4

9. Alternate Interior Angles
When a transversal cuts two lines, alternate interior angles are angles within the two lines on alternate sides of the transversal. m 1 3 4 2 n

10. Alternate Exterior Angles
When a transversal cuts two lines, alternate exterior angles are angles outside of the two lines on alternate sides of the transversal. 1 3 m n 4 2

11. Interior Angles on the Same Side of the Transversal
When a transversal cuts two lines, interior angles on the same side of the transversal are angles within the two lines on the same side of the transversal. m 1 3 2 4 n

12. Exterior Angles on the Same Side of the Transversal
When a transversal cuts two lines, exterior angles on the same side of the transversal are angles outside of the two lines on the same side of the transversal. 1 3 m n 2 4

13. Example In the figure and Find
Since angles 1 and 2 are vertical, they are congruent. So,
Since angles 1 and 3 are
corresponding angles,
they are congruent.
So, 1 3 2 m n

14. Example In the figure, and Find Consider as a transversal for the
parallel line segments.
Then angles B and D are
alternate interior angles
and so they are congruent.
So, A B C D E

15. Example In the figure, and If then find
Considering as a transversal, we see that angles A and B are interior angles on the same side of the transversal and so they are supplementary.
So,
Considering as a transversal, we see that angles B and D are interior angles on the same side of the transversal and so they are supplementary. A B ? C D

16. Example In the figure, bisects and Find Note that is twice So,
Considering as a transversal for the parallel line segments, we see that
are corresponding angles and so they are congruent. A D E B C

17. Example In the figure is more than and is less than twice Also, Find m
Let denote Then
Note that angles 2 and 4 are alternate interior angles and so they are congruent.
So, Adding 44 and subtracting from both sides gives
So, Note that angles 1 and 5 are alternate interior angles, and so m n 2 4 3 5 1

18. Proving Lines Parallel
So, far we have discussed that if we have a pair of parallel lines, then certain pairs of angles created by a transversal are congruent or supplementary.
Now we consider the converse.
If two lines are cut by a transversal so that corresponding angles are congruent, then the lines are parallel.
If the alternate interior or exterior angles are congruent, then the lines are parallel.
If the interior or exterior angles on the same side of the transversal are supplementary, then the lines are parallel.

19. Example In the figure, angles A and B are right angles and What is
Since these angles are supplementary. Note that they are interior angles on the same side of the transversal This means that
Now, since angles C and D are interior angles on the same side of the transversal they are supplementary.
So, A D B C