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If 2 parallel planes are cut by a third plane, then the lines of intersection are parallel.
Theorem 31

If 2 parallel lines are cut by a transversal, then corresponding angles are congruent.
Postulate 10

If 2 parallel lines are cut by a transversal, then alternate interior angles are congruent.
Theorem 32

If 2 parallel lines are cut by a transversal, then sameside interior angles are supplementary.
Theorem 33

If a transversal is perpendicular to one of two parallel lines, then it is perpendicular to the other one also.
Theorem 34

If 2 lines are cut by a transversal and corresponding angles are congruent, then the lines are parallel.
Postulate 11

If 2 lines are cut by a transversal and alternate interior angles are congruent, then the lines are parallel.
Theorem 35

If 2 lines are cut by a transversal and sameside interior angels are supplementary, then the lines are parallel.
Theorem 36

In a plane, 2 lines perpendicular to the same line are parallel.
Theorem 37

Through a point outside a line, there is exactly 1 line parallel to the given line.
Theorem 38

Through a point outside a line, there is exactly 1 line perpendicular to the given line.
Theorem 39

2 lines parallel to a third line are parallel to each other.
Theorem 310

Name 5 ways to prove two lines are parallel when all three lines are coplanar.
 Show that a pair of corresponding angles are congruent.
 Show that a pair of alternate interior angles are congruent.
 Show that a pair of sameside interior angles are supplementary.
 In a plane show that both lines are perpendicular to a third line.
 Show that both lines are parallel to a third line.

