Answer
1. Consider three lines, A, B, and C, that have the same transversal.
2. If line A is parallel to line B, then its corresponding angles are congruent
3. If line A is parallel to line C, then its corresponding angles are congruent.
4. If line A's corresponding angles are congruent to those of B and C, then it follows that the corresponding angles of lines B and C are congruent.
5. If the corresponding angles of lines B and C are congruent, then they are parallel.
6. Thus, if two lines are parallel to the same line, then they are parallel.
