1. a || b
1. given
2. c is a transversal of a and b.
2. given
3. ∠1 ≅ ∠3
3. corresponding angles theorem
4. ∠3 and ∠8 are vertical angles.
4. def. of vertical angles
5. ∠3 ≅ ∠8
5. vertical angles are congruent
6. ∠1 ≅ ∠8
6. transitive property