1. ∠1 and ∠3 are vert. ∠s
1. given
2. ∠1 and ∠2 are a lin. pr.
2. def. of lin. pr.
3. m∠1 + m∠2 = 180
3. ∠ add. post.
4. ∠2 and ∠3 are a lin. pr.
4. def. of lin. pr.
5. m∠2 + m∠3 = 180
5. ∠ add. post.
6. m∠1 + m∠2 = m∠2 + m∠3
6. subst. prop.
7. m∠1 = m∠3
7. subtr. prop.
8. ∠1 ≅ ∠3
8. def. of ≅