Answer: 1.∠1 and ∠2 are supplementary angles........... 1. Given
2. m∠1 + m∠2 = 180° ........................... 2. Definition of Supplementary Angles
3. ∠1 and ∠3 are supplementary angles.......... 3. Exterior sides in opposite rays
4. m∠1 + m∠3 = 180°.......................................4. (Angles forming a linear pair sum to 180° )
5. m∠1 + m∠2 = m∠1 + m∠3........................ 5. ( Transitive Property of Equality)
6. ( m∠2 =m∠3)..............................................6. (Algebra)
7. l || m ............................................................7. (Converse of Corresponding Angles Postulate)