6. ∠3 and ∠5 are supplementary because of the definition of supplementary. given: s | | t prove: 3, 5 are supplementary 1. s||t definition of supplementary 2. ∠1 and ∠3 are supplementary 4 if lines are ||, corresponding angles are equal 3. m∠1 + m∠3 = 180° substitution 4. m∠1 = m∠5 exterior sides in opposite rays 5. m∠5 + m∠3 = 180° 1 given