∠1 and ∠2 are supplementary // given∠3 and ∠4 are supplementary // given∠1 ≅ ∠3 // given m∠1 + m∠2 = 180° // definition of supplementary anglesm∠3 + m∠4 = 180° // definition of supplementary angles m∠1 + m∠2 = m∠3 + m∠4 // transitive property of equality m∠1 = m∠3 // definition of congruent angles m∠1 + m∠2 = m∠1 + m∠4 // substitution property of equality (replaced m∠3 with m∠1) m∠2 = m∠4 // subtraction property of equality (subtracted m∠1 from both sides) ∠2 ≅ ∠4 // definition of congruent angles
