1. ∠1 ≅ ∠3 1. given
∠1 and ∠6 are supplementary angles
∠3, ∠4, and ∠5 are supplementary angles
2. m∠1 = m∠3 2. definition of congruency
3. m∠1 + m∠6 = 180° 3. definition of supplementary angles
m∠3 + m∠4 + m∠5 = 180°
4. m∠3 + m∠4 + m∠5 = m∠1 + m∠6 4. transitive property
5. m∠3 + m∠4 + m∠5 = m∠3 + m∠6 5. substitution property
6. m∠4 + m∠5 = m∠6 6. subtraction property of equality
