Statement reason 1. let points a, b, and c form a triangle. given 2. let be a line passing through b, parallel to , with angles as labeled. defining a parallel line and labeling angles 3. 4. m∠1 = m∠4, and m∠3 = m∠5. congruent angles have equal measures. 5. m∠4 + m∠2 + m∠5 = 180° angle addition and definition of a straight angle 6. m∠1 + m∠2 + m∠3 = 180° substitution what is the missing step in this proof?