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. , and . congruent angles have equal measures. 5. angle addition and definition of a straight angle 6. substitution what is the missing step in this proof? statement: , and . reason: alternate interior angles theorem statement: is parallel to . reason: is a transversal cutting and . statement: , and . reason: alternate interior angles theorem statement: , and . reason: and , and and are pairs of supplementary angles.