Points a, b, and c form a triangle. complete the statements to prove that the sum of the interior angles of abc is 180°.
statement
reason
points a, b, and c form a triangle.
given
let de be a line passing through b and parallel to ac. definition of parallel lines
23 25 and 21 - 24
mz1 =m24 and m23 = m 25
m24+ m2 + m25 = 180°
m21+ m2 + m23 = 180°
angle addition and definition of a straight line
substitution

Statements reasons 1. ab ≅ cd; ad ≅ bc 1. given 2. ac ≅ ac 2. reflexive property 3. △adc ≅ △cba 3. ? 4. ∠dac ≅ ∠bca; ∠acd ≅ ∠cab 4. cpctc 5. ∠dac and ∠bca are alt. int. ∠s; ∠acd and ∠cab are alt. int. ∠s 5. definition of alternate interior angles 6. ab ∥ cd; ad ∥ bc 6. converse of the alternate interior angles theorem 7. abcd is a parallelogram 7. definition of parallelogram what is the missing reason in step 3?