Points a, b, and c form a triangle. Complete the statements to prove that the sum of the interior angles of δabc is 180°. Statement reason points a, b, and c form a triangle. Given let be a line passing through b and parallel to. Definition of parallel lines ∠3 ≅ ∠5 and ∠1 ≅ ∠4 m∠1 = m∠4 and m∠3 = m∠5 m∠4 + m∠2 + m∠5 = 180° angle addition and definition of a straight line m∠1 + m∠2 + m∠3 = 180° substitution.