It is given that ∠B ≅ ∠C. Assume AB and AC are not congruent. If AB > AC, then m∠C > m∠B by . If AC > AB, then m∠B > m∠C for the same reason. However, using the given statement and the definition of congruency, we know that m∠B = m∠C. Therefore, AB = AC and AB ≅ AC. What is the missing reason in the proof? converse of the triangle parts relationship theorem substitution definition of congruency converse of the isosceles triangle theorem