Consider the diagram and proof by contradiction. Given: △ABC with ∠B ≅ ∠C
Prove: AB ≅ AC

Triangle A B C is shown. Angles A B C and B C A are congruent.

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

The Angle-Side Relationships theorem (or triangle parts relationship theorem) states that if one side of a triangle is longer than another side, then the angle opposite the longer side will have a greater degree measure than the angle opposite the shorter side.

The converse to the Angle-Side Relationships theorem (or triangle parts relationship theorem) states that if one angle of a triangle has a greater degree measure than another angle, then the side opposite the greater angle will be longer than the side opposite the smaller angle.

Thus, from the proof if AB > AC, then m∠C > m∠B by the converse of the triangle parts relationship theorem.