1. **Problem Statement:** We are examining the reflexive property in geometry, which states that a segment or angle is equal to itself, often written as $AB = BA$. 2. **Context:** When two triangles share a common side or angle, this shared part is congruent to itself by the reflexive property. This is crucial in proving triangle congruence. 3. **Explanation:** In the given graph, two triangles share the side connecting points 1 and 3. This side is common to both triangles. 4. **Formula/Property Used:** The reflexive property states: $$AB = BA$$ This means the segment $\overline{13}$ in one triangle is exactly equal to the segment $\overline{31}$ in the other triangle. 5. **Application:** Since $\overline{13}$ is common to both triangles, it can be used as a congruent side in triangle congruence proofs such as SAS, SSS, or ASA. 6. **Summary:** The reflexive property allows us to state that the shared side $\overline{13}$ is congruent to itself, which helps establish the congruence of the two triangles sharing this side. **Final answer:** The reflexive property confirms that the common side $\overline{13}$ is equal in both triangles, i.e., $\overline{13} = \overline{31}$, supporting triangle congruence proofs.