If `triangle ABC cong triangle DEF` then
`AB` = ________

Answer

To solve the question, we will follow these steps:

1. **Understand the Given Information**: We are given that triangle ABC is congruent to triangle DEF. This means that all corresponding sides and angles of these triangles are equal.

2. **Identify the Corresponding Parts**: Since the triangles are congruent, we can use the concept of Corresponding Parts of Congruent Triangles (CPCT). This means:
- Side AB corresponds to side DE
- Side BC corresponds to side EF
- Side AC corresponds to side DF

3. **Write the Equality**: From the information above, we can conclude that:
- AB = DE

4. **Final Answer**: Therefore, if triangle ABC is congruent to triangle DEF, then AB = DE.