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

Answer

To solve the question, we need to understand the concept of congruent triangles and how their corresponding parts relate to each other.

### Step-by-Step Solution:

1. **Understanding Congruent Triangles**: 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. **Identifying Corresponding Parts**: In congruent triangles, the corresponding sides and angles are as follows:
- Side AB corresponds to side DE
- Side BC corresponds to side EF
- Side AC corresponds to side DF

3. **Finding the Corresponding Side**: Since we need to find the length of side BC in triangle ABC, we look at its corresponding side in triangle DEF. From the list above, we see that:
- BC (from triangle ABC) corresponds to EF (from triangle DEF).

4. **Conclusion**: Therefore, we can conclude that:
\[ BC = EF \]

### Final Answer:
BC = EF

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