In triangles ABC and DEF, AB = FD and ∠A = ∠D. The two triangles are congruent by SAS criterion if:

Question

Accepted Answer

**Step 1:** Recall the SAS (Side-Angle-Side) criterion:  
Two triangles are congruent by SAS if two sides and the included angle of one triangle are equal to the corresponding two sides and the included angle of another triangle.

**Step 2:** Given:  
- AB = FD  
- ∠A = ∠D

**Step 3:** For SAS, we need another side that forms the included angle with AB and ∠A in triangle ABC, and with FD and ∠D in triangle DEF.

- In triangle ABC, the sides forming ∠A are AB and AC.
- In triangle DEF, the sides forming ∠D are FD and DE.

So, the corresponding sides are:  
- AB ↔ FD  
- AC ↔ DE  
- ∠A ↔ ∠D

**Step 4:** Therefore, for the SAS criterion to be satisfied, we must have:  
**AC = DE**

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**Final Answer:**  
**Correct Answer:** 4. AC = DE

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