Geometry

Question Analysis:
This question focuses on the properties of triangles divided by a line segment.
The main idea is to use the given side - length and angle - relationship information to determine triangle congruence and angle equalities.

Key Concept Explanation:
- For triangle congruence, SSS (Side - Side - Side) requires three pairs of equal sides, and SAS (Side - Angle - Side) requires two pairs of equal sides and the included angle to be equal.
- In congruent triangles, corresponding angles are equal.

Step - by - Step Solution：
1. Since , (common side), and so , by the SAS criterion, .
Also, we can consider

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