Tam giác ABC cân tại A; M là trung điểm BC

The note appears to discuss properties of triangle ABC, which is isosceles at vertex A, with M being the midpoint of side BC. It also references points D, E, and the relationships involving various segments. The mathematical notation suggests a geometric proof or exploration regarding relationships between the segments.

If you need assistance with specific concepts or calculations in this problem, feel free to mention them!