请教我这道题得解题思路---**Extraction Content:** **Question:** 18. 求证: 三角形一边的两端到这边的中线所在直线的距离相等. **Diagram Description:** * Type: Geometric diagram. * Elements: * Triangle ABC. * A line passing through point A and intersecting side BC at point D. * Points are labeled as A, B, C, D. * Segments: AB, BC, CA, AD, BD, CD. * The line AD appears to be a median to side BC (although the question refers to a median line of *one side*). The diagram shows AD as a line passing through A and D, where D is on BC. The question asks about the median of a side, so if AD is the median, D would be the midpoint of BC. **Additional Text (Annotations):** * An annotation appears to be a statement or condition, possibly related to the problem or a previous problem, written vertically on the right side: * `2. ∠ABC = ∠ADC (对顶角相等, 两直线平行)` - This seems unrelated to question 18. * `AD = BC` * Handwritten notes near the diagram, possibly relating to the proof or concepts: * `.`. `中线是三角形顶点到对边中点的连线段` (Median is the line segment connecting the vertex of a triangle to the midpoint of the opposite side) * `.`. `线段两端到中点距离相等` (The distance from the two ends of a line segment to the midpoint is equal) * `.`. `三角形一边的两端到这条边的中线所在直线的距离相等` (The distance from the two ends of one side of a triangle to the line containing the median to this side is equal). - This is a restatement of the question. * A number '8' is circled in red below the question. * Other faint handwritten marks are present but difficult to discern clearly.