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定理三角形的中位线平行于第三边,且等于第三边的一半.已知:如图1,DE是△ABC的中位线.求证:DE∥BC,DE
The middle triangle of the theorem triangle is parallel to the third edge and equal to half of the third edge. Known: As in Fig. 1, DE is the median line of ABC. Proof: DE∥BC,DE