The double length method of the central line of congruent triangle: ad is the central line of triangle ABC, e and F are on AB and AC respectively, and De is perpendicular to DF, then what is the relationship among be, CF and ef?

In △ BDE and △ CDG, ∠ DBE = ∠ DCG, BD = CD, ∠ BDE = ∠ CDG, so, △ BDE ≌ △ CDG, we can get be = CG, de = DG. In △ def and △ DGF, de = DG, ∠ EDF = 90 °

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