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因為角A是直角、DE垂直DF得出四邊形EDFA為矩形, 因為D點是BC的中點所以E是AB的中點、F是AC的中點即BE=DE DF=CF 又因為DE垂直DF得出EF平方=DE的平方+DF的平方 所以EF平方=BE平方+CF平方
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