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因為DE⊥AC,BF⊥AC 所以DE平行於BF,角DEA=角BFC. 因為四邊形ABCD是平行四邊形, 所以AD平行於BC,AD=BC, 角DAE=角BCF, 所以三角形ADE全等於CBF, CE=BF, 所以四邊形DEBF是平行四邊形. 方法較多
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