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解题思路： 掌握平行四边形和菱形等特殊四边形的基本性质
解题过程：
证明：
∵　DE∥AC　　CE∥BD
∴　四边形OCED为平行四边形
又∵　ABCD为矩形，AC、BD为对角线且交于点O
∴DO＝CO
∴平行四边形OCED为菱形