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證明:因為PD⊥平面ABCD,所以PD⊥BC;因為四邊形ABCD為正方形,所以BC⊥DC;由PD⊥BC,BC⊥DC,可知BC⊥平面PDC,因為DE在平面PDC上,所以BC⊥DE.因為PD=DC,所以三角形PDC是等腰三角形,又因為E是PC中點,所以DE⊥PC.由BC⊥D...
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