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2.试证明?(A??B)?(?B?C)??C??A
证明:?(A??B)?(?B?C)??C
?(?A?B)?(?B?C)??C
?(?A?B)?(?B??C)?(C??C) ?(?A?B)?((?B??C)???0) ?(?A?B)?(?B??C)
?(?A? (?B??C))?(B?(?B??C)) ?(?A? (?B??C))?0 ??A? (?B??C) ??(A?B?C)
故由左边不可推出右边?A
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