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這個三位數是100a+10b+c 因為2a+3b+c能被7整除,囙此設2a+3b+c=7n 則c=7n-2a-3b 所以100a+10b+c=100a+10b+7n-2a-3b=98a+7b+7n=7(14a+b+n) 因為7(14a+b+n)能被7整除,所以100a+10b+c能被7整除 即abc這個三位數能被7整除. 祝你開心
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