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【OG20-P278-290题】 If S is a set of odd integers and 3 and -1 are in S, is -15 in S?
1. 5 is in S
2. Whenever two numbers are in S, their product is in S.
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5在S里,S里有3,所以15也在S里,
S里有-1,所以-1x15=-15也在S里
题目讨论 (3条评论)

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ziqi
忽略了两个条件一起的情况
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0 回复 2022-03-14 11:25:40
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Jeaninla
S中有5、3、-1,就有15。再推一次就有-15。
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1 回复 2021-02-09 07:57:59
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Miketoll
好吧,马虎了,
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0 回复 2020-01-05 18:42:15