请教

superkun · 发表于 2003-12-1 21:40:00

A recent survey showed that 50 percent of people polled believe that elected officials should resign if indicted for a crime, whereas 35 percent believe that elected officials should resign only if they are convicted of a crime. Therefore, more people believe that elected officials should resign if indicted than believe that they should resign if convicted
a.draws a conclusion about the population in general based only on a sample of that population
b.confuses a sufficient condition with a required condition
c.is based on an ambiguity of one of its terms
d.draws a conclusion about a specific belief based on responses to queries about two different specific beliefs
e.contains premises that cannot all be true
答案是b，我翻到相关的逻辑资料说 A if B 就是B--〉A， A only if B就是 A——〉B
既if 表示充分条件，only if 必要条件，现在不太明白两者区别，特别感觉翻译成中文
B都像是A的充分条件，既是“如果B成立则A成立“，“当且仅当B成立“，A成立“
请各位指点一下，我哪里想错了，现在头很晕
谢谢

fdkbaby · 发表于 2003-12-1 22:43:00

我是这么理解的：
A only if B是“当且仅当B成立的时候，A成立”，那么也就是“B不成立的时候，A也一点不成立”，也就是“B否推出A否”。取逆否命题成立，则为“A推出B”。所以这么看来，only if就是必要条件了。

superkun · 发表于 2003-12-2 13:59:00

非常感谢，想通了！

番茄炒蛋 · 发表于 2003-12-2 14:25:00

解释很棒哦，也来帮帮我的那几题嘛，呵呵

请教

请教

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