真心求教关于逆否命题，逆命题，否命题的问题-谢谢

flexpoly · 发表于 2015-5-19 15:14:39

我的理解，貌似搜百度也是这样解释的：

原命题与逆否命题等价，
原命题与逆命题，否命题不等价

如：原命题：天下雨，地上湿（下雨是地湿的必要条件，非充分条件：即下雨能推出地湿，而地湿，不能推出下雨，可以是他因导致地湿）
逆否命题：地上不湿润，天没下雨。可由与原命题推出

逆命题：地上湿润，天下雨不可由与原命题推出
否命题：天没下雨，地上不湿润不可由与原命题推出

以下疑问：

题一： Advertisement: Today's customers expect high quality. Every advance in the quality of manufactured products raises customer expectations. The company that is satisfied with the current quality of its products will soon find that its customers are not. At MegaCorp, meeting or exceeding customer expectations is our goal.

Which of the following must be true on the basis of the statements in the advertisement above?
（B ）A company that does not correctly anticipate the expectations of its customers is certain to fail in advancing the quality of its products.
（C） MegaCorp's goal is possible to meet only if continuing advances in product quality are possible.

逻辑链：提高产品质量→提高顾客期望
逆否命题： FAIL TO ANTICIPATE顾客期望→ FAIL TO 提高产品质量

此题答案是C可以理解，但B不是原逻辑链的逆否命题吗，为什么不对？

题二 Social scientists are underrepresented on the advisory councils of the National Institutes of Health (NIH). Since these councils advise NIH directors and recommend policy, the underrepresentation of social scientists results in a relative lack of NIH financial support for research in the social sciences.

If the statements above are correct, they most strongly support which of the following?
（B） A significant increase in the representation of social scientists on NIH advisory councils would result in an increase in NIH funding for social science research.
（D ）A significant increase in funding for the training of social scientists would result in an increase in the number of social scientists on NIH advisory councils.

案B （B貌似是原命题的否命题，为什么对）但 D不是逆否命题吗，为什么不对

逻辑链：underrepresented →lack of NIH financial support
逆否命题：financial support 充足→没有underrepresented （D)
否命题：没有underrepresented→ financial support 充足 (B)

flexpoly · 发表于 2015-5-20 15:38:45

那位大侠给指点指点，刚刚看了逻辑bible中关于sufficient, necessary condition的讲义，觉得对上述疑问更大了

jansonx · 发表于 2015-5-20 17:23:51

这道题我思考了很久，讲以下我的理解：
题目的逻辑链有两条
1. Every advance in the quality of manufactured products raises customer expectations: increase in quality -> rise in expectation
2. Today's customers expect high quality： meet expectation -> high quality

答案B并非1的逆否；错在用词“anticipate the expectation"；如果改为“raise the expectations", 则成立

flexpoly · 发表于 2015-5-21 09:24:27

第二个逻辑连我忽略了。如此看来，答案c正是第二个逻辑连。bible说only if 是necessary condition的表致辞。c说MegaCorp's goal is possible to meet only if continuing advances in product quality are possible. 则有， to meet goal - continuing advance in quality. 正是第二个逻辑连。

对b的错误之处，我也认为措辞有误。握手握手。请问对第二题有何高见？

alzn2765 · 发表于 2015-5-22 12:54:52

我试试。

这道题的逻辑是这样的，我先告诉你，有件事是错误的。然后说，这件事是underrepresented →lack of NIH financial support。然后我问你：我想表达的是什么意思？

我想表达的就是这件事的否命题是对的啊！所以B是正确的。

不信咱们试一试：
医生：喝水少会导致尿毒症的。
问你医生想表达什么意思？

你觉得医生是想让你多喝水还是想告诉你“没有尿毒症的人一定不会少喝水”？正常人都不会觉得是第二个吧。

我觉得你在看句子的时候没看出来作者的态度。

[逻辑小分队] 真心求教关于逆否命题，逆命题，否命题的问题-谢谢

所属分类: GMAT考试

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