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标题: 区分充分与必要条件 [打印本页]

作者: chelseayang 时间: 2004-12-3 11:43
标题: 区分充分与必要条件

针对这个比较麻烦的问题，我转抄了一点我认为简单但比较有用的东西，希望对大家有所帮助。

Sufficient and necessary conditions

Conditional reasoning is a fundamental component of both the Logical Reasoning and Logic Games section of the LSAT.

A sufficient condition can be defined as an event or circumstance whose occurrence indicates that a necessary condition must also occur.

A necessary condition can be defined as an event or circumstance whose occurrence is required in order for a sufficient condition to occur. In other words, if a sufficient condition occurs, you automatically know that the necessary condition also occurs.

Consider the following statement:

      To look sexy, you must smile.

We represent this statement as follows:

         Sufficient                Necessary

         Look sexy                   Smile

One of the confusing elements in recognizing conditional statements is that so many different terms can be used to introduce a sufficient or necessary condition. Consider the following statements:

If you want to look sexy, you must smile.

Smiling is necessary to look sexy.

When someone looks sexy, she must be smiling.

Only someone who is smiling can look sexy.

Unless you smile, you cannot look sexy.

You will look sexy only if you smile.

All the above would be diagrammed the same way:

         Sufficient          Necessary

         Look sexy          Smile

The following list may be helpful to you.

Sufficient Condition Indicators    Necessary Condition Indicators

      If                                                       Then

      When                                                 only

      Whenever                                        only if

      Every                                                 must

      All                                                    required

      Any                                                    unless

      eople who                                     except

      In order to                                        until

                                                               Without

You should be able to memorize the indicator words on each list, which could assist you in recognizing the types of situations faster.

[此贴子已经被作者于2004-12-3 11:49:07编辑过]

作者: swimmingfrog 时间: 2004-12-3 13:39
顶，太有帮助了！Thanks!

作者: forestsky 时间: 2004-12-22 00:34
this paragraph is wise, however, I prefer to declare the condition andconclusion. The sufficient condition mentions above should be, in myopinion, refered to as conclusion. While we want to discern thedifference between sufficient condition and necessary condition, webetter come with the conclusion. From my point of view, thesufficient condition is one that make the conclusion happen without anyfurther conditions while the necessary condition is one that when theconclusion occurs, it must present. This is my two cents and I like todiscuss this with you wise people. /Big smile

作者: entia 时间: 2004-12-23 09:33
ding！

作者: americanlaw 时间: 2010-4-1 16:49
貌似bible上的。。。

作者: americanlaw 时间: 2010-4-1 16:51
补充一句，记住必要和充分的引导词，比如if/unless，真的很有用，还有就是多练，到最后会有一种条件反射似的直觉。

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