其中最为经常用到的条件就是充分必要条件。可能有一些同学会问，充分必要条件是否就是我们数学中学到的充分必要条件，在逻辑题中充分必要条件又是怎样运用的呢?今天就为大家答疑解惑。
一、GMAT逻辑中的充分必要条件
1、GMAT逻辑中充分必要条件的概念。
A是B的充分条件：A→B只要有A就有B。
如果A成立，则一定有B;只要A，就会B。
英文表述：if A, B; if only A, B; whenever/whoever/wherever(=if) A, B。
2、GMAT逻辑中充分必要条件的表现形式。
If A, then B.( A→B)
If B, then C. ( B→C)
If C, then D. ( C→D)
If all of the statements above are true, which of the following must also be true?
(A) If D, then A.
(B) If not B, then not C.
(C) If not D, then not A.( A→D=非D→非A)
(D) If D, then E.(C)
(E) If not A, then not D.
二、GMAT逻辑中充分必要条件的标志
1、introduce a sufficient condition: if/when/whenever/every/all/any/people who/in order to/to等。
2、introduce a necessary condition: then/only/only if/must/require/no/none(在句子开头)等。
3、unless equation: unless/except/until/without(其后面部分变成必要条件，其它加NOT变充分条件)等。
4、特例：
the only way to achieve success is to work hard. 中achieve success(充分条件)
only a professional consultant can solve the organization’s problems. 中a professional consultant(必要条件)
no citizen can be denied the right to vote. denied the right to vote(充分条件). 中no citizen(必要条件)
(详情请看 GMAT逻辑中充分必要条件)
三、GMAT逻辑链充分必要条件举例分析
Whenever a major political scandal erupts before an election and voters blame the scandal on all parties about equally, virtually all incumbents, from whatever party, seeking reelection are returned to office.(即political scandal and voters blame是all incumbents returned to office的充分条件)However, when voters blame such a scandal on only one party, incumbents from that party are likely to be defeated by challengers from other parties. The proportion of incumbents who seek reelection is high and remarkably constant from election to election.
If the voters’ reactions are guided by a principle, which one of the following principles would best account for the contrast in reactions described above?
(A) Whenever one incumbent is responsible for one major political scandal and another incumbent is responsible for another, the consequences for the two incumbents should be the same.
(B) When a major political scandal is blamed on incumbents from all parties, that judgment is more accurate than any judgment that incumbents from only on party are to blame.
(C) Incumbents who are rightly blamed for a major political scandal should not seek reelection, but if they do, they should not be returned to office.
(D) Major political scandals can practically always be blamed on incumbents, but whether those incumbents should be voted out of office depends on who their challengers are.
(E) When major political scandals are less the responsibility of individual incumbents than of the parties to which they belong, whatever party was responsible must be penalized when possible.
题中充分必要条件分析：
A是B的必要条件：B→A只要有B就有A;
A对B是必要的;只有A，才会B;没有A，就没有B(实际上是B→A的逆否命题);
英文表述：A is necessary to B; only if A, B; not B unless(=if not) A。
例二：
Leachate is a solution, frequently highly contaminated, that develops when water permeates a landfill site. If and only if the landfill’s capacity to hold liquids is exceeded does the leachate escape into the environment, generally in unpredictable quantities(即如果landfill’s capacity exceeded，就有leachate escape into the environment，只要leachate escape into the environment，就有landfill’s capacity exceeded，在此是解题的关键). A method must be found for disposing of leachate. Most landfill leachate is send directly to sewage treatment plants, but not all sewage plants are capable of handling the highly contaminated water.
Which one of the following can be inferred from the passage?
(A) The ability to predict the volume of escaping landfill leachate would help solve the disposal problem.
(B) If any water permeates a landfill, leachate will escape into the environment.
(C) No sewage treatment plants are capable of handling leachate.
(D) Some landfill leachate is send to sewage treatment plants that are incapable of handling it.
(E) If leachate does not escape from a landfill into the environment, then the landfill’s capacity to hold liquids has not been exceeded.(逆否命题非A←→非B=原命题)
(详情请看 GMAT逻辑练习题)
GMAT逻辑中关于充分必要条件的讲解百利天下小编通过两个例题简单地讲解了一下特征，仅代表一些一般的题目，大家可以先体会一下逻辑中的充分必要条件的运用特点。
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