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LSAT逻辑题分析:原逆否命题

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2017-02-08 17:56立信编辑

Either Perrys faction or Tuckers faction, but not both, will win control of the government. If Perrys faction wins, the nation will suffer economically. If Tuckers faction wins, the nation will suffer militarily. Given the statement

  Either Perry’s faction or Tucker’s faction, but not both, will win control of the government. If Perry’s faction wins, the nation will suffer economically. If Tucker’s faction wins, the nation will suffer militarily.

  Given the statements in the passage, which one of the following statements must be true?

  (A) It is possible, but not certain, that the nation will neither suffer economically nor suffer militarily.

  (B) If the nation suffers economically, it is certain that Perry’s faction has won control of the government.

  (C) It is certain that the nation will suffer either economically or militarily, and also certain that it will not suffer both.

  (D) If the nation suffers militarily, it is possible, but not certain, that Tucker’s faction has won control of the government.

  (E) If the nation suffers both economically and militarily, it is certain that neither Perry’s faction nor Tucker’s has won control of the government.

  由原文,可得1。Perry’s faction wins——》the nation will suffer economically

  2。Tucker’s faction wins——》the nation will suffer militarily

  3。Either Perry’s faction or Tucker’s faction, but not both, will win control of the government可推出A错

  B的逻辑关系为the nation will suffer economically——》Perry’s faction wins,无法从原文的出,所以错

  同理。可推出其他。

  D强调了it is possible, but not certain。所以正确。

  原命题为:A-->B

  逆命题为:B-->A

  否命题为:非A-->非B

  逆否命题为:非B-->非A

  原命题和逆否命题为等价命题.如果原命题成立,逆否命题成立.vice versa.

  逆命题和否命题为等价命题,如果逆命题成立,否命题成立.vice versa.

  举例:

  原命题:吃多了-->肚子涨

  逆命题:肚子涨-->吃多了

  否命题:没吃多--> 肚子不涨

  逆否命题:肚子不涨-->没吃多

  原命题是正确的.所以逆否成立.

  但是逆命题和否命题不一定成立.肚子涨也许是水喝多了.

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