Guangdong Province Examination Judgment Reasoning in 2022: Hypothetical proposition, why "walk backwards"?

Candidates who are preparing for the national and provincial exams may be confused by the hypothetical proposition knowledge in the test, judgment and reasoning, which is similar to "sufficient conditions" and "necessary conditions"; "Forward and backward" and "backward and forward"; There are many concepts and rules, which make many candidates scratch their heads and complain. It is difficult to understand the principle, and as a result, another kind of question type appears, asking you to infer the necessary preconditions according to the conclusion. You haven't learned to "push forward" yet, and you have to "walk backward". You have to admire the proposer's superb propositional level. Today, Chinese public education takes candidates to solve this annoying "reverse push" problem.

example

The foreign language teaching and research section of a university recruited five foreign language teachers, each of whom taught only one foreign language. And the following conditions are met:

If Xiao Gan teaches German, Sun Xiao doesn't teach Russian.

Or Xiao Li teaches German, or small money teaches German.

If Sun Xiao does not teach Russian, Xiao Zhao does not teach French.

Either Xiao Zhao teaches French or Zhou Xiao doesn't teach English.

Which of the following options, if true, can lead to the conclusion that "Xiao Li teaches German"?

A. Russian is not taught in Sun Xiao. B. teach german with little money.

C. Zhou Xiao teaches English. French is not taught in Xiao Zhao.

Chinese public analysis C. The first way of thinking, reverse reasoning, will inevitably come to the conclusion that "Xiao Li teaches German". According to condition 3, it is necessary to ensure that the condition of "small money does not teach German" is established. The rule of "no, push one" is used here. In the same way, if you want to get "small money not to teach German", according to condition 1, you need to ensure that "Sun Xiao teaches Russian" is established. If you want "Sun Xiao teaches Russian", you need to ensure that "Xiao Zhao teaches French" is established, and then introduce "Zhou Xiao teaches English". Therefore, if you want to get "Xiao Li teaches German", you need to make sure that "Zhou Xiao teaches English". The answer is C.

After reading the analysis of this topic, do you think the whole person is not very good? Reverse reasoning really tests people's logical thinking. But don't worry, there is a relatively simple method, which needs to be answered by the negative proposition of hypothetical proposition.

First, list the general forms of these propositions:

Qian German? Sun can't speak Russian.

② Li Deyu or Chandler.

③ Sun can't speak Russian? Zhao can't speak French.

④ Zhao French or Zhou English.

Because the precondition is inferred from the conclusion, there are many hypothetical propositions in the stem, which can be used to deny the conclusion and bring it into conditional reverse reasoning. If we deny this conclusion, we will get Li Bude language from condition ②, and Chandler will get Sunbu Russian from condition ①, Zhaobu French from condition ③ and then from condition ④. The answer is C.

This question fully combines the reasoning rules related to compatible alternative propositions and hypothetical propositions. It is very difficult for candidates who have just come into contact with this kind of reasoning questions, but they have mastered the knowledge points and problem-solving skills involved. I believe this question must be a simple one for everyone. Chinese public education wishes all candidates greater progress!