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有关“充要条件”的判断,实际上总是归结为对命题真假的判定,一此带有否定性命题的真假的判定,若能转化为它的等价命题——逆否命题后,再作判断,较为容易。
The judgment on the “requisites for necessary and sufficient conditions” is actually always attributed to the determination of the truth and falsehood of the proposition. Once the judgment of true and false with a negative proposition, if it can be converted into its equivalent proposition - after the negative theorem It is easier to judge again.