在数学证明中,有时按常规思路从正面思考难以解决问题,而如果运用反证法,逆向思考,则可以化繁为简,化难为易.对命题结论的正确否定是运用反证法的关键一步,但这并非我们想象的那么简单.本文介绍一个重要的定律——De Morgen法则,它可以帮助我们快速而正确地对原命题进行“反设”. In mathematical proofs, it is sometimes difficult to solve the problem from the positive side according to the conventional thinking, and if we use the contradictory laws and reverse the thinking, we can simplify everything and turn it into a difficult task. Correctly denying the conclusions of the propositions is a crucial step in applying the law of reverse syndromes This is not an easy one to imagine. This article introduces an important law, De Morgen’s Law, that helps us to “reverse” the original proposition quickly and correctly.