反例指满足题设条件而结论不真的命题．立体几何判断题中的假命题，常可用反倒去应证．但由于有些学生想象能力欠佳，思维不严谨，解题时总是“想不到”反例．究其原因，主要是“不会想”．所以，教学时应回答“怎么想”这一关键问题．1将一般情形特殊化特殊化后得到命题的简单 Counterexamples are the propositions that satisfy the questions and the conclusions are not true. The false proposition in the three-dimensional geometric judgment question can often be used to reverse the evidence. However, because some students have poor imagination and rigorous thinking, when solving problems, they always “anticipate” counterexamples. The reason is mainly “do not think.” Therefore, teaching should answer the key question “How to Think”. 1 It is simple to obtain the proposition after specializing the general situation