有些较难的立体几何选填题,正确与否难以选择,选对吧,又没有充分的依据,选错吧,又举不出一个反例,这是我们的学生和有些初为人师的老师经常碰到的一个棘手的问题.若能够构造一个符合题意的理想图形,则可利用图形来说明理由,既直观,又简捷,而且能从理论上解释清楚.请看下面的例? Some of the more difficult three-dimensional geometry selection questions are difficult to choose, right or wrong, and they are not adequately based. There are no counterexamples. This is often the case for our students and some early teachers. A thorny problem encountered. If you can construct an ideal figure that fits your question, you can use the figure to explain the reason, which is both intuitive and simple, and can be clearly explained theoretically. See the example below.