关于分类讨论的教学─—以有向线段数量公式的教学为例王敬庚（北京师范大学数学系１００８７５）分类的思想或分类讨论的思想，是数学中的基本思想之一．由于解析几何是通过坐标系把几何问题转化为代数问题来研究的，而代数方法具有一般性，例如用一个字母表示实数时，这... Teaching on Classified Discussion--An Example of Teaching with Directed Quantities Formula Wang Jinggeng (Department of Mathematics, Beijing Normal University, 100875) The thought of categorizing ideas or categorizing ideas is one of the basic ideas in mathematics. Since analytic geometry is studied by transforming geometric problems into algebraic problems through coordinate systems, algebraic methods have a generality, for example, when a real number is represented by a single letter...