求二面角的大小 ,基本作法是算其平面角的大小 ,但平面角没有固定位置 ,高考中因其定位失误而丢分的现象颇多 .本文举例介绍几种常用的途径 ,帮助同学们掌握要领 .一、利用棱或两个面的垂面【例 1】　在三棱锥S-ABC中 ,SA⊥底面ABC ,AB⊥BC ,DE垂直平分SC ,且分别交AC、SC于D、 For the size of the dihedral angle, the basic method is to calculate the size of its plane angle, but there is no fixed position on the plane angle. The phenomenon of losing points because of its positioning errors is quite numerous. This article gives an example of several common ways to help the students Master the essentials. First, the use of edge or two surface of the vertical surface [Example 1] In the triangular pyramid S-ABC, SA ⊥ bottom ABC, AB ⊥ BC, DE vertical bisect SC, and AC, SC respectively to D,