求二面角时,通常要作其平面角,常用方法有:1)根据定义;2)通过三垂线定理;3)通过作棱的垂面,如图1.图1　三种方法示意图这三种方法是视已知点Ｐ的位置不同而出现的三种相应的作法.即当点Ｐ在二面角的棱上时,直接根据定义作出平面角;当点Ｐ在二面角的一个半平面内时,可利用三垂线 When calculating the dihedral angle, it is usually necessary to make its plane angle. Common methods are: 1) according to the definition; 2) through the triple vertical line theorem; 3) through the vertical surface of the edge, as shown in Fig. 1. The three methods are three kinds of corresponding methods that appear depending on the position of the known point P. That is, when the point P is on the edge of the dihedral angle, the plane angle is directly defined according to the definition; when the point P is in a dihedral angle, In the half plane, three vertical lines can be used