求二面角的大小是立体几何的重点和难点，也是多年来高考的热点之一．由三垂线定理作出二面角的平面角便是这一热点的中心；而对一些求二面角的复杂问题，学生往往不知所措；笔者根据多年的教学实践，提炼出一种由三垂线定理作二面角的平面角的简易方法——γ垂面法，收到较好 The size of the dihedral angle is the focus and difficulty of the three-dimensional geometry. It is also one of the highlights of the college entrance examination over the years. The plane angle of the dihedral angle made by the three vertical line theorem is the center of this hot spot; and for some complicated problems on the dihedral angle, the students are often at a loss; the author refined a three-by-one teaching practice based on many years. The vertical line theorem as a simple method of planar angle of dihedral angle - γ-plane method, received better