求二面角的大小是高中立体几何中的一项基本内容 ,也是高中立体几何的难点 .求二面角的方法一般是用定义法、三垂线定理法和垂面法作出二面角的平面角 ,但二面角的平面角可以不必作出 (有时往往也不易作出 ) ,此时可采用面积法 .面积法的依据是 :设一封闭平面图形面积为S ,它在 The size of the dihedral angle is a basic content in the high school stereo geometry, and it is also a difficulty in the high-dimensional solid geometry. The method of finding the dihedral angle is generally to use the definition method, the tri- perpendicular line theorem and the vertical surface method to make the dihedral angle. The plane angle, but the plane angle of the dihedral angle can not be made (sometimes it is not always easy to make). At this time, the area method can be used. The area method is based on: the area of ​​a closed plane is S, which is