求二面角的大小是立体几何的重点和难点,其求解方法通常有:(1)常规方法:作出二面角的平面角求之;(2)向量法:根据平面的法向量夹角来确定二面角的大小.但对于常规方法,有时二面角的平面角难以作出,或者二面角的平面角虽然作出,但求角计算繁琐;对于向量法,有时难以判断法向量夹角与二面角大小的关系.但是,如果运用下面的结论,则常能 The size of the dihedral angle is the key and difficult point of the three-dimensional geometry. The solution methods are: (1) the conventional method: the planar angle of the dihedral angle is obtained; (2) the vector method: based on the normal angle of the plane The size of the dihedral angle is determined. However, for the conventional method, the dihedral plane angle is sometimes difficult to make, or the dihedral plane angle is made, but the calculation of the angle is complicated; for the vector method, it is sometimes difficult to determine the angle between the normal vector and The dihedral size relationship. However, if the following conclusions are used, then often