First, the direct method: according to the conditions of the topic, calculate the plane angle of dihedral angle by using the known vertical conditions, especially the three perpendicular theorem, and then calculate the plane angle of dihedral angle by using the triangle (usually right angle) where dihedral angle is located. Second, the vector method: using the angle between the normal vectors m and n of two planes to find it, this is the most effective method in the college entrance examination, no matter how difficult it is. It is also the best way. However, it is necessary to judge whether the calculated result is the same as the actual angle according to the actual size of dihedral angle. Third, indirect method: area projection theorem method, that is, the projection area of a plane figure on another plane s projection area = s original figure area * *cos (dihedral angle formed by two planes)