How to find dihedral angle? Please explain all the methods in detail.

The first is the definition method, which is not common.

The second is the angle finding method of the three perpendicular theorem, which is also the most common. For example, to find dihedral angle A-CD-B, then A is the perpendicular of BCD plane, E is the perpendicular of CD, and F is the perpendicular of CD, connecting FA, then the angle AFE is the plane angle of dihedral angle.

The third clock is regional photography. If the area of triangle ABC on the plane 1 is S 1 and the area of triangle DEF projected by ABC on the plane 1 is S2, then the dihedral cosine of 1 2 is S2/S 1.

Finally, after knowing the method, we should do more problems and accumulate experience, so as to analyze specific problems and be handy when doing problems.