Establish a rectangular coordinate system and do it by vector method. Methods: An isosceles right triangle with a bottom is the key to find this point with point A as point O (the starting point of the vector), and the three faces must be perpendicular to each other! , AB is the x axis, AC' is the y axis and AA' is the z axis, and a rectangular coordinate system is established. Find the direction vector of A'B and the normal vector of plane MNC' (let's just write it as n), and then the relationship between vector A'B and vector n is several times (generally it can be clearly seen that China people are smart). Finally, be sure to explain that the line segment A'B is not in the plane MNC''! (Otherwise, points will be deducted in the regular exam). Dihedral angle is more troublesome. Of course, the normal vectors of two surfaces can be found first, and then the Cos value can be found by the included angle formula. Of course, the discerning person knows at a glance that the normal vector of plane MNC' is any vertical line at the bottom, so we use (0,0, 1). The normal vector of two faces is still an angle formula, and then the Cos value is calculated, and then the Tan value is converted. You should be able to do it. Of course, this shameless topic is to scare senior one into failing the college entrance examination. In addition, the method of rectangular coordinate system (although it is the method of science in senior three) can be used for such problems, and it is no trouble at all. The traditional method (the one with lines) will have no way to meet difficulties. The theory is also low and suitable for' actual combat'. Therefore, the best liberal arts can also be used, invincible! ! !