First of all, this is a high school math problem. Then, the first small problem, imagine the open side, that is, the diagonal length of a rectangle with length and width of 4 and 2, and the Pythagorean theorem will be known as soon as it is calculated. The second small problem is that m is obviously the midpoint and the ratio is 1. The third small problem, the stupid way is to establish a coordinate system, write down the coordinates of each point and find the plane normal vector. The modulus of two plane normal vector points multiplied by the two normal vectors is the cosine value, which is enough to be expressed by the inverted triangle. The simple method is to connect BC 1 and extend the intersection of C 1M to P. According to Pythagorean theorem, it can be proved that the triangle BPC 1 is a right triangle. According to the theorem of three perpendicular lines, the angle CBC 1 is the required plane included angle, which is obviously 45 degrees.