1, a pair of congruent triangles appears, and the included angle between the corresponding sides of the third group of △ AOC △ BOD (SAS) is equal to the vertex angle of the original triangle, that is, the included angle between AC and BD (or extension line) is α (obtained from the figure of 8). 2. The sum of the distances from a point CG = PE+PF on the bottom of an isosceles triangle to two waists is equal to the waist.