Assuming that the lengths of the triangles GNM and GM are less than or equal to the sum of the other two sides GN and NM of the triangle, the lines G, N, M*** hold. Similarly, for the triangle NOM, NM is less than or equal to the sum of ON and OM on the other two sides. When the equal sign holds, the straight lines of O, N, M***, that is, N and M are symmetrical about the origin. So when GM takes the maximum value, the straight line G, N, O, M***, and the triangle DNO are isosceles triangles (DN=ON), so there is ∠GND=2∠DON (an outer angle of a triangle is equal to the sum of two inner angles that are not adjacent to it).