Let be a shortest path from vertex to vertex such that all intermediate are contained in .
By (2) we know that a shortest path does not contain the same vertex twice, hence there are two possibilities to construct .
vertex is not a vertex on the path, then we have .
vertex is a vertex on the path, then the path consists of a subpath from to and a subpath from to . Each subpath can only contain intermediate vertices in , and also shortest paths from to and to respectively (otherwise a shorter path from to can be constructed), namely they have lengths and , respectively. Hence .