7. [10 marks] Let G = (V,E) be a 3-connected graph with at least 6 vertices. Let C be a cycle in G of length 5. We show how to find a longer cycle in G. (a) Let x be a vertex of G that is not on C. Show that there are three C-paths Po, P1, P2 that are disjoint except at the shared initial vertex and only intersect C at their final vertices. (b) Show that at least two of P0, P1, P2 have final vertices that are adjacent along C. (c) Combine two of Po, P1, P2 with C to produce a cycle in G that is longer than C.

Let G be a connected graph with n ≥ 2 vertices. Let A be the adjacency matrix of G. Prove that the diameter of G is the least number d such that all the non-diagonal entries of the matrix A are positive.

8. [10 marks] Suppose that 15 people are at a dinner and that each person knows at least 9 of the others. Can the diners be seated around a circular table so that each person knows both of their immediate neighbors? Explain why your answer is correct.