9.14 Let G be a connected plane graph of order n ≥ 5 and size m. (a) Prove that if the length of a smallest cycle in G is 5, then m≤ §³(n−2). (b) Use (a) to show that the Petersen graph is nonplanar. (c) Use Kuratowski's theorem to show that the Petersen graph is nonpla- nar. (d) Prove or disprove: If n < 20 and the length of a smallest cycle in G is 5, then G has a vertex of degree 2 or less.