Theorem 9.2 If G is a planar graph of order n>=3 and size m,then m <3n-6. 。Corollary9.3 Every planar graph contains a vertex of degree 5 or less. 。Corollary9.4 -K is nonplanar.Theorem 9.2 • If G is a planar graph of order n>=3 and size m, then m <= 3n-6. • Corollary 9.3 – Every planar graph contains a vertex of degree 5 or less. • Corollary 9.4 – K5 is nonplanar