Let G be a simple connected graph. Let $\displaystyle e(v)=Max_{u \in V(G)} d(u,v)$ in which V(G) is the set of vertices of G and d(u,v) is the distance between u and v. Let $\displaystyle rad(G)=Min_{v \in V(G)} e(v)$ and $\displaystyle diam(G)= Max_{v \in V(G)} e(v)$. Now prove that if $\displaystyle rad(G) \le k \le diam(G)$ then there is a vertex v such that $\displaystyle e(v)=k$.