这题很难吗?
只要知道2(n/3)^n * (n+1) = [n/(n+1)]^n * [(n+1)/3]^n * (n+1) > 1/3 * [(n+1)/3]^n * (n+1) = [(n+1)/3]^{n+1}
(n+1)!= n!(n+1) < (n/2)^n * (n+1) = [n/(n+1)]^n * [(n+1)/2]^n * (n+1) < 1/2 * [(n+1)/2]^n * (n+1) = [(n+1)/2]^{n+1}