比较(n^2)+2n与2^n的大小关系
问题描述：
比较(n^2)+2n与2^n的大小关系
答
Assume n is a positive integer.
Let f_n=n^2+2n and g_n=2^n.It is easy to check that f_n>g_n for n=1,2,3,4,5.For n>=6,we can prove that f_n