你好设第n个数为an依题意得a1=8,a2=13,a3=20那么an-a(n-1)=2n+1a(n-1)-a(n-2)=2n-1.a2-a1=5两边都加起来得an-a(n-1)+a(n-1)-a(n-2)+...+a2-a1=2n+1+2n-1+5an-a1=(n-1)(2n+1+5)/2an=(n-1)(2n+6)/2+a1an=(n-1)(n+3)+...