数列bn＝n²+2n，前n项和sn等于多少？∵Sn=n2+2n+1，∴当n≥2时，Sn-Sn-1=n2+2n+1-[（n-1）2+2（n-1）+1]=2n+1，...

智能穿戴 |ysladmin 2024-05-21 阅读(13)