关于高二等差数列的几道题1.在等差数列{an}中,若a11=20,则s21=?2.在等差数列{An}中已知A1=-20,An+1=An+4则｜A1｜+｜A2｜+｜A3｜+｜A4｜=___________________,则｜A1｜+｜A2｜+｜A3｜+...+｜A20｜=_________________.3.数列{an}的前项n和Sn=3n^2-5n,求a20的值4.已知数列的通项公式an=3^n+2n+1,求前n项的和.
问题描述：
关于高二等差数列的几道题
1.在等差数列{an}中,若a11=20,则s21=?
2.在等差数列{An}中已知A1=-20,An+1=An+4则｜A1｜+｜A2｜+｜A3｜+｜A4｜=___________________,则｜A1｜+｜A2｜+｜A3｜+...+｜A20｜=_________________.
3.数列{an}的前项n和Sn=3n^2-5n,求a20的值
4.已知数列的通项公式an=3^n+2n+1,求前n项的和.
答
1.由于a1+a21=2*a11,依此类推s21=21*a11＝210 2.an+1=an+4所以an=an-1+4.a2=a1+4累加所以an=a1+4(n-1)an=-24+4n当n0|an|=4n-24sn=|a1|+|a2|+|a3|+.|an|=20+16+12+8+4+0+a7+..+ansn=60+a7+...+ansn=60+(4+an)(n-6)/2=...