作业帮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.
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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.
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.
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.
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-24
sn=|a1|+|a2|+|a3|+.|an|=20+16+12+8+4+0+a7+..+an
sn=60+a7+...+an
sn=60+(4+an)(n-6)/2=60+(4n-20)(n-6)/2
｜A1｜+｜A2｜+｜A3｜+...+｜A20｜=60+60*14/2=480
3..因为Sn=3n^2-5n
所以an=Sn-Sn-1=3n^2-5n-[3(n-1)^2-5(n-1)]=6n-8(n≥2)
当n=1时,S1=a1=-2,满足上式
故an=6n-8,a20=112
4.Sn=(3^1+3^2+...+3^n)+2(1+2+3+...+n)+1*n
=3(3^n -1)/(3-1)+2*n(n+1)/2+n
=3^(n+1)/2 +n^2+2n-3/2