急!等差数列{an}的前n项和为Sn,公差d不等于0,且S3=9,a1,a3,a等差数列{an}的前n项和为Sn,公差d不等于0,且S3=9,a1,a3,a7成等比数列.(1)求数列{an}的通项公式(2)设bn=2^an,求数列{bn}的前n项和路过的救命啊
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![急!等差数列{an}的前n项和为Sn,公差d不等于0,且S3=9,a1,a3,a等差数列{an}的前n项和为Sn,公差d不等于0,且S3=9,a1,a3,a7成等比数列.(1)求数列{an}的通项公式(2)设bn=2^an,求数列{bn}的前n项和路过的救命啊](/uploads/image/z/2430742-22-2.jpg?t=%E6%80%A5%21%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%7Ban%7D%E7%9A%84%E5%89%8Dn%E9%A1%B9%E5%92%8C%E4%B8%BASn%2C%E5%85%AC%E5%B7%AEd%E4%B8%8D%E7%AD%89%E4%BA%8E0%2C%E4%B8%94S3%3D9%2Ca1%2Ca3%2Ca%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%7Ban%7D%E7%9A%84%E5%89%8Dn%E9%A1%B9%E5%92%8C%E4%B8%BASn%2C%E5%85%AC%E5%B7%AEd%E4%B8%8D%E7%AD%89%E4%BA%8E0%2C%E4%B8%94S3%3D9%2Ca1%2Ca3%2Ca7%E6%88%90%E7%AD%89%E6%AF%94%E6%95%B0%E5%88%97.%281%29%E6%B1%82%E6%95%B0%E5%88%97%7Ban%7D%E7%9A%84%E9%80%9A%E9%A1%B9%E5%85%AC%E5%BC%8F%282%29%E8%AE%BEbn%3D2%5Ean%2C%E6%B1%82%E6%95%B0%E5%88%97%7Bbn%7D%E7%9A%84%E5%89%8Dn%E9%A1%B9%E5%92%8C%E8%B7%AF%E8%BF%87%E7%9A%84%E6%95%91%E5%91%BD%E5%95%8A)
急!等差数列{an}的前n项和为Sn,公差d不等于0,且S3=9,a1,a3,a等差数列{an}的前n项和为Sn,公差d不等于0,且S3=9,a1,a3,a7成等比数列.(1)求数列{an}的通项公式(2)设bn=2^an,求数列{bn}的前n项和路过的救命啊
急!等差数列{an}的前n项和为Sn,公差d不等于0,且S3=9,a1,a3,a
等差数列{an}的前n项和为Sn,公差d不等于0,且S3=9,a1,a3,a7成等比数列.(1)求数列{an}的通项公式(2)设bn=2^an,求数列{bn}的前n项和
路过的救命啊
急!等差数列{an}的前n项和为Sn,公差d不等于0,且S3=9,a1,a3,a等差数列{an}的前n项和为Sn,公差d不等于0,且S3=9,a1,a3,a7成等比数列.(1)求数列{an}的通项公式(2)设bn=2^an,求数列{bn}的前n项和路过的救命啊
等差数列{an}的前n项和为Sn,公差d不等于0,且S3=9,a1,a3,a7成等比数列.(1)求数列{an}的通项公式(2)设bn=2^an,求数列{bn}的前n项和
S3=9
S3=a1+a2+a3=3a1+3d=9
a1+d=3=a2
a7=a1+6d=3+5d
a3=a2+d=3+d
a1=3-d
,a1,a3,a7成等比数列
a3^2=a1*a7
(3+d)^2=(3-d)(3+5d)
d^2-d=0
d=1
a1=3-1=2
an=a1+(n-1)d=2+(n-1)*1=2+n-1=n+1
b1=2^a1=2^2
b2=2^3
b3=2^4
b^n=2^(n+1)
Sn=2^2+2^3+2^4+...2^(n+1)
2Sn=2^3+2^4+...2^(n+1)+2^(n+2)
2Sn-Sn=2^(n+2)-2^2
Sn=2^(n+2)-4