(bn,an是数列,{an}=3n-5,an是首项为-2,公差为3的等差数列)bn=2^an,求数列bn的前n项S
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(bn,an是数列,{an}=3n-5,an是首项为-2,公差为3的等差数列)bn=2^an,求数列bn的前n项S
![(bn,an是数列,{an}=3n-5,an是首项为-2,公差为3的等差数列)bn=2^an,求数列bn的前n项S](/uploads/image/z/16235057-65-7.jpg?t=%EF%BC%88bn%2Can%E6%98%AF%E6%95%B0%E5%88%97%2C%7Ban%7D%3D3n-5%2Can%E6%98%AF%E9%A6%96%E9%A1%B9%E4%B8%BA-2%2C%E5%85%AC%E5%B7%AE%E4%B8%BA3%E7%9A%84%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%EF%BC%89bn%3D2%5Ean%2C%E6%B1%82%E6%95%B0%E5%88%97bn%E7%9A%84%E5%89%8Dn%E9%A1%B9S)
n=2^an
=2^(3n-5)
=2^(3n)*2^(-5)
b(n-1)=2^[3(n-1)]*2^(-5)
=2^(3n)*2^(-3)*2^(-5)
bn/b(n-1)=[2^(3n)*2^(-5)]/[2^(3n)*2^(-3)*2^(-5)]
=2^3
∴bn是以公比q=2^3的等比数列
b1=2^(3*1)*2^(-5)
=2^(-2)
Sn=b1(q^n-1)/(q-1)
=2^(-2)[(2^3)^n-1]/(2^3-1)
=2^(-2)(2^3n-1)/7
=[2^(3n-2)-2^(-2)]/7
=2^(3n-5)
=2^(3n)*2^(-5)
b(n-1)=2^[3(n-1)]*2^(-5)
=2^(3n)*2^(-3)*2^(-5)
bn/b(n-1)=[2^(3n)*2^(-5)]/[2^(3n)*2^(-3)*2^(-5)]
=2^3
∴bn是以公比q=2^3的等比数列
b1=2^(3*1)*2^(-5)
=2^(-2)
Sn=b1(q^n-1)/(q-1)
=2^(-2)[(2^3)^n-1]/(2^3-1)
=2^(-2)(2^3n-1)/7
=[2^(3n-2)-2^(-2)]/7
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