已知数列{an}的前n项和为sn,且sn=2n^2+n-3,n∈N*,数列{bn}满足an=4log2(bn)+3,n∈N*1

2025-12-18 10:14:35
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回答1:

  • 解:(1)由Sn=2n^2+n可得,当n=1时,a1=s1=3
    当n≥2时,an=sn-sn-1=2n^2+n-2(n-1)^2-(n-1)=4n-1
    而n=1,a1=4-1=3适合上式,
    故an=4n-1,
    又∵an=4log2bn+3=4n-1
    ∴bn=2^n−1

    (2)由(Ⅰ)知,anbn=(4n−1)•2^n−1
    Tn=3×2^0+7×2+…+(4n−1)•2^n−1
    2Tn=3×2+7×2^2+…+(4n-5)•2^n-1+(4n-1)•2^n
    ∴Tn=(4n−1)•2^n−[3+4(2+2^2+…+2^n−1)]

    =(4n-1)•2^n−[3+4•2(1−2n−1)/1−2]

    =(4n-1)•2^n-[3+4(2^n-2)]=(4n-5)•2^n+5