设数列an的各项均为正数,它的前n项和为Sn,点(an,Sn)在函数y=(1/8)x^2+(1/2)x+1/2的图像上,

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  • 据题意,有an>0,Sn=an²/8+an/2+1/2.

    (1)令n=1,得a1=2.

    S(n+1)-Sn=a(n+1)=(a²(n+1)-a²n)/8+(a(n+1)-an)/2.

    整理得(a(n+1)+an)(a(n+1)-an-4)=0.

    则a(n+1)=-an或a(n+1)=an+4.

    又an>0,故前者舍去,得a(n+1)=an+4.

    即数列{an}是首项a1=2,公差d=4的等差数列.

    an=a1+(n-1)d=4n-2.

    (2)代入an,得bn=(2n+1)/(2n-1)+(2n-1)/(2n+1).

    当n=1时,T1-2=4/3<2成立.①

    假设当n=k时成立,即Tk-2k<2.②

    当n=k+1时,T(k+1)-2(k+1)=Tk-2k+b(k+1)-2

    由于Tk-2k<2,故Tk-2k+b(k+1)-2<b(k+1)

    b(k+1)=(2k+3)/(2k+1)+(2k+1)/(2k+3)

    =2-2/(2k+3)+1/(2k+1)

    =2+(1-2k)/(2k+1)(2k+3)

    <2.

    故T(k+1)-2(k+1)<2.③

    联立①、②、③,得对任意n∈N*,Tn-2n<2.

    综上,命题得证.