已知各项均为正数的数列{an}的前n项和Sn满足S1>1,且6Sn=(an+1)(an+2),n∈N*.
(I)求数列{an}的通项公式;
(II)设数列{bn}满足,记Tn为数列{bn}的前n项和.求证:2Tn+1<log2(an+3)
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(I)解:n=1时,6a1=a12+3a1+2,且a1>1,解得a1=2.
n≥2时,6Sn=an2+3an+2,6Sn-1=an-12+3an-1+2,两式相减得(an+an-1)(an-an-1-3)=0,
∵an+an-1>0,
∴an-an-1=3,
∴{an}为等差数列,
∵a1=2,
∴an=3n-1.
(II)证明:∵数列{bn}满足,
∴
∴Tn=b1+b2+…+bn=
要证2Tn+1<log2(an+3),即证<log2(an+3)
即证
即证
令,
∴
∵cn>0,∴cn+1<cn,
∴{cn}是单调递减数列
∴
∴
故2Tn+1<log2(an+3).
解析分析:(I)n=1时,6a1=a12+3a1+2,且a1>1,解得a1=2.n≥2时,6Sn=an2+3an+2,6Sn-1=an-12+3an-1+2,两式相减得(an+an-1)(an-an-1-3)=0由此能求出an.(II)根据数列{bn}满足,可得,从而Tn=b1+b2+…+bn=,利用分析法证明.要证2Tn+1<log2(an+3),即证<log2(an+3),即证,构造函数,可得{cn}是单调递减数列,即可证出结论.
点评:本题考查数列的综合应用,解题时要认真审题,仔细解答,注意挖掘题设中的隐含条件,合理地进行等价转化.