问题
解答题
已知等比数列{an}各项均为正数,且2a1+3a2=8,a32=
(1)求数列{an}的前n项和Sn; (2)若数列{bn}满足bn=
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答案
(1)∵正项等比数列{an}中,a32=
a2•a6=1 4
a42,1 4
∴q2=
=4,q>0,a42 a32
∴q=2;
又2a1+3a2=8,即2a1+3a1q=8,
∴a1=1.
∴Sn=
=2n-1.1×(1-2n) 1-2
(2)∵bn=1 log2(Sn+1).log2(Sn+1+1)
=1 log2(2n-1+1).log2(2n+1-1+1)
=1 log22n.log22n+1
=1 n(n+1)
=
-1 n
,1 n+1
∴Tn=b1+b2+…+bn
=(1-
)+(1 2
-1 2
)+…+(1 3
-1 n
)1 n+1
=1-1 n+1
=
.n n+1