已知数列{an}中,Sn为前n项的和,2Sn=3an-1.(Ⅰ)求an;(Ⅱ)若数列{bn}满足bn=an+(-1)nlog3an,求

2025-01-21 15:26:30
推荐回答(1个)
回答1:

(Ⅰ)因为2Sn=3an-1,
所以2Sn-1=3an-1-1,(n≥2)
两式相减得2an=3an-3an-1
所以 an=3an-1
所以数列{an}是等比数列的公比q=3
当n=1,得2a1=3a1-1,解得a1=1.
则an=3n-1
(Ⅱ) bn=an+(-1)nlog3an=3n-1+(-1)nlog33n-1=3n-1+(-1)n(n-1),
则数列{bn}的前2n项和T2n=(1+3+32+…+32n-1)+[-0+1-2+3-…+(2n-1)]=

1?32n
1?3
+n=
32n
2
+n?
1
2