已知等差数列{an}的公差d>0,且a2,a5是方程x2-12x+27=0的两根,数列{bn}的前n项和为Tn,且满足b1=3,bn

2025-05-17 19:11:53
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回答1:

(Ⅰ)∵等差数列{an}的公差d>0,且a2,a5是方程x2-12x+27=0的两根,

a2+a5=12
a2a5=27
,解得a2=3,a5=9,或a2=9,a5=3(∵d>0,∴舍去)
a1+d=3
a1+4d=9
,解得a1=1,d=2,
∴an=1+(n-1)×2=2n-1.n∈N*
∵b1=3,bn+1=2Tn+3(n∈N*),①
∴bn=2Tn-1+3(n∈N*),②
两式相减并整理,得bn+1=3bn,n≥2,
bn3n,n∈N*
(Ⅱ)cn=
an
bn
=
2n?1
3n

Mn
1
3
+
3
32
+…+
2n+1
3n
,①
1
3
Mn
1
32
+
3
33
+…+
2n?1
3n+1