已知等差数列{an}的首项a1≠0,前n项和为Sn,且S4+a2=2S3;等比数列{bn}满足b1=a2,b2=a4(1)若a1=2,设

2025-05-15 19:18:32
推荐回答(1个)
回答1:

(1)设等差数列{an}的公差为d,
由S4+a2=2S3,得4a1+6d+a1+d=6a1+6d,
∴a1=d,…(2分)
则an=a1+(n-1)d=na1
∴b1=a2=2a1,b2=a4=4a1
等比数列{bn}中q=

b2
b1
=2,…(3分)
则bn=2a1?2n-1=2n?a1,…(4分)
当a1=2时,bn=2n+1cn
2
(n+1)(n+2)
=2(
1
n+1
?
1
n+2
)
…(6分)
则Tn=c1+c2+…+cn=2(
1
2
?
1
3
+
1
3
?
1
4
+…+
1
n+1
?
1
n+2
)

=2(
1
2
?
1
n+2
)=
n
n+2?
…(8分)
(2)f(n)=log3Tn=log3
n
n+2?

∴f(1)+f(2)+…+f(n)
=log3
1
3
+log3
2
4
+…+log3
n
n+2

=log3(
1
3
?
2
4
?…?
n
n+2
)
=log3
2
(n+1)(n+2)
≤log3
2
(1+1)(1+2)
=-1
即f(1)+f(2)+…+f(n)的最大值为-1.…(12分)