已知各项均不相等的等差数列{an}的前四项和S4=14,且a1,a3,a7成等比数列.(Ⅰ)求数列{an}的通项公式

2025-06-26 10:14:22
推荐回答(1个)
回答1:

(I)设公差为d,由已知得:

S4=14
a32a1a7

4a1+
4×3
2
d=14
(a1+2d)2a1(a1+6d)

解得:d=1或d=0(舍去),
∴a1=2,
故an=2+(n-1)=n+1;
(II)∵
1
anan+1
=
1
(n+1)(n+2)
=
1
n+1
-
1
n+2

∴Tn=
1
2
-
1
3
+
1
3
-
1
4
+…+
1
n+1
-
1
n+2
=
1
2
-
1
n+2
=
n
2(n+2)

∵Tn≤λan+1对?n∈N*恒成立,即
n
2(n+2)
≤λ(n+2),λ≥
n
2(n+2)2
?n∈N*恒成立,
n
2(n+2)2
=
1
2(n+
4
n
+4)
1
2(4+4)
=
1
16

∴λ的最小值为
1
16