magic_wolf
幼苗
共回答了17个问题采纳率:100% 举报
(1)∵等差数列{a n }的公差大于0,且a 3 ,a 5 是方程x 2 -14x+45=0的两根,
∴a 3 =5,a 5 =9,∴ d=
a 5 - a 3
5-3 =2
∴a n =a 5 +2(n-5)=2n-1
∵S n =
1 -b n
2 ,∴n≥2时,b n =S n -S n-1 =
b n-1 - b n
2 ,∴
b n
b n-1 =
1
3
∵n=1时,b 1 =S 1 =
1 -b 1
2 ,∴b 1 =
1
3
∴b n =
1
3 • (
1
3 ) n-1 = (
1
3 ) n ;
(2)证明:由(1)知c n =a n •b n =
2n-1
3 n
∴c n+1 -c n =
2n+1
3 n+1 -
2n-1
3 n =
4(1-n)
3 n+1 ≤0
∴c n+1 <c n
(3)T n =
1
3 +
3
3 2 +…+
2n-1
3 n
∴
1
3 T n =
1
3 2 +…+
2n-3
3 n +
2n-1
3 n+1
两式相减可得:
2
3 T n =
1
3 +
2
3 2 +…+
2
3 n -
2n-1
3 n+1 =
3
2 -
3
2 •
n+1
3 n
∴T n = 1-
n+1
3 n .
1年前
2