2 |
9 |
1 |
Sn |
1 |
Sn |
1 |
Sn-1 |
Sn-1-Sn |
SnSn-1 |
2 |
9 |
1 |
Sn |
2 |
9 |
1 |
Sn |
11 |
9 |
11-9n |
9 |
9 |
11-9n |
81 |
(11-9n)(20-9n) |
81 |
(11-9n)(20-9n) |
11 |
9 |
20 |
9 |
A.a3+a9>b4+b10 |
B.a3+a9≥b4+b10 |
C.a3+a9≠b4+b10 |
D.a3+a9与b4+b10的大小不确定 |
1 |
50 |
A.等差数列 | B.等比数列 |
C.从第二项起是等比数列 | D.从第二项起是等差数列 |
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