22π |
3 |
A.
| B.-
| C.±
| D.-
|
11(a1+a11) |
2 |
22π |
3 |
2π |
3 |
3 |
A.S10=S11 | B.S10>S11 | C.S9=S10 | D.S9<S10 |
1 |
bn |
1 |
2 |
1 |
4 |
1 |
6 |
1 |
20 |
n |
2 |
1 |
2 |
1 |
3 |
A.一定是等差数列 |
B.一定是等比数列 |
C.或者是等差数列,或者是等比数列 |
D.既不可能是等差数列,也不可能是等比数列 |
1 |
sn-1 |
A.a19>0,a21<0 | B.a20>0,a21<0 |
C.a19<0,a21>0 | D.a19<0,a20>0 |
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