A.30 | B.29 | C.28 | D.27 |
(a1+a2n+1) (n+1) |
2 |
580 |
n+1 |
(a1+a2n+1)(2n+1) |
2 |
S1 |
S2 |
| ||
|
2n+1 |
n+1 |
290 |
551 |
A.6 | B.5 | C.4 | D.3 |
A.
| B.
| C.
| D.
|
1 |
2 |
a9+a10 |
a7+a8 |
A.1+
| B.1-
| C.3+2
| D.3-2
|
A.以7为首项,公差为2的等差数列 |
B.以7为首项,公差为5的等差数列 |
C.以5为首项,公差为2的等差数列 |
D.不是等差数列 |
1 |
4 |
A.30 | B.45 | C.50 | D.80 |
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