A.14 | B.15 | C.16 | D.17 |
(a1+a9)×9 |
2 |
(a1+ an)×n |
2 |
(a5+an-4)×n |
2 |
A.既不是等差数列又不是等比数列 |
B.是等比数列但不是等差数列 |
C.既是等差数列又是等比数列 |
D.是等差数列但不是等比数列 |
A.1 | B.-1 | C.-2 | D.2 |
A.12 | B.10 | C.8 | D.6 |
A.26,16,8, | B.25,17,8 | C.25,16,9 | D.24,17,9 |
A.20 | B.26 | C.30 | D.32 |
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