题目
题型:不详难度:来源:
A.18 | B.17 | C.16 | D.15 |
答案
9(a1+a9) |
2 |
9×2a5 |
2 |
解得a5=2,故a5+an-4=32,
而Sn=
n(a1+an) |
2 |
n(a5+an-4) |
2 |
故选D
核心考点
举一反三
n |
m |
m |
n |
A.小于4 | B.等于4 |
C.大于4 | D.大于2且小于4 |
A.18 | B.17 | C.16 | D.15 |
9(a1+a9) |
2 |
9×2a5 |
2 |
n(a1+an) |
2 |
n(a5+an-4) |
2 |
n |
m |
m |
n |
A.小于4 | B.等于4 |
C.大于4 | D.大于2且小于4 |