试题:
设Sn是等差数列{an}的前n项和,若 S4≥10,S5≤15,S7≥21,则a7的取值区间为 [ ] |
A.(﹣∞,7] B.[3,4] C.[4,7] D.[3,7] |
等差数列的定义及性质, 等差数列的前n项和
2016-05-24
答案:
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