来自刘弢的问题
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]
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]
1回答
2020-04-22 19:34