设等差数列{an}的前n项和为Sn,若S4≥10,S5≤15,则a4的最大值为_.

问题描述:

设等差数列{an}的前n项和为Sn,若S4≥10,S5≤15,则a4的最大值为______.

∵等差数列{an}的前n项和为Sn,且S4≥10,S5≤15

S4=4a1+
4×3
2
d≥10
S5=5a1+
5×4
2
d≤15

2a1+3d≥5
a1+2d≤3

a4a1+3d≥
5−3d
2
+3d=
5+3d
2
a4a1+3d=(a1+2d)+d≤3+d

5+3d
2
a4≤3+d
,5+3d≤6+2d,d≤1
∴a4≤3+d≤3+1=4故a4的最大值为4,
故答案是4