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______.
答
∵等差数列{an}的前n项和为Sn,且S4≥10,S5≤15∴S4=4a1+4×32d≥10S5=5a1+5×42d≤15即2a1+3d≥5a1+2d≤3∴a4=a1+3d≥5−3d2+3d=5+3d2a4=a1+3d=(a1+2d)+d≤3+d∴5+3d2≤a4≤3+d,5+3d≤6+2d,d≤1∴a4≤3+d...