设Sn是等差数列{an}的前n项和,且S5S8,则a1
问题描述:
设Sn是等差数列{an}的前n项和,且S5S8,则a1
答
S6=S7
S7-S6=a7=0
又S7>S8
S8-S7=a8<0
故a8=a7+d,故d<0
a7=a1+6d=0
a1=-6d >0
a1 >0