设Sn是等差数列{an}的前n项和,若S7=35,则a4=_.
问题描述:
设Sn是等差数列{an}的前n项和,若S7=35,则a4=______.
答
S7=
=35,(a1+a7)•7 2
∴a1+a7=10
∴2a4=a1+a7=10,a4=5
故答案为5.