设Sn是等差数列{an}的前n项和,已知a2=3,a6=11,则S7=_.

问题描述:

设Sn是等差数列{an}的前n项和,已知a2=3,a6=11,则S7=______.

∵a2+a6=a1+a7
s7

7(a1+a7
2
=49
故答案是49