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=______.
答
∵a2+a6=a1+a7
∴s7=
=497(a1+a7) 2
故答案是49