已知等差数列{an}中,a3+a8=a5,则s11=

问题描述:

已知等差数列{an}中,a3+a8=a5,则s11=

a3=a1+2d
a8=a1+7d
a5=a1+4d
a1+2d+a1+7d=a1+4d
a1+5d=0
a6=0
a5+a7=a4+a8=a3+a9=a2+a10=a1+a11=2*a6=0
s11=0