等差数列{an}中,已知a3+a15=40,则S17=_.
问题描述:
等差数列{an}中,已知a3+a15=40,则S17=______.
答
S17=
=17(a1+a17) 2
=17(a3+a15) 2
=34017×40 2
故答案为:340