已知(an)为等差数列,a3+a4+a8=9.求S9
问题描述:
已知(an)为等差数列,a3+a4+a8=9.求S9
答
a1+2d+a1+3d+a1+7d=9
3a1+12d=9
s9=9a1+9*8d/2
=9a1+36d
=27