Question

求教一道等差数列（难）

已知等差数列a2+a5+a6=12,求此等差数列的前9项和S9

Answer 1

题目肯定错了!解不出来,这道题应该是a2+a5+a8=12

a2+a5+a8=12,
(a1+d)+(a1+4d)+(a1+7d)=12
3a1+12d=12
a1+4d=4
S9
=a1+a2+a3+.....+a9
=a1+(a1+d)+(a1+2d)+(a1+3d)+.....+(a1+8d)
=9a1+36d
=9(a1+4d)
=9*4
=36