9=a2+a5+a8=3a5, a5=3
-21=a2a5a7=(a5-3d)a5(a5+2d)
a5^2-a5d-6d^2=-7
9-3d-6d^2=-7
6d^2+3d-16=0
d=(-3+/-√393)/12,
an=a5+(n-5)d 题目数字有错吧?不然答案这么怪?