{an}各项为整数,所以d为整数,且d≠0
a5=6 →
a2=6-3d
a10=6+5d
a2*a10=(6-3d)(6+5d)>10→
36+2d-15d^2>10→
15d^2-2d-260
d=-1时,a2=9,a10=1,a2×a10>0