依题意,a1=f(x+1)=3(x+1)-2=3x+1,
a3=f(x-1)=3(x-1)-2=3x-5,
由{an}是等差数列,可知a1+a3=2a2,即3x+1+3x-5=2×2,解得x=4/3
所以a1=5,公差=a2-a1=-3,an=5+（n-1）×-3=-3n+8