a2=a1+d a4=a1+3d a6=a1+5d
a2,a4－2,a6 成等【比】数列
(a1+3d-2)^2=(a1+d)(a1+5d)
(3d-1)^2=(1+d)(1+5d)
9d^2-6d+1=5d^2+6d+1
4d^2=12d 公差不为0
d=3
an=a1+(n-1)d=3n-2