a3=a1+2d=a1+4
a4=a1+3d=a1+6
因为a1,a3,a4成等比数列,则
a4/a3=a3/a1
(a1+4)^2=a1(a1+6)
解之,a1=-8
则 a2=a1+d=-8+2
=-6