a3^2=a1*a9
(a1+2d)^2=a1*(a1+8d)
(1+2d)^2=(1+8d)
4d^2+4d+1=8d+1
4d^2-4d=0
d^2-d=0
d(d-1)=0
d=1或d=0(舍去)
an=a1+(n-1)d
=1+(n-1)*1
=n