(a4)^2=a1*a13
(a1+3d)^2=3*(a1+12d)
(3+3d)^2=3*(3+12d)
9(1+d)^2=9*(1+4d)
(1+d)^2=1+4d
d^2+2d+1=1+4d
d^2-2d=0
d(d-2)=0
d=0或d=2
当d=0时
an=a1+(n-1)d
an=3+(n-1)*0
an=3
Sn=3+3+3+...+3=3n
当d=2时
an=a1+(n-1)d
an=3+(n-1)*2
an=2n+1
Sn=(a1+an)*n/2
=(3+2n+1)*n/2
=n(2n+4)/2
=n(n+2)