an=a1+d(n-1)
(a2)^2=(a1+d)^2=a1(a1+3d)
a1=d
an=dn
a1,a3,ak1,ak2,……,akn的公比q=a3/a1=3,k1=3*3=9
akn/ak(n-1)=a3/a1=3
即kn/k(n-1)=3
kn=9*(1-3^n)/(1-3)=(9/2)*(3^n)-9/2
an=a1+d(n-1)
(a2)^2=(a1+d)^2=a1(a1+3d)
a1=d
an=dn
a1,a3,ak1,ak2,……,akn的公比q=a3/a1=3,k1=3*3=9
akn/ak(n-1)=a3/a1=3
即kn/k(n-1)=3
kn=9*(1-3^n)/(1-3)=(9/2)*(3^n)-9/2