an为等差数列,有设an=an-1+d
所以q=bn/bn-1=3^an/3^an-1=3^(an-1 +d)/3^an-1=3^d
bn为等比数列.
2.、
a2=4,a3+a4=14
a3=a2+d,a4=a2+2d a3+a4=2a2+3d=2*4+3d=14
3d=14-8=6 d=3
a1=a2-3=1
an=1+3(n-1)=3n-2
bn=3^(3n-2) q=3^d=3^3
Sn=[b1-bnq]/(1-q)
=[3-3^(3n-2)*3^3]/(1-3^3)
= (3^(3n-2)*27-3 )/26