Question

急阿,已知公差不为零的等差数列｛an｝中,a1=1,且a1,a3,a13成等比数列,（1）求数列｛an｝的通项公式.（2

Answer 1

1.

a3^2=a1a13

(1+2d)^2=1+12d

d=2

an=2n-1

2.

bn=2^(2n-1)

b[n-1]=2^(2n-3)

bn/b[n-1]=4

b1=1

Sn=(4^n-1)/3