Question

数列{an}是公差不为零的等差数列,并且a5,a8,a13是等比数列{bn}的相邻三项,（1）求等比数列{bn}的公比

Answer 1

an=a1+(n-1)d

a5=a1+4d

a8=a1+7d

a13=a1+12d

a5*a13=(a8)²

(a1+4d)*(a1+12d)=(a1+7d)²

d=2a1

a5=a1+4d=9a1

a8=a1+7d=15a1

a13=a1+12d=25

公比q=15：9=5/3