等差数列(an)公差d≠0,如果,a5,a9,a15成等比数列,则公比为?
问题描述:
等差数列(an)公差d≠0,如果,a5,a9,a15成等比数列,则公比为?
答
a5*a15=(a9)²
(a1+4d)(a1+14d)=(a1+8d)²
a1=4d
q=a9/a5
=(a1+8d)/(a1+4d)
=(12)d/(8d)
=3/2