． 已知等差数列{an},公差d≠0,a1,a5,a17成等比数列,则 /=

题目

． 已知等差数列{an},公差d≠0,a1,a5,a17成等比数列,则 /=
完全正确

答案

a1,a5,a17成等比数列
a5^2=a1a17
(a1+4d)^2=a1(a1+16d)
a1^2+8a1d+16d^2=a1^2+16a1d
16d^2=8a1d
a1=2d
所以
/
=/
=/
=/
=26/29