Question

已知公差不为零的等差数列{an}满足a1，a3，a4成等比数列，Sn为{an}的前n项和，则(S3

-S2）／（S5-S3)的值为多少

Answer 1

a1,a3,a4成等比数列
a3^2=a1*a4
a3^2=(a3-2d)*(a3+d)
a3^2=a3^2-2d^2-d*a3
d*(2d+a3)=0
d1=0
d2=-a3/2

d=0时
(s3-s2)/(s5-s3)
=a3/(a4+a5)
=1/2

d=-a3/2时
(s3-s2)/(s5-s3)
=a3/(a4+a5)
=a3/(2a3-3a3/2)
=1/(1/2)=2
看完了好评我哦~~