Question

设等比数列{an}的公比为q,q>0且q≠1,Sn为{an}的前n项和。记Tn=an/Sn,则：A.T3≤T6 B.T3＜T6 C.T3≥T6 D.T3

Answer 1

Sn=an(1-q^n)/(1-q)
Tn=an/Sn=(1-q)/(1-q^n)
T3=(1-q)/(1-q^3)
T6=(1-q)/(1-q^6)=(1-q)/(1-q^3)(1+q^3)
因为q>0，所以T6分母的(1+q^3)>1
所以T6的分母比T3大，所以T3>T6
所以应该选还没打全的D。