1 等比数列{an}中,a1-a5=-15/2,S4=-5,则a4为多少 2 等比数列{an}中,a(n+2)=an,求实数q若a(n+3)=an时求q

题目

1 等比数列{an}中,a1-a5=-15/2,S4=-5,则a4为多少 2 等比数列{an}中,a(n+2)=an,求实数q若a(n+3)=an时求q

答案

S4=(a1-a1*q^4)/(1-q)=(a1-a5)/(1-q)=-15/2(1-q)=-5解得q=-1/2又因为a1-a5=a1-a1*q^4=-15/2,把q=-1/2代入解得a1=-8,a4=a1*q^3=1第一问因a(n+2)=an得q^2=1,解得q=1或-1第二问因a(n+3)=an得q^3=1解得q=1...