Question

15.设等比数列{an}的公比q

Answer 1

S4=a1(1-q4)/(1-q),S2=a1(1-q2)/(1-q),

已知S4=5S2,则a1(1-q4)/(1-q)=5a1(1-q2)/(1-q),

即q=±2,

又公比q