设数列为{an},公比为q,共2n项,则偶数项是以a2为首项,q²为公比的等比数列,共n项.a1[q^(2n) -1]/(q-1)=4a2[(q²)^n -1]/(q-1)a1[q^(2n)-1]/(q-1)=4a1q[q^(2n-1)]/(q-1)4q=1q=1/4a2×a4=9(a3+a4)a3²=9(a3...