设公比为q,S3/S5=(a1+a1q+a1q^2)/(a1+a1q+……+a1q^4)=27/28,∴28(1+q+q^2)=27(1+q+q^2+q^3+q^4),整理得27q^4+27q^3-q^2-q-1=0,解得q1≈0.3427857，q2≈-1.0346388，∴a5/a3=q^2≈0.1175020或1.0704777.
