6Sn=an^2+3an+26Sn+1=an+1^2+3an+1+2两式作差 因式分解得（an+1+an)*(an+1-an-3)=0已知数列{An}的各项均为正数an+1-an-3)=0an=3n-3+a1 a2,a4,a9成等比数列a4^2=a2*a9(a+9)^2=(a+3)(a+24)a1=1...