等比数列钱n项和Sn=[a1*(1-q^n)]/(1-q)
=a1/(1-q)-a1q^n/(1-q)
令a1/(1-q)=m 则Sn=m(1-q^n)
由已知得 Sn=3^(n+1）+a=3*3∧n+a=-3(-a/3-3^n)
则得 q=3 a=-3