由等比数列求和公式s(n)=a(1)(q^n-1)/(q-1)得a(1)(q^n-1)/(q-1)=3^n+a化简得a(1)q^n-a(1)=(q-1)3^n+a(q-1)左右两边对应可得q=3 a(1)=q-1=2 a(q-1)=-a(1)2a=-2即a=-1
