因为是等比数列,所以Sn=(a1-a1*q^n)/(1-q)=[a1/(1-q)]-[a1/(1-q)]*q^n=4^n+a,所以-[a1/(1-q)]*q^n=4^n,且[a1/(1-q)]=a,即q=4,则[a1/(1-q)]=-a1/3=1,a1=-3,所以a=1