等比数列的和sn=a1*(1-q^n)/(1-q)或na1(显然不符,不予考虑)sn=a1/(1-q)-a1*q^n/(1-q)与S(n)=a·3的n次方+b对比,可以看出a=-a1/(1-q),b=a1/(1-q)所以a+b=0