a1(1+q^3)=18a1q(1+q)=12(1+q^3)/q(1+q)=3/2(1-q+q²)/q=3/22q²-5q+2=0q=2或q=1/2(舍去)a1=2an=2*2^(n-1)=2^nS8=a1(1-q^8)/(1-q)=2×(1-2^8)/(1-2)=2^9-2=510
