设首项为a1,公比为q,｜q｜<1,
a1+a2+...+an=a1(1-q^n)/(1-q)=a1/(1-q)=12
a1^2+a2^2+...+an^2=a1^2(1+q^2+...+q^2(n-1))=a1^2/(1-q^2)=48
可解得,a1=6,q=1/2
a1^3+a2^3+...+an^3=a1^3(1+q^3+...+q^3(n-1))=a1^3/(1-q^3)=1728/7