设公比为q，a2=a1q=1-a1，a1(1+q)=1a4=9-a3，a1q^3=9-a1q^2，a1q^2(1+q)=q^2=9，q=3或q=-3(舍）a1=1/(1+q)=1/4a4+a5=a1q^3+a1q^4=a1q^3(1+q)=q^3=3^3=27
