令a2=a1q
令a3=a1q^2
则a2/(a1-a3)=q/(1-q^2)
因a3=a1+(2a2)
则a1q^2=a1+(2a1q)
即q^2=1+2q
即q/(1-q^2)=-1/2
所以a2/(a1-a3)=-1/2