由a^2-a-1=0得,a^2=a+1,于是a^4=(a+1)^2=a^2+2a+1=3a+2 a^8=(3a+2)^2=9a^2+12a+4=21a+13 a^8+7a^-4=21a+13+7/(3a+2) =(63a^2+81a+26+7)/(3a+2) =(63a+63+81a+33)/(3a+2) =(144a+96)/(3a+2)=48