前式可化为：x^2=5x-1,后式可化为 x^2(x-4)-1=(5x-1)(x-4)-1=5x^2-21x＋3=3*(x^2-5x＋1)＋(2x^2-6x)=2x^2-6x=2(5x-1)-6x=4x-2.而由前式求得x=(5正负根号21)/2,代入4x-2中得3正负根号21.