得：x2-4x=-1
x^4-5x^3+6x^2-5x
=x^2（x^2-4x)-x^3+6x^2-5x
=-x^2-x^3+6x^2-5x
=-x^3+5x^2-5x
=-x(x^2-4x)+x^2-5x
=x+x^2-5x
=x^2-4x=-1