x^2-4x=-1 (x≠0)
x^4-5x^3+6x^2-5x
=x^4-4x^3-x^3+4x^2+2x^2-8x+3x
=x^2(x^2-4x)-x(x^2-4x)+2(x^2-4x)+3x
=-x^2+x-2+3x
=-x^2+4x-2
=-(x^2-4x)-2
=-1
x^2-4x=-1 (x≠0)
x^4-5x^3+6x^2-5x
=x^4-4x^3-x^3+4x^2+2x^2-8x+3x
=x^2(x^2-4x)-x(x^2-4x)+2(x^2-4x)+3x
=-x^2+x-2+3x
=-x^2+4x-2
=-(x^2-4x)-2
=-1