x^4-4x^3+6x^2-4x+1=0

x(x^3-4x^2+6x)=-1

x=-1 или  x^3-4x^2+6x=-1

                  x(x^2-4x+6)=-1

                   x=-1 или x^2-4x+6=-1

                                    x^2-4x=-7

                                    x(x-4)=-7

                                    x=-7 или x-4=-7

                                                     x=-3