x^4-4x^3+5x^2-4x+1
=x^2(x^2+1/(x^2)-4x-4/x+5)
=x^2[(x+1/x)^2-4(x+1/x)+3]
=x^2(x+1/x-3)(x+1/x-1)
=(x^2-3x+1)(x^2-x+1)
系数对称的四次因式都可以用这种办法分解