x^6+4x^5+2x^4-6x^3-3x^2+2x+1 =(x^3+2*x^2-x-1)^2 f(x) =正负（ x^3+2*x^2-x-1）,5,已知x^6+4x^5+2x^4-6x^3-3x^2+2x+1=[f(x)]^2,其中f(x)是关于X的多项式,求这个多项式
