x^5-1
=x^5-x+(x-1)
=x(x^4-1)+(x-1)
=x(x^2+1)(x^2-1)+(x-1)
=x(x^2+1)(x+1)(x-1)+(x-1)
=(x-1)[x(x^2+1)(x+1)+1)
=(x-1)(x^4+x^3+x^2+x+1)