6x^5+11x^4+5x^3+5x^2-x-6
=6x^5+6x^3+11x^4-x^3+5x^2-x-6
=6x^3(x^2+1)+11x^4+11x^2-x^3-6x^2-x-6
=6x^3(x^2+1)+11x^2(x^2+1)-(x^3+x)-(6x^2+6)
=6x^3(x^2+1)+11x^2(x^2+1)-x(x^2+1)-6(x^2+1)
所以,得证.