[QUOTE=FernandoRevilla;620861]Right if you change $\displaystyle P$ by $\displaystyle Q$ .

One final question: What do you mean change P by Q? If P is in Rn-1[X] then Q is in Rn[X] such that fn(Q)=P. Why chanige P by Q in my reasoning?

Thanks for all the information, guidance until now Dr. Revilla! I appreciate everything!