First Author:   Mohamadou Aminou University of Ngaoundere mohamad.benamin@yahoo.fr   Ngaoundere, Cameroun   +237699870018(Phone) +237699870018(Fax) Second Author: Amadou Bobbo University of Yaounde II-Soa amadou_bobbo@yahoo.fr   Yaounde, Cameroon       Third Author: Mohammadou Nourou University of Garoua nourouacademy@gmail.com   Garoua, Cameroon