Ecuatia de grad III     are forma:

 

                                

 

                    Facem substitutia:     

    avand      ,  

 

Se noteaza cu      si  

 

                  Radacinile ecuatiei sunt:

                 

                 

                 

                                - discriminantul ecuatiei.

 

                  Daca ecuatia are toate radacinile reale,distincte.

                 Daca ecuatia are toate radacinile reale ,dintre care cel putin 2 egale.

                  Daca ecuatia are o singura radacina reala si 2 complexe  conjugate.