equ3(a, b, c, d)

            function x  = equ3(a, b, c, d)

            if a == 0
              x = equ2(b,c,d);
            else            
              x = newton(a,b,c,d);
              beta =  b + a*x;
              gamma = c + beta*x;
              x = [x equ2(a, beta, gamma)]; 
            end;