

 The  matrix mA :

            1/1  1/1  2/1
       mA = 3/2  3/4  3/1
            4/3  2/1  1/2

 If you copy all the values below the pivot
 in the identity matrix. And take the opposite.

              1/1  0/1  0/1       
       mID = -3/2  1/1  0/1          
             -4/3  0/1  1/1        

 And now if you multiply this matrix, 
 by mA for example,  all the values 
 below the pivot will be equal to 0.
             
            1/1  1/1   2/1
 mID * mA = 0/1 -3/4   0/1
            0/1  2/3 -13/6

