Consider  = (a + i b) (a - i b)-(c + i b)(- c + i d)
                      = (a2 - i2 b2 ) - (i2 d2 - c2)
                       = (a2 + b2 ) - (- d2 - c2)             [  i2 = - 1 ]
                      = (a2 + b2 + c2 + d2) Â
                     Â