(1) x - y = 0 ==> x = y xy = 1 ==> x^2 = 1 ==> x = y = 1 ,-1 (2) xy = 0 x = 0 x - y = 1 ==> y = -1 x - y = -1 ==> y = 1 y= 0 x - y = 1 ==> x = 1 x - y = -1 ==> x = -1 x = 0,0,1,- 1,1,-1 y= 1,-1,0,0,1,-1