xy-x-y+1=3
x(y-1)-(y-1)=3
(x-1)(y-1)=3=1×3=-1×﹙-3)
∴x-1=1,y-1=3或x-1=3,y-1=1或x-1=-1,y-1=-3或x-1=-3,y-1=-1
x=2,y=4或x=4,y=2或x=0,y=-2或x=-2,y=0