(x+y)(x-y)=3*1=(-3)*(-1)
x+y=3
x-y=1
则x=2,y=1
或x+y=1,x-y=3
x+y=-3,x-y=-1
x+y=-1,x-y=-3
x=2,y=1
x=2,y=-1
x=-2,y=-1
x=-2,y=1