x2-y2
--------------
x2-2xy+y2
Now x2-y2 factors to (x+y)(x-y), and x2-2xy+y2 factors to (x-y)(x-y). So:
x2-y2 (x+y)(x-y) (x+y)
-------------- = --------------- = ---------, for all x≠y
x2-2xy+y2 (x-y)(x-y) (x-y)