2xy-x^2-y^2+1 =-(x^2-2xy+y^2-1)=-[(x-y)^2-1]=-(x-y+1)(x-y-1)=(y-x-1)(x-y-1)
=-(-2xy+x^2+y^2-1 )=-((x-y)^2-1)=-(x-y+1)(x-y-1)
[1+(x-y)][1-(x-y)]
2xy-x²-y²+1=-(x²-2xy+y²-1)=-[(x-y)²-1]=-(x-y+1)(x-y-1)
