x-2y分之x-y+x2-4xy+4y的2次方分之x2-y2
=x-y/x-2y+x^2-y^2/(x^2-4xy+4y^2)
=x-y/x-2y+x^2-y^2/(x-2y)^2
=((x-y)(x-2y)+x^2-y^2)/(x-2y)^2
=(x^2-3xy+2y^2+x^2-y^2)/(x-2y)^2
=(2x^2-3xy+y^2)/(x-2y)^2
=(2x-y)(x-y)/(x-2y)^2