【x^2+2yz-y^2-z^2】 /【x^2-2xy-z^2+y^2】
=【x^2-(y-z)^2】/【(x-y)^2-z^2】
=(x+y-z)(x-y+z)/(x-y+z)/(x-y-z)
=(x+y-z)/(x-y-z)