x·y/(-z) · (-x)·y^2/z^2 · x^2·(-y)/z
= - x·y/z · x·y^2/z^2 · x^2·y/z
= - x·y·z^{-1} · x·y^2·z^{-2} · x^2·y·z^{-1}
= - x^4·y^4·z^{-4}
= - (xy/z)^4
(x - y)^{-1}/(x + y)^2 · (x + y)^{-2}/(x - y)^3
= 1/((x + y)^4·(x - y)^4) = 1/((x + y)·(x - y))^4 = ((x + y)·(x - y))^{-4} = ((x + y)·(x - y))^{-4}