Rút gọn phân thức:
1.\(\dfrac{\left(x-y\right)^{3^{ }}-3xy\left(x+y\right)+y^3}{x-6y}\)
2. \(\dfrac{x^2+y^2+z^2-2xy+2xz-2yz}{x^2-2xy+y^2-z^2}\)
3.\(\dfrac{\left(n+1\right)!}{n!\left(n+2\right)}\)
4. \(\dfrac{n!}{\left(n+1\right)!-n!}\)
5. \(\dfrac{\left(n+1\right)!-\left(n+2\right)!}{\left(n+1\right)!+\left(n+2\right)!}\)