Factorise the following expression:

$a^4-16x^4$

Factorise the following expression:

$x^2-y^2+2x+1$

Factorise the following expression:

$x^2+4y^2-z^2-4xy$

$25{\left(x+y\right)}^2-36{\left(x-2y\right)}^2$ can be factorised as:

One of the factors of $x^2+y^2+2xy-z^2$ is

Factorise: $\left(256x^{16}-1\right)$