Factorise the following expression:

$a^2{x}^2-y^2$

Factorise the following expression:

$49c^2-25d^2$

Factorise the following expression:

$100-r^2$

Factorise the following expression:

$49a^2-1$

Factorise the following expression:

$100b^2-81$

Factorise the following expression:

$x^2{y}^2-4$

Factorise the following expression:

$1-d^2$

Factorise the following expression:

$b^4-100$

Factorise the following expression:

$25-a^2{x}^2$