YOu can also apply the rule when you have (and by the way, the signs are not important, you can also apply this when you have )

and going from to is simple, you'll see why

assume that x goes to 0 and y goes to 0 (but different from 0 !)

is in the form

now you can divide both numerator and denominator by xy :

and this is in the form , since x and y go to 0 !