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 !