twesterm on 13 April 2008
l'Hospital's Rule
(it would be too painful to type this out here so just clicky the linky)
This was one of the fun ones I had back in college since it can let you solve limits in indeterminate forms such as 0/0. 
-edit-
Actually, shouldn't be too hard to type since we can do symbols.
If you have a limit
lim f(x)/g(x)
where both f(x) and g(x) equal 0 or +/- ∞ then if f'(x) and g'(x) are both of a determinate form you can have the following
lim f'(x)/g'(x) = lim f(x)/g(x)
So that way you can solve some indeterminate limits. And there are still more troubles you have to deal with after that but it's still a pretty neat little rule.
