There is a very
silly, but handy,
mnemonic for remembering this
formula. It runs as follows:
If you pretend you're Santa Claus, you could call f(x) "Hi" (as in, high, on top), and g(x) "Ho" (rhymes with "low"). Then the formula is "HoDeeHi minus HiDeeHo over HoHo".
(dy / dx) = HoDeeHi - HiDeeHo
(Ho)(Ho)
You will probably feel like an
idiot reciting this to
calculus students, but hey, whatever works.