At some level I think you have to do both. Either by explicit drilling or frequent use, you end up memorizing a lot of formulas anyway. And for the longer ones,

At some level I think you have to do both. Either by explicit drilling or frequent use, you end up memorizing a lot of formulas anyway. And for the longer ones, it takes too long to derive from first principles every time, so whichever way it happens, it's good to have them in mental RAM. You can always look them up, but just knowing is faster.