is it necessary to know where to derive all my formulas, as a general rule? i always try to follow them and understand more or less where they came from, because i hate hand waving. i can derive a good chunk of them, especially the ones of mechanics, but some of the messier ones, like the ones of thermo or em sometimes go over my mind. i can tell you where most of the ones i dont know how to derive come from, but not do the derivation itself. should i try to memorize them?