I've never had to memorize them. After doing them a few times, you just know them. I'm only speaking for myself, but I just know all the differentiation rules, trig identities by heart. Actually pretty much any formula, I've never had to really memorize, but rather just understand the justification behind them. I can't explain it but I pretty much know any mathematical formula, rules, etc, by heart. I guess that's why I love MATH!