I actually do know offhand that I use the Taylor series to calculate it, and that the Carmack method works by using a couple tricks of binary floating point format to do a really good starting point and then run the Taylor series through a single round of iteration. I've also implemented it myself before since library methods were too slow. This probably puts me in the top 0.001% of developers world wide in terms of what I happen to have memorized and from personal experience about square roots. However, I don't offhand remember the details at all and wouldn't pretend to. I would never use this an an interview question because it doesn't test for anything at all. 99% of candidates would have no clue regardless of skill level, and of the 1% who knew anything, most would be because they happened to study the right "interview tricks" site while cramming for the interview. So I still think these sorts of questions are absolutely terrible for the purpose of finding competent developers.
