On Sun, Jan 6, 2019 at 1:00 PM Fred Cisin via cctalk <[email protected]> wrote:
> Few people (but most are right here) can recite PI to enough digits to > reach the level of inaccuracy. And those who believe that PI is exactly > 22/7 are unaffected by FDIV. (YES, some schools do still teach that!) > Really? I find it hard to believe any schools taught that as anything other than an approximation. And as an approximation it's not good for much unless you are multiplying PI in your head by a factors of 7 a lot (i.e 21*PI ~ 66). Personally, I use the PI^2 ~ 10 approximation far more often when doing math in my head. But since I'm almost always sitting at a screen, approximations are less useful than they used to be. If I need PI to 600 places, "scale=600;a(1)*4" is always there for the asking. The exception is the approximations that provide physical scale. For example, 1 km/s ~ 1 parsec/million years, or v=4.74 PM/parallax, which provides the conversion from proper motion to parallax, 5 magnitudes is a factor of 100. And of course, "c" in whatever units you need. They are just easier to remember than to look up. -- Eric Korpela [email protected] AST:7731^29u18e3
