Nice to see so much interest in a numerics thread!
In practice it is good to avoid flt. point "/"  ... it's usually
10 to 50 times slower than "*".  One trick is to calculate
Inverse_Sqrt followed by one call to "/".  THere are
no "/"'s in the Newton-Raphson calculation for Inverse_Sqrt.

Inverse_Sqrt is extremely useful indepepently of Sqrt.
The following Inverse_Sqrt is usually faster than 1.0 / Sqrt on the
machines I tested, (but 1.0 / Inverse_Sqrt is not necessarily as fast or
as accurate as the machine's Sqrt).

--  Jonathan