Cheap algorithm to find measure of angle between vectors

Question

Finding the angle between two vectors is not hard using the cosine rule. However, because I am programming for a platform with very limited resources, I would like to avoid

Answer 1

Back in the day of a few K of RAM and machines with limited mathematical capabilities I used lookup tables and linear interpolation. The basic idea is simple: create an array with as much resolution as you need (more elements reduce the error created by interpolation). Then interpolate between lookup values.

Here is an example in processing (original dead link).

You can do this with your other trig functions as well. On the 6502 processor this allowed for full 3D wire frame graphics to be computed with an order of magnitude speed increase.