I wrote a module which contains some methods for doing math functions.
I want the module to be included in one of my rails models.
Where is the best place to put it?
if you put it under RAILS_ROOT/lib directory and assuming the name of the file and the name of the module match, then it will be automatically added by rails to the load_path and you can use it directly throughout your rails application.
in recent versions of rails you can also add directories to the load_path directly by tweaking environment.rb (look at the code, there is a commented out example in there). But if we want convention over configuration i'd recommend lib.
about making it a gem/plugin, I'd only do it if it's going to be something really generic that you are going to be reusing in several projects. If not, it's not worth the overwork.