# How can I extract equations from one document for use in another?

I have a LaTeX document that includes many equations. I would like to create a separate "quick reference" sheet that includes many of these equations. Instead of copy-and-pasting the equations from the first document to the second document, I would like to be able to reference the equations by name and have them be automatically included (so that if I later change an equation in the first document, that change will be automatically reflected in the second document after I recompile it). What's a good way to do this?

-