# Export mathematical expressions for further reuse [duplicate]

I am dealing with a program with Mathematica where I compute expressions that take a very long time to compute. I need these results for further computation and/or display. I would like not to have to redo all the computations all the time, so save the results in files that can be imported again, without computation. But I need Mathematica to be able to read the expressions as mathematical expressions again, not just as text files.

For example, if I export the expression GR*Gl into a file on my hard drive, and I import it later in another program and assign it the name GA, I want that, if I assign further GR = 2 and Gl = 3, GA to be equal to 2*3 = 6. In other words, I want to be able to use it for further mathematical computation, as I would do directly in the original program.

Which commands should I use for that? I am not finding how to do this very simple thing.