I am new to Mathematica so the my questions might be pretty much at the pupil's level.
If I want to use differentiation to derive:
(fg)' = f'g + g'f
How can I keep empty symbol
f, g from been evaluated for derivative and get 0? For example if I use
D[f g, x]
I immediately get 0 result. I guess it might be related to hold, but couldn't find out the correct way to get it work.