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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.

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    $\begingroup$ Try Dt[f g, x]. $\endgroup$ – J. M. will be back soon Jun 4 '13 at 16:02
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    $\begingroup$ Or perhaps, D[f[x] g[x], x]. $\endgroup$ – Michael E2 Jun 4 '13 at 16:05
  • $\begingroup$ Or try D[f[x] g[x], x] $\endgroup$ – user21 Jun 4 '13 at 16:05
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    $\begingroup$ Michael wins this round $\endgroup$ – Jacob Akkerboom Jun 4 '13 at 16:06
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    $\begingroup$ Closely related: how to differentiate formally $\endgroup$ – Jens Jun 4 '13 at 16:11
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Some alternatives (picked from the comments):

D[f[x] g[x], x]

Dt[f g, x]

f[#] g[#] &'
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  • $\begingroup$ Great it is solved! $\endgroup$ – JimmyLin Jun 4 '13 at 17:09
  • $\begingroup$ Thanks! Didn't think of something like f[#] g[#] &'. $\endgroup$ – Silvia Jun 4 '13 at 19:43

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