# Momentum Flux: Quick Question (Product Rule)

1. ### andrew.c

47
1. The problem statement, all variables and given/known data
Heya,
I can expand the term on the LHs and get the term on the RHS no problem :)
But,
I don't understand how the bottom line is arrived at; I think its basically just a backward engineering of the product rule, but I can't get it!

Any help would be useful

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2. ### clamtrox

939
$$\frac{\partial(\rho u u)}{\partial x} = \rho u \frac{\partial u}{\partial x} + u \frac{\partial (\rho u)}{\partial x}.$$

3. ### andrew.c

47
Thanks
:surprised That was easier than I thought!

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