If F(x,y,z,t) the "total derivative of F is:
[tex]\frac{\partial F}{\partial x}dx+\frac{\partial F}{\partial y}dy+\frac{\partial F}{\partial z}dz+\frac{\partial F}{\partial t}dt[/tex]
Odd. Your second post telling us that it is "problem 2 not problem 3" appears after my post but with an earlier "time stamp".
In any case, I have already given the general formula for "total differential". You might also want to know that, for a vector valued function E, [tex]\frac{\partial(E.E)}{\partial x}= 2E.\frac{\partial E}{\partial x}[/tex]. That should be all you need.