Hi again Chipset3600!
The proper formula is:
I have some trouble understanding your scans, but they do not look right.
Let's define .
And let's define .
What is ?
And ?
Find the total differential:
My teacher give to us this formule:
but in this exercice he just dont give the point "P" and the valous of
i use this formule:
and the result was:
part 1: http://i.imgur.com/W46hP.jpg
part 2: http://i.imgur.com/uYTek.jpg
is this correct??
Well, I couldn't make sense of your first scan, so just now I skipped to the second one.
And yes!
Your partial derivatives are correct (except for that should have a minus sign).
However, for the total derivative du you shouldn't have added them as you did.
Each of them should be multiplied by dx, dy, and dz respectively.