Chemical Forums
Specialty Chemistry Forums => Other Sciences Question Forum => Topic started by: zeshkani on August 30, 2007, 01:49:21 PM
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can somebody explain on how to do a total differentiation on this, i have the formula but iam just lost
z=x^2+2y^2-2xy+2x-4y-8
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You can look at this term by term to make things simpler. I'll do the first couple of terms to start things off:
x2
The differential of this term is fairly easy since it involves only one variable. I like to think of a differential this way. You know the derivative (dz/dx) of z = x2:
(dz/dx) = 2x
Well, if you treat the derivative like a fraction, and multiply through by dx, you get:
dz = 2x dx
which is the differential of x2.
-2xy
This term is a more difficult term. Here you have to make use of the product rule:
d(uv) = u dv + v du
So in this case,
d(-2xy) = -2 d(xy) = -2(xdy + ydx) = -2x dy - 2y dx
You should be able to do the rest now.
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would this be the answer:
dz=2x-2y+2dx + (4y-2x-4 dy)
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Do you mean:
dz = (2x-2y+2) dx + (4y-2x-4) dy
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yeah thats what i mean i just forgot to put () around the dx part
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if you don't want to think like Yggdrasil you could form the partial derivatives:
dz/dx = 2x - 2y + 2
dz/dy = 4y - 2x - 4
then the differential is defined as:
dz = (2x - 2y + 2)dx + (4y - 2x - 4)dy
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thx for all the help, i finally got it ;D
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FeLiXe's method is probably the easier way to think of things. Conceptually, though it is important to know the product rule for differentials (i.e. d(uv) = u dv + v du).