Calculating Partial Derivative of F(u,v) w.r.t u

In summary, the derivative of f(x) = F(x, 6) is F_u(x, 6) + F_v(x, 6), where F_u and F_v are the partial derivatives of F with respect to the first and second variables respectively. This can also be written as the limit of the change in F over the change in the first variable, with the second variable held constant. The example given is F(u, v) = u^2 - u*v, with f(x) = F(x, x) = 0 and f'(x) = 0. However, for a general function F(u, v), the derivative of f(x) will depend on both F_u and F_v. This is
  • #1
-EquinoX-
564
1

Homework Statement



Let [tex]F(u,v)[/tex] be a function of two variables. Find f '(x) for [tex]f(x) = F(x, 6)[/tex].

Homework Equations


The Attempt at a Solution



I need to find the answer in terms of [tex]F_u[/tex], how can I do this?
 
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  • #2
Remember what the definition of F_u is.
 
  • #3
I am confused what are we supposed to find the derivative of F_u with respect to what? x or y?
 
  • #4
F_u is the derivative of F(u,v) with respect to the first variable with the second variable held constant, right? f'(x) looks like pretty much the same thing.
 
  • #5
my guess is that it will be something like:

F_u * v(6)

is this right?
 
  • #6
What does that mean? Try an example f(u,v)=u^2-u*v. What's F_u? What is f'(x)?
 
  • #7
I assume that F_u is the derivative of f(u,v)

f(u,v)=u^2-u*v

is just:

(u -v) is this correct?
 
  • #8
-EquinoX- said:
I assume that F_u is the derivative of f(u,v)
No, [itex]F_u[/itex] is the partial derivative of F(u, v). You calculate it by assuming that v is constant and that only u is changing. [itex]F_u[/itex] can also be written as [itex]\partial F/ \partial u[/itex]
-EquinoX- said:
f(u,v)=u^2-u*v

is just:

(u -v) is this correct?
No.
One way to to calculate this is to take the limit:
[tex]\lim_{h \rightarrow 0} \frac{F(u + h, v) - F(u, v)}{h}[/tex]
 
  • #9
You mean F_u(u,v)=2u-v, yes? Now what's are f(x) and f'(x)?
 
  • #10
f(x) is therefore 2x-6 right? and f'(x) is 2 ?
 
  • #11
Equinox,
Don't forget that you're answering Dick's question about a specific example (post 6), not the question you first posted. He's trying to get you to think about this the right way.
 
  • #12
Yes I am aware of that.. I am supposed to find the relation between the example he's given and the real answer to my question. I believe so the answer is then F_u(x,6) ?
 
  • #13
If you mean f'(x)=F_u(x,6), yes. f(x)=2x-6, and f'(x)=2 are NOT right.
 
  • #14
so f'(x)=F_u(x,6) is not correct?
 
  • #15
-EquinoX- said:
so f'(x)=F_u(x,6) is not correct?

Yes it is. The other two things aren't.
 
  • #16
what if a variation of the question asks for f(x) = F(x, x), is it just then 0?
 
  • #17
No, why would you say that? Wouldn't you need to think about F_v as well? v isn't fixed anymore.
 
  • #18
Hmm..is x here actually a number or is x another function?
 
  • #19
Actually, sorry, I meant yes. For the specific function F(u,v)=u^2-u*v, sure f(x)=F(x,x)=0. f'(x)=0. Sorry, I misspoke. But you can't express f'(x) purely in terms of F_u.
 
  • #20
hmm.. I tried to input 0 as the answer and it didn't accept it..
 
  • #21
Why did you input 0? I said that was only for that particular function that I made up in my head. For a general function F(u,v), f'(x) is going to depend on F_u AND F_v.
 
  • #22
so it is F_u + F_v ?
 
  • #23
Yes it is. But why do you say that? It would be really helpful if you would give reasons or show a calculation instead of just pulling the answer out of a hat.
 
  • #24
I think it's just because of the definition of the chain rule. As now u and v varies with x and both are not constants therefore I have that conclusion in mind
 
  • #25
Good. Yes, it's the chain rule. More specifically the answer is F_u(x,x)+F_v(x,x), right?
 
  • #26
that is correct! thanks for helping me out Dick I appreciate it
 

FAQ: Calculating Partial Derivative of F(u,v) w.r.t u

What is a partial derivative?

A partial derivative is a mathematical concept used to measure the rate of change of a function with respect to a specific variable while holding all other variables constant. It is denoted by ∂ (pronounced "del").

How do you calculate a partial derivative?

To calculate a partial derivative of a function F(u,v) with respect to a variable u, you must hold all other variables (in this case, v) constant and apply the standard rules of differentiation. This means taking the derivative of the function as if u was the only variable, treating all other variables as constants.

What does it mean to calculate a partial derivative "with respect to u"?

Calculating a partial derivative with respect to u means finding the rate of change of the function F(u,v) as u changes, while keeping all other variables (in this case, v) constant. It can be thought of as finding the slope of the function in the u direction.

Why do we need to calculate partial derivatives?

Partial derivatives are useful in many fields of science and engineering, including physics, economics, and engineering. They allow us to understand how a function changes in response to changes in specific variables, which is crucial for modeling and predicting real-world phenomena.

Can you calculate partial derivatives for any function?

In most cases, yes. However, there are certain functions that may not be differentiable or may require more advanced techniques to calculate their partial derivatives. It is always important to check for any potential issues, such as discontinuities or undefined points, before attempting to calculate partial derivatives.

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