How to find this rate of change?

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Homework Help Overview

The problem involves finding the rate of change of the function f(x, y, z) = 3x + 2y + 4z at the point (1, 2, 3) constrained to the plane y = 2. The discussion centers around the implications of this constraint and how it affects the derivatives involved.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants discuss whether to calculate the partial derivatives ∂f/∂x or ∂f/∂z given that y is constant. There is consideration of the concept of maximal change and how it relates to gradients. Some participants express confusion over the definitions of slope and gradient.

Discussion Status

The discussion is ongoing, with participants exploring different interpretations of the problem. Some guidance has been offered regarding the use of gradients to find the rate of change, but there is no explicit consensus on the best approach or the implications of the definitions being discussed.

Contextual Notes

Participants question the clarity of the problem statement and whether it was well-written. There is also a discussion about the nature of the gradient and its relationship to the rate of change in multiple dimensions.

supermiedos
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Homework Statement



Given f(x, y, z) = 3x + 2y + 4z, find its rate of change at (1, 2, 3) on the plane y = 2.

Homework Equations


The Attempt at a Solution


Since y is constant, I don't know if I have to find ∂f/∂x or ∂f/∂z. It's a homework problem, but I don't know if it was bad written by the teacher. Could you help me please?
 
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Rate of change with respect to what?
x and z? I think I would calculate those derivatives and evaluate them at the given point.
Maximal change for any direction? This can be evaluated, too (it has to be in the xz-plane here).
 
mfb said:
Rate of change with respect to what?
x and z? I think I would calculate those derivatives and evaluate them at the given point.
Maximal change for any direction? This can be evaluated, too (it has to be in the xz-plane here).

That's exactly what I tought. There are two answers here, right? If i remember, the maximal change can be calculated using gradients, right?

Btw, my teacher says slope and gradients are the same thing... How is that possible? A slope is a number, and the gradient is a vector!
 
Well, you could define "slope" as the gradient... looks odd, but it is just a definition issue.
If i remember, the maximal change can be calculated using gradients, right?
This is possible, right.
 
supermiedos said:

Homework Statement



Given f(x, y, z) = 3x + 2y + 4z, find its rate of change at (1, 2, 3) on the plane y = 2.

Homework Equations





The Attempt at a Solution


Since y is constant, I don't know if I have to find ∂f/∂x or ∂f/∂z. It's a homework problem, but I don't know if it was bad written by the teacher. Could you help me please?
No, this is not "bad written" (or even "badly written":-p). Since you are asked about its rate on change on y= 2, replace y by 2: f(x, z)= 3x+ 4+ 4z. The rate of change of that, at (1, 2, 3) is the length of the gradient vector evaluated at (1, 2, 3). (If the function really is what you wrote without any powers, its gradient vector is a constant and the point (1, 2, 3) is irrelevant.)
 
HallsofIvy said:
No, this is not "bad written" (or even "badly written":-p). Since you are asked about its rate on change on y= 2, replace y by 2: f(x, z)= 3x+ 4+ 4z. The rate of change of that, at (1, 2, 3) is the length of the gradient vector evaluated at (1, 2, 3). (If the function really is what you wrote without any powers, its gradient vector is a constant and the point (1, 2, 3) is irrelevant.)

I see. So, if I have a function f(x, y, z) and I'm asked to find its rate of change at (a, b, c), when, let's say, y = b. I have to find the gradient of f(x, b, y) at that point? But the length of the gradient represents the highest rate of change, right? So the problem must be more specific (since I could also find the lowest rate of change)
 
supermiedos said:
(since I could also find the lowest rate of change)
This is always zero with 2 or more variables, if the function is differentiable.
But the length of the gradient represents the highest rate of change, right?
Right
 

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