Relationship between partial derivatives

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Discussion Overview

The discussion revolves around the relationship between partial derivatives at a point, particularly in the context of functions defined implicitly by level sets. Participants explore the mathematical formulation and geometric interpretations of these relationships.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Conceptual clarification

Main Points Raised

  • One participant asks how to derive the relationship between partial derivatives, specifically questioning the expression dy/dx|x = - df/dx|y / df/dy|x.
  • Another participant suggests a correction to the notation, proposing dy/dx|f = - df/dx|y / df/dy|x.
  • A participant introduces the concept that for a function f(x,y) = c, the total differential leads to the equation df/dx dx + df/dy dy = 0, which can be solved for dy/dx.
  • There is a geometric interpretation presented, stating that the curve defined by x(t), y(t) on the level set where f = c has a velocity vector that is perpendicular to the gradient vector of f.
  • One participant expresses gratitude for the clarification but indicates confusion regarding the introduction of the parameter t in the explanation.
  • Another participant elaborates on the dot product interpretation of the equation df/dx dx/dt + df/dy dy/dt = 0, noting the challenge of understanding the similar equation in terms of differentials.

Areas of Agreement / Disagreement

Participants express differing levels of understanding regarding the geometric interpretations and the introduction of parameters, indicating that the discussion remains somewhat unresolved with varying perspectives on the clarity of the explanations provided.

Contextual Notes

Some assumptions about the definitions of the variables and the context of the level sets may not be fully articulated, which could affect the clarity of the relationships discussed.

Niishi
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Hello,
Can anyone please tell me how to get the relationship between partial derivatives at a point, that is, dy/dx|x = - df/dx|y / df/dy|x ?
 
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You maybe meant dy/dx|f = - df/dx|y / df/dy|x ?
 
Last edited:
f(x,y) = c, implies df/dx dx + f/dy dy = 0, now solve for dy/dx.
 
the geometry is that a curve x(t),y(t) moving in the level set where f= c, has velocity vector (dx/dt, dy/dt) which is perpendicular to the gradient vector of f: (df/dx, df/dy).
 
Thanks mathwonk i was able to get it. I did not understand the explanation in the second the post particularly why t has been introduced etc.
 
well saying df/dx dx/dt + df/dy dy/dt = 0, is a dot product statement involving grad f and the velocity vector of (x(t),y(t)).

it is a little harder to make geometric sense out of the similar equation df/dx dx + df/dy dy = 0, in terms of differentials.
 

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