Multi variable partial differentiation, cant solve

In summary, the chain rule is used to find the partial derivatives of z with respect to x and y. This results in dz/dx = fu and dz/dy = -fu, which cancel each other out when added together, proving that dz/dx + dz/dy = 0.
  • #1
LovePhysics
16
0

Homework Statement



If z = f(x-y), show that dz/dx + dz/dy = 0

2. The attempt at a solution
I thought:

dz/dx = fx
dz/dy = -fy

which doesn't make sense really... because its not equal to 0.

or maybe it should be:
dz/dx = dz/df * df/dx = fx * ??
dz/dy = dz/df * df/dy = -fy * ??
 
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  • #2
LovePhysics said:

Homework Statement



If z = f(x-y), show that dz/dx + dz/dy = 0

2. The attempt at a solution
I thought:

dz/dx = fx
dz/dy = -fy

which doesn't make sense really... because its not equal to 0.

or maybe it should be:
dz/dx = dz/df * df/dx = fx * ??
dz/dy = dz/df * df/dy = -fy * ??

Use the chain rule. If z= f(u) and u= x- y then [itex]\partial f/\partial x= df/du \partial u/\partial x[/itex] and [itex]\partial f/\partial y= df/du \partial u/\partial y[/itex].

(If z= f, then "dz/df" is just 1.)
 
  • #3
HallsofIvy said:
Use the chain rule. If z= f(u) and u= x- y then [itex]\partial f/\partial x= df/du \partial u/\partial x[/itex] and [itex]\partial f/\partial y= df/du \partial u/\partial y[/itex].

(If z= f, then "dz/df" is just 1.)
Thats great thx.

dz/dx = df/du du/dx = fu * 1 = fu
dz/dy = df/du du/dy = fu * -1 = -fu


dz/dx + dz/dy = fu - fu = 0
 

FAQ: Multi variable partial differentiation, cant solve

What is multi variable partial differentiation?

Multi variable partial differentiation is a mathematical concept used to calculate the rate of change of a function with respect to multiple independent variables. It is an extension of ordinary differentiation, which only involves one independent variable.

What is the purpose of multi variable partial differentiation?

The purpose of multi variable partial differentiation is to help us understand how a function changes when multiple independent variables are varied simultaneously. It is especially useful in fields such as physics and economics, where multiple variables are often involved in a single equation.

Why am I having trouble solving problems involving multi variable partial differentiation?

Solving problems involving multi variable partial differentiation can be challenging because it requires a solid understanding of basic calculus principles and the ability to manipulate multiple variables and equations simultaneously. It may also involve complex mathematical concepts, such as partial derivatives and chain rule.

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Can multi variable partial differentiation be used in real-life applications?

Yes, multi variable partial differentiation has many real-life applications. It is commonly used in fields such as physics, engineering, economics, and statistics to model and analyze complex systems. For example, it can be used to optimize production processes, analyze market trends, and determine the trajectory of a projectile.

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