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Sum of partial derivatives

  • #1

Homework Statement


I need to prove that [itex]x\frac{ \partial^2z}{ \partial x^2} + y\frac{\partial^2z}{\partial y\partial x} = 2\frac{\partial z}{\partial x}[/itex]

Homework Equations



[itex] z = \frac{x^2y^2}{x+y} [/itex]

The Attempt at a Solution



I actually did it the long way and I got the right answer but here is my teacher's solution :

[itex] z = \frac{x^2y^2}{x+y} [/itex]

[itex]\Rightarrow x\frac{ \partial z}{ \partial x} + y\frac{ \partial z}{ \partial x} = 3\frac{ \partial z}{ \partial x} [/itex]

[itex]\Rightarrow \frac{ \partial z}{ \partial x} +x\frac{ \partial^2z}{ \partial x^2} + y\frac{ \partial z}{ \partial x} = 3\frac{ \partial z}{ \partial x} [/itex]

Answer follows.

To be honest, I have absolutely no idea about what technique he actually uses there. Is there any "rule" or "trick" that I am not aware of here?
 

Answers and Replies

  • #2
lurflurf
Homework Helper
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It follows from observing z is homogeneous of degree 3, Euler's homogeneous function theorem, and interchanging operators.

x zxx+y zyx=(x zx+y zy-z)x
by commuting operators
=(3-1)zx=2zx
by Euler's homogeneous function theorem
thus
zx is homogeneous of degree 2
or we could go backwards and just show zx is homogeneous of degree 2
 
  • #3
Thank you! Our teacher didn't ever mention homogenous functions though, I assume this ain't going to be in the exam. At least I'll have that tool in my arsenal :tongue:
 

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