# Sum of partial derivatives

1. Oct 29, 2011

### Mr.Rockwater

1. The problem statement, all variables and given/known data
I need to prove that $x\frac{ \partial^2z}{ \partial x^2} + y\frac{\partial^2z}{\partial y\partial x} = 2\frac{\partial z}{\partial x}$

2. Relevant equations

$z = \frac{x^2y^2}{x+y}$

3. 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 :

$z = \frac{x^2y^2}{x+y}$

$\Rightarrow x\frac{ \partial z}{ \partial x} + y\frac{ \partial z}{ \partial x} = 3\frac{ \partial z}{ \partial x}$

$\Rightarrow \frac{ \partial z}{ \partial x} +x\frac{ \partial^2z}{ \partial x^2} + y\frac{ \partial z}{ \partial x} = 3\frac{ \partial z}{ \partial x}$

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?

2. Oct 30, 2011

### lurflurf

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. Oct 31, 2011

### Mr.Rockwater

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: