# Differential equation help!

1. May 27, 2013

### llorgos

Hi! I would like to ask what is the general solution of the following differential equation
$\frac{\partial X_x}{\partial t} = - \frac{\partial X_t}{\partial x}$

Thank you very much.

2. May 27, 2013

### CompuChip

What is Xx - does that stand for $\frac{\partial X}{\partial x}$ ?
Because then you are basically asking about $X_{xt} = -X_{tx}$.

In that case you should be looking at "weird" functions - given Clairaut's theorem at least the partial derivatives should not be continuous.

3. May 27, 2013

### llorgos

Hi.

If you want I can write it as $\frac{\partial X}{\partial t} = -\frac{\partial T}{\partial x}$ where $T = T(x,t)$ and $X = X(x,t)$ in general.

I know they must be equal to a constant. Please correct me if I am wrong.

Thank you.

4. May 27, 2013

### Office_Shredder

Staff Emeritus
So the functions on the left and right hand side are not equal in general? And you're asking what the general form for X and T is as separate functions?

5. May 28, 2013

### JJacquelin

$\frac{\partial X}{\partial t} = -\frac{\partial X}{\partial x}$
$X=f(t-x)$ any derivable function $f$

6. May 28, 2013

### HallsofIvy

Staff Emeritus
Let X(x, t)= f(x, t) be any differentiable function of x and t and define T(x, t)= -f(t, x).
For example, take X(x, t)= x+ t^2, T(x, t)= -t- x^2. Then $\partial X/\partial x= 1= -\partial T/\partial t$.

I don't know what you mean by "they must be equal to a constant".