# Proving an exponential function obeys the wave equation

1. Jan 9, 2014

### LiamG_G

1. The problem statement, all variables and given/known data
Prove that $y(x,t)=De^{-(Bx-Ct)^{2}}$ obeys the wave equation

2. Relevant equations
The wave equation:
$\frac{d^{2}y(x,t)}{dx^{2}}=\frac{1}{v^{2}}\frac{d^{2}y(x,t)}{dt^{2}}$

3. The attempt at a solution
1: $y(x,t)=De^{-u^{2}}; \frac{du}{dx}=B; \frac{du}{dt}=-C$
2: $\frac{dy(x,t)}{dx}=-2uBDe^{-u^{2}}; \frac{d^{2}y(x,t)}{dx^{2}}=4u^{2}B^{2}De^{-u^{2}}$
3: $\frac{dy(x,t)}{dt}=2uCDe^{-u^{2}}; \frac{d^{2}y(x,t)}{dt^{2}}=4u^{2}C^{2}De^{-u^{2}}$
Then I'm stuck, I think I might have done something wrong but I can't see what.
I think v=C (from (x-vt)) and that would cancel the $C^{2}$ when I substitute into the wave equation, but then I would be left with a $B^{2}$

2. Jan 9, 2014

### nasu

The C in the exponent is not the speed of the wave. It has dimensions of s^(-1).
The speed of the wave will be a function of C and D. Which you can find from the equation.

3. Jan 9, 2014

### BruceW

also, the equation
$$\frac{dy(x,t)}{dx}=-2uBDe^{-u^{2}}$$
is correct, but the next equation
$$\frac{d^{2}y(x,t)}{dx^{2}}=4u^{2}B^{2}De^{-u^{2}}$$
is not correct. It looks like you've done the derivative of the exponential, but what about the $u$ (in the previous equation), doesn't it depend on x also?

4. Jan 11, 2014

### LiamG_G

Sorry it has taken so long. I will come back to this question I'm just in the middle of moving right now :S
Thanks for replies :)

5. Jan 11, 2014

### BruceW

no worries :) hope the move goes well

6. Jan 11, 2014

### HallsofIvy

You did not differentiate the first "u". It should be
$\frac{\partial y(x,t)}{\partial x}= -2B^2De^{-u^2}- u^2B^2De^{-u^2}$

7. Jan 11, 2014

### dextercioby

I know that you;re supposed to differentiate like crazy to get to the expected solution, but you can surprise your professor if you make the following change of variables in your PDE: x-vt = u; x+vt = v. Then you can fully solve the original PDE and make certain simplifying assumptions to find the function you're given in the statement.