## Partial Diff qn

1. The problem statement, all variables and given/known data
Does the following differential equation for u(x,y) have solutions which have the form of a product of functions of each independent variable:

$$\partial$$2u/$$\partial$$x$$\partial$$y=u

[b]3. The attempt at a solution[/]

=>$$\int$$$$\int$$u dx dy =uxy=f(x)f(y) ???
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 im not sure if that even makes sense, but if u(x,y)=f(x)f(y) and d*f(x)/dx=u/constant and d*f(y)/dy=constant... i think, i don't know im confused..
 Recognitions: Homework Help Science Advisor Set u(x,y)=f(x)*g(y). What is your partial derivative in terms of f(x), f'(x), g(y), and g'(y)?

## Partial Diff qn

so $$\partial$$2u/$$\partial$$x$$\partial$$y=f(x)g(y)??
so in terms of f'(x) and g'(y) im not sure..
 Recognitions: Homework Help You have an equation that equates the derivative of u to u itself. You can already guess what u will be, but lets not guess. Setting u(x,y)=f(x)g(y), then the equation becomes $$\frac{\partial^2 f(x)g(y)}{\partial x \partial y}=f(x)g(y)$$. Now what Dick asks you to do is to calculate $$\frac{\partial^2 f(x)g(y)}{\partial x \partial y}$$. Hint: $$\frac{d}{dx}f(x)= ?$$ In terms of f'(x)
 Recognitions: Homework Help Science Advisor I actually meant u(x,y)=f(x)g(y). There's no reason why the function of x would have to be the same as the function of y. And yes, evaluate the derivative.
 Recognitions: Homework Help Sorry, that was sloppy of me I actually meant f(x)g(x). I will fix it.
 hmm d/dx of f(x).. isn't that just f'(x)??.... is it something like the integral of f'(x)g'(y)??
 so... $$\frac{\partial^2 f(x)g(y)}{\partial x \partial y}$$ =$$\int$$$$\int$$f'(x)g'(y) dx dy??

Recognitions:
Homework Help
What you've written down now is the equivalent to $\frac{d}{dx}f(x)=\int f'(x) dx$. Which is wrong. Lets test it, f(x)=x then $\frac{d}{dx}x=1 \neq \int 1 dx=x+c$.

 hmm d/dx of f(x).. isn't that just f'(x)??
This is right, so if you evaluate $$\frac{\partial^2 f(x)g(y)}{\partial x \partial y}$$ you get....?
 u get f'(x)g'(y)?? Im confused becasue when u take partial of f(x), u get d/dy(d/dx*f(x))=0??
 Recognitions: Homework Help Yes that's correct, but you're not taking the partial of f(x), you're talking the partial derivative of f(x)g(y). $$\frac{\partial^2 f(x)g(y)} {\partial x \partial y}=\frac{\partial}{\partial x} f(x) \frac{\partial}{\partial y}g(y)=f'(x)g'(y)$$
 ok so since f(x)g(y)=f'(x)g(y)=u(x,y)=??
 Recognitions: Homework Help You forgot a prime it should be $f'(x)g'(y)=f(x)g(y)=u(x,y)$. You can instantly see which function satisfies the equation, but you can also separate the variables.
 e^x+y?? so f(x)=e^x and g(y)=e^y same as the derivativeS?
 Recognitions: Homework Help That would be a solution indeed, you're on the right track. it is not the general solution however. The general solution will involve integration constants and a constant in the argument of the exponential. Can you see how to integrate the differential equation?
 e^xy?? so f(x)=e^x and g(y)=e^y same as the derrivativeS? srry this was an accident

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