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Yagoda
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Homework Statement
Let [itex]h(u,v) = f(u+v, u-v)[/itex]. Show that [itex]f_{xx} - f_{yy} = h_{uv}[/itex] and [itex]f_{xx} + f_{yy} = \frac12(h_{uu}+h_{vv}) [/itex].
Homework Equations
The Attempt at a Solution
I'm always confused on how to tackle these types of questions because there isn't an actual function to differentiate.
So I am assuming here that x = u+v and y = u-v and going from there. So I need to find the first partial with respect to x, which might be something like [itex]f_x = \frac{\partial f}{\partial(u+v)}[/itex] since I need to get it in terms of u and v, but this doesn't seem right. What sort of approach do I need to use on these questions?
Edit: What I've done now is write [itex]h_{uv} = \frac{\partial}{\partial v}\frac{\partial h}{\partial u} = \frac{\partial}{\partial v} ( \frac{\partial f}{\partial x} \frac{\partial x}{\partial u} + \frac{\partial f}{\partial y} \frac{\partial y}{\partial u}) =\frac{\partial}{\partial v} (\frac{\partial f}{\partial x} + \frac{\partial f}{\partial y}) [/itex]. I'm having trouble converting from u's and v's to x's and y's.
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