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**malsch****Member**- Registered: 2012-12-15
- Posts: 2

Hi, I have a function f(x,y). u = ax + by and v = bx - ay. a and b are constants. i need to prove that d2f/du2 + d2f/dv2 = 0.

I have started out by finding df/du using chain rule:

df/du = df/dx * dx/du + df/dy * dy/du

Hence, df/du = df/a*dx + df/b*dy

My problem is that now i need to find d^2f/du^2 but i do not know how to continue from df/du (ie. i do not know how i can differentiate df/a*dx + df/b*dy with respect to u).

Thanks for reading

*Last edited by malsch (2012-12-15 23:12:24)*

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**John E. Franklin****Member**- Registered: 2005-08-29
- Posts: 3,588

You said ". i need to prove that d2f/dx2 + d2f/dv2 = 0.".

I see f, x, f, v, but no u, why no u ?

**igloo** **myrtilles** **fourmis**

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**malsch****Member**- Registered: 2012-12-15
- Posts: 2

sorry i miswrote dx2 instead of du2. i've edited the post

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