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-   -   Quick math question! (http://www.physicsforums.com/showthread.php?t=582912)

 btbam91 Mar1-12 12:03 PM

Quick math question!

How do I evaluate (d^2)/(dxdy)?

Is it just d/dx*d/dy?

Thanks!

 tiny-tim Mar1-12 12:24 PM

hi btbam91! :smile:
Quote:
 Quote by btbam91 (Post 3793009) Is it just ∂/∂x*∂/∂y?

(and it's also ∂/∂y*∂/∂x)

 btbam91 Mar1-12 12:35 PM

Re: Quick math question!

Hmmm, double check my work because I must be doing something wrong!

I'm looking to evaluate -(d^2)/(dxdy) of F, where F = ax^2+bxy+cy^2

So for dF/dx, I get 2ax+by. For dF/dy, I get bx+2cy.

So, multiplying both together and accounting for the negative sign:

-[(2ax+by)*(bx+2cy)]= -[2abx^2+4acxy+b^2xy+2bcy^2]

The answer is supposed to be -b according to the solution manual (that doesn't show the solution :p)

Am I missing something here? Thanks!

 Integral Mar1-12 12:43 PM

Re: Quick math question!

Do not multiply, but apply each differential sequentially. So after finding the first differential, do the second on the resulting expression. Order is not critical.

 tiny-tim Mar1-12 12:43 PM

ohhh! :smile:

i assumed you meant ∂/∂x of ∂/∂y …

you ∂/∂y it first, then you ∂/∂x it …
∂(∂F/∂y)/∂x :wink:

 btbam91 Mar1-12 12:48 PM

Re: Quick math question!

Oh! I got it now! Thanks fellas!

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