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Given the function:$$ f(x,y) = x^5y^4+\sin\left({x\over{y}}\right)$$ How can I find such partial derivatives as $f_{xx}$, $f_{xy}$, $f_{yx}$, and $f_{yy}$?

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You meant partial derivatives right? –  EuYu Nov 26 '12 at 4:34
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Assuming, as EuYu suggests, that you mean partial derivatives, you find them the same way you always find partial derivatives --- by differentiating with respect to one variable, while treating the other as a constant. Where are you getting stuck? –  Gerry Myerson Nov 26 '12 at 4:36
    
Yes, derivatives. I fixed it now. –  StealzHelium Nov 26 '12 at 4:38
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Think about what $f_{xx}$ means. Let $F := f_x$. Then $f_{xx} = F_x$. Just take the partial derivative of the partial derivative. –  anonymous Nov 26 '12 at 4:39
    
I'm getting stuck in what exactly the notation is asking me to do. But if all it's asking is to basically take it twice then I think I got it. –  StealzHelium Nov 26 '12 at 4:41
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up vote 1 down vote accepted

You have $f(x,y) = x^5y^4+\sin\left({x\over{y}}\right)$:

  • For $f_{xx}$ its asking you to take the derivative -- of the function -- twice of $x$ and treat $y$ as a constant. Same logic with $f_{yy}$

  • For $f_{xy}$ its asking you to take the derivative -- of the function -- of $x$ treat $y$ as a constant, then take the derivative of $y$ and treat $x$ as a constant. Same logic $f_{yx}$

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