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I have the function $f(x,y)=(x^3)*y - (y^3)*x$.

I have to find $[(df/dx) +(df/dy)]/[(df/dx) *(df/dy)]$.

So,what I dont get is how to find $df/dx$ or $df/dy$..

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The function $f$ depends on two variables $x$ and $y$ and when you want to derive $f$ relative to $x$ you must treat $y$ as a constant so you have: $$\frac{\partial}{\partial x}f(x,y)=3yx^2-y^3$$ Now I think you can calculate $\frac{\partial}{\partial y}f(x,y)$ by the same method.

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