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**polymerase** Let $\displaystyle f(x,y)=x^2\arctan\frac{x}{y}-y^2\arctan\frac{x}{y}$ if $\displaystyle xy\neq0$

Let $\displaystyle f(x,y)=0$ if $\displaystyle xy=0$

Compute $\displaystyle \frac{\partial}{\partial y}\frac{\partial f}{\partial x}(0,0) $ and $\displaystyle \frac{\partial}{\partial x}\frac{\partial f}{\partial y}(0,0)$ if they exist.

Ive proven that f(x,y) is continuous at 0, but I'm not sure which function I should be using to calculate the derivative, any help?

Thanks