Let [math]f(x ,y)= x^2 \arctan \left(\frac{y}{x}\right) - y^2\arctan\left(\frac{x}{y}\right).[/math] How do I show that  [math]\frac{\partial^2 f}{ \partial x \partial y}=\frac{x^2 -y^2}{x^2 + y^2}?[/math]
How do I draw the graph of [math]y = -\frac{3}{x-2} + 1[/math]?