Use the method of Example 5 to calculate $\int_{C} \mathbf{F} \cdot d \mathbf{r},$ where
$$\mathbf{F}(x, y)=\frac{2 x y \mathbf{i}+\left(y^{2}-x^{2}\right) \mathbf{j}}{\left(x^{2}+y^{2}\right)^{2}}$$
and $C$ is any positively oriented simple closed curve that encloses the origin.