
I'm tring to prove the following statement:
Suppose $f:\mathbb{R}^2\rightarrow \mathbb{R}$ is continuously differentiable, and for any $(x_0,y_0)\in \mathbb{R}^2$, we have
$$
\frac{\partial f}{\partial x}(x_0,y_0)+\frac{\partial f}{\partial y}(x_0,y_0)\neq 0
$$
then show that:
$$
E=\{(x,y):f(x,y)...