Can you help with the following question?
Let $f: \mathbb{R}^2 \to \mathbb{R}$ be a function with continuous derivatives. It is given that $f\left(\dfrac{\cos(t)}{t},\dfrac{\sin(t)}{t}\right)$ is growing for $t > 0$.
Proof that $Df(0,0)=(0,0)$. (The differential of $f$ in the point $(0,0)$ is $(0,0)$).
Thanks!
