Find a differential of second order of a function $u=f(x,y)$ with continuous partial derivatives up to third order at least. Hint: Take a look at $du$ as a function of the variables $x$, $y$, $dx$, $dy$: $du= F(x,y,dx,dy)=u_xdx +u_ydy$.
Can someone please explain me what should I do in this question?
Thanks in advance!