Prove that the differentiable function $\displaystyle F(x,y,z)=\left ( f(x,y,z) ,g(x,y,z) , f(x,y,z)+g(x,y,z) \right)$ can never have a differentiable inverse.

My attempt : I notice that $\displaystyle \dim dom F=3$ and that $\displaystyle \dim Im F =2$. Hence $\displaystyle F$ is not injective, hence $\displaystyle F$ doesn't have an inverse, so it cannot have a differentiable inverse.

I'm not sure my attempt is good.

Maybe I should have done it using the Inverse Function Theorem, but if the Jacobian of $\displaystyle F$ is not invertible, I can't say anything whether the existence of an inverse of $\displaystyle F$, I believe. Hmm but according to my memory of Linear Algebra, if the matrix of a function is not invertible then the function is not injective so it doesn't have an inverse.

What do you think?