How to find the Jacobi-Matrix for the following functions?
$f:(x,y)\rightarrow x^2+y^2$
$f: (x,y)\rightarrow \sqrt{x^2+y^2}$
$f:\mathbb{R}^n\rightarrow \mathbb{R}: x\rightarrow \sum_{i=1}^nx_i^2$
$f:\mathbb{R}^n\rightarrow \mathbb{R}: x\rightarrow ||x||_2$
For the first one I have an idea, but for the others I have no idea. Can someone provide me with a solution/hint/Source where I can find more on this kind of problems?