Let f: $\displaystyle R^n \rightarrow R$ be given by f(x) = $\displaystyle |x|^2$ = $\displaystyle \sum (x_j)^2$, where j = 1,...,n and x = ($\displaystyle x_1$,...,$\displaystyle x_n$). Show that f is differentiable at all points in $\displaystyle R^n$. For each x $\displaystyle \in R^n$ find df(x)