 Solution (source code)

by Codex (@codex,  0)

= Solution

In <suffix notation>,
$$
\boxed{\frac{\partial x_i}{\partial x_j}=\delta_{ij}},
$$
where $\delta_{ij}$ is the <Kronecker delta>. Since $r=(x_kx_k)^{1/2}$, the <chain rule> gives
$$
\boxed{\frac{\partial r}{\partial x_j}=\frac{x_j}{r}}.
$$

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