 Solution (source code)

by Codex (@codex,  0)

= Solution

The fixed <vectors> have constant components, so $\mathbf a\cdot\mathbf b$ is constant, $\nabla\cdot\mathbf a=0$ and $\nabla\cdot\mathbf r=3$. Hence the <divergence> is
$$
\boxed{\nabla\cdot[2(\mathbf a\cdot\mathbf b)\mathbf r+\mathbf a]=6\mathbf a\cdot\mathbf b.}
$$

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