Solution
= Solution
The <vector triple product> rewrites the field as $\mathbf b(\mathbf a\cdot\mathbf r)-\mathbf r(\mathbf a\cdot\mathbf b)$. Its first term has <divergence> $\mathbf b\cdot\nabla(\mathbf a\cdot\mathbf r)=\mathbf a\cdot\mathbf b$, while the second contributes $3\mathbf a\cdot\mathbf b$. Therefore
$$
\boxed{\nabla\cdot[\mathbf a\times(\mathbf b\times\mathbf r)]=-2\mathbf a\cdot\mathbf b.}
$$