Solution
= Solution
Since $\mathbf r\cdot\mathbf r=x^2+y^2+z^2$, its <gradient> is
$$
\boxed{\nabla(\mathbf r\cdot\mathbf r)=(2x,2y,2z)=2\mathbf r}.
$$
= Solution
Since $\mathbf r\cdot\mathbf r=x^2+y^2+z^2$, its <gradient> is
$$
\boxed{\nabla(\mathbf r\cdot\mathbf r)=(2x,2y,2z)=2\mathbf r}.
$$