Let $\mathbf{F}$ be a differentiable vector field, and let $g(x, y, z)$ be a differentiable scalar function. Verify the following identities.
a. $\nabla \cdot(g \mathbf{F})=g \nabla \cdot \mathbf{F}+\nabla g \cdot \mathbf{F}$
b. $\nabla \times(g \mathbf{F})=g \nabla \times \mathbf{F}+\nabla g \times \mathbf{F}$