Suppose that vectors $\mathbf{u}, \mathbf{v},$ and $\mathbf{w}$ are mutually orthogonal-that is, $\mathbf{u} \perp \mathbf{v}, \mathbf{u} \perp \mathbf{w},$ and $\mathbf{v} \perp \mathbf{w} .$ Prove that $(\mathbf{u} \times \mathbf{v}) \times \mathbf{w}=\mathbf{0}$ and $\mathbf{u} \times(\mathbf{v} \times \mathbf{w})=\mathbf{0}.$