In Problems 15-22, find (a) $\mathbf{v} \times \mathbf{w},($ b) $\mathbf{w} \times \mathbf{v},(c) \mathbf{w} \times \mathbf{w}$, and (d) $\mathbf{v} \times \mathbf{v}$.
$\begin{aligned} \mathbf{v} & =\mathbf{i}+\mathbf{j} \\ \mathbf{w} & =2 \mathbf{i}+\mathbf{j}+\mathbf{k}\end{aligned}$