(a) If $\mathbf{u} \cdot \mathbf{r}=\mathbf{v} \cdot \mathbf{r}$ for every vector $\mathbf{r}$ in $V_{\mathbf{r}}$, show that $\mathbf{u}=$
(b) Prove Property 3 of the cross product
$$a \times(b+c)=a \times b+a \times c$$
by showing that
$$[a \times(b+c)] \cdot r=[a \times b+a \times c] \cdot r$$
for every vector $\mathbf{r}$ in $V$,