In Problems 9-18, (a) find the dot product $\mathbf{v} \cdot \mathbf{w} ;(b)$ find the angle between $\mathbf{v}$ and $\mathbf{w} ;(c)$ state whether the vectors are parallel, orthogonal, or neither.
$\mathbf{v}=\mathbf{i}-\mathbf{j}, \quad \mathbf{w}=\mathbf{i}+\mathbf{j}$