Remember $$ \theta=\cos ^{-1}\left(\frac{\mathbf{u} \cdot \mathbf{v}}{\|\mathbf{u}\|\|\mathbf{v}\|}\right) $$ It follows from this that $\theta=\pi / 2$ if and only if $\mathbf{u} \cdot \mathbf{v}=0$. Thus, Two nonzero vectors $\mathbf{u}$ and $\mathbf{v}$ in $R^{n}$ are said to be orthogonal (or perpendicular) if $\mathbf{u} \cdot \mathbf{v}=0 .$ We will also agree that the zero vector in $R^{n}$ is orthogonal to every vector in $R^{n}$.