Chapter 9 - Section 9.2 - The Dot Product - 9.2 Exercises - Page 646: 2

a) $\cos \theta=\frac{\textbf{u}\cdot\textbf{v}}{|\textbf{u}||\textbf{v}|}$ b) perpendicular.

$\cos \theta=\frac{\textbf{u}\cdot\textbf{v}}{|\textbf{u}||\textbf{v}|}$ If $\textbf{u}\cdot\textbf{v}$=0, this implies that $\cos \theta= 0$. $\cos \theta$ is 0 when $\theta= 90^{\circ}$, that is, when the vectors are perpendicular.