Given two vectors, how can I denote the angle between them? Given the vectors $\vec{a}$ and $\vec{b}$, how can I denote the angle between them?
I like the following method:


  ..., where $\theta$ is the angle between vectors $\vec{a}$ and $\vec{b}$...


Seriously, words can really improve proof legibility, and should be used.
What's wrong with using the identity that $a\cdot b = |a||b|\cos{\theta}$ and solving for theta?