Angle by Trigonometric Functions, $\underline{a},\underline{b} \neq \underline{0}$:

\[
  \cos{\theta}
  =
  \frac
  {
     \underline{a} \cdot \underline{b}
  }
  {
     |\underline{a}|~|\underline{b}|
  },
  \qquad
  \sin{\theta}
  =
  \frac
  {
     \underline{\widehat{a}} \cdot \underline{b}
  }
  {
     |\underline{a}|~|\underline{b}|
  }
\]