Cartesian Points within the Unit Disc: $x^2+y^2<1$.
<BR>
Examples:

\[
   \overrightarrow{OP}
   =
   r_P\begin{pmatrix}\cos{\theta_P}\\\sin{\theta_P}\end{pmatrix}
   =
   \underline{e}_P,
   \qquad
   \overrightarrow{OQ}
   =
   r_Q\begin{pmatrix}\cos{\theta_Q}\\\sin{\theta_Q}\end{pmatrix}
   =
   \underline{e}_Q
\]

Hat vectores:

\[
   \underline{f}_P
   =
   \underline{\widehat e}_P
   =
   \begin{pmatrix}-\sin{\theta_P}\\\cos{\theta_P}\end{pmatrix},
   \qquad
   \underline{f}_Q
   =
   \underline{\widehat e}_Q
   =
   \begin{pmatrix}-\sin{\theta_Q}\\\cos{\theta_Q}\end{pmatrix}
\]

Note, com $\theta=\theta_Q-\theta_P$:

\[
   \underline{e}_P \cdot~\underline{e}_Q
   =
   \underline{f}_P\cdot~\underline{f}_Q
   =
   \cos{\theta}
\]
\[
   \underline{e}_P \cdot~\underline{f}_Q
   =
   -\underline{e}_Q\cdot~\underline{f}_P
   =
   \sin{\theta}
\]