Geometri: Sudut Garis dan Garis

\begin{equation*} \begin{split} \cos \theta & = \frac {\frac 12 \sqrt{2}}{\frac 12 \sqrt{3}} \\\\ \cos \theta & = \frac {\sqrt{2}}{ \sqrt{3}} \quad {\color {blue} \times \frac {\sqrt{3}}{\sqrt{3}}} \\\\ \cos \theta & = \frac 12 \sqrt{6} \end{split} \end{equation*}