Geometry
# 3D Coordinate Geometry

$ABCDEF,$ $M$ and $N$ are the midpoints of $\overline{BC}$ and $\overline{CD},$ respectively. If $\theta$ is the angle between $\overrightarrow{NF} \text{ and } \overrightarrow{MA},$ what is the value of $\cos \theta?$

In the above regular octahedronCount the number of pairs of edges in the above cuboid that are skew.

Find the distance between the following two lines:

$\begin{cases} l_1: x=-y=8-z \\\\ l_2: x=y \text{ and } z=-10. \end{cases}$