$ABCDEFGH$ is a cube with edge length equal to 1. The space is equipped with the orthonormal coordinate system ($D; \overrightarrow{DC}, \overrightarrow{DA}, \overrightarrow{DH}$). In this coordinate system, we have: $D(0;0;0)$, $C(1;0;0)$, $A(0;1;0)$, $H(0;0;1)$ and $E(0;1;1)$. Let $I$ be the midpoint of $[AB]$. Let $\mathscr{P}$ be the plane parallel to the plane $(BGE)$ and passing through the point $I$. It is admitted that the section of the cube by the plane $\mathscr{P}$ is a hexagon whose vertices $I, J, K, L, M$, and $N$ belong respectively to the edges $[AB], [BC], [CG], [GH], [HE]$ and $[AE]$.
- a. Show that the vector $\overrightarrow{DF}$ is normal to the plane $(BGE)$. b. Deduce a Cartesian equation of the plane $\mathscr{P}$.
- Show that the point $N$ is the midpoint of the segment $[AE]$.
- a. Determine a parametric representation of the line $(HB)$. b. Deduce that the line $(HB)$ and the plane $\mathscr{P}$ intersect at a point $T$ whose coordinates you will specify.
- Calculate, in units of volume, the volume of the tetrahedron $FBGE$.