18. A net of a cube and a schematic diagram of the cube are shown in the figure. In the cube, let $M$ be the midpoint of $BC$ and $N$ be the midpoint of $GH$.\\
(1) Mark the letters $F$, $G$, $H$ at the corresponding vertices of the cube (no explanation needed).\\
(2) Prove: line $MN \parallel$ plane $BDH$.\\
(3) Find the cosine of the dihedral angle $A - E G - M$.\\
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