1. Given a regular tetrahedron $ABCD$, the midpoints of $AB$, $BC$, and $CD$ are $E$, $F$, and $G$ respectively. Find the dihedral angle $C-FG-E$.
(Answer: $\pi-\arccos \frac{\sqrt{3}}{3$.

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(Answer: $\pi-\arccos \frac{\sqrt{3}}{3}$.