9. In a tetrahedron $ABCD$ with all edges of equal length, $E, F$ are the midpoints of $AD, BC$ respectively, and $AF, CE$ are connected. (I) Find the angle formed by the skew lines $AF$ and $CE$; (II) Find the angle formed by $CD$ and the plane $BCD$.
9. ( I ) Connect $F D$, in the plane $A F D$, draw $E G / / A F$ intersecting $E D$ at $K$, then $\angle C E G$ is the planar angle (or supplementary angle) formed by the skew lines $A F$ and $C E$. Let the edge length of the regular tetrahedron be $a$, then $E G=\frac{1}{2} A F=\frac{\sqrt{3}}{4} a, C E=\frac{\sqrt{3}}{2} a, C G=\frac{\sqrt{7}}{4} a$, the required angle is $\arccos \frac{2}{3}$.
(II) In the plane $A F D$, draw $E H \perp F D$, then $\angle E C H$ is the planar angle formed by $C E$ and the plane $B C D$ (note that plane $A F D \perp$ plane $B C D$), it can be found that $E H=\frac{\sqrt{6}}{6} a$, hence the required angle is $\arcsin \frac{\sqrt{2}}{3}$.