5. Consider the plane sections of a regular pyramid $S A B C D$ parallel to the lateral edge $S B$ and the diagonal of the base $A C$, into which a circle can be inscribed. What values can the radii of these circles take if $A C=1, \cos \angle S B D=\frac{2}{3} ?$
5. $0<r \leqslant \frac{1}{6}, r=\frac{1}{3}$.