The line of sight of a circular cone is 1, and the angle it forms with the base is $\theta$. The edge length of the inscribed cube in this cone is $\qquad$
$\qquad$ (4 vertices of the cube are on the base of the cone, and the other 4 vertices are on the side surface of the cone).
9. $\frac{2 l \sin \theta}{2+\sqrt{2} \tan \theta}$.

If we make an axial section, we have $S A=l, \angle S A B=\theta, A O=l \cos \theta$.
Now, let the edge length of the cube be $x$, then $D O=\frac{\sqrt{2}}{2} x, A D=\iota \cos \theta-\frac{\sqrt{2}}{2} x$.
By the definition of trigonometric functions, we know $C D=A D \cdot \tan \theta$, which can be solved to get, $x=$
$$
\frac{2 l \sin \theta}{2+\sqrt{2} \tan \theta} .
$$