9. Half-planes $\alpha_{1}, \alpha_{2}, \ldots, \alpha_{n}$ have a common edge $l$. What is the sum of the dihedral angles $\alpha_{1} \widehat{l \alpha_{2}}, \alpha_{2} l \alpha_{3}, \ldots, \alpha_{n-1} l \alpha_{n}$, $\widehat{\alpha_{n} l} l \alpha_{1}$, which together fill the entire space?
9. By intersecting the entire figure with a plane perpendicular to the common edge $l$, we obtain the linear angles of these dihedral angles in the section. These planar angles have a common vertex and fill the entire plane, so the sum of their magnitudes is $2 \pi$ (or $360^{\circ}$).