9 In an octagon, if three vertices are chosen at random from the eight vertices, the probability that these three points form the vertices of a right-angled triangle is $\qquad$ .
9 Connecting three vertices of a regular octagon can yield $\mathrm{C}_{8}^{3}=56$ (triangles), among which the triangles with one side being one of the 4 diameters of the circumcircle are right-angled triangles, totaling $4 \times 6=24$ (right-angled triangles), so the required probability is $\frac{3}{7}$.