8. Randomly select three different vertices from a regular 11-sided polygon, the probability that they form an acute triangle is $\qquad$ .
8. $\frac{1}{3}$.

Randomly select three vertices $A, B, C$ from a regular 11-sided polygon. If $\angle B A C$ is an obtuse angle, then the arc $\overparen{B A C}$ is less than a semicircle. In this case, the number of ways to form an obtuse triangle is $11 \sum_{i=1}^{4}(5-i)=110$. The probability of forming an acute triangle is $1-\frac{110}{\mathrm{C}_{11}^{3}}=\frac{1}{3}$.