6. Randomly select 3 vertices from a regular 11-sided polygon, the probability that they form an acute triangle is

保留源文本的换行和格式，直接输出翻译结果如下：

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

Analysis: By symmetry, we only need to consider the probability of forming an acute triangle among the triangles containing a fixed vertex of the regular undecagon. The number of triangles containing vertex $A$ is $C_{10}^{2}=45$, among which there are $1+2+3+4+5=15$ acute triangles, hence $P=\frac{1}{3}$