7. Four points are randomly chosen from the vertices of a regular 12-sided polygon. Find the probability that the four chosen points form a rectangle (including square).從一個正十二邊形的頂點中隨意選四點, 求該四點成一長方形（包括正方形）的概率。
7. $\frac{1}{33}$
7. The four points form a rectangle if and only they are two pairs of diametrically opposite points. Hence among the $C_{4}^{12}=495$ possible outcomes, there are $C_{2}^{6}=15$ favourable outcomes (by choosing 2 out of the 6 pairs of diametrically opposite points). Therefore the answer is $\frac{15}{495}=\frac{1}{33}$.