题目

       One of China’s most famous art forms is the 200-year-old Jingxi (Beijing Opera ). It is not only a treasure（财富）of the Chinese nation but also a cultural symbol of China.        Beijing Opera is an art on stage that includes singing, dancing ,acting and acrobatics（杂技动作）. Beijing Opera uses special imagery（意象）in creating the characters. All roles are divided according to sex , personality, age, profession and social status（地位）. Hangdang is the general term for role types in Beijing Opera .There are four types of roles in Beijing Opera toda.设表示等差数列的前n项和，且，若，则n=     .