2 $\sqrt{3}$ ，则 $\frac{1}{a^2}$ + $\frac{1}{b^2}$ ﹣ $\frac{a^2}{b^2}$ 的最小值为____．

    Beijing Opera is also called Peking Opera. It came into being after 1790 when the famous four Anhui opera troupes(戏班) came to Beijing. Its music and singing(唱腔) come from Xipi and Erhuang in Anhui and Hubei. Its costumes are all fascinating and artistic. It is the highest expression of the Chinese culture. It's full of famous stories, beautiful facial paintings, and wonderful gestures and fighting. This kind of opera is very popular with Chinese people.
    There are four main roles in Beijing Opera: Sheng, Dan, Jing and Chou. Sheng is the leading male(男性) actor. For example, a Wusheng is a soldier or fighter. A Xiaosheng is a young man. A Laosheng is an old man. Dan is the female(女性) role. Jing, mostly male, is the face-painted role and Chou is the comedy actor or clown.
    Stories in Beijing Opera are very interesting. Some of them are from the history book, but most of them are from the literature, especially famous novels. The people in the story usually have some disagreements. They become angry and unhappy. They are sad and lonely. Sometimes they are nervous and worried. Then they find a way to make peace. The stories usually end with happiness and laughter and people are all happy in the end.

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