已知弧度数为2的圆心角所对的弦长也是2，则这个圆心角所对的弧长是（　　）

    十年春，齐师伐我。公将战。曹刿请见。其乡人曰：“肉食者谋之，又何间焉？”刿曰：“肉食者鄙，未能远谋。”乃入见。问：“何以战？”公曰：“衣食所安，弗敢专也，必以分人。”对曰：“小惠未遍，民弗从也。”公曰：“牺牲玉帛，弗敢加也，必以信。”对曰：“小信未孚，神弗福也。”公曰：“小大之狱，虽不能察，必以情，”对曰：“忠之属也。可以一战。战则请从。”

    公与之乘。战于长勺。公将鼓之。刿曰：“未可。”齐人三鼓。刿曰：“可矣。”齐师败绩。公将驰之。刿曰：“未可。”下视其辙，登轼而望之，曰：“可矣。”遂逐齐师。

    既克，公问其故。对曰：“夫战，勇气也。一鼓作气，再而衰，三而竭。彼竭我盈，故克之。夫大国，难测也，惧有伏焉。吾视其辙乱，望其旗靡，故逐之。”

${\int }_0^{\frac{\pi }{2}}{\sin }^2\frac{x}{2}dx=$(   )

    An American in Beijing lives his Chinese dream

    Gao Tianrui walks along streets in Beijing's Xicheng district as____public security volunteer.____ (wear) his bright red armband (红袖章) and speaking fluent Mandarin, 62-year-old Gao Tianrui could ____(take) for any regular Chinese man. But on ____(close) inspection, things are not quite____they seem: Gao's real name is Terry Crossman and he is from the United States. He is now a member of “Xicheng Dama”, that is, volunteers, usually women in late middle-age,____walk along the streets of downtown Beijing's Xicheng district. Terry said that, at first, he saw the group as busybodies, but ____(gradual) he saw that it was a good thing for retired seniors____(help) others.

    He is often seen giving tourists____(direction), getting water for a baby and even helping a neighbor sell yogurt. “I like helping others,” he said, “I live in the hutong and my neighbors and I usually help each other ... This is where I live, where my friends are and where I call home.” Crossman ____(become) interested in Chinese culture as a teenager when he read Tao Te Ching by Laozi.