已知扇形的周长为20，当扇形的圆心角为____ 弧度时，它有最大的面积．

    Do you know Chen Yixun? He is a great singer from Hong Kong. He has a lot of beautiful songs, so many people love him. This year Chen Yixun gives concerts (演唱会) in some cities in China. And on November 7 the concert is in Jiaxing.

    Chen Yixun wants to give great concerts, and he does a lot of work for that. First, he practises singing every day. And he always drinks warm water. There are also dances at the concerts, so he needs to practice dancing, too. It's a hard job. Then, he also needs to do exercise like running and swimming. He has to be healthy and strong for the concerts.

    “I have a busy life, but I love it. I enjoy singing and I wish to give great shows to people,” says Chen Yixun. Wish him good luck!

下列运算正确的是（　　）

李商隐的一联诗“秋阴不散霜飞晚，留得残荷听雨声”，引发人们无限的遐想。请根据诗意，发挥自己的联想和想象，写一段文字展现这联诗的意境。要求：100个字左右，至少运用两种修辞手法。

西汉中朝，唐朝的三省六部，明朝的内阁，清朝的军机处等机构的设立，本质上反映了