补写出下列各句中的空缺部分。

①杜牧在《阿房宫赋》中告诫人们要牢记历史的教训，“秦人不暇自哀，而后人哀之；____，____”，语重心长，耐人寻味。

②《庄子•逍遥游》中，上承对蝉和斑鸠嘲笑大鹏的叙写，下启对朝菌、寒蝉等的论说的句子是____，____。

③陶渊明《归去来兮辞》中描写拄着拐杖出去走走，随时随地休息的一句是 ____。

张华在一次数学活动中，利用“在面积一定的矩形中，正方形的周长最短”的结论，推导出“式子x+$\frac{1}{x}$（x＞0）的最小值是2”．其推导方法如下：在面积是1的矩形中设矩形的一边长为x，则另一边长是$\frac{1}{x}$，矩形的周长是2（x+$\frac{1}{x}$）；当矩形成为正方形时，就有x=$\frac{1}{x}$（x＞0），解得x=1，这时矩形的周长2（x+$\frac{1}{x}$）=4最小，因此x+$\frac{1}{x}$（x＞0）的最小值是2．模仿张华的推导，你求得式子$\frac{x^2+9}{x}$（x＞0）的最小值是（　　）

阅读短文,下列问题

    Today is Saturday. It is Children's Day, I get up very early. I go to school at about six. Our school is very cleanand beautiful. Birds in the trees sing all the time. Every student wears a white coat. Girls wears beautiful skirts. We sing and dance, run and jump. We look like happy birds.

    Our party begins at half past seven. First, we listen to our teachers, and then the programme(节目) begins. Many people sing together on the stage(舞台) . They sing and dance very well. All the teachers and students like watching and listening to them. After the party we play games in the classroom. All of us enjoy the day very much.