圆形的操场的一周全长是1800米．如果沿着这一圈每隔12米安装一盏灯，一共需要装几盏灯？

如图，在△ABC中，分别以AC，BC为边作等边△ACD和等边△BCE．设△ACD、△BCE、△ABC的面积分别是S₁、S₂、S₃，现有如下结论：

①S₁：S₂=AC²：BC²；

②连接AE，BD，则△BCD≌△ECA；

③若AC⊥BC，则S₁•S₂= $\frac{3}{4}$ S₃²．

其中结论正确的序号是____．

某品牌牛奶每盒装有牛奶的质量约为275____ （填上合适的单位），体积为0.25____ （填上合适的单位），由此可以估算出牛奶的密度约为____kg/m³。喝掉一半后，牛奶的密度将____ 。(填“变大”、“变小”或“不变”)

有一种芳华，叫“正值青春”。适值毕业季，小华在日记中写下：“青春，是生命旅途中一个崭新的起点。初中三年的学习生活，在我的成长中留下难忘的印记。”

短文填空先通读下面的短文，然后根据短文内容和所给首字母，在空格内填入一个适当的词，使短文意思完整。
      American people like to say “Thank you” when others help them or say something kind to them. People of many other countries do so, too. It is a very good habit.
      You should say “Thank you” when someone p____  you the salt on the table, when someone walking before you k____  the door open for you, when someone s____  you have done your work well, or you have b____  a nice thing, or your city is very beautiful. “Thank you”is used not only between friends, but also between parents and children, brother and sisters, husbands and wives.
    “Excuse me”is another s____  sentence they use. When you hear someone say so behind you, you know that someone wants to walk p____ you. It is not polite to b____  others while they are talking. If you want to speak to one of them, say “Excuse me” first, and then begin talking. You should also do so when you want to cough(咳嗽) or make any u____  noise before others.