在香港海洋公园的游乐场中，有一台大型游戏机叫“跳楼机”．参加游戏的游客被安全带固定在座椅上，由电动机将座椅沿光滑的竖直轨道提升到离地面40m高处，然后由静止释放．座椅沿轨道自由下落一段时间后，开始受到压缩空气提供的恒定阻力而紧接着做匀减速运动，下落到离地面4.0m高处速度刚好减小到零，这一下落全过程经历的时间是6s．求：（取g=10m/s²）

①9 $\frac{2}{5}$ －（3 $\frac{3}{7}$ ＋0.4）

②1.8× $\frac{1}{4}$ ＋2.2×25％

③ $\frac{1}{1\times 3}+\frac{1}{3\times 5}+\frac{1}{5\times 7}+\cdots +\frac{1}{17\times 19}+\frac{1}{19\times 21}$

    Will you be happy when you leave your mother? Or will you miss your mother? If you do, you are homesick. “homesick” is a word made up of HOME and SICK. You know each word, right? But do you know the meaning when you put them together? It is Sick For Home. It means you miss your home. You miss the delicious food at home.

    Now think for a minute about SEASICK. Is Seasick the same as Homesick? Does seasick mean Sick For Sea? It means something quite different. “Seasick” means Sick By The Movement On The Sea. If you travel by ship and you feel seasick, you just want to go back home and have a good rest. Why? Because you feel sick on a ship.

    Have you ever heard of a person being heartsick? If you're heartsick, you are sad and your heart is broken. And we also have other words made like this, such as “handshake” and “handbag”. Isn't it interesting? It is also helpful to know something like this in your English study.