课内阅读。

      当人类的胚胎成长到5周左右时，手就出现了，但非常小，如同鱼鳍一般。在随后的发育中，手指慢慢开始成长，手指之间的蹼渐渐退化。到了11周的时候，手的关节、肌肉甚至指甲都已经发育完全。所有的功能已经齐备。一个20周大的宝宝，在母亲的肚子里，已经会用幼嫩的手指给自己的耳朵挠痒了。

       在400万年的进化史中，人类的手逐渐演变成了大自然所能创造出的最完美的工具。这一器官是力量与智慧的象征，是一个精美的机械结构。每只手有29块骨头，一些六指人还会多几块。这些骨头由123条韧带联系在一起，由35条强劲的肌肉来牵引，而控制这些肌肉的是48条神经。整个手掌结构由30多条动脉以及数量众多的小血管来滋养。

    Young people can also have problems with their minds. Some students become worried because they have to study very hard．Others have trouble getting on well with people like their parents and classmates.

    Liu Wei, a Junior 2 student from Hefei, could not understand his teacher and was doing badly in his lessons. He became so worried about it that he started to cut his finger with a knife.

    Another student, 14-year-old Yan Fang from Guangzhou, was afraid of exams. She got very worried, and when she looked at the exam paper. She couldn't think of anything to write.

    A recent report from Jiefang Daily says about 18% of Shanghai teenagers have mental(精神的)problems. Their troubles include being worried and very unhappy and having problems in learning and getting on with people. Many students who have problems won't go for advice or help. Some think they will look foolish if they go to see a doctor. Others don't want to talk about their secret.

    Liang Yuezhu, an expert on teenagers from Beijing Anding Hospital has the following advice for teenagers:

    Talk to your parents or teachers often.

    Take part in group activities and play sports.

    Go to see a doctor if you feel unhappy or unwell.

①是宪法和法律规定公民必须履行的法定义务

②体现了志愿者主动亲近社会，自觉服务社会

③说明道德义务是法律要求我们做的，我们必须去做

④说明公民要自觉承担对他人、对社会的道德责任和道德义务

若a=1﹣$\sqrt{2}$，先化简再求$\frac{a^2-1}{a^2+a}$+$\frac{\sqrt{a^2-2a+1}}{a^2-a}$的值．