[例1] 已知：如图，C是线段AB上一点，M、N分别是线段AC、BC的中点，AB=11，求MN。
[例2] 已知：C是线段AB的中点，D是CB上一点，E是DB的中点，若CE=4，，求线段AB的长。
[例3] 如图，线段AB 上有C、D两点，点C将AB分成两部分，点D将线段AB 分成两部分，若，求AB。
[例4] 已知：如图线段MN，P为MN中点，Q为PN中点，R是MQ中点，则。[例5] 已知：B是线段AC上一点，且，又D是线段AC延长线上一点，且，若，求AB、BC的长。
[例6] 如图：，F是BC的中点，，求EF。
[例7] 如图：E、F是线段AC、AB的中点，且，求线段EF的长。
[例8] 已知A、B、C、D为直线上四点且满足，M、N分别为AB
和CD的中点，，求AB、AC、AD。
【模拟试题】（答题时间：30分钟）
2. 如图，已知，CD的长为10cm，求AB的长。
3. 如图，B、C两点，把AD分成三部分，E是线段AD中点，，求：（1）EC的长；（2）的值。
4. 如图，M是AC中点，N是BC中点，O为AB中点，求证：MC=ON。
5. 一条直线上顺次有A、B、C、D四点，且C为AD中点，，求
的值。
6. 已知线段AB、CD的公共部分，线段AB、CD的中点E、F的距离是6cm，求AB、CD的长。
7. 已知线段，点C在直线AB上，点M、N分别是AC、BC的中点，求MN的长度。
8. 同一直线上A、B、C、D四点，已知，且，求AB的长。
A Virtual Life
Maia Szalavitz, formerly a television producer, now spends her time as a writer. In this essay she explores digital reality and its consequences. Along the way, she compares the digital world to the "real" world, acknowledging the attractions of the
electronic dimension.
迈亚·塞拉维茨曾是电视制片人，目前从事写作。她在本文中探索了数字化世界及其后果。与此同时，她将数字化世界与真实世界做了比较，承认电子空间自有其魅力。
Maia Szalavitz 1 After too long on the Net, even a phone call can be a shock. My boyfriend's Liverpool accent suddenly becomes impossible to interpret after his easily understood words on screen; a secretary's clipped tone seems more rejecting than I'd imagined it would be. Time itself becomes fluid -- hours become minutes, or seconds stretch into days. Weekends, once a highlight of my week, are now just two ordinary days.
虚拟世界的生活
迈亚·塞拉维茨
在网上呆了太久，听到电话铃声也会吓一大跳。显示屏上看多了我男朋友那些一目了然的文字，他的利物浦口音一下子变得难以听懂；而秘书的清脆快速的语调听上去比我想象的要生硬。时间本身变得捉摸不定——几小时变成几分钟，或几秒钟延伸为几天。周末原本是我一周的黄金时段，现在却不过是平平常常的两天。
2 For the last three years, since I stopped working as a television producer, I have done much of my work as a telecommuter. I submit articles and edit them via email and communicate with colleagues on Internet mailing lists. My boyfriend lives in England, so much of our relationship is also computer-assisted.
在我不再当电视制片人的这三年间，我的大部分工作都是在家里使用计算机终端进行的。我通过电子邮件投稿和校订，利用互联网上的人名地址与同行交流。我男朋友住在英国，因此两人的关系也在很大程度上借助于电脑维系。
3 If I desired, I could stay inside for weeks without wanting anything. I can order food, and manage my money, love and work. In fact, at times I have spent as long as three weeks alone at home, going out only to get mail and buy newspapers and groceries. I watched most of the endless snowstorm of '96 on TV.
我要是愿意的话，可以一连几个星期不出门而什么也不缺。我可以在网上订购食品、网上理财、网上恋爱、网上工作。事实上我有时独自呆在家里长达三个星期，只偶尔出去拿信、买报纸及日用品。1996年那一场接一场的暴风雪我大都是在电视上看到的。
4 But after a while, life itself begins to feel unreal. I start to feel as though I've become one with my machines, taking data in, spitting them back out, just another link in the Net. Others on line report the same symptoms. We start to feel an aversion to outside forms of socializing. We have become the Net critics' worst nightmare.
