如图，点C在线段AB上，点M、N分别是AC、BC的中点．

（1）若AC=8cm，CB=6cm，求线段MN的长；

（2）若C为线段AB上任一点，满足AC+CB=a，其它条件不变，你能猜想MN的长度吗？写出你的结论并说明理由；

（3）若C为直线AB上线段AB之外的任一点，且AC=m，CB=n，则线段MN的长 ．

阅读理解。判断对错。

      Hi. I'm Tom. I'm a boy. I get up at 6:30. I eat breakfast at 7:00. I go to school at 7:00. I have English class at 8:00. At 9:00, it's time for match class. I have music class at 10:00. I eat lunch at 12:00. It's time for PE class at 1:30. I go home at 4:00. It's time to go to bed at 9:00 p.m. it's my day. How about you？