四 计算下面各题,怎样简便就怎样算。
$\frac{6}{13}+\frac{7}{9}+\frac{7}{13}$ $\frac{11}{15}+(\frac{1}{12}+\frac{4}{15})$ $\frac{8}{9}+\frac{3}{8}-\frac{8}{9}$
$\frac{8}{21}+\frac{7}{9}+\frac{13}{21}+\frac{2}{9}$ $3-\frac{2}{5}-0.6$ $1\frac{17}{20}-(\frac{2}{7}+\frac{17}{20})$

$\frac {6}{13}+\frac {7}{9}+\frac {7}{13}$ 
$=(\frac {6}{13}+\frac {7}{13})+\frac {7}{9}$ 
$=\frac {16}{9}$
$\frac {11}{15}+(\frac {1}{12}+\frac {4}{15})$
$=(\frac {11}{15}+\frac {4}{15})+\frac {1}{12}$
$=\frac {13}{12}$
$\frac {8}{9}+\frac {3}{8}-\frac {8}{9}$ 
$=\frac {8}{9}-\frac {8}{9}+\frac {3}{8}$ 
$=\frac {3}{8}$
$\frac {8}{21}+\frac {7}{9}+\frac {13}{21}+\frac {2}{9}$
$=(\frac {8}{21}+\frac {13}{21})+(\frac {7}{9}+\frac {2}{9})$
$=1+1$
$=2$
$3-\frac {2}{5}-0.6$ 
$=3-0.4-0.6$ 
$=3-1$ 
$=2$
$1\frac {17}{20}-(\frac {2}{7}+\frac {17}{20})$
$=1\frac {17}{20}-\frac {17}{20}-\frac {2}{7}$
$=1-\frac {2}{7}$
$=\frac {5}{7}$

五 长江各区段航程分布表如下：
|航段|上海—南京|南京—武汉|武汉—重庆|重庆—宜宾|
|约占全程的|$\frac{1}{7}$|$\frac{1}{4}$|$\frac{12}{25}$| |
1. 李叔叔从上海到重庆出差,他走了全程的几分之几？
2. 重庆到宜宾的路程大约占全程的几分之几？
3. 王亮从上海出发到武汉旅游,周兰从武汉到重庆旅游,他们谁走的路程长些？

1. $\frac {1}{7}+\frac {1}{4}+\frac {12}{25}=\frac {611}{700}$
2. $1-\frac {611}{700}=\frac {89}{700}$
3. $\frac {11}{28}＜\frac {12}{25}$，周兰走的路程长些。

六 先观察,再计算。
$1-\frac{1}{2}= \frac{2-1}{1×2}= \frac{1}{2}$ $\frac{1}{2}-\frac{1}{3}= \frac{3-2}{2×3}= \frac{1}{6}$ $\frac{1}{3}-\frac{1}{4}= \frac{4-3}{3×4}= \frac{1}{12}$
$\frac{1}{4}-\frac{1}{5}= \frac{5-4}{4×5}= \frac{1}{20}$ $\frac{1}{5}-\frac{1}{6}= \frac{6-5}{5×6}= \frac{1}{30}$ $\frac{1}{6}-\frac{1}{7}= \frac{7-6}{6×7}= \frac{1}{42}$……
根据上面的方法计算：$\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}$。

原式$=1-\frac {1}{2}+\frac {1}{2}-\frac {1}{3}+\frac {1}{3}-\frac {1}{4}+... +\frac {1}{8}-\frac {1}{9}=1-\frac {1}{9}$=$\frac {8}{9}$