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第69页

信息发布者：

$=3-(\frac {6}{13}+\frac {7}{13})$

$=3-1$

$=2$

$=(\frac {4}{9}+\frac {5}{9})+(\frac {3}{10}+\frac {7}{10})$

$=1+1$

$=2$

$=\frac {9}{7}-\frac {1}{3}-\frac {2}{7}$

$=1-\frac {1}{3}$

$=\frac {2}{3}$

$=\frac {7}{8}-\frac {2}{3}+\frac {1}{8}$

$=1-\frac {2}{3}$

$=\frac {1}{3}$

$1 \frac{2} {3}$

$\frac{1} {n}+\frac{1} {n+1} = \frac{n+( n+1 )} {n ×( n+1 )}(n为非零自然数)$

$\frac{199} {9900}$

$\frac{403} {40602}$

$=1+\frac {1}{2}-(\frac {1}{2}+\frac {1}{3})+\frac {1}{3}+\frac {1}{4}-(\frac {1}{4}+\frac {1}{5})+\frac {1}{5}-\frac {1}{6}-(\frac {1}{6}+\frac {1}{7})$

$=1+\frac {1}{2}-\frac {1}{2}-\frac {1}{3}+\frac {1}{3}-\frac {1}{4}+\frac {1}{4}-\frac {1}{5}+\frac {1}{5}-\frac {1}{6}-\frac {1}{7}$

$=1-\frac {1}{7}$

$=\frac {6}{7}$

$\frac{3} {5} + \frac{1} {10} = \frac{7} {10}(升)\ $

$\frac{7} {10}+\frac{7} {10}=\frac{7} {5}(升)\ $

$答：这瓶果汁原来有\frac {7}{5}升。$