1. 计算。
$ \frac{1}{5} + \frac{1}{3} $
$ \frac{1}{3} + \frac{1}{8} $
$ \frac{2}{7} + \frac{2}{5} $
$ \frac{5}{8} + \frac{5}{12} $
$ \frac{1}{3} - \frac{1}{5} $
$ \frac{1}{9} - \frac{1}{18} $
$ \frac{2}{5} - \frac{2}{7} $
$ \frac{7}{8} - \frac{7}{12} $

$​=\frac {8}{15}​$
$​=\frac {11}{24}​$
$​=\frac {24}{35}​$
$​=\frac {25}{24}​$
$​=\frac {2}{15}​$
$​=\frac {1}{18}​$
$​=\frac {4}{35}​$
$​=\frac {7}{24}​$

2. 计算。
$ \frac{5}{6} + \frac{1}{2} + \frac{2}{3} $
$ \frac{1}{3} + \frac{2}{4} + \frac{2}{5} $
$ \frac{9}{10} + \frac{7}{15} + \frac{11}{20} $
$ \frac{7}{8} - \frac{1}{4} - \frac{1}{2} $
$ \frac{11}{12} - \frac{3}{8} - \frac{1}{6} $
$ 1 - \frac{1}{7} - \frac{2}{3} $

$​=\frac {7}{24}​$
$​=\frac {4}{3}+\frac {3}{4}​$
$​=\frac {25}{12}​$
$​=\frac {5}{6}+\frac {2}{5}​$
$​=\frac {37}{30}​$
$​=\frac {41}{30}+\frac {11}{20}​$
$​=\frac {23}{12}​$
$​=\frac {5}{8}-\frac {1}{2}​$
$​=\frac {1}{8}​$
$​=\frac {13}{24}-\frac {1}{6}​$
$​=\frac {3}{8}​$
$​=\frac {6}{7}-\frac {2}{3}​$
$​=\frac {4}{21}​$

3. 计算。
$ \frac{2}{7} + \frac{5}{8} + \frac{3}{8} $
$ \frac{4}{15} + \frac{6}{13} + \frac{7}{13} + \frac{3}{5} $
$ 1 - \frac{4}{15} - \frac{11}{15} $
$ \frac{8}{9} - \frac{7}{15} + \frac{1}{9} $

$​=\frac {2}{7}+(\frac {5}{8}+\frac {3}{8})​$
$​=\frac {2}{7}+1​$
$​=1\frac 27​$
$​=(\frac {4}{15}+\frac {3}{5})+(\frac {6}{13}+\frac {7}{13})​$
$​=\frac {13}{15}+1​$
$​=1\frac {13}{15}​$
$​=1-(\frac {4}{15}+\frac {11}{15})​$
​=1-1​
​=0​
$​=(\frac {8}{9}+\frac {1}{9})-\frac {7}{15}​$
$​=1-\frac {7}{15}​$
$​=\frac {8}{15}​$