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​
$​=(\frac {8}{9}+\frac {1}{9})-\frac {7}{15}​$
$​=1-\frac {7}{15}​$
$​=\frac {8}{15}​$