1. 直接写出得数。
$\frac{2}{7}+\frac{3}{5}=$
$\frac{1}{5}+\frac{1}{8}=$
$\frac{1}{6}+\frac{1}{10}=$
$\frac{4}{9}+\frac{1}{36}=$
$\frac{1}{4}-\frac{1}{5}=$
$1-\frac{1}{2}=$
$\frac{1}{2}-\frac{3}{10}=$
$\frac{8}{9}-\frac{8}{9}=$
$\frac{3}{4}+\frac{5}{4}=$
$\frac{3}{8}+\frac{3}{2}=$
$\frac{4}{5}+\frac{1}{6}=$
$\frac{2}{9}-\frac{1}{11}=$

$\frac{2}{7}+\frac{3}{5}=\frac{10}{35}+\frac{21}{35}=\frac{31}{35}$
$\frac{1}{5}+\frac{1}{8}=\frac{8}{40}+\frac{5}{40}=\frac{13}{40}$
$\frac{1}{6}+\frac{1}{10}=\frac{5}{30}+\frac{3}{30}=\frac{8}{30}=\frac{4}{15}$
$\frac{4}{9}+\frac{1}{36}=\frac{16}{36}+\frac{1}{36}=\frac{17}{36}$
$\frac{1}{4}-\frac{1}{5}=\frac{5}{20}-\frac{4}{20}=\frac{1}{20}$
$1-\frac{1}{2}=\frac{2}{2}-\frac{1}{2}=\frac{1}{2}$
$\frac{1}{2}-\frac{3}{10}=\frac{5}{10}-\frac{3}{10}=\frac{2}{10}=\frac{1}{5}$
$\frac{8}{9}-\frac{8}{9}=0$
$\frac{3}{4}+\frac{5}{4}=\frac{8}{4}=2$
$\frac{3}{8}+\frac{3}{2}=\frac{3}{8}+\frac{12}{8}=\frac{15}{8}$
$\frac{4}{5}+\frac{1}{6}=\frac{24}{30}+\frac{5}{30}=\frac{29}{30}$
$\frac{2}{9}-\frac{1}{11}=\frac{22}{99}-\frac{9}{99}=\frac{13}{99}$

2. 填空。
$\frac{2}{9}+\frac{2}{11}+\frac{7}{9}$
$1-\frac{1}{8}-\frac{7}{8}$
$=\frac{2}{11}+(\quad+\quad)$
$=1-(\quad+\quad)$
$=$
$=$
$=$
$=$

第一题括号依次填$\frac{2}{9}$、$\frac{7}{9}$，结果依次为$\frac{2}{11}+1$、$1\frac{2}{11}$；第二题括号依次填$\frac{1}{8}$、$\frac{7}{8}$，结果依次为$1-1$、$0$。

1. 计算$\frac{2}{9}+\frac{2}{11}+\frac{7}{9}$时，运用加法交换律和结合律，将同分母的$\frac{2}{9}$与$\frac{7}{9}$结合先算：
$\frac{2}{9}+\frac{2}{11}+\frac{7}{9}$
$=\frac{2}{11}+(\frac{2}{9}+\frac{7}{9})$
$=\frac{2}{11}+1$
$=1\frac{2}{11}$
2. 计算$1-\frac{1}{8}-\frac{7}{8}$时，运用减法的性质，将连续减去的两个数求和后再减：
$1-\frac{1}{8}-\frac{7}{8}$
$=1-(\frac{1}{8}+\frac{7}{8})$
$=1-1$
$=0$

3. 脱式计算。
$\frac{4}{7}-\frac{4}{15}+\frac{3}{7}$
$\frac{9}{5}-\frac{5}{6}-\frac{1}{6}$
$\frac{7}{4}-(\frac{3}{4}-\frac{1}{6})$
$1-(\frac{5}{19}+\frac{3}{19})$
$\frac{3}{8}+\frac{1}{21}+\frac{5}{8}+\frac{20}{21}$
$\frac{23}{24}-\frac{3}{4}+\frac{1}{24}$

$\frac{4}{7}-\frac{4}{15}+\frac{3}{7}$
$=\frac{4}{7}+\frac{3}{7}-\frac{4}{15}$
$=1-\frac{4}{15}$
$=\frac{11}{15}$
$\frac{9}{5}-\frac{5}{6}-\frac{1}{6}$
$=\frac{9}{5}-(\frac{5}{6}+\frac{1}{6})$
$=\frac{9}{5}-1$
$=\frac{4}{5}$
$\frac{7}{4}-(\frac{3}{4}-\frac{1}{6})$
$=\frac{7}{4}-\frac{3}{4}+\frac{1}{6}$
$=1+\frac{1}{6}$
$=\frac{7}{6}$
$1-(\frac{5}{19}+\frac{3}{19})$
$=1-\frac{8}{19}$
$=\frac{11}{19}$
$\frac{3}{8}+\frac{1}{21}+\frac{5}{8}+\frac{20}{21}$
$=(\frac{3}{8}+\frac{5}{8})+(\frac{1}{21}+\frac{20}{21})$
$=1+1$
$=2$
$\frac{23}{24}-\frac{3}{4}+\frac{1}{24}$
$=\frac{23}{24}+\frac{1}{24}-\frac{3}{4}$
$=1-\frac{3}{4}$
$=\frac{1}{4}$