1.口算。
$\frac{8}{17}+\frac{9}{17}=$ $\frac{1}{2}-\frac{1}{4}=$ $\frac{7}{39}×\frac{13}{28}=$ $\frac{3}{14}÷\frac{12}{7}=$
$\frac{7}{5}×\frac{20}{21}=$ $\frac{21}{25}÷\frac{3}{5}=$ $\frac{4}{15}+\frac{7}{30}=$ $2-\frac{7}{18}=$

$\frac{8}{17}+\frac{9}{17}=1$
$\frac{1}{2}-\frac{1}{4}=\frac{1}{4}$
$\frac{7}{39}×\frac{13}{28}=\frac{1}{12}$
$\frac{3}{14}÷\frac{12}{7}=\frac{1}{8}$
$\frac{7}{5}×\frac{20}{21}=\frac{4}{3}$
$\frac{21}{25}÷\frac{3}{5}=\frac{7}{5}$
$\frac{4}{15}+\frac{7}{30}=\frac{1}{2}$
$2-\frac{7}{18}=1\frac{11}{18}$

2.计算。
$2 - \frac{6}{7} × \frac{7}{18}$
$\frac{3}{4} ÷ \frac{5}{8} - \frac{5}{9}$
$( \frac{7}{8} - \frac{5}{16} ) × ( \frac{7}{9} - \frac{2}{3} )$

$2 - \frac{6}{7}×\frac{7}{18}$
$= 2 - \frac{1}{3}$
$= \frac{5}{3}$
$\frac{3}{4}÷\frac{5}{8} - \frac{5}{9}$
$= \frac{3}{4}×\frac{8}{5} - \frac{5}{9}$
$= \frac{6}{5} - \frac{5}{9}$
$= \frac{54}{45} - \frac{25}{45}$
$= \frac{29}{45}$
$(\frac{7}{8} - \frac{5}{16})×(\frac{7}{9} - \frac{2}{3})$
$= (\frac{14}{16} - \frac{5}{16})×(\frac{7}{9} - \frac{6}{9})$
$= \frac{9}{16}×\frac{1}{9}$
$= \frac{1}{16}$

3.按要求添括号,改变$\frac{2}{9}÷2+\frac{1}{6}×\frac{3}{4}$的运算顺序,并计算。
(1)加法→除法→乘法 $\frac{2}{9}÷2+\frac{1}{6}×\frac{3}{4}$
(2)乘法→加法→除法 $\frac{2}{9}÷2+\frac{1}{6}×\frac{3}{4}$
(3)加法→乘法→除法 $\frac{2}{9}÷2+\frac{1}{6}×\frac{3}{4}$

(1)
添括号：$\frac{2}{9}÷(2+\frac{1}{6})×\frac{3}{4}$
计算：
$\begin{aligned}\frac{2}{9}÷(2+\frac{1}{6})×\frac{3}{4}&=\frac{2}{9}÷\frac{13}{6}×\frac{3}{4}\\&=\frac{2}{9}×\frac{6}{13}×\frac{3}{4}\\&=\frac{1}{13}\end{aligned}$
(2)
添括号：$\frac{2}{9}÷(2+\frac{1}{6}×\frac{3}{4})$
计算：
$\begin{aligned}\frac{2}{9}÷(2+\frac{1}{6}×\frac{3}{4})&=\frac{2}{9}÷(2+\frac{1}{8})\\&=\frac{2}{9}÷\frac{17}{8}\\&=\frac{16}{153}\end{aligned}$
(3)
添括号：$\frac{2}{9}÷[(2+\frac{1}{6})×\frac{3}{4}]$
计算：
$\begin{aligned}\frac{2}{9}÷[(2+\frac{1}{6})×\frac{3}{4}]&=\frac{2}{9}÷[\frac{13}{6}×\frac{3}{4}]\\&=\frac{2}{9}÷\frac{13}{8}\\&=\frac{16}{117}\end{aligned}$

4.添上适当的运算符号和括号,使等式成立。
$\frac{1}{4}$ $\frac{1}{4}$ $\frac{1}{4}$ $\frac{1}{4}$=$\frac{1}{3}$
$\frac{1}{4}$ $\frac{1}{4}$ $\frac{1}{4}$ $\frac{1}{4}$ $\frac{1}{4}$=$\frac{1}{8}$

$\frac{1}{4} ÷ ( \frac{1}{4} ÷ \frac{1}{4} - \frac{1}{4} ) = \frac{1}{3}$
$( \frac{1}{4} + \frac{1}{4} ) × \frac{1}{4} × ( \frac{1}{4} ÷ \frac{1}{4} ) = \frac{1}{8}$
答：等式成立的添法如上，答案不唯一。