1.
|$\dfrac{5}{7}$|$+\dfrac{1}{4}$| |$-\dfrac{3}{10}$| |
|----|----|----|----|----|
|$\dfrac{3}{5}$| | | | |
|$\dfrac{5}{8}$| | | | |
|$\dfrac{5}{6}$|$-\dfrac{3}{8}$| |$+\dfrac{1}{2}$| |
|----|----|----|----|----|
|$\dfrac{7}{8}$| | | | |
|$\dfrac{4}{5}$| | | | |

$\frac{5}{7}+\frac{1}{4}-\frac{3}{10}$
$=\frac{100}{140}+\frac{35}{140}-\frac{42}{140}$
$=\frac{135}{140}-\frac{42}{140}$
$=\frac{93}{140}$
$\frac{3}{5}+\frac{1}{4}-\frac{3}{10}$
$=\frac{12}{20}+\frac{5}{20}-\frac{6}{20}$
$=\frac{17}{20}-\frac{6}{20}$
$=\frac{11}{20}$
$\frac{5}{8}+\frac{1}{4}-\frac{3}{10}$
$=\frac{25}{40}+\frac{10}{40}-\frac{12}{40}$
$=\frac{35}{40}-\frac{12}{40}$
$=\frac{23}{40}$
$\frac{5}{6}-\frac{3}{8}+\frac{1}{2}$
$=\frac{20}{24}-\frac{9}{24}+\frac{12}{24}$
$=\frac{11}{24}+\frac{12}{24}$
$=\frac{23}{24}$
$\frac{7}{8}-\frac{3}{8}+\frac{1}{2}$
$=\frac{4}{8}+\frac{4}{8}$
$=\frac{8}{8}$
$=1$
$\frac{4}{5}-\frac{3}{8}+\frac{1}{2}$
$=\frac{32}{40}-\frac{15}{40}+\frac{20}{40}$
$=\frac{17}{40}+\frac{20}{40}$
$=\frac{37}{40}$

2. 在$□$里填数，在$◯$里填运算符号，在()里填变形的依据。
$\dfrac{6}{7}+\dfrac{7}{9}+\dfrac{8}{9}=\dfrac{6}{7}+(□◯□)$ ()
$\dfrac{2}{3}-\dfrac{1}{4}-\dfrac{1}{5}=\dfrac{2}{3}◯(□◯□)$ ()

$\dfrac{6}{7}+\dfrac{7}{9}+\dfrac{8}{9}=\dfrac{6}{7}+(\dfrac{7}{9}+\dfrac{8}{9})$（加法结合律）
$\dfrac{2}{3}-\dfrac{1}{4}-\dfrac{1}{5}=\dfrac{2}{3}-(\dfrac{1}{4}+\dfrac{1}{5})$（减法的性质）

1. 解方程。
$x-\dfrac{5}{12}=\dfrac{3}{8}$
$x+\dfrac{7}{15}=2$
$\dfrac{7}{8}+x=\dfrac{17}{16}$

解：
$x-\dfrac{5}{12}=\dfrac{3}{8}$
$x=\dfrac{3}{8}+\dfrac{5}{12}$
$x=\dfrac{9}{24}+\dfrac{10}{24}$
$x=\dfrac{19}{24}$
解：
$x+\dfrac{7}{15}=2$
$x=2-\dfrac{7}{15}$
$x=\dfrac{30}{15}-\dfrac{7}{15}$
$x=\dfrac{23}{15}$
解：
$\dfrac{7}{8}+x=\dfrac{17}{16}$
$x=\dfrac{17}{16}-\dfrac{7}{8}$
$x=\dfrac{17}{16}-\dfrac{14}{16}$
$x=\dfrac{3}{16}$

2. 下面各题，怎样算简便就怎样算。
$\dfrac{1}{8}+\dfrac{5}{12}-\dfrac{1}{2}$
$\dfrac{8}{9}-\dfrac{3}{8}+\dfrac{9}{1}$
$\dfrac{7}{8}+\dfrac{5}{7}-\dfrac{7}{8}+\dfrac{5}{7}$
$1-\dfrac{1}{8}-\dfrac{7}{8}$
$\dfrac{13}{8}-\dfrac{5}{6}+\dfrac{3}{8}$
$\dfrac{1}{8}+\dfrac{4}{7}-\dfrac{4}{7}+\dfrac{1}{8}$

$\dfrac{1}{8}+\dfrac{5}{12}-\dfrac{1}{2}$
$=\dfrac{3}{24}+\dfrac{10}{24}-\dfrac{12}{24}$
$=\dfrac{3+10-12}{24}$
$=\dfrac{1}{24}$
$\dfrac{8}{9}-\dfrac{3}{8}+\dfrac{1}{9}$
$=\dfrac{8}{9}+\dfrac{1}{9}-\dfrac{3}{8}$
$=1-\dfrac{3}{8}$
$=\dfrac{5}{8}$
$\dfrac{7}{8}+\dfrac{5}{7}-\dfrac{7}{8}+\dfrac{5}{7}$
$=(\dfrac{7}{8}-\dfrac{7}{8})+(\dfrac{5}{7}+\dfrac{5}{7})$
$=0+\dfrac{10}{7}$
$=1\dfrac{3}{7}$
$1-\dfrac{1}{8}-\dfrac{7}{8}$
$=1-(\dfrac{1}{8}+\dfrac{7}{8})$
$=1-1$
$=0$
$\dfrac{13}{8}-\dfrac{5}{6}+\dfrac{3}{8}$
$=\dfrac{13}{8}+\dfrac{3}{8}-\dfrac{5}{6}$
$=2-\dfrac{5}{6}$
$=1\dfrac{1}{6}$
$\dfrac{1}{8}+\dfrac{4}{7}-\dfrac{4}{7}+\dfrac{1}{8}$
$=(\dfrac{1}{8}+\dfrac{1}{8})+(\dfrac{4}{7}-\dfrac{4}{7})$
$=\dfrac{2}{8}+0$
$=\dfrac{1}{4}$