三、慎思妙算。
1. 直接写出得数。
$\frac{5}{9} + \frac{2}{9} =$ $\frac{7}{8} - \frac{5}{8} =$ $\frac{9}{10} - \frac{1}{2} =$ $\frac{7}{18}×\frac{3}{14} =$
$\frac{3}{8} - \frac{1}{4} =$ $\frac{1}{6} + \frac{7}{12} =$ $\frac{3}{7} + \frac{1}{2} =$ $\frac{17}{45}×15 =$

$\frac{5}{9} + \frac{2}{9} = \frac{7}{9}$
$\frac{7}{8} - \frac{5}{8} = \frac{2}{8} = \frac{1}{4}$
$\frac{9}{10} - \frac{1}{2} = \frac{9}{10} - \frac{5}{10} = \frac{4}{10} = \frac{2}{5}$
$\frac{7}{18}×\frac{3}{14} = \frac{7×3}{18×14} = \frac{21}{252} = \frac{1}{12}$
$\frac{3}{8} - \frac{1}{4} = \frac{3}{8} - \frac{2}{8} = \frac{1}{8}$
$\frac{1}{6} + \frac{7}{12} = \frac{2}{12} + \frac{7}{12} = \frac{9}{12} = \frac{3}{4}$
$\frac{3}{7} + \frac{1}{2} = \frac{6}{14} + \frac{7}{14} = \frac{13}{14}$
$\frac{17}{45}×15 = \frac{17×15}{45} = \frac{255}{45} = \frac{17}{3}$

2. 计算下面各题，能简算的要简算。
$\frac{2}{3} + \frac{1}{5} - \frac{3}{10}$         $\frac{18}{11} - (\frac{7}{11} + \frac{3}{8})$         $\frac{2}{7} + \frac{3}{11} + \frac{8}{11} + \frac{4}{7}$

$\frac{7}{10}×15×\frac{8}{21}$         $\frac{5}{9}×\frac{7}{12}×\frac{18}{35}$         $\frac{4}{11}×\frac{17}{18}×\frac{33}{34}$

第一部分：分数加减混合运算
1. $\frac{2}{3} + \frac{1}{5} - \frac{3}{10}$
$=\frac{20}{30} + \frac{6}{30} - \frac{9}{30}$
$=\frac{26}{30} - \frac{9}{30}$
$=\frac{17}{30}$
2. $\frac{18}{11} - (\frac{7}{11} + \frac{3}{8})$
$=\frac{18}{11} - \frac{7}{11} - \frac{3}{8}$
$=1 - \frac{3}{8}$
$=\frac{5}{8}$
3. $\frac{2}{7} + \frac{3}{11} + \frac{8}{11} + \frac{4}{7}$
$=(\frac{2}{7} + \frac{4}{7}) + (\frac{3}{11} + \frac{8}{11})$
$=\frac{6}{7} + 1$
$=1\frac{6}{7}$
第二部分：分数乘法运算
4. $\frac{7}{10}×15×\frac{8}{21}$
$=\frac{7×15×8}{10×21}$
$=\frac{1×3×8}{2×3}$
$=4$
5. $\frac{5}{9}×\frac{7}{12}×\frac{18}{35}$
$=\frac{5×7×18}{9×12×35}$
$=\frac{1×1×2}{1×12×1}$
$=\frac{1}{6}$
6. $\frac{4}{11}×\frac{17}{18}×\frac{33}{34}$
$=\frac{4×17×33}{11×18×34}$
$=\frac{4×3}{18×2}$
$=\frac{1}{3}$

3. 解方程。
$x - 5.2 + 4.8 = 67$ $2x÷3.6 = 0.4$ $2.5×8 + 10x = 25.5$

解方程
1. $ x - 5.2 + 4.8 = 67 $
解：$ x - 0.4 = 67 $
$ x = 67 + 0.4 $
$ x = 67.4 $
2. $ 2x÷3.6 = 0.4 $
解：$ 2x = 0.4×3.6 $
$ 2x = 1.44 $
$ x = 1.44÷2 $
$ x = 0.72 $
3. $ 2.5×8 + 10x = 25.5 $
解：$ 20 + 10x = 25.5 $
$ 10x = 25.5 - 20 $
$ 10x = 5.5 $
$ x = 5.5÷10 $
$ x = 0.55 $