1. 直接写出得数。
$1.1 - 20\% =$         $70×\frac{17}{35} =$         $\frac{7}{15}×\frac{5}{21} =$         $\frac{4}{5}×\frac{5}{12}×\frac{3}{7} =$
$\frac{3}{20}×\frac{5}{7} =$         $33÷99 =$         $45\%×\frac{2}{3} =$         $6÷\frac{6}{7} + 3 =$
$3.6 + 5.04 =$         $1 - 77\% =$         $1.59÷30\% =$         $2×35\% + 65\% =$
$102÷17\% =$         $0.9×90\% =$         $36÷24 =$ ()$\%$         $9÷\frac{3}{10}×3\% =$
$\frac{5}{9}×5\% =$         $80\%÷16 =$         $24÷800 =$ ()$\%$         $12×(20\% - \frac{1}{6}) =$

1. 0.9 34 $\dfrac{1}{9}$         $\dfrac{1}{7}$         $\dfrac{3}{28}$         $\dfrac{1}{3}$ 0.3 10 8.64 0.23 5.3 1.35 600 0.81 150 0.9 $\dfrac{1}{36}$ 0.05 3 $\dfrac{2}{5}$

2. 计算下面各题，能简算的要简算。
$(\frac{1}{6} + \frac{1}{7})×6×7$         $\frac{5}{8}×20\% + \frac{3}{5}×62.5\%$         $\frac{17}{18}×[\frac{9}{5} - (80\% + \frac{6}{17})]$
$(10 - \frac{5}{12}÷\frac{1}{6})×\frac{1}{3}$         $38.96 - 7.34 - 2.66$         $\frac{14}{13}×\frac{1}{22} - \frac{1}{13}÷22$

1. 计算$(\frac{1}{6} + \frac{1}{7})×6×7$
解：
$\begin{aligned}&(\frac{1}{6} + \frac{1}{7})×6×7\\=&\frac{1}{6}×6×7 + \frac{1}{7}×6×7\\=&7 + 6\\=&13\end{aligned}$
2. 计算$\frac{5}{8}×20\% + \frac{3}{5}×62.5\%$
解：
$\begin{aligned}&\frac{5}{8}×20\% + \frac{3}{5}×62.5\%\\=&\frac{5}{8}×\frac{1}{5} + \frac{3}{5}×\frac{5}{8}\\=&\frac{5}{8}×(\frac{1}{5} + \frac{3}{5})\\=&\frac{5}{8}×\frac{4}{5}\\=&\frac{1}{2}\end{aligned}$
3. 计算$\frac{17}{18}×[\frac{9}{5} - (80\% + \frac{6}{17})]$
解：
$\begin{aligned}&\frac{17}{18}×[\frac{9}{5} - (80\% + \frac{6}{17})]\\=&\frac{17}{18}×[\frac{9}{5} - (\frac{4}{5} + \frac{6}{17})]\\=&\frac{17}{18}×(\frac{9}{5} - \frac{4}{5} - \frac{6}{17})\\=&\frac{17}{18}×(1 - \frac{6}{17})\\=&\frac{17}{18}×\frac{11}{17}\\=&\frac{11}{18}\end{aligned}$
4. 计算$(10 - \frac{5}{12}÷\frac{1}{6})×\frac{1}{3}$
解：
$\begin{aligned}&(10 - \frac{5}{12}÷\frac{1}{6})×\frac{1}{3}\\=&(10 - \frac{5}{12}×6)×\frac{1}{3}\\=&(10 - \frac{5}{2})×\frac{1}{3}\\=&\frac{15}{2}×\frac{1}{3}\\=&\frac{5}{2}\end{aligned}$
5. 计算$38.96 - 7.34 - 2.66$
解：
$\begin{aligned}&38.96 - 7.34 - 2.66\\=&38.96 - (7.34 + 2.66)\\=&38.96 - 10\\=&28.96\end{aligned}$
6. 计算$\frac{14}{13}×\frac{1}{22} - \frac{1}{13}÷22$
解：
$\begin{aligned}&\frac{14}{13}×\frac{1}{22} - \frac{1}{13}÷22\\=&\frac{14}{13}×\frac{1}{22} - \frac{1}{13}×\frac{1}{22}\\=&(\frac{14}{13} - \frac{1}{13})×\frac{1}{22}\\=&1×\frac{1}{22}\\=&\frac{1}{22}\end{aligned}$
综上，答案依次为$13$；$\frac{1}{2}$；$\frac{11}{18}$；$\frac{5}{2}$；$28.96$；$\frac{1}{22}$。

计算：$\frac{1}{2×4} + \frac{1}{4×6} + \frac{1}{6×8} + ··· + \frac{1}{48×50}$。

解：
观察原式，可发现$\frac{1}{n(n + 2)}=\frac{1}{2}×(\frac{1}{n}-\frac{1}{n + 2})$（$n$为偶数）。
则$\frac{1}{2×4} + \frac{1}{4×6} + \frac{1}{6×8} + ··· + \frac{1}{48×50}$
$=\frac{1}{2}×(\frac{1}{2}-\frac{1}{4})+\frac{1}{2}×(\frac{1}{4}-\frac{1}{6})+\frac{1}{2}×(\frac{1}{6}-\frac{1}{8})+···+\frac{1}{2}×(\frac{1}{48}-\frac{1}{50})$
$=\frac{1}{2}×[(\frac{1}{2}-\frac{1}{4})+(\frac{1}{4}-\frac{1}{6})+(\frac{1}{6}-\frac{1}{8})+···+(\frac{1}{48}-\frac{1}{50})]$
去括号后，中间项可相互抵消：
$=\frac{1}{2}×(\frac{1}{2}-\frac{1}{50})$
$=\frac{1}{2}×(\frac{25}{50}-\frac{1}{50})$
$=\frac{1}{2}×\frac{24}{50}$
$=\frac{6}{25}$
综上，原式的结果为$\frac{6}{25}$。