1. $\frac{5}{8} × \frac{1}{3} = \frac{□ × □}{□ × □} = \frac{□}{□}$ $\frac{3}{5} × \frac{2}{7} = \frac{□ × □}{□ × □} = \frac{□}{□}$
$\frac{2}{5} × 4 = \frac{2}{5} × \frac{4}{□} = \frac{□ × □}{□ × □} = \frac{□}{□}$
$8 × \frac{5}{6} = \frac{8}{□} × \frac{5}{6} = \frac{□ × □}{□ × □} = \frac{□}{□}$

$\frac{5×1}{8×3} = \frac{5}{24}$；$\frac{3×2}{5×7} = \frac{6}{35}$；$\frac{2}{5} × \frac{4}{1} = \frac{2×4}{5×1} = \frac{8}{5}$；$\frac{8}{1} × \frac{5}{6} = \frac{8×5}{1×6} = \frac{20}{3}$

分数乘分数，分子相乘的积作分子，分母相乘的积作分母；整数乘分数，先把整数看成分母是1的分数，再按分数乘分数的方法计算。
$\frac{5}{8} × \frac{1}{3} = \frac{5×1}{8×3} = \frac{5}{24}$；
$\frac{3}{5} × \frac{2}{7} = \frac{3×2}{5×7} = \frac{6}{35}$；
$\frac{2}{5} × 4 = \frac{2}{5} × \frac{4}{1} = \frac{2×4}{5×1} = \frac{8}{5}$；
$8 × \frac{5}{6} = \frac{8}{1} × \frac{5}{6} = \frac{8×5}{1×6} = \frac{40}{6} = \frac{20}{3}$

2. 计算。
$\frac{3}{4} × \frac{8}{9}$ $\frac{7}{12} × \frac{3}{5}$ $\frac{7}{8} × \frac{5}{6}$
$\frac{5}{12} × \frac{3}{10}$ $\frac{4}{12} × \frac{5}{8}$ $\frac{4}{9} × \frac{27}{28}$
$\frac{5}{14} × \frac{21}{25}$ $\frac{5}{36} × \frac{9}{15}$ $\frac{13}{24} × \frac{12}{39}$

$​=\frac {3×8}{4×9}​$
$​=\frac {2}{3}​$
$​=\frac {7×3}{12×5}​$
$​=\frac {7}{20}​$
$​=\frac {7×5}{8×6}​$
$​=\frac {35}{48}​$
$​=\frac {5×3}{12×10}​$
$​=\frac {1}{8}​$
$​=\frac {4×5}{12×8}​$
$​=\frac {5}{24}​$
$​=\frac {4×27}{9×28}​$
$​=\frac {3}{7}​$
$​=\frac {5×21}{14×25}​$
$​=\frac {3}{10}​$
$​=\frac {5×9}{36×15}​$
$​=\frac {1}{12}​$
$​=\frac {13×12}{24×39}​$
$​=\frac {1}{6}​$

3. 从$\frac{5}{6}$，$\frac{4}{5}$，$\frac{3}{8}$和$\frac{2}{3}$这四个分数中选两个，先组成乘法算式，再计算。

$​\frac {5}{6}×\frac {4}{5}=\frac {2}{3}​$