五、计算。
$0.4×(12×2.5)$
$(1.5 - 0.6)×(3 - 1.8)$
$\frac{1}{4}×4÷\frac{1}{4}×4$
$(3.8 - 1.8÷2)×0.5$
$\frac{8}{13}÷7 + \frac{1}{7}×\frac{5}{13}$
$\frac{9}{10}÷[\frac{1}{2}×(\frac{6}{5} - \frac{3}{10})]$

1. $0.4×(12×2.5)$
$=0.4×2.5×12$
$=1×12$
$=12$
2. $(1.5 - 0.6)×(3 - 1.8)$
$=0.9×1.2$
$=1.08$
3. $\frac{1}{4}×4÷\frac{1}{4}×4$
$=1÷\frac{1}{4}×4$
$=4×4$
$=16$
4. $(3.8 - 1.8÷2)×0.5$
$=(3.8 - 0.9)×0.5$
$=2.9×0.5$
$=1.45$
5. $\frac{8}{13}÷7 + \frac{1}{7}×\frac{5}{13}$
$=\frac{8}{13}×\frac{1}{7} + \frac{1}{7}×\frac{5}{13}$
$=(\frac{8}{13} + \frac{5}{13})×\frac{1}{7}$
$=1×\frac{1}{7}$
$=\frac{1}{7}$
6. $\frac{9}{10}÷[\frac{1}{2}×(\frac{6}{5} - \frac{3}{10})]$
$=\frac{9}{10}÷[\frac{1}{2}×(\frac{12}{10} - \frac{3}{10})]$
$=\frac{9}{10}÷[\frac{1}{2}×\frac{9}{10}]$
$=\frac{9}{10}÷\frac{9}{20}$
$=\frac{9}{10}×\frac{20}{9}$
$=2$

六、找规律并计算。
1. 用计算器计算左边的题，找出规律，并直接写出右边题的得数。
$9×9 - 1 =$
$9876×9 - 4 =$
$98×9 - 2 =$
$98765×9 - 5 =$
$987×9 - 3 =$
$987654×9 - 6 =$
2. 先观察前两题的计算过程，再试着计算后两题。
$9×9 - 1 = 9×(10 - 1) - 1 = 9×10 - 9 - 1 = 80$
$98×9 - 2 = 98×(10 - 1) - 2 = 980 - 98 - 2 = 880$
$987×9 - 3 =$
$9876×9 - 4 =$

1. $9×9 - 1 = 80$
$98×9 - 2 = 880$
$987×9 - 3 = 8880$
$9876×9 - 4 = 88880$
$98765×9 - 5 = 888880$
$987654×9 - 6 = 8888880$
2. $987×9 - 3 = 987×(10 - 1) - 3 = 9870 - 987 - 3 = 8880$
$9876×9 - 4 = 9876×(10 - 1) - 4 = 98760 - 9876 - 4 = 88880$