《2025年启东中学作业本七年级数学上册苏科版盐城专版》

9. 计算:
(1)$-2\frac {1}{2}÷1\frac {1}{4}×(-4)$;
(2)$28\frac {7}{8}÷(-7)$;
(3)$(\frac {2}{9}-\frac {1}{4}-\frac {1}{18})÷(-\frac {1}{36})$;
(4)$-(-8)÷\frac {9}{4}×\frac {4}{9}÷|-16|$;
(5)$1\frac {7}{8}÷(-10)×(-3\frac {1}{3})÷(-3\frac {3}{4})$;
(6)$-8÷(-\frac {1}{2})×\frac {5}{16}÷1\frac {3}{5}×(-\frac {6}{5})$.

答案：
(1)8
(2)$-4\frac{1}{8}$
(3)3
(4)$\frac{8}{81}$
(5)$-\frac{1}{6}$
(6)$-\frac{15}{4}$

10. 阅读下列材料:
计算$50÷(\frac {1}{3}-\frac {1}{4}+\frac {1}{12})$.
解法一:原式$=50÷\frac {1}{3}-50÷\frac {1}{4}+50÷\frac {1}{12}= 50×3-50×4+50×12= 550$.
解法二:原式$=50÷(\frac {4}{12}-\frac {3}{12}+\frac {1}{12})= 50÷\frac {2}{12}= 50×6= 300$.
解法三:原式的倒数为$(\frac {1}{3}-\frac {1}{4}+\frac {1}{12})÷50= (\frac {1}{3}-\frac {1}{4}+\frac {1}{12})×\frac {1}{50}= \frac {1}{3}×\frac {1}{50}-\frac {1}{4}×\frac {1}{50}+\frac {1}{12}×\frac {1}{50}= \frac {1}{300}$,故原式= 300.
(1) 上述解法得到的结果不同,你认为解法一是错误的;
(2) 请你选择合适的解法计算:$(-\frac {1}{2})÷(-\frac {1}{2}-\frac {3}{4}+\frac {5}{8})$.
解:$(-\frac{1}{2})÷(-\frac{1}{2}-\frac{3}{4}+\frac{5}{8})$
$=(-\frac{1}{2})÷(-\frac{4}{8}-\frac{6}{8}+\frac{5}{8})$
$=(-\frac{1}{2})÷(-\frac{5}{8})$
$=(-\frac{1}{2})×(-\frac{8}{5})$
$=\frac{4}{5}$.

答案：
(1)一
(2)解:$(-\frac{1}{2})÷(-\frac{1}{2}-\frac{3}{4}+\frac{5}{8})$$=(-\frac{1}{2})÷(-\frac{4}{8}-\frac{6}{8}+\frac{5}{8})$$=(-\frac{1}{2})÷(-\frac{5}{8})$$=(-\frac{1}{2})×(-\frac{8}{5})$$=\frac{4}{5}$.