7. 下列计算正确的是 （）

A.$-3.5 ÷ \frac{7}{8} × (-\frac{3}{4}) = -3$
B.$-2 ÷ 3 × 3 = -\frac{2}{9}$
C.$(-6) × (-\frac{1}{4}) ÷ (+\frac{6}{5}) = \frac{5}{4}$
D.$-\frac{1}{30} ÷ (\frac{1}{6} ÷ \frac{1}{5}) = -1$

7. C

8.(易错题)若$|a|=5$，$|b|=\frac{1}{5}$，则$a÷ b×\frac{1}{b}$的值为（）

A.$\pm5$
B.$\pm25$
C.$\pm125$
D.$-25$或$-5$

8. C

9. 计算：
(1) $\frac{2}{5} ÷ (-2\frac{1}{2}) × |-3\frac{3}{4}|$；
(2) $(-10\frac{1}{8}) ÷ \frac{9}{4} × \frac{4}{9} ÷ (-2)$；
(3) $3\frac{1}{7} × (-\frac{3}{7}) ÷ 7\frac{1}{3} × \frac{7}{22} ÷ (-\frac{9}{22})$；
(4) $-65\frac{5}{13} ÷ \frac{5}{2} × \frac{1}{2}$；
(5) $1\frac{7}{8} ÷ (-10) × (-3\frac{1}{3}) ÷ (-3\frac{3}{4})$。

解:(1) 原式$=\dfrac{2}{5}÷(-\dfrac{5}{2})×\dfrac{15}{4}=-\dfrac{2}{5}×\dfrac{2}{5}×\dfrac{15}{4}=-\dfrac{3}{5}$。(2) 原式$=\dfrac{81}{8}×\dfrac{4}{9}×\dfrac{4}{9}×\dfrac{1}{2}=1$。(3) 原式$=\dfrac{22}{7}×\dfrac{3}{7}×\dfrac{3}{22}×\dfrac{7}{22}×\dfrac{22}{9}=\dfrac{1}{7}$。(4) 原式$=-65\dfrac{5}{13}×\dfrac{2}{5}×\dfrac{1}{2}=-65\dfrac{5}{13}×(\dfrac{2}{5}×\dfrac{1}{2})=-65\dfrac{5}{13}×\dfrac{1}{5}=(-65-\dfrac{5}{13})×\dfrac{1}{5}=(-65)×\dfrac{1}{5}+(-\dfrac{5}{13})×\dfrac{1}{5}=-13-\dfrac{1}{13}=-13\dfrac{1}{13}$。(5) 原式$=\dfrac{15}{8}×(-\dfrac{1}{10})×(-\dfrac{10}{3})×(-\dfrac{4}{15})=-[(\dfrac{15}{8}×\dfrac{4}{15})×(\dfrac{1}{10}×\dfrac{10}{3})]=-(\dfrac{1}{2}×\dfrac{1}{3})=-\dfrac{1}{6}$。

10.(1)若$a$的相反数是$5\frac{1}{2}$，$b$的倒数是$-\frac{4}{11}$，求$a$与$b$的商的5倍的值；
(2)已知两个数－4，＋6，它们的相反数的和为$a$，倒数的和为$b$，和的倒数为$c$，求$a÷b×c$的值。

解:(1)因为$a$的相反数是$5\dfrac{1}{2}$,$b$的倒数是$-\dfrac{4}{11}$,所以$a=-5\dfrac{1}{2}$,$b=-\dfrac{11}{4}$.所以$a$与$b$的商的5倍的值为$(-5\dfrac{1}{2})÷(-\dfrac{11}{4})×5=\dfrac{11}{2}×\dfrac{4}{11}×5=10$。(2)由题意,得$a=4+(-6)=-2$,$b=-\dfrac{1}{4}+\dfrac{1}{6}=-\dfrac{1}{12}$,$c=\dfrac{1}{-4+6}=\dfrac{1}{2}$.所以$a÷b×c=-2÷(-\dfrac{1}{12})×\dfrac{1}{2}=2×12×\dfrac{1}{2}=12$。