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1. 计算下列各式,积为正数的是 （
A.$2×3×5×(-4)$
B.$2×(-3)×(-4)×(-3)$
C.$(-2)×0×(-4)×(-5)$
D.$(-2)×(-3)×(-4)×(-5)$
D
）A.$2×3×5×(-4)$
B.$2×(-3)×(-4)×(-3)$
C.$(-2)×0×(-4)×(-5)$
D.$(-2)×(-3)×(-4)×(-5)$
答案
1. D 2×3×5×(-4)=-120,2×(-3)×(-4)×(-3)=-72,(-2)×0×(-4)×(-5)=0,(-2)×(-3)×(-4)×(-5)=120. 故选项D符合题意.
2. 下列变形不正确的是 （
A.$5×7×(-6)=(-6)×5×7$
B.$(\frac{1}{4}-\frac{1}{2})×(-12)=(-12)×(\frac{1}{4}-\frac{1}{2})$
C.$(-\frac{1}{6}+\frac{1}{3})×(-4)=(-4)×(-\frac{1}{6})+\frac{1}{3}×4$
D.$(-0.125)×(-\frac{1}{3})×(-8)=[(-0.125)×(-8)]×(-\frac{1}{3})$
C
）A.$5×7×(-6)=(-6)×5×7$
B.$(\frac{1}{4}-\frac{1}{2})×(-12)=(-12)×(\frac{1}{4}-\frac{1}{2})$
C.$(-\frac{1}{6}+\frac{1}{3})×(-4)=(-4)×(-\frac{1}{6})+\frac{1}{3}×4$
D.$(-0.125)×(-\frac{1}{3})×(-8)=[(-0.125)×(-8)]×(-\frac{1}{3})$
答案
2. C $(-\frac{1}{6}+\frac{1}{3})×(-4)=(-4)×(-\frac{1}{6})+\frac{1}{3}×(-4)$.故选项C的变形不正确.
3. 已知$abc＜0,ac＜0,a＞c$,则下列结论中,正确的是（
A.$a＜0,b＜0,c＞0$
B.$a＞0,b＞0,c＜0$
C.$a＜0,b＜0,c＜0$
D.$a＞0,b＞0,c＞0$
B
）A.$a＜0,b＜0,c＞0$
B.$a＞0,b＞0,c＜0$
C.$a＜0,b＜0,c＜0$
D.$a＞0,b＞0,c＞0$
答案
3. B 由$ac＜0$,得$a$与$c$异号.由$a＞c$,得$a＞0,c＜0$.由$abc＜0$,得$b＞0$.
4. 计算：$-\dfrac{13}{17} × 19 - \dfrac{13}{17} × 15 =$
-26
.答案
4. -26 原式$=-\dfrac{13}{17}×(19+15)=-\dfrac{13}{17}×34=-26$.
5. 计算：
(1) $(-0.125) × (-\dfrac{4}{7}) × 8 × (-7)$.
(2) $(-24) × (-1\dfrac{1}{3} + \dfrac{5}{6} - \dfrac{7}{8}) - 1.4 × 6 + 3.9 × 6$.
(3) $0.7 × 1\dfrac{4}{9} + 2\dfrac{3}{4} × (-15) + 0.7 × \dfrac{5}{9} + \dfrac{1}{4} × (-15)$.
(4) $3\dfrac{1}{7} × \dfrac{21}{22} × (3\dfrac{1}{7} - 7\dfrac{1}{3}) × (-\dfrac{7}{22})$.
(1) $(-0.125) × (-\dfrac{4}{7}) × 8 × (-7)$.
(2) $(-24) × (-1\dfrac{1}{3} + \dfrac{5}{6} - \dfrac{7}{8}) - 1.4 × 6 + 3.9 × 6$.
(3) $0.7 × 1\dfrac{4}{9} + 2\dfrac{3}{4} × (-15) + 0.7 × \dfrac{5}{9} + \dfrac{1}{4} × (-15)$.
(4) $3\dfrac{1}{7} × \dfrac{21}{22} × (3\dfrac{1}{7} - 7\dfrac{1}{3}) × (-\dfrac{7}{22})$.
答案
5. (1) 原式$=-\dfrac{1}{8}×8×\dfrac{4}{7}×7=-4$.
(2) 原式$=(-24)×(-\dfrac{4}{3})+(-24)×\dfrac{5}{6}+(-24)×(-\dfrac{7}{8})+6×(3.9-1.4)=32-20+21+15=48$.
(3) 原式$=0.7×(1\dfrac{4}{9}+\dfrac{5}{9})+(-15)×(2\dfrac{3}{4}+\dfrac{1}{4})=0.7×2+(-15)×3=1.4+(-45)=-43.6$.
(4) 原式$=[\dfrac{22}{7}×(-\dfrac{7}{22})]×[\dfrac{21}{22}×(\dfrac{22}{7}-\dfrac{22}{3})]=-1×(\dfrac{21}{22}×\dfrac{22}{7}-\dfrac{21}{22}×\dfrac{22}{3})=-1×(3-7)=-1×(-4)=4$.
(2) 原式$=(-24)×(-\dfrac{4}{3})+(-24)×\dfrac{5}{6}+(-24)×(-\dfrac{7}{8})+6×(3.9-1.4)=32-20+21+15=48$.
(3) 原式$=0.7×(1\dfrac{4}{9}+\dfrac{5}{9})+(-15)×(2\dfrac{3}{4}+\dfrac{1}{4})=0.7×2+(-15)×3=1.4+(-45)=-43.6$.
(4) 原式$=[\dfrac{22}{7}×(-\dfrac{7}{22})]×[\dfrac{21}{22}×(\dfrac{22}{7}-\dfrac{22}{3})]=-1×(\dfrac{21}{22}×\dfrac{22}{7}-\dfrac{21}{22}×\dfrac{22}{3})=-1×(3-7)=-1×(-4)=4$.
