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\(\dfrac{36}{64}=\dfrac{9}{16}\) ; \(-\dfrac{12}{81}=-\dfrac{4}{27}\)
Quy đồng:
\(\dfrac{9}{16}=\dfrac{243}{432}\) ; \(-\dfrac{4}{27}=-\dfrac{64}{432}\)
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a: 5/14=15/42
4/21=8/42
b: 4/5=48/60
7/12=35/60
8/15=32/60
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Bài 1:
\(\dfrac{-5}{18}=\dfrac{-20}{72};\dfrac{7}{-24}=\dfrac{-21}{72}.\)
\(\dfrac{-15}{-40}=\dfrac{3}{8}=\dfrac{9}{24};\dfrac{24}{-72}=\dfrac{-1}{3}=\dfrac{-8}{24}.\)
Bài 3:
a) \(\dfrac{2}{3}h=\dfrac{8}{12}h;\dfrac{3}{4}h=\dfrac{9}{12}h.\Rightarrow\dfrac{2}{3}h< \dfrac{3}{4}h.\)
b) \(\dfrac{4}{5}km/h=\dfrac{8}{10}km/h;\dfrac{9}{10}km/h.\Rightarrow\dfrac{4}{5}km/h< \dfrac{9}{10}km/h.\)
![](https://rs.olm.vn/images/avt/0.png?1311)
a; \(\dfrac{-12}{30}\) = \(\dfrac{-2}{5}\) = \(\dfrac{-2.4}{5.4}\) = \(\dfrac{-8}{20}\)
\(\dfrac{18}{-40}\) = \(\dfrac{9}{20}\)
b; \(\dfrac{-12}{81}\) = \(\dfrac{-4}{27}\); \(\dfrac{36}{64}\) = \(\dfrac{9}{16}\); \(\dfrac{25}{-10}\) = \(\dfrac{-5}{2}\)
27 = 33; 16 = 24; 2 = 2
BCNN(27; 16; 2) = 24.33= 432
\(\dfrac{-12}{81}\) = \(\dfrac{-4}{27}\) = \(\dfrac{-4.16}{27.16}\) = \(\dfrac{-64}{432}\)
\(\dfrac{36}{64}\) = \(\dfrac{9}{16}\) = \(\dfrac{9.27}{16.27}\) = \(\dfrac{243}{432}\)
\(\dfrac{25}{-10}\) = \(\dfrac{-5}{2}\)= \(\dfrac{-5.216}{2.216}\) = \(\dfrac{-1080}{432}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
a) Ta có: \(12 = 2^2 . 3; 15 = 3.5\)
\(BCNN(12, 15) = 2^2.3.5 = 60\) nên chọn mẫu chung là 60.
\(\begin{array}{l}\frac{9}{{12}} = \frac{{9.5}}{{12.5}} = \frac{{45}}{{60}}\\\frac{7}{{15}} = \frac{{7.4}}{{15.4}} = \frac{{28}}{{60}}\end{array}\)
b) Ta có: \(10 = 2.5; 4 = 2^2; 14=2.7\)
\(BCNN(10, 4, 14) =2^2.5.7= 140\) nên chọn mẫu chung là 140.
\(\begin{array}{l}\frac{7}{{10}} = \frac{{7.14}}{{10.14}} = \frac{{98}}{{140}}\\\frac{3}{4} = \frac{{3.35}}{{4.35}} = \frac{{105}}{{140}}\\\frac{9}{{14}} = \frac{{9.10}}{{14.10}} = \frac{{90}}{{140}}\end{array}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
13/30=138/180
38/45=152/180
7/12=156/180
17/20=153/180
13/18=130/180
Thứ tự tăng dần là : 130/180;138/180;152/180;153/180;156/180
=>13/18;13/30;38/45;17/20;7/12
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