21/45.25/28+7/4-5/9=
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a) \(\dfrac{7}{-25}+\dfrac{8}{25}=\dfrac{-7}{25}+\dfrac{8}{25}=\dfrac{1}{25}\)
\(\dfrac{4}{5}+\dfrac{4}{-18}=\dfrac{4}{5+\left(-18\right)}=\dfrac{4}{-13}=\dfrac{-4}{13}\)
\(\dfrac{7}{21}+\dfrac{9}{-36}=\dfrac{1}{3}+\dfrac{1}{-4}=\dfrac{1}{3+\left(-4\right)}=\dfrac{1}{-1}=1\)
b) \(\dfrac{12}{20}-\dfrac{2}{5}=\dfrac{12}{20}+\left(\dfrac{-2}{5}\right)=\dfrac{3}{5}+\dfrac{-2}{5}=\dfrac{3+\left(-2\right)}{5}=\dfrac{1}{5}\)
\(\dfrac{2}{3}-\dfrac{-5}{6}=\dfrac{2}{3}+\dfrac{5}{6}=\dfrac{4}{6}+\dfrac{5}{6}=\dfrac{9}{6}=\dfrac{3}{2}\)
\(6-\dfrac{3}{20}=6+\dfrac{-3}{20}=\dfrac{60}{60}+\dfrac{-9}{60}=\dfrac{51}{60}\)
c) \(\dfrac{2}{21}.\dfrac{-7}{3}=\dfrac{2.\left(-1\right)}{7.3}=\dfrac{-2}{21}\)
\(\dfrac{27}{28}.\left(-21\right)=\dfrac{27.\left(-21\right)}{28}=\dfrac{-567}{28}\)
\(\dfrac{-15}{7}.\dfrac{-14}{25}=\dfrac{-3.\left(-2\right)}{1.5}=\dfrac{6}{5}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
A) 7/38 x 9/11 +7/38 x 4/11 -7/38 x 2/11
=7/38.(9/11+4/11-2/11)
=7/38
B) 5/31 x 21/25 + 5/31 x -7/10 - 5/31 x 9/20
=5/31.(21/25-7/10-9/20)
=5/31.(-31/100)
=-1/20
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Học sinh nhẩm và ghi kết quả như sau:
35 : 5 = 7
28 : 7 = 4
24 : 6 = 4
21 : 3 = 7
35 : 7 = 5
28 : 4 = 7
24 : 4 = 6
21 : 7 = 3
35 : 5 = 7
28 : 7 = 4
24 : 6 = 4
21 : 3 = 7
35 : 7 = 5
28 : 4 = 7
24 : 4 = 6
21 : 7 = 3
![](https://rs.olm.vn/images/avt/0.png?1311)
câu A cách 1
\(\dfrac{17}{19}\times\dfrac{12}{15}\times\dfrac{19}{17}\)
=\(\dfrac{17\times12\times19}{19\times15\times17}\)
=\(\dfrac{12}{15}\)
câu A cách 2
\(\dfrac{17}{19}\times\dfrac{12}{15}\times\dfrac{19}{17}\)
=\(\dfrac{17}{19}\times\dfrac{19}{17}\times\dfrac{12}{15}\)
=1 x \(\dfrac{12}{15}\)
=\(\dfrac{12}{15}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Cách 1:
A. \(\dfrac{17}{19}\) x \(\dfrac{12}{15}\) x \(\dfrac{19}{17}\)
= (\(\dfrac{17}{19}\) x \(\dfrac{19}{17}\)) x \(\dfrac{12}{15}\)
= \(1\) x \(\dfrac{12}{15}\)
= \(\dfrac{4}{5}\)
Cách hai:
A; \(\dfrac{17}{19}\) x \(\dfrac{12}{15}\) x \(\dfrac{19}{17}\)
= \(\dfrac{68}{95}\) x \(\dfrac{19}{17}\)
= \(\dfrac{4}{5}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
7 x 3 = 21 7 x 6 = 42 7 x 5 = 35 7 x 4 = 28
21 : 7 = 3 42 : 7 = 6 35 : 7 = 5 28 : 7 = 4
21 : 3 = 7 42 : 6 = 7 35 : 5 = 7 28 : 4 = 7
7 × 3 = 21 7 × 6 = 42 7 × 5 = 35. 7 × 4 = 28
21 : 7 = 3. 42 : 7 = 6. 35 : 7 = 5. 28 : 7 = 4
21 : 3 = 7. 42 : 6 = 7. 35 : 5 = 7. 28 : 4 = 7
![](https://rs.olm.vn/images/avt/0.png?1311)
\(\frac{9}{12}+\frac{4}{7}\)
\(=\frac{3}{4}+\frac{4}{7}\)
\(=\frac{37}{28}\)
\(\frac{5}{10}+\frac{21}{28}\)
\(=\frac{1}{2}+\frac{3}{4}\)
\(=\frac{5}{4}\)
\(\frac{7}{9}+\frac{15}{27}\)
\(=\frac{7}{9}+\frac{5}{9}\)
\(=\frac{12}{9}\)
\(\dfrac{21}{45}\cdot\dfrac{25}{28}+\dfrac{7}{4}-\dfrac{5}{9}\\ =\dfrac{5}{12}+\dfrac{7}{4}-\dfrac{5}{9}\\ =\dfrac{15}{36}+\dfrac{63}{36}-\dfrac{20}{36}\\ =\dfrac{58}{36}=\dfrac{29}{18}\)