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\(\dfrac{2}{5}+\dfrac{3}{7}+\dfrac{3}{5}+\dfrac{4}{7}\\ =\left(\dfrac{2}{5}+\dfrac{3}{5}\right)+\left(\dfrac{3}{7}+\dfrac{4}{7}\right)\\ =\dfrac{5}{5}+\dfrac{7}{7}\\ =1+1\\ =2.\)
![](https://rs.olm.vn/images/avt/0.png?1311)
1)
a) 21/23 x 5/11 x 4/35
= \(\frac{21.5.4}{23.11.35}\)
= \(\frac{7.3.5.4}{23.11.5.7}\)
= \(\frac{3.4}{23.11}\) ( ta đã rút gọc 7 và 5 )
= \(\frac{12}{253}\)
b) 26 x 13/42 : 13
= \(\frac{26.13}{42.13}\)
= \(\frac{13.2.13}{21.2.13}\)
= \(\frac{13}{21}\)
c) 5/9 : 2/9 + 4/9 : 2/3
= \(\frac{5.9}{9.2}+\frac{4.3}{9.2}\)
= \(\frac{5}{2}+\frac{2.2.3}{3.3.2}\)
= \(\frac{5}{2}+\frac{2}{3}\)
= \(\frac{19}{6}\)
2 ) tìm x
a ) 2 - x = 1/3 + 1/6
2 - x = 1/2
x = 2 - 1/2
x = 3/2
b ) x + 5/8 = 1 + 3/4
x + 5/8 = 7/4
x = 7/4 - 5/8
x = 9/8
3)
a) x < 3/4 + 4/5
=> x < 31/20
=> x = 1
b) 1/2 + 3/5 < x < 5/4
=> 11/10 < x < 5/4
ta quy đồng phân số 11/10 và 5/4
\(\frac{11}{10}=\frac{11.4}{10.4}=\frac{44}{40}\)
\(\frac{5}{4}=\frac{5.10}{4.10}=\frac{50}{40}\)
=> 44/40 < x < 50/40
=> x không thỏa mãn
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(a,\dfrac{5}{9}\times\dfrac{5}{2}+\dfrac{4}{9}\times\dfrac{5}{2}=\dfrac{5}{2}\times\left(\dfrac{5}{9}+\dfrac{4}{9}\right)=\dfrac{5}{2}\times1=\dfrac{5}{2}\)
\(12\times\dfrac{3}{4}-\dfrac{3}{4}\times\dfrac{12}{7}=\dfrac{3}{4}\times\left(12-\dfrac{12}{7}\right)=\dfrac{3}{4}\times\dfrac{72}{7}=\dfrac{54}{7}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Bài làm
- \(\frac{4}{5}+\frac{3}{7}\)
= \(\frac{28}{35}+\frac{15}{35}=\frac{43}{35}\)
\(\frac{4}{5}+\frac{3}{7}=\frac{43}{35}\)
\(\frac{5}{6}-\frac{1}{3}=\frac{1}{2}\)
\(\frac{4}{9}.3=\frac{4}{3}\)
\(\frac{1}{2}:\frac{3}{2}=\frac{1}{3}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(\dfrac{2}{3}\times\dfrac{4}{9}+\dfrac{2}{3}\times\dfrac{5}{9}\\ =\dfrac{2}{3}\times\left(\dfrac{4}{9}+\dfrac{5}{9}\right)\\ =\dfrac{2}{3}\times\dfrac{9}{9}\\ =\dfrac{2}{3}\times1\\ =\dfrac{2}{3}.\)
`2/3 xx 4/9 + 2/3 xx 5/9 `
`= 2/3 xx (4/9 + 5/9)`
`= 2/3 xx 1`
`= 2/3`