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Câu 13: 

`a) 3/4 + 9/5 : 3/2 - 1`

`=3/4 + 9/5 . 3/2 - 1`

`= 3/4 + 6/5 - 1`

`= 19/20` 

`b) 6/7 . 8/13 + 6/13 . 9/7 - 4/13 . 6/ 7`
`= 6/7 . (8/13 - 4/13) + 6/13 . 9/7`
`= 6/7 . 4/13 + 6/13 . 9/7`
`=` $\frac{6. 4 + 6. 9}{13.7}$ 
`=6/7`

Câu `14`

`a) 1/3 + 2/3 : x = -7`

`<=> 2/3 . 1/x = -7 - 1/3`

`<=> 2/3 . 1/x = -22/3`

`<=> 1/x = -22/3 : 2/3`

`<=> 1/x = -11`

`=> x = -1/11`

`b) (9/2 - 2x) . 11/7 = 11/4 `

`<=> 9/2 - 2x = 11/4 : 11/7`

`<=> 9/2 - 2x = 7/4` 

`<=> -2x = 9/2 - 7/4`

`<=> -2x = 11/4` 

`=> x = 11/4 : 2` 

`=> x = 11/4 . 1/2`

`=> x = 11/8`

Đáp án+Giải thích các bước giải:

 Câu 13

`a. 3/4 + 9/5 : 3/2 -1`

`= 3/4 + 9/5 . 2/3 -1`

`= 3/4 + 6/5 -1`

`= {15+24-20}/{20}`

`= {19}/{20}`

`b. 6/7 . 8/{13} + 6/{13} . 9/7 - 4/{13} . 6/7`

`= 6/7 . 8/{13} + 9/{13} . 6/7 - 4/{13} . 6/7`

`= 6/7 ( 8/{13} + 9/{13} - 4/{13} )`

`= 6/7 ( {8+9-4}/{13} )`

`= 6/7 . {13}/{13}`

`= 6/7 .1`

`= 6/7`

Câu 14

`a. 1/3 + 2/3 :x=-7`

`2/3 :x= -7- 1/3`

`2/3 :x ={-21-1}/3`

`2/3 :x= {-22}/3`

`x= 2/3 : {-22}/3`

`x= 2/3 . {-3}/22`

`x= {-1}/{11}`

`b.( 9/2 -2x). {11}/7 = {11}/4`

`9/2 -2x= {11}/4 . 7/{11}`

`9/2 -2x= 7/4`

`2x= 9/2 - 7/4`

`2x = {18-7}/4`

`2x= {11}/4`

`x= {11}/4 . 1/2`

`x= {11}/8`

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