(7*13+8*13):(8+21-x)=39
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`@` `\text {Ans}`
`\downarrow`
`1,`
`a)`
`-7/25 + (-8)/25`
`= (-7 - 8)/25`
`= -15/25`
`= -3/5`
`b)`
`6/13 + (-15)/39`
`= 18/39 + (-15)/39`
`= (18 - 15)/39`
`= 3/39`
`= 1/13`
`c)`
`5/7 + 4/(-14)`
`= 10/14 + (-4)/14`
`= (10 - 4)/14`
`= 6/14`
`= 3/7`
`d)`
`-8/18 + (-15)/27`
`= -4/9 + (-5)/9`
`= (-4-5)/9`
`= -9/9 = -1`
`2,`
`a)`
`3/5 + (-7)/4`
`= 12/20 + (-35)/20`
`= (12 - 35)/20`
`=-23/20`
`b)`
`(-2) + (-5)/8`
`= (-16)/8 + (-5)/8`
`= (-16 - 5)/8`
`= -21/8`
`c)`
`1/8 + (-5)/9`
`= 9/72 + (-40)/72`
`= (9-40)/72`
`= -31/72`
`d)`
`6/13 + (-14)/39`
`= 18/39 + (-14)/39`
`= (18 - 14)/39`
`= 4/39`
`e)`
`(-18)/24 + 15/21`
`= (-3)/4 + 5/7`
`= (-21)/28 + 20/28`
`= (-21 + 20)/28`
`= -1/28`
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\(\dfrac{26}{81}+\dfrac{4}{27}=\dfrac{26+12}{81}=\dfrac{38}{81}\)
\(\dfrac{5}{64}+\dfrac{7}{8}=\dfrac{5+56}{64}=\dfrac{61}{64}\)
\(\dfrac{26}{81}+\dfrac{4}{27}=\dfrac{26}{81}+\dfrac{4\times3}{27\times3}=\dfrac{26}{81}+\dfrac{12}{81}=\dfrac{38}{81}\)
\(\dfrac{5}{64}+\dfrac{7}{8}=\dfrac{5}{64}+\dfrac{7\times8}{8\times8}=\dfrac{5}{64}+\dfrac{56}{64}=\dfrac{61}{64}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(=\dfrac{49}{13\cdot15}-\dfrac{105}{13\cdot3\cdot4\cdot3}+\dfrac{320}{15\cdot13\cdot7}\)
\(=\dfrac{49\cdot7+320}{15\cdot13\cdot7}-\dfrac{105}{13\cdot9\cdot4}\)
\(=\dfrac{663}{13\cdot105}-\dfrac{105}{13\cdot36}\)
\(=\dfrac{1427}{5460}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
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1, 476 : x =28
x=476 : 28
x=17
Vậy x= 17
,2, x : 26 = 39
x=39 . 26
x=1014
Vậy x= 1014
Thôi nhiều quá mình không trả lời được
~Thôi chúc bạn học giỏi~
(7 . 13 + 8 . 13) / (8 + 21 - x) = 39
13 (7 + 8) / (8 + 21 - x) = 39
13 . 15 / (8 + 21 - x) = 39
13 . 3 . 5 / (8 + 21 - x) = 39
39 . 5 / (8 + 21 - x) = 39
5 / (29 - x) = 1
29 - x = 5
x = 24