Bài `12`

`a)`

`x/6 =  5/24`

`=> 24x = 5.6`

`=> 24x = 30`

`=> x = 30 : 24`

`=> x = 5/4`

Vậy `x = 5/4`

`b)`

`(-4)/y = 20/14` `( y` $\neq$ `0 )`

`=> 20y = -4 . 14`

`=> 20y = -56`

`=> y = (-56)/20`

`=> y = (-14)/5`

Vậy `y = (-14)/5`

`c)`

`4/7 = 12/x` `( x` $\neq$ `0 )`

`=> 4x = 12 . 7`

`=> 4x = 84`

`=> x = 84 : 4`

`=> x = 21`

Vậy `x = 21`

Bài `13`

`a)`

`(-5)/6` và `(-7)/8`

$*$ Trước hết ta so sánh `5/6` và `7/8`

Ta có : 

`+)` `5/6 = 1 - 1/6`

`+)` `7/8 = 1 - 1/8`

Ta thấy : `1/6 > 1/8` 

`=> 5/6 < 7/8`

`=> (-5)/6 > (-7)/8`

Vậy `(-5)/6 > (-7)/8`

`b)` 

`(-11)/24` và `(-21)/56`

Ta có :

`(-11)/24 = ( -11 . 7 )/( 24 . 7 ) = (-77)/ 168`

`(-21)/56 = ( -21 . 3 )/( 56 . 3 ) = ( -63)/168`

Do `(-77) < ( -63 )`

`=> (-77)/ 168 < ( -63)/168`

`=> (-11)/24` < `(-21)/56`

Vậy `(-11)/24` < `(-21)/56`

Bài 12:
$\dfrac{x}{6}$ $\text{=}$ $\dfrac{5}{24}$ $\text{=> x =}$ $\dfrac{5}{24}$ $\text{=}$ $\dfrac{5.6}{24}$ $\text{=}$ $\dfrac{30}{24}$ $\text{=}$ $\dfrac{5}{4}$ $\text{=}$ $\text{1,25}$
$\dfrac{-4}{y}$ $\text{=}$ $\dfrac{20}{14}$ $\text{=>}$ $\text{y =}$ $\dfrac{-4.14}{20}$ $\text{=}$ $\dfrac{-56}{20}$ $\text{=}$ $\dfrac{-14}{5}$ $\text{=}$ $\text{-2,8}$
$\dfrac{4}{7}$ $\text{=}$ $\dfrac{12}{x}$ $\text{=>}$ $\text{x =}$ $\dfrac{7.12}{4}$ $\text{=}$ $\text{-21}$