Đáp án:

Áp dụng $ab=cd\Rightarrow\left\{\begin{array}{l}\dfrac ad=\dfrac cb\\\dfrac da=\dfrac bc\\\dfrac ac=\dfrac db\\\dfrac ca=\dfrac bd\end{array}\right.$

a) $-10.12=-3.40\Rightarrow\dfrac{-10}{40}=\dfrac{-3}{12}$.

$-10.12=-3.40\Rightarrow\dfrac{40}{-10}=\dfrac{12}{-3}$

$-10.12=-3.40\Rightarrow\dfrac{-3}{-10}=\dfrac{12}{40}$

$-10.12=-3.40\Rightarrow\dfrac{-10}{-3}=\dfrac{40}{12}$.

b) $2AB=7MN\Rightarrow\dfrac{AB}{MN}=\dfrac72$

$2AB=7MN\Rightarrow\dfrac{MN}{AB}=\dfrac27$

$2AB=7MN\Rightarrow\dfrac{MN}2=\dfrac{AB}7$

$2AB=7MN\Rightarrow\dfrac2{MN}=\dfrac7{AB}$

c) $1.125=5.25\Rightarrow\dfrac15=\dfrac{25}{125}$

$1.125=5.25\Rightarrow5=\dfrac{125}{25}$

$1.125=5.25\Rightarrow\dfrac1{25}=\dfrac5{125}$

$1.125=5.25\Rightarrow25=\dfrac{125}5$.

d) $AB.CD=2.3\Rightarrow\dfrac{AB}3=\dfrac 2{CD}$

$AB.CD=2.3\Rightarrow\dfrac3{AB}=\dfrac{CD}2$

$AB.CD=2.3\Rightarrow\dfrac3{CD}=\dfrac{AB}2$

$AB.CD=2.3\Rightarrow\dfrac{CD}3=\dfrac2{AB}$.

`a)` `-10.12=-3.40`

Tỉ lệ thức : `12/(-3)=40/(-10)`

`b)` `2.AB=7.MN`

Tỉ lệ thức : `(AB)/7=(MN)/2`

`c)` `1.125=5.25`

Tỉ lệ thức : `125/5=25/1`

`d)` `AB.CD=2.3`

Tỉ lệ thức : `(CD)/2=3/(AB)`