7/x=y/27=-42/54=-7/9

7/x=-7/9 nên x=-9

y/27=-7/9=-21/27 nên y=-21

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`A)2/3=x/60`

`=>40/60=x/60`

`=>x=40`

`B)-1/2=y/18`

`=>-9/18=y/18`

`=>y=-9`

`C)3/x=y/35=-36/84`

Mà `-36/84=(-3 xx 12)/(7 xx 12)=-3/7`

`=>3/x=-3/7`

`=>x=-7`

`y/35=-3/7=-15/35`

`=>y=-15`

`D)7/x=y/27=-42/54`

Mà `-42/54=(-7 xx 6)/(9 xx 6)=-7/9`

`=>7/x=-7/9`

`=>x=-9`

`y/27=-7/9=-21/27`

`=>y=-21`

a) Ta có: \(\dfrac{2}{3}=\dfrac{x}{60}\)

\(\Leftrightarrow x=\dfrac{2\cdot60}{3}=40\)

Vậy: x=40

\(\frac{7}{x}=\frac{y}{27}=\frac{-42}{54}\)

+) => \(\frac{y\cdot2}{27\cdot2}=\frac{-42}{54}\)

=> \(y\cdot2=-42\)

\(\Rightarrow y=-21\)

+) \(\frac{7}{x}=\frac{-21}{27}\)

=> \(\frac{7\cdot\left(-3\right)}{x\cdot\left(-3\right)}=\frac{-21}{27}\)

=> \(x\cdot\left(-3\right)=27\)

=> \(x=-9\)

* Lớp 6 chưa học đến tỉ lệ thức nên đây là cách đơn giản nhất r *

\(\frac{y}{27}\)=\(\frac{-42}{54}\)=>\(\frac{2y}{54}\)=\(\frac{-42}{54}\)=>2y= -42                                                                                                                                                                                                                   y= \(\frac{-42}{2}\)= -21                                                                                                                           \(\frac{7}{x}\)=\(\frac{-21}{27}\)=> -21x=7*27=189                                                                                                                                                                                                     x=\(\frac{189}{-21}\)= -9                                                                                                                                                              Vậy x= -9, y= -21

\(\frac{7}{x}=\frac{y}{21}=\frac{-42}{54}\)

Ta có : \(\frac{-42}{54}=\frac{-42:6}{54:6}=\frac{-7}{9}\)

+) \(\frac{7}{x}=\frac{-7}{9}\)=> 7.9 = (-7).x => (-7).x = 63 => x = -9

+) \(\frac{y}{21}=\frac{-7}{9}\)=> y.9 = 21.(-7) => y.9 = -147 => y = -49/3

Vậy x = 9,y = -49/3

x=-9, y=-49/3

Ta có 

7/x = -42/54 => x= 7.54/-42 =>x=-9 (tmđk)

y/21 = -42/54 => y= -42.21/54 => y= -49/3 

\(\dfrac{4}{x}=\dfrac{y}{21}=\dfrac{28}{49}=\dfrac{28:7}{49:7}=\dfrac{4}{9}\\ Vậy:x=\dfrac{4.9}{4}=9\\ y=\dfrac{4.21}{9}=\dfrac{28}{3}\)

\(\dfrac{x}{2}=\dfrac{3}{y}\\ \Leftrightarrow x.y=2.3=6\\ Vậy:\left[{}\begin{matrix}\left(x;y\right)=\left(1;6\right)=\left(6;1\right)\\\left(x;y\right)=\left(2;3\right)=\left(3;2\right)\end{matrix}\right.\)

\(3,\\ \dfrac{42}{54}=\dfrac{42:6}{54:6}=\dfrac{7}{9}\\ \dfrac{42}{54}=\dfrac{7}{x}=\dfrac{7}{9}\\ Vậy:x=\dfrac{7.9}{7}=9\)

Ta có:

\(\frac{y}{27}=\frac{-42}{54}\)

\(\Rightarrow y=\frac{-42.27}{54}=-21\)

Lại có:

\(\frac{7}{x}=\frac{-21}{27}\)

\(\Rightarrow x=\frac{7.27}{-21}=-9\)

Vậy .....

Tìm x:

 \(\frac{7}{x}\)= \(\frac{-42}{54}\)

\(\Rightarrow\)\(\frac{7.\left(-6\right)}{x.\left(-6\right)}\) = \(\frac{-42}{54}\)

\(\Rightarrow\)\(x.\left(-6\right)=54\)

\(\Rightarrow x=54\div\left(-6\right)\)

\(\Rightarrow x=-9\)

Tìm y:

\(\frac{y}{27}=\frac{-42}{54}\)

\(\Rightarrow\frac{y.2}{27.2}=\frac{-42}{54}\)

\(\Rightarrow y.2=-42\)

\(\Rightarrow y=\left(-42\right)\div2\)

\(\Rightarrow y=-21\)

\(Họctốtnhé\)

\(\frac{7}{x}=\frac{y}{27}=\frac{-42}{54}\)

Ta có:

\(\frac{7}{x}=\frac{-42}{54}\)

\(\Rightarrow\frac{7}{x}=\frac{-7}{9}\)

\(\Rightarrow-7.x=7.9\)

\(\Rightarrow-7.x=63\)

\(\Rightarrow x=63:\left(-7\right)=-9\)

\(\frac{y}{27}=\frac{-42}{54}\)

\(\Rightarrow\frac{y}{27}=\frac{-7}{9}\)

\(\Rightarrow9.y=-7.27\)

\(\Rightarrow9.y=-189\)

\(\Rightarrow y=-189:9=-21\)

Vậy x = -9; y = -21

a) (x+5)+(x+10)+.........+(x+60)=450 

   12x +(5+10+.........+60)=450

  12x+390=450

   12x=60

    x=5

b) Gọi n là thương của phép chia a cho 54;              =>54n+38=252+r                  =>r-2 chia hết cho 54

r là dư của phép chia a cho 18 (n,r thuộc N;r<14)    =>54n =214+r                      =>r-2=0

=>a=54n + 38                                                       =>n=(214+r):54                     =>r =2

   a=18x14+r                                                          =>214+r chia hết cho 54       =>a=18x14+2=254                                    

=>54n+38=18x14+r                                               =>216+r-2 chia hết cho 54