`y : 5/11 = 2/5 : 2`

`=> y : 5/11 = 2/5 xx1/2`

`=> y : 5/11 = 2/10`

`=> y : 5/11 =1/5`

`=> y= 1/5 xx 5/11`

`=> y= 5/55`

`=> y=1/11`

\(y:\dfrac{5}{11}=\dfrac{2}{5}:2\)
\(y:\dfrac{5}{11}=\dfrac{2}{5}\times\dfrac{1}{2}\)
\(y:\dfrac{5}{11}=\dfrac{1}{5}\)
\(y=\dfrac{1}{5}\times\dfrac{5}{11}\)
\(y=\dfrac{1}{11}\)

vậy 

`y xx1,7+y xx2,1=9,12`

`y xx(1,7+2,1)=9,12`

`y xx3,8=9,12`

`y=9,12:3,8`

`y=2,4`

\(y\times1,7+y\times2,1=9,12\)

\(y\times\left(1,7+2,1\right)=9,12\)

\(y\times3,8=9,12\)

\(y=9,12:3,8\)

\(y=2,4\)

Yx3,8=9,12

Y=2,4

y+5 = 2 => y= 2-5 = -3

7-y= 5 => y= 7-5 = 2

duyệt đi

\(\dfrac{1}{5}+y=7\)

       \(y=7-\dfrac{1}{5}\)

      \(y=\dfrac{35}{5}-\dfrac{1}{5}\)

      \(y=\dfrac{34}{5}\)

_________________________

\(y:\dfrac{3}{4}=\dfrac{5}{7}\)

\(y=\dfrac{5}{7}\times\dfrac{3}{4}\)

\(y=\dfrac{15}{28}\)

Chúc bạn học tốt

\(\dfrac{1}{5}+Y=7\\ Y=7-\dfrac{1}{5}\\ Y=\dfrac{35}{5}-\dfrac{1}{5}\\ Y=\dfrac{34}{5}\)

\(Y\div\dfrac{3}{4}=\dfrac{5}{7}\\ Y=\dfrac{5}{7}\times\dfrac{3}{4}\\ Y=\dfrac{15}{28}\)

\(19+68+\left(y-19\right)=105\)

        \(68+y=105\)

                \(y=105-68\)

                \(y=37\)

Chúc bạn học tốt

trí với dương

cái này là y mà

có phải x đâu

\(\left(y+1\right)+\left(y+3\right)+\left(y+5\right)+..+\left(y+19\right)\\ 18y\left(1+3+5+...+19\right)=250\)

Đến đây bn tính tổng rồi tìm x nhé.

\(...\Rightarrow\left\{\left[\left(y+19\right)-\left(y+1\right)\right]:2+1\right\}\left(x+1+x+19\right):2=250\)

\(\Rightarrow\left[\left(y+19-y-1\right):2+1\right]\left(2x+20\right):2=250\)

\(\Rightarrow10.2\left(x+10\right):2=250\)

\(\Rightarrow10.\left(x+10\right)=250\)

\(\Rightarrow x+10=250:10\)

\(\Rightarrow x+10=25\)

\(\Rightarrow x=25-10\)

\(\Rightarrow x=15\)

Số các số lẻ từ 1 đến 19 là :

   [19-1] : 2 +1 = 10 [số]

(Y+1)+(Y+3)+(Y+5)+.........+(Y+19)=250

=> 10 . Y +1+3+5+...+19 = 250

=> 10.Y +10 : 2 x [1+19] = 250

=> 10.Y +100 = 250 => 10.Y= 150 => Y= 15

(y+1)+(y+3)+.....+(y+19)=250

=>(y+y+....+y)+(1+3+....+19)=250 (có 10 số y)

=>10y+100=250

=>10y=150

=>y=15