có x/y=3/5suyra x/3=y/5

đặt x,y=k suy ra x=3k;y=5k

mà xy=1215suyra5k.3k=1215

15k mũ 2=1215

k mũ 2=81

k=9 hoặc -9

vsk=9 suy ra x=27;y=45

vậy x=27;y=45

Từ \(\frac{x}{y}=\frac{3}{5}\)\(\Rightarrow\frac{x}{3}=\frac{y}{5}\)

Đặt \(\frac{x}{3}=\frac{y}{5}=k\left(k\ne0\right)\)

\(\Rightarrow x=3k\); \(y=5k\)

mà \(xy=1215\)

\(\Rightarrow3k.5k=15k^2=1215\)

\(\Rightarrow k^2=81\)\(\Rightarrow k=\pm9\)

+) TH1: Nếu \(k=-9\)

\(\Rightarrow x=\left(-9\right).3=-27\); \(y=\left(-9\right).5=-45\)

+) TH2: Nếu \(k=9\)

\(\Rightarrow x=9.3=27\); \(y=9.5=45\)

Vậy các cặp giá trị \(\left(x;y\right)\)thỏa mãn đề bài là: \(\left(-27;-45\right)\), \(\left(27;45\right)\)

1215:x:5=3

1215:x=3x5

1215:x=15

x=1215:15

x=81

1215:x:5=3

1215:5:x=3

243:x=3

x=243:3

x=81

a, Áp dụng dãy tỉ số bằng nhau, ta có:
\(\frac{x}{y}=\frac{5}{7}\Rightarrow\)\(\frac{x}{5}=\frac{y}{7}=\frac{x+y}{5+7}=\frac{4,05}{12}=\frac{27}{80}\)

\(\Rightarrow\)\(x=\frac{5.27}{80}=\frac{27}{16}; y=\frac{7.27}{80}=\frac{189}{80}\)

b, Có \(\frac{x}{3}=\frac{y}{5}\)\(\Rightarrow\)\(\frac{xy}{3.5}=\frac{y^2}{25}\)\(\Rightarrow\)\(\frac{1215}{15}=\frac{y^2}{25}\)

\(\Rightarrow\)\(y^2=2025\Rightarrow y=\pm45\)

y=45 => x 27

y=-45 => x=-27

6/13:12=1/26

15:5/22=66

1/3:2/5:3/4=5/6:3/4=10/9

6/13 : 12 = \(\dfrac{6}{13}x\dfrac{1}{12}=\dfrac{1}{26}\)

15 : 5/22= \(15x\dfrac{22}{5}=66\)

1/3 : 2/5 : 3/4=\(\dfrac{1}{3}x\dfrac{5}{2}x\dfrac{4}{3}=\dfrac{10}{9}\)

\(\left(Xx216\right):8=1215\)

\(X=\left(1215x8\right):216\)

\(X=9720:216\)

 \(X=45\)

(x*216):8=1215

x*216    =1215*8=9720

x          =9720:216

x          =45

\(\frac{x}{y}=\frac{5}{7}=\frac{x}{7}=\frac{y}{5}\) và x + y = 4,08

Áp dụng tính chất dãy tỉ số bằng nhau,ta có: 

   \(\frac{x}{7}=\frac{y}{5}=\frac{x+y}{7+5}=\frac{4,08}{12}=\frac{17}{50}\)

\(\frac{x}{7}=\frac{17}{50}\Rightarrow x=\frac{17.7}{50}=\frac{119}{50}\)

\(\frac{y}{5}=\frac{17}{50}\Rightarrow y=\frac{17.5}{50}=\frac{17}{10}\)

Vậy..

Còn 2 cách kia là j??? 

a, \(\frac{x}{y}=\frac{5}{7}\)và x+y=4,08

Ta có: 4,08=\(\frac{102}{25}\)

 \(\frac{x}{y}=\frac{5}{7}\Rightarrow7x=5y\)

\(\Rightarrow\frac{x}{5}=\frac{y}{7}\)và x+y=\(\frac{102}{25}\)

theo t/c dãy tỉ số bằng nhau ta có:

\(\frac{x}{5}=\frac{y}{7}=\frac{x+y}{5+7}=\frac{\frac{102}{25}}{12}=\frac{17}{50}\)

\(\Rightarrow\frac{x}{5}=\frac{17}{50}\Rightarrow x=\frac{17}{10}\)

\(\frac{y}{7}=\frac{17}{50}\Rightarrow y=\frac{119}{50}\)

vậy x=

      y=

4(x-12)-2=50

4(x-12)=50+2

4(x-12)=52

x-12=52÷4

x-12=13

x=25

4(x-12)-2=50

4(x-12)=50+2

4(x-12)=52

x-12=52÷4

x-12=13

x=25